Plant disease resistance
Encyclopedia
Plant disease resistance is crucial to the reliable production of food, and it provides significant reductions in agricultural use of fuel, land, water and other inputs. There are numerous examples of devastating plant disease impacts (see Irish Potato Famine, Chestnut blight
), as well as recurrent severe plant disease issues (see Rice blast, Soybean cyst nematode
, Citrus canker
). However, disease control measures are reasonably successful for most crops. Across large regions and many crop species, it is estimated that diseases typically reduce plant yields by 10% every year in more developed settings, but yield loss to diseases often exceeds 20% in less developed settings.
Plant disease resistance derives both from pre-formed defenses and from infection-induced responses mediated by the plant immune system. Relative to a disease-susceptible plant, disease resistance is often defined as reduction of pathogen growth on or in the plant, while the term disease tolerance
describes plants that exhibit less disease damage despite similar levels of pathogen growth. Disease outcome is determined by the three-way interaction of the pathogen, the plant, and the environmental conditions (an interaction known as the disease triangle). Defense-activating compounds can move cell-to-cell and systemically through the plant vascular system, but plants do not have circulating immune cells so most cell types in plants retain the capacity to express a broad suite of antimicrobial defenses. Although obvious qualitative differences in disease resistance can be observed when some plants are compared (allowing classification as “resistant” or “susceptible” after infection by the same pathogen strain at similar pathogen inoculum levels in similar environments), a gradation of quantitative differences in disease resistance is more typically observed between plant lines or genotypes. Plants are almost always resistant to certain pathogens but susceptible to other pathogens; resistance is usually pathogen species-specific or pathogen strain-specific.
Note that plant defense against herbivory
(plant resistance to insect pests) exhibits some mechanistic similarities to, but also differences from, plant disease resistance (plant resistance to microscopic organisms).
Plants, like animals, have a basal immune system that includes a small number of pattern recognition receptors that are specific for broadly conserved microbe-associated molecular patterns (MAMPs, also called pathogen-associated molecular patterns or PAMPs). Examples of these microbial compounds that elicit plant basal defense include bacterial flagellin or lipopolysaccharides, or fungal chitin. The defenses induced by MAMP perception are sufficient to repel most potentially pathogenic microorganisms. However, pathogens express effector proteins that are adapted to allow them to infect certain plant species; these effectors often enhance pathogen virulence by suppressing basal host defenses.
Importantly, plants have evolved R gene
s (resistance genes) whose products allow recognition of specific pathogen effectors, either through direct binding of the effector or by recognition of the alteration that the effector has caused to a host protein. R gene products control a broad set of disease resistance responses whose induction is often sufficiently rapid and strong to stop adapted pathogens from further growth or spread. Plant genomes each contain a few hundred apparent R genes, and the R genes studied to date usually confer specificity for particular strains of a pathogen species. As first noted by Harold Flor in the mid-20th century in his formulation of the gene-for-gene relationship
, the plant R gene and the pathogen “avirulence gene” (effector gene) must have matched specificity for that R gene to confer resistance. The presence of an R gene can place significant selective pressure on the pathogen to alter or delete the corresponding avirulence/effector gene. Some R genes show evidence of high stability over millions of years while other R genes, especially those that occur in small clusters of similar genes, can evolve new pathogen specificities over much shorter time periods.
The use of receptors carrying leucine-rich repeat
(LRR) pathogen recognition specificity domains is common to plant, insect, jawless vertebrate and mammal immune systems, as is the presence of Toll/Interleukin receptor (TIR) domains in many of these receptors, and the expression of defensins, thionins, oxidative burst and other defense responses.
Some of the key endogenous chemical mediators of plant defense signal transduction include salicylic acid
, jasmonic acid
or jasmonate
, ethylene
, reactive oxygen species
, and nitric oxide
. Numerous genes and/or proteins have been identified that mediate plant defense signal transduction. Cytoskeleton and vesicle trafficking dynamics help to target plant defense responses asymmetrically within plant cells, toward the point of pathogen attack.
Plant immune systems can also respond to an initial infection in one part of the plant by physiologically elevating the capacity for a successful defense response in other parts of the plant. These responses include systemic acquired resistance
, largely mediated by salicylic acid-dependent pathways, and induced systemic resistance, largely mediated by jasmonic acid-dependent pathways. Against viruses, plants often induce pathogen-specific gene silencing mechanisms mediated by RNA interference
. These are primitive forms of adaptive immunity.
In a small number of cases, plant genes have been identified that are broadly effective against an entire pathogen species (against a microbial species that is pathogenic on other genotypes of that host species). Examples include barley MLO against powdery mildew, wheat Lr34 against leaf rust, and wheat Yr36 against stripe rust. An array of mechanisms for this type of resistance may exist depending on the particular gene and plant-pathogen combination. Other reasons for effective plant immunity can include a relatively complete lack of coadaptation (the pathogen and/or plant lack multiple mechanisms needed for colonization and growth within that host species), or a particularly effective suite of pre-formed defenses (see above).
, tillage, planting density, purchase of disease-free seeds and cleaning of equipment, but plant varieties with inherent (genetically determined) disease resistance are generally the first choice for disease control. Breeding for disease resistance has been underway since plants were first domesticated, but it requires continual effort. This is because pathogen populations are often under natural selection
for increased virulence, new pathogens can be introduced to an area, cultivation methods can favor increased disease incidence over time, changes in cultivation practice can favor new diseases, and plant breeding for other traits can disrupt the disease resistance that was present in older plant varieties. A plant line with acceptable disease resistance against one pathogen may still lack resistance against other pathogens.
Plant breeding for disease resistance typically includes:
Each of the above steps can be difficult to successfully accomplish, and many highly refined methods in plant breeding
and plant pathology are used to increase the effectiveness and reduce the cost of resistance breeding.
Resistance is termed durable if it continues to be effective over multiple years of widespread use, but some resistance “breaks down” as pathogen populations evolve to overcome or escape the resistance. Resistance that is specific to certain races or strains of a pathogen species is often controlled by single R gene
s and can be less durable; broad-spectrum resistance against an entire pathogen species is often quantitative and only incompletely effective, but more durable, and is often controlled by many genes that segregate in breeding populations. However, there are numerous exceptions to the above generalized trends, which were given the names vertical resistance
and horizontal resistance
, respectively, by J.E. Vanderplank.
Crops such as potato, apple, banana and sugarcane are often propagated by vegetative reproduction
to preserve highly desirable plant varieties, because for these species, outcrossing seriously disrupts the preferred plant varieties. See also asexual propagation. Vegetatively propagated crops may be among the best targets for resistance improvement by the biotechnology
method of plant transformation
to add individual genes that improve disease resistance without causing large genetic disruption of the preferred plant varieties.
from a related or relatively unrelated organism.
), and constant surveillance for disease problems to facilitate early initiation of appropriate responses. Some pathogen species are known to have a much greater capacity to overcome plant disease resistance than others, often because of their ability to evolve rapidly and to disperse broadly.
Hammond-Kosack, K. and Jones, J.D.G. "Responses to plant pathogens." In: Buchanan, Gruissem and Jones, eds. Biochemistry and Molecular Biology of Plants. 2000 Amer.Soc.Plant Biol., Rockville, MD. ISBN 0-943088-39-9
Jones JD and Dangl JL. 2006 The plant immune system . Nature 444:323-329.
Schumann, G. Plant Diseases: Their Biology and Social Impact. 1991 APS Press, St. Paul, MN ISBN 0-89054-16-7
Chestnut blight
The pathogenic fungus Cryphonectria parasitica is a member of the ascomycota category, and is the main cause of chestnut blight, a devastating disease of the American chestnut tree that caused a mass extinction in the early 1900s of this once plentiful tree from its historic range in the eastern...
), as well as recurrent severe plant disease issues (see Rice blast, Soybean cyst nematode
Soybean cyst nematode
The soybean cyst nematode , Heterodera glycines, is a plant-parasitic nematode and a devastating pest of the soybean worldwide. The nematode infects the roots of soybean, and the female nematode eventually becomes a cyst...
, Citrus canker
Citrus canker
Citrus canker is a disease affecting citrus species that is caused by the bacterium Xanthomonas axonopodis. Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefruit...
). However, disease control measures are reasonably successful for most crops. Across large regions and many crop species, it is estimated that diseases typically reduce plant yields by 10% every year in more developed settings, but yield loss to diseases often exceeds 20% in less developed settings.
Plant disease resistance derives both from pre-formed defenses and from infection-induced responses mediated by the plant immune system. Relative to a disease-susceptible plant, disease resistance is often defined as reduction of pathogen growth on or in the plant, while the term disease tolerance
Tolerance to infections
Tolerance to infections, or disease tolerance, is one of the mechanisms host organisms can fight against parasites, pathogens or herbivores that attack the host. Tolerance to infections is defined as the ability of a host to limit the impact of parasites, pathogens or herbivores on host health,...
describes plants that exhibit less disease damage despite similar levels of pathogen growth. Disease outcome is determined by the three-way interaction of the pathogen, the plant, and the environmental conditions (an interaction known as the disease triangle). Defense-activating compounds can move cell-to-cell and systemically through the plant vascular system, but plants do not have circulating immune cells so most cell types in plants retain the capacity to express a broad suite of antimicrobial defenses. Although obvious qualitative differences in disease resistance can be observed when some plants are compared (allowing classification as “resistant” or “susceptible” after infection by the same pathogen strain at similar pathogen inoculum levels in similar environments), a gradation of quantitative differences in disease resistance is more typically observed between plant lines or genotypes. Plants are almost always resistant to certain pathogens but susceptible to other pathogens; resistance is usually pathogen species-specific or pathogen strain-specific.
Note that plant defense against herbivory
Plant defense against herbivory
Plant defense against herbivory or host-plant resistance describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants use several strategies to defend against damage caused by herbivores...
(plant resistance to insect pests) exhibits some mechanistic similarities to, but also differences from, plant disease resistance (plant resistance to microscopic organisms).
Pre-formed structures and compounds that contribute to resistance
- Plant cuticle/surface
- Plant cell walls
- Antimicrobial chemicals (for example: glucosides, saponins)
- Antimicrobial proteins
- Enzyme inhibitors
- Detoxifying enzymes that break down pathogen-derived toxins
- Receptors that perceive pathogen presence and activate inducible plant defenses
Inducible plant defenses that are generated after infection
- Cell wall reinforcement (callose, lignin, suberin, cell wall proteins)
- Antimicrobial chemicals (including reactive oxygen species such as hydrogen peroxide, or peroxynitrite, or more complex phytoalexins such as genistein or camalexin)
- Antimicrobial proteins such as defensins, thionins, or PR-1
- Antimicrobial enzymes such as chitinases, beta-glucanases, or peroxidases
- Hypersensitive responseHypersensitive responseThe hypersensitive response is a mechanism, used by plants, to prevent the spread of infection by microbial pathogens. The HR is characterized by the rapid death of cells in the local region surrounding an infection. The HR serves to restrict the growth and spread of pathogens to other parts of...
- a rapid host cell death response associated with defense mediated by “Resistance genes.”
Plant Immune Systems and Plant Defense Signal Transduction
Plant immune systems show some mechanistic similarities and apparent common origin with the immune systems of insects and mammals, but also exhibit many plant-specific characteristics. As in most cellular responses to the environment, defenses are activated when receptor proteins directly or indirectly detect pathogen presence and trigger ion channel gating, oxidative burst, cellular redox changes, protein kinase cascades, and/or other responses that either directly activate cellular changes (such as cell wall reinforcement), or activate changes in gene expression that then elevate plant defense responses.Plants, like animals, have a basal immune system that includes a small number of pattern recognition receptors that are specific for broadly conserved microbe-associated molecular patterns (MAMPs, also called pathogen-associated molecular patterns or PAMPs). Examples of these microbial compounds that elicit plant basal defense include bacterial flagellin or lipopolysaccharides, or fungal chitin. The defenses induced by MAMP perception are sufficient to repel most potentially pathogenic microorganisms. However, pathogens express effector proteins that are adapted to allow them to infect certain plant species; these effectors often enhance pathogen virulence by suppressing basal host defenses.
Importantly, plants have evolved R gene
R gene
Resistance genes are genes in plant genomes that convey plant disease resistance against pathogens by producing R proteins. The main class of R-genes consist of a nucleotide binding domain and a leucine rich repeat domain and are often referred to as R-genes. Generally, the NB domain binds...
s (resistance genes) whose products allow recognition of specific pathogen effectors, either through direct binding of the effector or by recognition of the alteration that the effector has caused to a host protein. R gene products control a broad set of disease resistance responses whose induction is often sufficiently rapid and strong to stop adapted pathogens from further growth or spread. Plant genomes each contain a few hundred apparent R genes, and the R genes studied to date usually confer specificity for particular strains of a pathogen species. As first noted by Harold Flor in the mid-20th century in his formulation of the gene-for-gene relationship
Gene-for-gene relationship
The gene-for-gene relationship was discovered by Harold Henry Flor who was working with rust of flax . Flor was the first scientist to study the genetics of both the host and parasite and to integrate them into one genetic system...
, the plant R gene and the pathogen “avirulence gene” (effector gene) must have matched specificity for that R gene to confer resistance. The presence of an R gene can place significant selective pressure on the pathogen to alter or delete the corresponding avirulence/effector gene. Some R genes show evidence of high stability over millions of years while other R genes, especially those that occur in small clusters of similar genes, can evolve new pathogen specificities over much shorter time periods.
The use of receptors carrying leucine-rich repeat
Leucine-rich repeat
A leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
(LRR) pathogen recognition specificity domains is common to plant, insect, jawless vertebrate and mammal immune systems, as is the presence of Toll/Interleukin receptor (TIR) domains in many of these receptors, and the expression of defensins, thionins, oxidative burst and other defense responses.
Some of the key endogenous chemical mediators of plant defense signal transduction include salicylic acid
Salicylic acid
Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin...
, jasmonic acid
Jasmonic acid
Jasmonic acid is derived from the fatty acid linolenic acid. It is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway....
or jasmonate
Jasmonate
Jasmonate and its derivatives are lipid-based hormone signals that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herbivory and plant responses to poor environmental...
, ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
, and nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
. Numerous genes and/or proteins have been identified that mediate plant defense signal transduction. Cytoskeleton and vesicle trafficking dynamics help to target plant defense responses asymmetrically within plant cells, toward the point of pathogen attack.
Plant immune systems can also respond to an initial infection in one part of the plant by physiologically elevating the capacity for a successful defense response in other parts of the plant. These responses include systemic acquired resistance
Systemic acquired resistance
The systemic acquired resistance is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and there is evidence that SAR in plants and innate immunity in animals may be evolutionarily...
, largely mediated by salicylic acid-dependent pathways, and induced systemic resistance, largely mediated by jasmonic acid-dependent pathways. Against viruses, plants often induce pathogen-specific gene silencing mechanisms mediated by RNA interference
RNA interference
RNA interference is a process within living cells that moderates the activity of their genes. Historically, it was known by other names, including co-suppression, post transcriptional gene silencing , and quelling. Only after these apparently unrelated processes were fully understood did it become...
. These are primitive forms of adaptive immunity.
In a small number of cases, plant genes have been identified that are broadly effective against an entire pathogen species (against a microbial species that is pathogenic on other genotypes of that host species). Examples include barley MLO against powdery mildew, wheat Lr34 against leaf rust, and wheat Yr36 against stripe rust. An array of mechanisms for this type of resistance may exist depending on the particular gene and plant-pathogen combination. Other reasons for effective plant immunity can include a relatively complete lack of coadaptation (the pathogen and/or plant lack multiple mechanisms needed for colonization and growth within that host species), or a particularly effective suite of pre-formed defenses (see above).
Plant Breeding for Disease Resistance
Plant breeders focus a significant part of their effort on selection and development of disease-resistant plant lines. Plant diseases can also be partially controlled by use of pesticides, and by cultivation practices such as crop rotationCrop rotation
Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...
, tillage, planting density, purchase of disease-free seeds and cleaning of equipment, but plant varieties with inherent (genetically determined) disease resistance are generally the first choice for disease control. Breeding for disease resistance has been underway since plants were first domesticated, but it requires continual effort. This is because pathogen populations are often under natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
for increased virulence, new pathogens can be introduced to an area, cultivation methods can favor increased disease incidence over time, changes in cultivation practice can favor new diseases, and plant breeding for other traits can disrupt the disease resistance that was present in older plant varieties. A plant line with acceptable disease resistance against one pathogen may still lack resistance against other pathogens.
Plant breeding for disease resistance typically includes:
- Identification of resistant breeding sources (plants that may be less desirable in other ways, but which carry a useful disease resistance trait). Ancient plant varieties and wild relatives are very important to preserve because they are the most common sources of enhanced plant disease resistance.
- Crossing of a desirable but disease-susceptible plant variety to another variety that is a source of resistance, to generate plant populations that mix and segregate for the traits of the parents.
- Growth of the breeding populations in a disease-conducive setting. This may require artificial inoculation of pathogen onto the plant population. Careful attention must be paid to the types of pathogen isolates that are present, as there can be significant variation the effectiveness of resistance against different isolates of the same pathogen species.
- Selection of disease-resistant individuals. Breeders are trying to sustain or improve numerous other plant traits related to plant yield and quality, including other disease resistance traits, while they are breeding for improved resistance to any particular pathogen.
Each of the above steps can be difficult to successfully accomplish, and many highly refined methods in plant breeding
Plant breeding
Plant breeding is the art and science of changing the genetics of plants in order to produce desired characteristics. Plant breeding can be accomplished through many different techniques ranging from simply selecting plants with desirable characteristics for propagation, to more complex molecular...
and plant pathology are used to increase the effectiveness and reduce the cost of resistance breeding.
Resistance is termed durable if it continues to be effective over multiple years of widespread use, but some resistance “breaks down” as pathogen populations evolve to overcome or escape the resistance. Resistance that is specific to certain races or strains of a pathogen species is often controlled by single R gene
R gene
Resistance genes are genes in plant genomes that convey plant disease resistance against pathogens by producing R proteins. The main class of R-genes consist of a nucleotide binding domain and a leucine rich repeat domain and are often referred to as R-genes. Generally, the NB domain binds...
s and can be less durable; broad-spectrum resistance against an entire pathogen species is often quantitative and only incompletely effective, but more durable, and is often controlled by many genes that segregate in breeding populations. However, there are numerous exceptions to the above generalized trends, which were given the names vertical resistance
Vertical resistance
The term vertical resistance was first used by J.E. Vanderplank to describe single-gene resistance. This contrasted the term horizontal resistance which was used to describe many-gene resistance. Raoul A...
and horizontal resistance
Horizontal resistance
In genetics, the term horizontal resistance was first used by J.E. Vanderplank to describe many-gene resistance. This contrasts with the term vertical resistance which was used to describe single-gene resistance. Raoul A. Robinson further refined the definition of horizontal resistance...
, respectively, by J.E. Vanderplank.
Crops such as potato, apple, banana and sugarcane are often propagated by vegetative reproduction
Vegetative reproduction
Vegetative reproduction is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores...
to preserve highly desirable plant varieties, because for these species, outcrossing seriously disrupts the preferred plant varieties. See also asexual propagation. Vegetatively propagated crops may be among the best targets for resistance improvement by the biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...
method of plant transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
to add individual genes that improve disease resistance without causing large genetic disruption of the preferred plant varieties.
Host Range
There are thousands of species of plant pathogenic microorganisms (see Plant Pathology), but only a small minority of these pathogens have the capacity to infect a broad range of plant species. Most pathogens instead exhibit a high degree of host-specificity. Non-host plant species are often said to express non-host resistance. The term host resistance is used when a pathogen species can be pathogenic on the host species but certain strains of that plant species resist certain strains of the pathogen species. There can be overlap in the causes of host resistance and non-host resistance. Pathogen host range can change quite suddenly if, for example, the capacity to synthesize a host-specific toxin or effector is gained by gene shuffling/mutation, or by horizontal gene transferHorizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
from a related or relatively unrelated organism.
Epidemics and Population Biology
Plants in native populations are often characterized by substantial genotype diversity and dispersed populations (growth in a mixture with many other plant species). They also have undergone millions of years of plant-pathogen coevolution. Hence as long as novel pathogens are not introduced from other parts of the globe, natural plant populations generally exhibit only a low incidence of severe disease epidemics. In agricultural systems, humans often cultivate single plant species at high density, with numerous fields of that species in a region, and with significantly reduced genetic diversity both within fields and between fields. In addition, rapid travel of people and cargo across large distances increases the risk of introducing pathogens against which the plant has not been selected for resistance. These factors make modern agriculture particularly prone to disease epidemics. Common solutions to this problem include constant breeding for disease resistance, use of pesticides to suppress recurrent potential epidemics, use of border inspections and plant import restrictions, maintenance of significant genetic diversity within the crop gene pool (see Crop diversityCrop diversity
Crop diversity is the variance in genetic and phenotypic characteristics of plants used in agriculture. Crops may vary in seed size, branching pattern, in height, flower color, fruiting time, or flavor. They may also vary in less obvious characteristics such as their response to heat, cold or...
), and constant surveillance for disease problems to facilitate early initiation of appropriate responses. Some pathogen species are known to have a much greater capacity to overcome plant disease resistance than others, often because of their ability to evolve rapidly and to disperse broadly.
Further reading
Lucas, J.A., "Plant Defence." Chapter 9 in Plant Pathology and Plant Pathogens, 3rd ed. 1998 Blackwell Science. ISBN 0-632-03046-1Hammond-Kosack, K. and Jones, J.D.G. "Responses to plant pathogens." In: Buchanan, Gruissem and Jones, eds. Biochemistry and Molecular Biology of Plants. 2000 Amer.Soc.Plant Biol., Rockville, MD. ISBN 0-943088-39-9
Jones JD and Dangl JL. 2006 The plant immune system . Nature 444:323-329.
Schumann, G. Plant Diseases: Their Biology and Social Impact. 1991 APS Press, St. Paul, MN ISBN 0-89054-16-7
See also
- Disease resistance in fruit and vegetablesDisease resistance in fruit and vegetablesThere are a number of lines of defence against pests and diseases in the organic garden, principal among these being the practice of good husbandry, creating healthy soil and ensuring high standards of garden hygiene...
- Gene-for-gene relationshipGene-for-gene relationshipThe gene-for-gene relationship was discovered by Harold Henry Flor who was working with rust of flax . Flor was the first scientist to study the genetics of both the host and parasite and to integrate them into one genetic system...
- Plant defense against herbivoryPlant defense against herbivoryPlant defense against herbivory or host-plant resistance describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants use several strategies to defend against damage caused by herbivores...
- Plant Pathology
- Systemic acquired resistanceSystemic acquired resistanceThe systemic acquired resistance is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and there is evidence that SAR in plants and innate immunity in animals may be evolutionarily...