Term
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Definition
| A phytoalexin found in sorghum (A monocot). A flavonoid from the side branch of the phenylpropanoid pathway. Its precursor is colourless, but it has a red colour. It is visible wherever it forms, and its concentration can be measured with photospectrometry. Not necessarily an important phytoalexin, but it is easy to study because of this property. Accumulates at the site of fungal penetration. The precursor accumulates in vesicles near the appressorium. The vesicles merge with each other and then turn red as they convert precursor into 3-deoxyanthocyanidin, before bursting open. The phytoalexin is released into the cytoplasm near the appressorium and its toxicity kills the fungus as well as the plant cell. Levels are much higher than that needed to kill the fungus. Neighboring cells are protected from infection. An efficient cell-by-cell response. |
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Term
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Definition
| Produced by chalcone synthase in the phenylpropanoid pathway. Can be turned into glyceollin in a side branch of the phenylpropanoid pathway. |
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Term
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Definition
| Encoded by a PR gene. Directly antimicrobial. Primed during ISR. |
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Term
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Definition
| A component of the cell wall of fungi. A PAMP. |
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Term
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Definition
| PR gene products with low pH. Secreted into the apoplast. Related to SAR. |
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Term
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Definition
| A commercial product containing benzothiadiazole. Produced by Syngenta. Used on pome fruits and other crops. |
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Term
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Definition
| The immune systems found only in jawed animals. Antibodies and T-cells recognize the pathogen and fight it. Not found in plants. You can insert transgenes into plants to make animal antibodies, but they won't work in the plant to fight pathogens. |
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Term
| Agrobacterium tumefaciens |
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Definition
| Some strains have genes for breaking down tartaric acid, which other strains lack. This determines the host range. |
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Term
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Definition
| Bacteria can quickly evolve resistance to antibiotics. Resistance can become fixed in the population due to horizontal transfer. Used to be sprayed indiscriminately. Effective until resistance evolves. |
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Term
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Definition
| A penetration mechanism found only in fungal pathogens. A germ tube emerges from the spore (first hyphae), grows a short distance, and forms an appressorium on the plant surface. A large, flattened cell with polysaccharides underneath that stick it to the plant surface. The germ tube empties its cytoplasmic contents into the appressorium and then dies. At the bottom centre, a penetration peg forms. |
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Term
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Definition
| An element in the promoter of PR genes. Binds to NPR1 monomers. |
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Term
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Definition
| The pathogen must stick to the surface of the plant in order to infect it. Mucilage is critical for this step. |
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Term
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Definition
| A saponin produced by oats. It is absorbed into the plasma membrane of fungal pathogens, and together the molecules form large pores. The fungus cytoplasm spills out, killing the pathogen. Some fungi can detoxify it with secreted enzymes. A large complex molecule with attached sugars. Oat plants have a layer of avenacin-producing cells in the root to stop fungal infections. |
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Term
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Definition
| Genes of the pathogen which are responsible for the differences between races. Some have inserted transposons, deleted sections, or mutations. Protein products are highly variable. Includes the effectors Avr2 and Avr9. The name "avirulence" is misleading; they encode virulence factors. |
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Term
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Definition
Non-pathogenic
There is no virulence or pathogenicity. |
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Term
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Definition
| A fungal metalloprotease enzyme. Has a metal atom. Digests plant proteins including PTI genes. Has direct binding with R receptors. |
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Term
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Definition
| A 50 AA effector produced by Cladosporium fulvum. A modified gene, found only in some isolates. |
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Term
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Definition
| A 28 AA effector produced by Cladosporium fulvum. Has a cysteine knot with disulphide bonds. |
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Term
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Definition
| An effector produced by Xanthomonas campestris pv. vesicatoria. Has a highly conserved 34 AA motif repeat. Occurs in different forms. Structure of the repeats is variable; most commonly repeated 17.5 times. Other strains have different numbers of repeats. If the repetitive region is knocked out, it affects virulence. When secreted into the cell, it targets the nucleus and binds directly to promoter elements, causing transcriptional activation, and preventing signal transduction. Causes the plant to become susceptible to the disease. |
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Term
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Definition
| A bacterial kinase. An inhibitor of FLS2. Binds to the kinase domain of FLS2, shutting down the resistance response. In some plants it may bind to a Prf/Pto complex, and resistance response is triggered. Binds directly to Pto in the cytoplasm, resulting in conformational change, and resistance. |
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Term
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Definition
| A bacterial cysteine protease. Cleaves RIN4 proteins in the host for digestion. Recognized by Rps2, the guard of RIN4. |
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Term
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Definition
| A large group of plant pathogens. Pathogenic bacteria do not belong to any unique clade. Bacterial cells are much smaller than plant cells, and many can fit inside one plant cell. Digests food outside of their body by releasing enzymes, and absorbing nutrients. |
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Term
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Definition
| PR gene products with high pH. Secreted into the vacuole. Related to ISR. |
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Term
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Definition
| A class of chemicals. Commercial products. An analog of salicylic acid, with less phytotoxicity. Inhibits NADH:ubiquinone oxidoreductase, an enzyme of the electron transport chain. Causes increases in ROS. Includes Actigard. |
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Term
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Definition
| A relationship between host and pathogen. The pathogen obtains nutrients from living host cells. The host may remain alive while infected, but it can result in cell death. Either the plant fails to recognize the pathogen, and/or the pathogen suppresses its recognition or resistance response. Analogous to a con man or pickpocket, who robs you without you knowing. Grows intercellularly, in the apoplast. Biotrophic fungi include rusts, which may not kill the plant. |
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Term
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Definition
| An R gene present in some plants, which can bind to avrBs3. It is not the target of avrBs3, and acts like a decoy. Produces a monooxygenase when it binds to avrBs3. Activates ETI, but doesn't increase resistance. Triggers a signal transduction. |
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Term
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Definition
| A part of signal transduction. The first metabolic response to a pathogen. Membrane pore proteins open, allowing Ca2+ from the apoplast to flow into the cytoplasm. This activates many proteins, including NADPH oxidase. |
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Term
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Definition
β-1-3 glucan
Found in the papilla. Strengthens the cell wall. |
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Term
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Definition
Closed coil NB-LRR
Leucine zipper (LZ) NB-LRR
A possible third domain for NB-LRR immune receptors. Orients the protein strands for dimerization or protein interaction. Good for forming dimers; can bind to other proteins and DNA. A cytoplasmic protein. |
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Term
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Definition
| An enzyme in the phenylpropanoid pathway. Produces the intermediate of glyceollin: 4-coumaryl-CoA. |
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Term
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Definition
| A component of the cell wall of fungi. A PAMP. |
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Term
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Definition
| Enzymes that break down chitin. Primed during ISR. |
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Term
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Definition
| A biotrophic fungal pathogen with hyphae that grow in the apoplast of tomato. Produces several effectors, including Avr2 and Avr9. One race secretes avr9 peptide, 28 AA, into the apoplast. This reacts with the cf9 gene in tomato. This race is the only race incompatible with the cf9 gene. No homologous DNA is found in other races; the gene is not recessive. |
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Term
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Definition
| A pathogen which attacks the bulbs of white and yellow onions. Cannot attack red or purple onions due to the antimicrobial activity of phenol pigments. Uses an appressorium to penetrate its host. |
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Term
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Definition
| A precursor of lignin formed in the phenylpropanoid pathway. |
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Term
| Constitutive antimicrobial compounds |
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Definition
Phytoanticipins
Pre-formed defences
A disease defence mechanism which is always present in the plant. Constitutive, and active even when the microorganism is absent. Important in primitive plants such as mosses and ferns. Chemicals are not anti-microbial before activation. Includes phenols and lactones. Precursors are recognizable as an anti-microbial before they are activated. Nothing changes in the plant in response to the pathogen. |
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Term
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Definition
| A wax found in the cuticle. Made from ester bonds. Broken down by cutinases. Not a good nutrient source for the pathogen; it digests the cuticle in order to reach the more nutritious tissue underneath. |
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Term
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Definition
| An esterase enzyme secreted by fungal pathogens. Breaks down cutin in the cuticle of plants. Some are necessary for penetration to occur. |
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Term
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Definition
| A poisonous mushroom used in some religious ceremonies. When a tiny dose is consumed the person has vivid dreams. Red and white in colour; may be the origin of Santa Claus? |
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Term
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Definition
| A hypothesis for how effectors are recognized by the plant. A further development of the guard hypothesis, where the receptor is very similar to the target of the effector. The effector binds to the R receptor instead of the target, and triggers PTI. Includes AvrPto. |
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Term
| Defense response (DR) genes |
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Definition
Defence related genes
Disease responsible genes
Genes which encode disease defence mechanisms that fight off microorganisms; the resistance response. Encode proteins and enzymes that produce antimicrobial compounds and structures. |
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Term
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Definition
Receptor-ligand hypothesis
A hypothesis for how effectors are recognized by the plant. The same interaction as what happens in PTI. Effectors have direct interaction with R proteins, resulting in resistance. The effector is released by the pathogen, and binds to the receptor R protein which triggers signal transduction in the nucleus. Includes Avr-Pita. |
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Term
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Definition
Includes epidemiology and pathogenesis.
Reproduction (begins epidemiology) -> Release -> Dissemination -> Arrival -> Germination -> Penetration (begins pathogenesis) -> Recognition -> Infection -> Invasion -> Reproduction |
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Term
| Disease defence mechanisms |
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Definition
| Plants may release chemicals to degrade the chemicals of pathogens. Mechanisms which fight the attacking organism. Encoded by DR genes. Plants would die quickly from disease without these defences. Include pre-formed and triggered mechanisms. Different plant species have a huge range of different defence mechanisms and chemistries. |
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Term
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Definition
| The sixth step in the infection process. The pathogen releases spores which may infect other plants. |
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Term
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Definition
| A disease of turf grass. In a disease trial, the organism was inoculated on grass. An ISR agent gave resistance, but it was not as good as the fungicide. |
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Term
| Effector triggered immunity (ETI) |
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Definition
Race-specific resistance
A type of resistance. A triggered response set off by pathogen effectors. Some strains of the pathogen can infect, but others cannot. HR is common, but not in all cases. Related to gene-for-gene concept. Similar to PTI, but stronger and faster. Begins after PTI has begun. Occurs in the cytoplasm, and sometimes the nucleus. Intracellular NB-LRR immune receptors recognize the effector in the cytoplasm. Not a conserved process; highly variable. Theories for how receptors recognize effectors include direct binding, guard hypothesis, and decoy hypothesis. |
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Term
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Definition
Avirulence factors
Elicitors
Molecules produced by pathogens that manipulate their host to facilitate infection, but can also trigger ETI. Can have a positive or negative effect on disease. Intended to damage the plant cell, but may act like a PAMP, triggering resistance response. There may be several versions. Structure is important for resistance. May be secreted by the pathogen directly into the host cell. May be detected by R proteins, resulting in resistance via MAP cascade. If not, it can reduce or eliminate the resistance process, leaving the plant susceptible. Can only be produced by a living cell. |
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Term
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Definition
| Lipid transfer proteins. Small lipid-binding proteins secreted by oomycetes. A PAMP. |
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Term
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Definition
| The fifth step in the infection process. The pathogen has enough mass and energy to emerge through the host epidermis. |
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Term
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Definition
| The portion of the disease cycle which occurs outside of the host. Includes dissemination, arrival, and germination. |
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Term
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Definition
| A lipid found in the membranes of fungi, but not plant cells. A PAMP. |
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Term
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Definition
| A pathogen of wheat, rye, and wild grasses. A fungus which produces a chemical which may be processed into LSD. The natural product is poisonous, causing burning in the hands and feet, and hallucinations. The Salem witch trials were caused by ergot-infected bread; whole villages were affected. |
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Term
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Definition
| A gaseous plant hormone. Has a lot of functional overlap with jasmonic acid, activating many of the same genes. A wound-related hormone. Protects from necrotrophic pathogens; appears when cells are damaged. Activates callose deposition for papillae. Activates ISR. Phloem-mobile. Does not accumulate during ISR, but cells' sensitivity to it is increased. |
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Term
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Definition
| A PAMP of gram-negative bacteria. Part of the flagellum. Includes FLG2. Proteins form a stacked ring structure. |
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Term
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Definition
| A common component of flagellin, the protein polymer which forms the flagella of gram-negative bacteria. Has highly conserved regions and variabe regions. Proteins form a ring structure. Has a highly conserved 22 AA region near the N-terminus (FLG22) which is exposed, and can be recognized by the plant as a PAMP. One of the easiest PAMPs to study. Once bound to its receptor-like kinase, FLS2, endocytosis brings FLG2 into the cell via an endosome. The outside of the endosome is lined with activated FLS2, triggering a MAP cascade. Eventually the FLG2 is deposited into the vacuole, or digested in a lysosome. |
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Term
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Definition
| A plant PRR which recognizes FLG2 in the apoplast. An RLK. Membrane-bound. When FLG2 binds to it, endocytosis brings FLS2/FLG2 into the cell; the endosome must contain BAK1 in order for this to occur. Sends signals to activate PTI. Activates two MAP cascades. |
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Term
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Definition
| Molecules with unpaired electrons. Unstable and reactive. Peroxidase turns lignin precursor alcohols into free radicals which form lignin polymers. Can come into contact with the pathogen, and polymerize with it, killing it. |
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Term
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Definition
| The largest group of plant pathogens. Almost half of all plant diseases are fungal. Attack plants more than they attack animals. Fungal hyphae are larger than other microorganisms, but can still fit inside a plant cell, and between plant cells. Digest food outside of their body by releasing enzymes, and absorbing nutrients. Many have haustoria to feed on living plant cells. Have unique metabolisms. May be necrotrophic, biotrophic, or hemibiotrophic. |
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Term
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Definition
| A necrotrophic fungal pathogen that attacks cereal crops including wheat, oats, and barley. Broken down by the pre-formed defence avenacin, which breaks down the sterols in the pathogen's membrane. Races that attack oats produce avenacinase, which breaks down avenacin. |
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Term
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Definition
| An older term from before molecular mechanisms for plant breeding were understood, in the 1940s. A single pathogen avirulence (AVR) gene interacts with a single host resistance (R) gene for resistance to occur. AVR genes are dominant in diploid pathogens but bacteria and ascomycetes are haploid. AVR genes are not allelic to virulence genes, but involved in recognition. Results in complete resistance due to recognition of AVR product of the pathogen. Resistance creates selection pressure on the pathogen for mutated AVR genes that avoid recognition, and thus cause disease. Change can occur easily; only one or a few genes need to be mutated to produce a new race. |
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Term
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Definition
| A long-distance calcium-based plant defence signal. GCaMP3 is a genetically encoded fluorescent calcium stain, enabling the signal to be visualized. L-glutamate causes its expression throughout the plant within only minutes. |
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Term
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Definition
| A phytoalexin found in soybeans (a legume). A flavonoid from a side branch of the phenylpropanoid pathway. Produced by chalcone synthase from 4-coumaryl-CoA through a series of steps. There are three versions with minor chemical differences: glyceollin 1 - 3. |
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Term
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Definition
| A hypothesis for how effectors are recognized by the plant. An immune receptor guards the target of a pathogen effector, usually involved in PTI. R proteins guard the target of the effector. The R protein forms a complex with the target of the effector, and binding of the effector alters conformation, triggering the resistance response. Degradation, modification, or conformational changes in structure can be sensed. Includes AvrRpt2. |
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Term
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Definition
| A bacterial molecule secreted by TTSS into the apoplast. Forms pores in the host plasma membrane, making it "leaky"; nutrients flow into the apoplast to feed the bacteria. A PAMP. |
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Term
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Definition
Temporary biotrophy
A relationship between host and pathogen. The pathogen feeds as a biotroph, on living host cells, and then switches to necrotrophy at some point, and obtains nutrients from dead cells. The pathogen builds up biomass in the apoplast (intercellularly) during its biotrophic stage, and then has rapid growth into the cytoplasm (intracellularly), and host death during the necrotrophic stage. Breaks down host cells with enzymes before digesting. Used to be considered a minor category of pathogen, but now recognized in many fungi and bacteria. Includes Anthracnose, which used to be considered a necrotroph. |
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Term
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Definition
| A type of non-host resistance. The plant recognizes several effectors of the pathogen, and triggers ETI and HR. Cells undergo HR during attempted penetration, preventing infection. The plant is kept safe, with visible HR spots. May be related to race-specific resistance. Occurs on non-host plants. |
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Term
| Hypersensitive response (HR) |
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Definition
Programmed cell death
Similar to apoptosis in animal cells. A genetically programmed response to infection where affected cells are killed quickly, resulting in disease resistance. The first stage is membrane depolarization and electrolyte leakage. DNA fragments, vesicles proliferate, the nucleus and cytoplasm condenses, and phytoalexins and/or ROS are released, killing the cell. Phytoalexins may be secreted by neighboring cells. Triggers phytoalexins, and may be triggered by phytoalexins. Certain chemicals merge in the vacuole, triggering cell death. The cell dies within 24 hours, killing the pathogen: an "over-reaction". The faster the cell dies, the more likely to succeed. During human fetal development, the cells in the webbing between fingers die to separate the fingers: sometimes cell death is necessary. In plants it protects against attack from pathogens. Related to one type of recognition. Often the key defence from a pathogen. Stops biotrophic pathogens, and sometimes necrotrophic pathogens. Quick cell death stops the spread of the pathogen. Appears as small necrotic spots on the plant. Used by plant breeders to form high levels of resistance. Has some disadvantages. |
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Term
| Hypersensitive response and pathogenicity gene cluster (HRP) |
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Definition
| Needed by gram-negative bacteria to elicit resistance or contribute to virulence. Effectors that lack a signal peptide, and are not secreted. HRP encodes a Type III Secretion System (TTSS) in almost all gram-negative pathogenic bacteria. TTSS gene products enter the host cell by bacterial pilus, and effectors elicit HR in the plant cell. TTSS were once virulence factors, but the host evolved to recognize it and trigger resistance. |
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Term
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Definition
Latent phase
The fourth step in the infection process. There are no visible symptoms, but the disease is present. |
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Term
| Induced antimicrobial compounds |
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Definition
Triggered defences
A disease defence mechanism which are not found in uninfected plants. When the pathogen is recognized, precursors are transformed into the active defence compound. An advantage for higher plants because resources aren't wasted when the pathogen is absent, and there is more energy for growth and reproduction. The amount of defence produced can be fine-tuned to the level of infection. Produced from remote precursors, such as simple sugars or amino acids derived primarily from the phenylpropanoid pathway. The plant must be precise in when and where defence occurs. Requires recognition of the pathogen. A false positive (such as dust or avirulent spores) wastes energy, but a false negative leaves the plant susceptible to disease. Requires energy and resources. Plants have evolved to be very efficient, and there is great diversity in response to pathogens. Broadly generalized to higher plants. |
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Term
| Induced disease resistance |
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Definition
| Pre-triggered defences present before the pathogen is present. A form of plant innate immunity. Involves defence compounds. Includes SAR and ISR. Interactions between SAR and ISR may be antagonistic or complementary, depending on the rates of inoculant, plant physiology, and pathogens involved. High rates of inoculant cause antagonism. Highly dependent on the physiological state of the plant. If there is any stress, it may have a negative effect. Does not always work the same way, because of varying conditions. Different cultivars may react differently, some not reacting at all. |
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Term
| Induced systemic resistance (ISR) |
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Definition
| A form of induced disease resistance. Pre-triggered JA- or ethylene-dependent resistance. A systemic response triggered by JA and ethylene. An ancestral trait for many plants, evolved a very long time ago. Effective against bacteria and fungi, but not viruses; necrotrophs and hemibiotrophs (some exceptions). Effective against pathogens that can be resisted through JA- or ethylene-dependent defences. Requires the host to be alive. Activation generally takes longer than SAR. Activated by PGPRs and PGPFs. First discovered with bacteria on plants. Isolated compounds can be made into treatments that have the same effect as the organisms: bacterial PAMPs, lipopolysaccharides, antibiotics, and volatile compounds. ISR-eliciting determinants are redundant; one PGPF/R may produce many at the same time, so that if one fails, another will work. Genes are primed, but not up-regulated. The cell prepares to express certain genes, including chitnases and β-1-3 glucanase, so that when needed they can be produced rapidly. May be an epigenetic change. Precursors of products may accumulate. A less "expensive" form of induced disease resistance than SAR. Arabidopsis primes 80 genes during ISR. Genes primed differ from those activated during SAR: different, mostly unknown functions. Non-phosphorylated mitogen-activated kinases accumulate, and when attack is sensed they are phosphorylated and activated. |
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Term
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Definition
| A stage in the disease cycle. The disease establishes and obtains nutrients from the host. |
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Term
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Definition
| Immunity conserved to all eukaryotes. Includes receptors and antimicrobial gene products. The only immunity plants can have to pathogens. |
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Term
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Definition
| The first step in the infection process. The spore comes into contact with the host. |
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Term
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Definition
| A route which pathogens can take once inside a plant. Between the plant cells, in the apoplast. The plant cells may remain alive. |
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Term
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Definition
| A route which pathogens can take once inside a plant. Through the plant cells. Results in plant cell death. |
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Term
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Definition
| A stage in the disease cycle. The disease spreads from its initial infection site, and grows in the plant. A prolonged interaction. Produces disease symptoms. Biotrophs do not kill at this stage. The pathogen secretes enzymes that digest the plant cells, and take up nutrients through their mucilage. |
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Term
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Definition
| A plant hormone which activates ISR. Phloem-mobile. Does not accumulate during ISR, but cells' sensitivity to it is increased. |
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Term
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Definition
| The cytoplasmic component of an RLK. Triggers PTI activity within the cell. Moves phosphate groups in order to activate other enzymes. |
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Term
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Definition
| A type of pre-formed disease defence mechanism. Contain a five-sided ring with four carbons and one oxygen. Highly antimicrobial and toxic to the plant cell. The plant's detoxification mechanisms keep the chemicals safe until the pathogen attacks. Some fungi have enzymes that can break down lactones. Includes tuliposide. |
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Term
| Leucine rich repeat (LRR) |
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Definition
| The leucine rich repeat for protein-protein interaction in an NB-LRR immune receptor. Creates unique pocket structures into which the effector fits. Gives specificity to the receptor. If mutated, it changes the shape of the binding site, and may no longer recognize the same effectors. |
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Term
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Definition
| A product of the phenylpropanoid pathway. Related to disease. A polymer of phenols. A normal constituent of plants, providing structural support and waterproofing in wood and in the walls of the xylem. Allowed plants to move onto the land. Wood is 25% lignin. Ubiquitous to all land plants, and a common defence mechanism. Normally restricted to the cell wall and middle lamella, packaged an exported to the cytoplasm. A common defence response that can be a pre-formed or triggered defence. Found in cucumber, potato, wheat, rice, and other plants. Has three precursors from the phenylpropanoid pathway: p-couaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The ratio of the alcohols differs with species. Vesicles of these alcohols builds up near the infection site, and are exported to the cell wall. Peroxidase in the cell wall converts them into free radicals non-enzymatically. These pair with each other to form lignin polymers. Lignin polymers form randomly, producing irregular structures that fill the empty spaces between cell wall cellulose and sugars. If the plant cell is broken or if a pathogen is detected, the vesicles of alcohol precursors are released and the cell fills up with lignin. Causes rapid cell death and cytoplasmic peroxidases. Acts as a mechanical barrier, strengthening the cell wall, resisting pathogen penetration. Hydrophobic, keeping pathogen enzymes and toxins out, and keeping nutrients in. Stops the pathogen from spreading to the rest of the pant, if defence is triggered efficiently. Pathogens may suppress the lignification, or kill cells too quickly for it to occur. |
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Term
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Definition
| Found in the membranes of gram-negative bacteria, but not plants. A PAMP. |
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Term
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Definition
| Produce psilocybin. Just a small dose can produce a positive effect, but too much can be very bad. |
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Term
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Definition
Microbe associated molecular pattern
A type of PAMP. |
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Term
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Definition
Mitogen-activated phosphorylation cascade
A signal transduction induced by FLG22. |
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Term
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Definition
| Possible defences against pathogens include: mechanical barriers, chemically modified cell walls resistant to degrading enzymes, increased resistance to toxins, increased resistance to loss of nutrients, lignification of the pathogen, and release of toxins, precursors and/or free radicals. |
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Term
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Definition
Phytoplasmas
A group of pathogens. Specialized gram-positive bacteria that have specialized attacks on plant tissue. Small in comparison to a plant cell. |
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Term
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Definition
| A mucous-like secretion produced by the pathogen to aid in sticking to the host's surface, and retention of water. Generally contains a disaccharide or exopolysaccharide. Nutrients are absorbed through the mucilage. |
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Term
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Definition
| A membrane-bound protein. Activated by the Ca2+ burst. Releases ROS into the apoplast. |
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Term
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Definition
| An R gene. Includes an NB and an LRR region, at least. There may be a third domain, of which there are two types: TIR and CC. Found in the cytoplasm, nucleus, chloroplast, and mitochondria. Has tandem copies; rare to have a single copy. There are 500 copies in rice, and 146 in Arabidopsis. The repeats were duplicated next to their original forms and continued to increase a series of tandem copies, which may cross-over and intermix. Can have multiple specificities. The diverse and repetitive nature of the gene allows it to evolve faster than the rest of the genome. It is able to change considerably during meiosis when crossing-over occurs between parental chromosomes; there may be double-crossovers, deletions, or duplications. |
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Term
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Definition
| A relationship between host and pathogen. The pathogen kills host tissue, and then obtains nutrients from the dead cells. The plant recognizes the pathogen, but fails to express resistance, or dies before it is able to. Analogous to a thug or mugger who kills you before robbing you. Grow inter- or intracellularly. Break down the plant cell wall with enzymes. Destroys plant cells with enzymes and toxins before digesting the nutrients. Grows through and around dead cells. Includes bacteria and fungi. Necrotrophic fungi cause tissue damage, and don't produce effectors (just kill the cell). Must deal with pre-formed and triggered defences. May use phytotoxins or enzymes to break down the plant cell wall. |
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Term
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Definition
Roundworms
A group of pathogens. Not microorganisms. Huge in comparison to plant cells; it can only fit its head into the cell. |
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Term
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Definition
| When the pathogen is unable to infect the host because the host species is not in its host range. The pathogen of one plant species is unable to infect other species of plants. The pathogen has limited options when it lands on the plant, and usually dies. Some pathogens have a host range of just one plant species. There may be several mechanisms. Includes non-HR type and HR type. |
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Term
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Definition
| A type of non-host resistance. Pre-formed defences are able to overcome the pathogen, without HR occurring. There is absence of virulence factors suppressing recognition and countering numerous early-induced defences. Neither ETI nor PTI are triggered. Includes tartaric acid catabolism in A. tumefaciens strains that attack grape. Mutations limit the host range. You cannot breed for non-host resistance. |
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Term
| Normal plant constituents |
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Definition
| A product of the phenylpropanoid pathway. Phenols with functions not related to disease. Lines the xylem, provides anti-insect chemicals, pigments, structural matrices, allelopathic compounds, etc. |
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Term
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Definition
| A cytoplasmic protein oligomer. Converted into a monomer due to salicylic acid. Changes the cell oxidative state, favouring disulphide bonds. The monomer enters the nucleus and activates TGA transcription factors, affecting expression of PR genes. |
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Term
| Nucleotide binding site (NB) |
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Definition
| The nucleotide binding site for ATP-and GTP-binding proteins in an NB-LRR immune receptor. Allows nucleotides to bind to the molecule. |
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Term
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Definition
| A phenol. The brown colour of wounded tissue is caused by oxidation and polymerization of O-diphenols. A non-specific enzyme inhibitor. |
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Term
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Definition
| A plant polymer that has been damaged by a pathogen. Originally produced by the host, but acts like a PAMP. |
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Term
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Definition
| A release of ROS that is related to HR. A part of signal transduction. Membrane-bound oxidase enzymes are activated by rapid pathogen recognition, and produce ROS from proteins, releasing them into the cytoplasm, membrane, and cell wall. The free radicals bind to form anti-microbial cross-linking conglomerates, which can damage membranes and cause signals for disease resistance. Leads to secretion of JA, a defensive hormone signal. Changes the redox state of the cell so that cytoplasm is more oxidized. Defence genes are activated within 4 hours. Occurs quicker than phytoalexin synthesis. Conserved between plants and animals. |
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Term
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Definition
| A precursor of lignin formed in the phenylpropanoid pathway. |
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Term
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Definition
Pathogen associated molecular patterns
Includes MAMPs. Molecular patterns recognized by the plant during pathogen recognition, inducing basal resistance (PTI). Highly conserved to each group of pathogenic organisms. Conserved because they provide an indispensable function to the pathogen which allows it to be pathogenic; the pathogen cannot avoid having PAMPs. Can be detected on a dead pathogen. Not produced by plant cells, or found in uninfected plants: only when the pathogen is present. Detected by PRRs including RLKs. Once bound to plant receptors, PAMPs are destroyed or deposited in the vacuole, but never secreted (would activate neighboring cells). Includes β-glucans and chitins of fungi. Not necessarily important molecules for the pathogen; just important for the signalling pathway of the host. Many different types of pathogens have PAMPs, with variable effects on the host. Can include plant-produced molecules that have been damaged in pathogen-specific ways. The PAMPs of some beneficial microbes are the same as those of some pathogens; special effectors allow symbiotic bacteria to live inside the plant, preventing defence response. |
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Term
| PAMP triggered immunity (PTI) |
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Definition
Basal resistance
Pathogen triggered immunity
Plant triggered immune response
Occurs when PAMPs are present. Includes an influx of Ca2+ ions into the cytoplasm, MAPK cascades, callose growth, and transcriptional changes. ETI begins shortly afterwards. The host plant has stronger or different defence response genes, so the pathogen is less able to attack. Not related to race-specific resistance. Involves many genes. Almost never triggers HR. Sometimes strong PTI may seem like ETI. |
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Term
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Definition
| A triggered response common in grasses (Graminae), especially in epidermal cells. A deposition of cell wall material localized on the inside of the cell wall, just ahead of the fungal hyphae. A response to infection. Has callose and lignin, which both strengthen the cell wall. Stops the spore germ tube from penetrating the cell wall by pushing against the appressorium. If successful, the fungus spore will run out of energy before it is able to break through. |
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Term
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Definition
| An organism which lives inside the host and obtains nutrients from the host. A nutritional relationship. |
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Term
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Definition
| A group of pathogens. Plants that do not photosynthesize, and instead feed on other plants. Not microorganisms. |
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Term
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Definition
| An organism which is associated with the host and causes damage to the host. Not necessarily a nutritional relationship. Includes fungi, bacteria, mollicutes, parasitic plants, viruses, nematodes, and protozoans. Not every organism has the potential to be a pathogen, and those which do usually have limited host range. Special features are required to be able to attack plants. The pathogen and plant react and respond to each other, resulting in resistance or susceptibility. Plants have many disease defence mechanisms. |
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Term
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Definition
| The development of disease. How a disease develops or does not develop. The portion of the disease cycle which occurs within the host. There is a molecular interaction, where the plant reacts to the pathogen, and the pathogen counter-reacts to the plant until a balance is reached. A complicated process. |
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Term
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Definition
| Ability to cause disease. A yes/no situation; either pathogenic or not. |
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Term
| Pattern recognition receptor (PRR) |
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Definition
| A receptor that binds to PAMPs. Most PRRs are RLKs with kinase activity. |
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Term
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Definition
| The third stage in the disease cycle. Varies depending on the pathogen. The pathogen must penetrate from the outside of the plant, to the inside cells. There are several mechanisms, including natural openings, wounds, or an appressorium. Natural openings might include stomata. Even microscopic wounds are vulnerable to infection. The pathogen enters the host often at a joint between two cells. The spore on the surface then empties all its cytoplasmic contents into the appressorium. Enzymes are secreted to digest the cuticle. Trophic level doesn't matter at this stage. |
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Term
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Definition
| A narrow hyphae that grows out of the bottom of an appressorium. Grows through the cuticle, into the cell wall, and then into the cell. Punches through plant tissues with mechanical and enzymatic action. Melanin is produced that makes the appressorium less permeable to water. Stored nutrients break down into glycerol, which causes osmosis that increases the hydrostatic pressure of the cell. Water must be present on the plant surface for this to occur. At its peak, the pressure in the appressorium is 40 times that of a car tire. Enzymes break down cutin, pectin, and epidermal cells. |
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Term
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Definition
| Found in the cell wall. An enzyme that converts lignin precursor alcohols into free radicals near the infection site. The alcohols then polymerize with each other in a non-enzymatic reaction. Ratios and types of alcohols used in response to a disease vary. |
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Term
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Definition
| A triggered disease mechanism. Produced in the phenylpropanoid pathway. Includes salicylic acid. Involved in biosynthesis of lignin and phytoalexins which may kill microorganisms. Very important for defence. Some pathogens may feed on them or nullify them. |
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Term
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Definition
| A type of pre-formed disease defence mechanism. Common in a range of plants. The first pre-formed disease defence discovered. Includes catechol and protocatecholic acid, both of which have a phenol ring (6 carbons in a ring with alternating double bonds), red colour, and antimicrobial activity. Produced in the phenylpropanoid pathway. Can be complex or simple. Causes the brown colour of wounded tissue, and pigmentation in onion bulbs infected by Colletotrichum circinans. Produced from phenylalanine and tyrosine. Wounding stimulates PAL expression and a general increase in plant metabolism. Does not build up in cells; it is converted into anti-microbial compounds after the pathogen is recognized. |
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Term
| Phenylalanine ammonia lyase (PAL) |
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Definition
| One of the first committed enzymes in the phenylpropanoid pathway. Converts phenylalanine into a phenol by removing the ammonia group and adding a phenol group. Has a regulatory function. More common than TAL. Released during plant wounding, causing browning where tissue is attacked, and reducing risk of infection. Increases after infection, though cells must be alive to produce it. In resistant plants, PAL mRNA increases not at the infection site, but in the surrounding uninfected cells. This blocks the pathogen in, and then later declines. In susceptible plants, the mRNA increases continually, diffused around the tissue, not localized at the infection site. There are multiple genes encoding several PALs with differing responses to pathogens, and differing kinetic properties. |
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Term
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Definition
| The metabolic process which produces phenolic compounds. The amino acids phenylalanine and tyrosine are acted upon by the enzymes PAL or TAL, respectively. A series of intermediates produce multiple compounds. Can produce phenols, lignin, phytoalexins, alcohol precursors of lignin, and normal plant constituents. This pathway is important because many intermediates have biological activity and can be precursors for other pathways. A many-branched pathway. The "Swiss Army Knife" of pathways. Very useful for the plant: can produce many chemicals while economizing on enzymes. |
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Term
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Definition
"Plant protectors"
Plant-protecting substances. Related to disease. Phenols with high antimicrobial action. Triggered defence compounds with low molecular weight; not enzymes or polymers. Synthesized by and accumulate in plants which have been exposed to microorganisms. Not a chemical definition; there is a wide range of chemicals. Not all plants produce phytoalexins. Types of phytoalexins depends on the plant family. Many are produced in the phenylpropanoid pathway. Has de novo synthesis from remote precursors in other pathways. Very toxic, but can only be produced by living cells. Produced during HR, and can cause HR. Includes terpenoids, wyerone, glyceollin, and 3-deoxyanthocianidin. |
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Term
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Definition
| The plant R receptor which recognizes Avr-Pita. A CC-NB-LRR immune receptor. Has direct binding with the effector. Confers immunity to the fungal pathogen. Triggers HR and PTI when activated. |
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Term
| Plant growth-promoting fungi (PGPF) |
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Definition
| Arbuscular mycorrhizal fungi that can activate ISR. Includes Phoma and Trichoderma. Non-pathogenic fungi that live on the epidermis and in the cortex of roots without causing damage. |
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Term
| Plant growth-promoting rhizobacteria (PGPR) |
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Definition
| Rhizobacteria which can induce ISR. Promote plant growth. Includes Bacillus and Pseudomonas. Non-pathogenic bacteria that colonize the root surface without causing damage. |
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Term
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Definition
Pathogenesis related
Protein products that are absent or low in healthy tissues. Associated with disease. Accumulates before an infection when SAR is activated. Important for SAR and ISR. Some PR products are directly antimicrobial, such as β-1-3 glucanase. There are different classes, with different isoelectric points and pH. Can have many different properties. There are over 17. |
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Term
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Definition
| The second step in the infection process. The spore absorbs water and germinates a germ tube which looks for natural openings and secretes enzymes. |
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Term
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Definition
| An LN-NB-LRR immune receptor. An R gene. Forms a complex with Pto. Acts as a guard to protect the target of AvrPto, FLS2. |
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Term
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Definition
| A fluorescent chromosome stain. Allowed imaging of ISR-activating microorganisms colonizing the root surface without causing harm. |
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Term
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Definition
| A group of plant pathogens. Only attack tropical latex trees. Small in comparison to plant cells. |
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Term
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Definition
| A bacterial kinase very similar to FLS2, produced in some plants. Forms a complex with Prf, which acts as a guard. When AvrPto binds to Pto, a conformational change occurs. Guarded by the LZ-NB-LRR protein Prf, which must be present in order for Pto to provide resistance. |
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Term
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Definition
| A genetically distinct group within a pathogen species that infects a given set of plant varieties. The pathogen is specialized towards different genotypes within the host species. Some varieties of a plant species are susceptible, while others are resistant. May or may not be classified as subspecies. Determined by the pathogen's collection of effectors. Most microorganisms have many effectors, encoded by avirulence (AVR) genes of the pathogen. Not determined by PAMPs, which are conserved sequences. |
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Term
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Definition
| An older breeding term. A group of isolates do or don't cause a disease, producing a pattern of virulence based on plant genotype. There may be more than one effector, requiring more than one receptor. A strong short-term source of resistance for breeding; only one gene is changed. Creates a strong selection pressure for the pathogen to change effectors so they won't be detected; only a small mutation is necessary. Not stable resistance, especially if the effector is highly variable. Lasts only for a few growing seasons. |
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Term
| Reactive oxygen species (ROS) |
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Definition
| Molecules with unpaired electrons. Can bind to other molecules, possibly damaging them. Also found in animals, where they trigger programmed cell death. Involved in the oxidative burst during HR. Released into the apoplast by NADPH oxidase. Has multiple effects. Some are directly antimicrobial. May be involved in membrane damage during HR and defence signal transduction. Acts with lipoxygenase to damage plant cell membranes. Includes superoxide anion and hydrogen peroxide. |
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Term
| Receptor-like kinase (RLK) |
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Definition
| A receptor that binds to PAMPs; the first event in PTI. Contains three domains: an extracellular leucine-rich repeat domain that binds to PAMPs, a transmembrane domain, and an internal kinase enzyme domain. The leucine-rich domain allows for highly variable protein folding and binding, creating unique pockets with different shapes and charges. The PAMP fits into the pocket like a lock-and-key. |
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Term
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Definition
| A stage in the disease cycle. The first step of resistance. The host recognizes the pathogen. A necessary step for triggered responses. Determines when and where defences are triggered. Interaction of either surface components or extracellular materials of the pathogen with receptors or other molecules on the host plasma membrane or in the cytoplasm. Correct and rapid recognition is key for plant efficiency. Breeding for plant resistance is mainly breeding for better pathogen recognition; the defence mechanisms are already present in the plant. Some encoded by resistance (R) genes. |
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Term
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Definition
Heterologous interaction
Incompatible
An outcome of an infection. There are limited or no symptoms. The host can still have the disease, but the pathogen has difficult compatibility with the host. Plants with resistance stop the pathogen from spreading, with quick and minimal defences. Determined by histochemistry. There are three steps: recognition, signal transduction, and resistance response. Includes non-host resistance, basal resistance, and race-specific resistance.
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Term
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Definition
| Genes involved in one step of resistance. Affect a large range of plants and pathogens, including viruses, bacteria, and some insects. |
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Term
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Definition
| The third step of resistance. A wide variety of compounds and structures are produced that probably act synergistically. Encoded by DR genes. |
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Term
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Definition
| The difference between two rice varieties, one susceptible to a race of bacterial pathogen, and the other resistant, was examined using lignin stain. Bacteria was inoculated in the centre of the leaf. After 16 hours, lignin built up at the infection site in the resistant variety, and at 120 the lignin was still localized. In the susceptible variety, not much lignification was noticed at 16 hours, and at 120 hours lignification was occurring in areas surrounding the necrotic lesion, as the bacteria spread. |
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Term
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Definition
| A plant protein important for PTI and ETA. Guarded by Rps2, which is released and activated when RIN4 is cleaved by AvrRpt2. |
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Term
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Definition
| A CC-NB-LRR immune receptor which guards RIN4. When AvrRpt2 cleaves RIN4, Rps2 is released and activated, triggering PTI. |
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Term
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Definition
| A plant hormone which occurs naturally. Causes burning if applied directly to the plant. Accumulates in response to the oxidative burst. A component of induced resistance. Produced in the phenylpropanoid pathway, and by chrosimate in the chloroplast. A phenolic compound with an aromatic benzene ring. Activates different genes from ethylene and jasmonic acid. Activates SAR. It is also the medicinal ingredient in aspirin; prevents prostaglandin production and blood vessel dilation in humans by inhibiting cyclooxygenase. Induced by ROS. Accumulates in the cytoplasm during SAR to reduce the oxidative state of the cell. Longer distant signalling may be by methyl-SA. Carried in the phloem. |
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Term
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Definition
| A pre-formed defence mechanism. Includes avenacin. A membrane cholesterol. |
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Term
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Definition
| When microorganisms obtain nutrients from tissue which is already dead. Not a type of pathogen. |
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Term
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Definition
Kinase cascade
The second step of resistance. The cascade of phosphorylation which triggers cell-wide immunity. Triggered by the recognition of PAMPs. A signal is sent to the cell nucleus, and gene expression is altered. Key for innate and non-adaptive immunity. Proteins are phosphorylated to be activated/inactivated, in a series, ending with DNA transcription factors. Some are encoded by resistance (R) genes. With each step, the number of proteins phosphorylated increases exponentially, causing a large effect from just one PAMP binding event. Acts like an amplification system. |
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Term
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Definition
| A precursor of lignin formed in the phenylpropanoid pathway. |
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Term
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Definition
| Many pathogens show host species specificity, but are biochemically and morphologically identical. Subspecies of pathogens are specialized to certain plant species in its host range. In bacteria known as pathovar (pv.), and in fungi known as forma specialis (f.sp.). |
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Term
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Definition
Compatible
Homologous interaction
An outcome of an infection. There are normal symptoms in the interaction. The pathogen is compatible with the host, and disease occurs. Susceptible plants have a defence response, but it is slower and less effective than a resistant plant. Recognition of the pathogen occurs too late, or not at all. |
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Term
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Definition
| Possible symptoms of plant diseases can include overgrowth, rotting, cracking, wilting, and discolouration of tissues. Depends on the type of pathogen, and the plant's response. Altered growth and rotting are typical. Has a negative impact on crop yield. |
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Term
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Definition
| A glycolipid synthesized by Pseudomonas syringae pv. tomato. Not an elicitor, but the avrD gene is involved with its synthesis. Exported directly and enters the cell without an HRR transport system. |
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Term
| Systemic acquired resistance (SAR) |
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Definition
| A form of induced disease resistance. Pre-triggered SA-dependent resistance. Occurs throughout the plant, after inoculation with something, and before the pathogen attacks. A viral or bacterial chemical may be applied to the plant. Can be activated by some beneficial organisms. Activated by salicylic acid, PAMPs, and effectors, and the resulting oxidative burst. Good for pathogens that can be resisted through SA-dependent defences. Effective against many pathogens including bacteria, fungi, and viruses; especially those which cause necrosis; mostly biotrophs and hemibiotrophs. Requires time to start up; expression of PR genes is coordinated and PR products accumulate in the cytoplasm; activated about 3 - 4 days after inoculation. Lasts for weeks or months, and then stops. The system becomes ready before the pathogen attacks. Comes at a cost to plant growth, especially if the plant is weakened or under environmental stress. An uninfected plant will do better without SAR. Lignification of the cell wall is accelerated as a defence response. Arabidopsis has 300 SAR genes. First discovered in rhizobacteria which improved plant growth. |
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Term
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Definition
| An antimicrobial produced by the plant. May be broken down by the pathogen. It is the key reason why grapes are the best fruit for making wine. The tartaric acid in grapes kills bacteria, and leaves yeast. As a result, less sugar and more alcohol is produced during fermentation. |
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Term
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Definition
| A phytoalexin common in the Solanaceae family. |
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Term
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Definition
Toll interleukin 1 receptor NB-LRR
A possible third domain for NB-LRR immune receptors. Originally discovered in mammals. Has homology to mammalian toll. Binds to transcription factors. TIR is the largest domain. A cytoplasmic protein. Activates transcription factors and oxygen production. |
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Term
| Transcription factors (TF) |
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Definition
| Bind to gene cis elements (promoters), turning on/off the expression of the gene. Has a positive (on) or negative (off) effect. Often shared for defence. During disease resistance, WRKY genes are affected by transcription factors. |
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Term
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Definition
| A lactone produced in tulips. There are several versions with small chemical differences. The precursor has a sugar group and is non-toxic to the plant. Its precursor is stored in the vacuole of skin cells in the bulb. When the plant cell is broken by disease damage, the contents of the vacuole and cytoplasm mix, and the overall pH raises. The precursor is unstable at pH above 5.0. Cytoplasmic enzymes turn the precursor into an acid by removing a sugar, which is converted into tuliposide. A pre-formed defence, because the immediate precursor is present in healthy plants. Can be detoxified by some fungi. |
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Term
| Tyrosine ammonia lyase (TAL) |
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Definition
| One of the first committed enzymes in the phenylpropanoid pathway. Converts tyrosine into a phenol by removing the ammonia group and adding a phenol group. Has a regulatory function. Less common than PAL, found only in grasses (Graminae). |
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Term
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Definition
| A large vesicle within a plant cell. May contain pre-formed disease defences. The pH is low. If the vacuole breaks from pathogen damage, it mixes with the neutral pH of the cytoplasm, decreasing pH of the whole cell. Vacuole contents become unstable, and enzymes from the cytoplasm convert vacuole precursors into active forms. |
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Term
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Definition
| The relative degree of pathogenicity. On a spectrum; not a yes/no situation. |
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Term
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Definition
| A large group of plant pathogens. Work the same as they do in animals. Very small, the size of large biomolecules. All viruses are biotrophic. Move through the host cytoplasm, and through plasmodesmata. ISR is an ineffective defence. Sometimes PGPRs activate SA synthesis, which gives resistance against viruses; this might be considered SAR rather than ISR. |
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Term
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Definition
| Genes which are affected by transcription factors during disease resistance. Arabidopsis has 74. Not all related to defence: may be used for different pathogens or functions. |
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Term
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Definition
| A phytoalexin found in beans (a legume). Similar to a phenol. Accumulates at the site of infection. Visible through staining and microscopy. Only cells well enough to make wyerone can produce it, and then the chemical kills them. Cells that died from wyerone accumulation stop fungus spread, trapping the pathogen. Found only in one layer of cells. |
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Term
| Xanthomonas campestris pv. vesicatoria |
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Definition
| A bacterial pathogen of tomato and pepper. Grows biotrophically in the apoplast. One race has avrBs1, while avrBs3 is in many races and pathovars. The latter has a 34 AA motif repeated 17.5 times. If the repeat number or position of repeats is changed, it will change the race of the pathogen. There is a transposon in the AVR gene that inactivates itself in order to change race. |
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Term
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Definition
| A fungal PAMP that can activate ISR. |
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