Plant Defences
There are many kinds of defences available to a plant
Members of every class of pathogen which infect humans also infect plants. Although the exact pathogenic species vary with the infected species, bacteria, fungi, viruses, nematodes and insects can all cause plant disease. As with animals, plants attacked by insects or other pathogens use a set of complex metabolic responses that lead to the formation of defensive chemical compounds that fight infection or make the plant less attractive to insects and other herbivores.
Like invertebrates, plants neither generate antibody or T-cell responses nor possess mobile cells that detect and attack pathogens. In addition, in case of infection, parts of some plants are treated as disposable and replaceable, in ways that very few animals are able to do. Walling off or discarding a part of a plant helps stop spread of an infection.
Most plant immune responses involve systemic chemical signals sent throughout a plant. Plants use pattern-recognition receptors to identify pathogens and to start a basal response, which produces chemical signals that aid in warding off infection. When a part of a plant becomes infected with a microbial or viral pathogen, in case of an incompatible interaction triggered by specific elicitors, the plant produces a localised hypersensitive response (HR), in which cells at the site of infection undergo rapid programmed cell death to prevent the spread of the disease to other parts of the plant. HR has some similarities to animal pyroptosis, such as a requirement of caspase-1-like proteolytic activity of (VPEγ), a cysteine protease that regulates cell disassembly during cell death.
Resistance (R) proteins, encoded by R genes, are widely present in plants and detect pathogens. These proteins contain domains similar to the NOD Like Receptors and Toll-like receptors utilized in animal innate immunity. Systemic acquired resistance (SAR) is a type of defensive response that renders the entire plant resistant to a broad spectrum of infectious agents. SAR involves the production of chemical messengers, such as salicylic acid or jasmonic acid. Some of these travel through the plant and signal other cells to produce defensive compounds to protect uninfected parts, e.g., leaves. Salicylic acid itself, although indispensable for expression of SAR, is not the translocated signal responsible for the systemic response. Recent evidence indicates a role for jasmonates in transmission of the signal to distal portions of the plant. RNA silencing mechanisms are also important in the plant systemic response, as they can block virus replication. The jasmonic acid response, is stimulated in leaves damaged by insects, and involves the production of methyl jasmonate.
Wounding response
Auxin induces the formation and organisation of phloem and xylem. When the plant is wounded, the auxin may induce the cell differentiation and regeneration of the vascular tissues.
Traumatin is a plant hormone produced in response to wound. Traumatin is a precursor to the related hormone traumatic acid.
Jasmonic acid (JA) is derived from the fatty acid linolenic acid. It is a member of the jasmonate class of plant hormones. It is biosynthesised from linolenic acid by the octadecanoid pathway.
The major function of JA in regulating plant growth includes growth inhibition, senescence, and leaf abscission. It is also responsible for tuber formation in potatoes, yams, and onions. It has an important role in response to wounding of plants and systemic acquired resistance. When plants are attacked by insects, they respond by releasing JA which inhibits the insects' ability to digest protein.
Jasmonic acid is also converted to a variety of derivatives including esters such as methyl jasmonate; it may also be conjugated to amino acids.
Systemin
Systemin is a plant peptide hormone involved in the wound response in the Solanaceae plant family; it was the first plant hormone that was proven to be a peptide. It was discovered in tomato leaves in 1991, since then other peptides, with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPEPs were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for Systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of Systemins and is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores; other peptides activate defencins and modify root growth.
In tomato, systemin causes the synthesis of over 20 defence-related proteins; these are mainly anti-nutritional proteins, signaling pathway proteins and proteases. Systemin plays a critical role in defense signalling in tomato. When systemin was silenced, production of protease inhibitors in tomato was severely impaired and larvae feeding on the plants grew three times as fast. HypSys caused similar changes in gene expression in tobacco, for example polyphenol oxidase activity increased ten-fold in tobacco leaves and protease inhibitors caused a 30% decrease in chemotropism activity within three days of wounding. When HypSys was over expressed in tobacco, larvae feeding on transgenic plants weighed half as much after ten days feeding, as those feeding on normal plants. The concentration of hydrogen peroxide increased in the vasculature tissues when the production of systemin, HypSys or AtPep1 is induced, this may also be involved in initiating systemic acquired resistance.
Over expression of systemin and HypSys has been found to improve plants' resistance to abiotic stresses including salt stress and UV radiation. When systemin was over-expressed in tomato, transgenic plants had lower stomata conductance than normal plants. When grown in salt solutions, transgenic plants had higher stomata conductances, lower leaf concentrations of abscisic acid and proline and a higher biomass. These findings suggest that the over-expression of systemin either allowed the plants to adapt to salt stress more efficiently or that they perceived a less stressful environment. Similarly, wounded tomato plants were less susceptible to salt stress than non-wounded plants. This may be because wounding decreases the growth of the plant and therefore slows the uptake of toxic ions into the roots. An analysis of salt-induced changes in gene expression found that the differences measured between the transgenic and normal plants could not be accounted for by changes in conventional salt stress-induced pathways. Instead it was proposed that jasmonic acid may be responsible by activating defence responses which include the plant minimising water loss and diverting pathways away from producing compatible solutes towards defensive compounds.
Dehydrins
Dehydrins are a family of proteins present in plants that are produced in response to low temperatures and drought stress. They may do this through protecting membranes from damage. Their production is induced by ABA and in response to salt. Dehydrins in barley and maize are extremely hydrophilic and glycine rich. They may also play a role in allowing plants to tolerate high salt concentrations.
Common Mechanisms of Disease Resistance
Pre-formed structures and compounds that contribute to resistance
Plant cuticle/surface
Plant cell walls
Antimicrobial chemicals (glucosides and 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 response - a rapid host cell death response associated with defense mediated by resistance genes.
Plant Defense Signal Transduction & Immune Systems
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, defences 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 defence include bacterial flagellin or lip polysaccharides, or fungal chitin. The defences induced by MAMP perception are sufficient to repel most potentially pathogenic microorganisms. However, pathogens express effecter proteins that are adapted to allow them to infect certain plant species; these effectors often enhance pathogen virulence by suppressing basal host defences.
Importantly, plants have evolved R genes whose products allow recognition of specific pathogen effectors, either through direct binding of the effecter or by recognition of the alteration that the effecter 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” (effecter 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 effecter 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 defence 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 defence signal transduction. Cytoskeleton and vesicle trafficking dynamics help to target plant defence 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 defence 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. 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 co-adaptation (the pathogen and/or plant lack multiple mechanisms needed for colonisation and growth within that host species), or a particularly effective suite of pre-formed defences.
Bacteria
Most bacteria that are associated with plants are actually apostrophic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.
Most plant pathogenic bacteria are rod shaped. In order to be able to colonize the plant they have specific pathogenic factors. Five main types of bacterial pathogenic factors are known:
1. Cell wall degrading enzymes – used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot.
2. Toxins - These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant.
3. Effecter proteins - These can be secreted into the extracellular environment or directly into the host cell, often via the Type three secretion system. Some effectors are known to suppress host defense processes.
4. Phytohormones – for example Agrobacterium changes the level of auxins to cause tumours.
5. Exopolysaccharides – these are produced by bacteria and block xylem vessels, often leading to the death of the plant.
Significant bacterial plant pathogens include:
Burkholderia
Proteobacteria
Xanthomonas spp.
Pseudomonas spp.
Phytoplasmas and spiroplasmas
Phytoplasmas and Spiroplasmas are a genre of bacteria that lack cell walls, and are related to the mycoplasmas which are human pathogens. Together they are referred to as the mollicutes. They also tend to have smaller genomes than true bacteria. They are normally transmitted by sap-sucking insects, being transferred into the plants phloem where it reproduces.
Fungi
The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes.
The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil borne. Many soil borne spores, normally zoospores and capable of living saprotrophically, carrying out the first part of their lifecycle in the soil.
Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides.
Significant fungal plant pathogens:
Ascomycetes
Fusarium spp. (causal agents of Fusarium wilt disease)
Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
Verticillium spp.
Magnaporthe grisea causes blast of rice and gray leaf spot in turf grasses
Plant pathology department of Infectious diseases
Basidiomycetes
Rhizoctonia spp.
Phakospora pachyrhizi Sydow; causes soybean rust
Puccinia spp.; causal agents of severe rusts of virtually all cereal grains and cultivated grasses
Oomycetes
The oomycetes are not true fungi but are fungal-like organisms. They include some of the most destructive plant pathogens including the genus Phytophthora which includes the causal agents of potato late blight and sudden oak death.
Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens.
Significant oomycete plant pathogens
Pythium spp.
Phytophthora spp.
Viruses and Viroids
There are many types of plant virus, and some are even asymptomatic. Normally plant viruses only cause a loss of crop yield. Therefore it is not economically viable to try to control them, the exception being when they infect perennial species such as fruit trees.
Most plant viruses have small, single stranded RNA genomes. These genomes may only encode three or four proteins: a replicase, a coat protein, a movement protein to allow cell to cell movement though plasmodesmata and sometimes a protein that allows transmission by a vector.
Plant viruses must be transmitted from plant to plant by a vector. This is often by an insect, but some fungi, nematodes and protozoa have been shown to be viral vectors.
Nematodes
Nematodes are small, muilticelluar wormlike creatures. Many live freely in the soil, but there are some species which parasitise plant roots. They are a problem in tropical and subtropical regions of the world, where they may infect crops. Potato cyst nematodes (Globodera pallida and G. rostochiensis) are widely distributed in Europe and North and South America and cause £160 million worth of damage in Europe every year. Root knot nematodes have quite a large host range whereas cyst nematodes tend to only be able to infect a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.
Protozoa
There are a few examples of plant diseases caused by protozoa. They are transmitted as zoospores which are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses. When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots.
Parasitic Plants
Parasitic plants such as mistletoe and dodder are included in the study of phytopathology. Dodder, for example, is used as a conduit for the transmission of viruses or virus-like agents from a host plant to either a plant that is not typically a host or for an agent that is not graft-transmissible.
Plant Disorders
Significant abiotic disorders can be caused by:
· Flooding and poor drainage
· Frost damage by snow and hail
· Drought
· Nutrient deficiency
· Salt deposition and other soluble mineral excesses
· Wind (windburn, and breakage by hurricanes and tornadoes)
· Lightning, wildfire and manmade
· Man-made (arguably not abiotic, but usually regarded as such)
· Soil compaction
· Pollution of air, soil, or both
· Salt from winter road salt application or irrigation
· Herbicide over-application
· Poor education and training of people working with plants
Management of diseases
Quarantine
Wherein a diseased patch of vegetation or individual plants are isolated from other, healthy growth. Specimens may be destroyed or relocated into a greenhouse for treatment/study. Another option is to avoid introduction of harmful non-native organisms by controlling all human traffic and activity although legislation and enforcement are key in order to ensure lasting effectiveness.
Cultural
Farming in some societies is kept on a small scale, tended by peoples whose culture includes farming traditions going back to ancient times. (An example of such traditions would be lifelong training in techniques of plot terracing, weather anticipation and response, fertilisation, grafting, seed care, and dedicated gardening.) Plants that are intently monitored often benefit not only from active external protection, but a greater overall vigour as well. While primitive in the sense of being the most labour-intensive solution by far, where practical or necessary it is more than adequate.
Plant resistance
Sophisticated agricultural developments now allow growers to choose from among systematically cross-bred species to ensure the greatest hardiness in their crops, as suited for a particular region's pathological profile. Breeding practices have been perfected over centuries, but with the advent of genetic manipulation even finer control of a crop's immunity traits is possible. The engineering of food plants may be less rewarding however, as higher output is frequently offset by popular suspicion and negative opinion about this tampering with nature.
Chemical
Many natural and synthetic compounds exist that could be employed to combat the above threats. This method works by directly eliminating disease-causing organisms or curbing their spread; however it has been shown to have too broad an effect, typically, to be good for the local ecosystem. From an economic standpoint all but the simplest natural additives may disqualify a product from organic status, potentially reducing the value of the yield.
Biological
Crop rotation may be an effective means to prevent a parasitic population from becoming well established, as an organism affecting leaves would be starved when the leafy crop is replaced by a tuberous type, etc. Other means to undermine parasites without attacking them directly may exist.
Integrated
The use of two or more of these methods in combination offers a higher chance of effectiveness.