Plant Immune System Flashcards
What are modes of infection used by pathogens?
Direct penetration.
Penetration through natural openings.
Penetration through wounds.
Structural barriers to infection.
How do plants fend off attack?
Susceptible host plant- infection.
Non-host plant- toxin production restricts haustorial growth.
Non-host plant- formation of papillae prevents cell penetration.
Host plant with R gene- mediated resistance: penetrated cells die, other cells remain healthy.
Plants carry both preformed (constitutive) and inducible defences.
What types of resistance do plants have to pathogen infection?
Passive (constitutive) defence: cuticle, preformed inhibitors, antimicrobial proteins, antimicrobial compounds, phytoanticipins.
Active (inducible) defences: local and systemic signals, resistance responses.
There are compounds that are present inplant cells that are not toxic, but become activated when immune response is triggered.
If a microbial species is unable to overcome these barriers in a particular plant species, this is termed non-host resistance.
Plants carry various classes of immune receptors, which are induced in the presence of a particular pathogen.
How to plants percieve microbes?
What is the strucure of these?
PAMP recognition receptors.
External leucine-rich repeat domain.
Membrane spanning domain.
Internal protein kinase domain.
What are PAMPs?
Pathogen associated molecular patterns.
Conserved across microbial genera.
Not required for infection, but important biological roles.
Not strictly found in pathogens (also other microbes).
Often surface exposed.
Not present in host.
What are some examples of specificity of perception?
The flg22 epitope from flagella and the lipid A component of LPS are perceived by all plant species. These may be ancient PAMPs.
These are also perceived by our cells – by toll-like receptors 5 and 4, respectively.
Cold shock proteins (CSPs) from bacteria are perceived by the Solanaceae (potato, tomato, pepper, tobacco etc).
Ef-Tu from bacteria is perceived only by the Brassicacea (broccoli, cauliflower, Arabidopsis etc).
Different genera of plants have different receptors. Some PAMPs detected by all plants, some are specific to certain plants.
How are PAMPs percieved and activated?
Through signalling mechanisms. Other cofactors help in the signalling process.
Association of factors with receptor after PAMP deetction causes activation, signalling and attenuation.
How does PAMP perception affect the host?
PAMP perception leads to drastic cellular changes in the host (PTI).
Perception leads to activation of a wide range of processes.
Responses are highly regulated.
Antimicrobial compounds produced (protein & metabolites).
Collectively, these responses lead to immunity to most microbes (Therefore called PAMP-Triggered Immunity).
Massive changes going on- gene expression.
Big commitment to plant to defend itself cause of all the gene expression changes.
Need to be tightly regulated, so that the plant doesn’t waste energy and resources every time it think it detects a pathogen.
What are pathogenesis related proteins?
PR proteins - transcriptionally induced specifically by pathogen attack as a defence response.
PR-1: unknown PR-2: β-1,3-glucanase PR-3: chitinase type I, II, IV, V, VI, VII PR-4: chitinase type I, II PR-5: thaumatin-like PR-6: proteinase-inhibitor PR-7: endoproteinase
Some have enzymatic activities.
These are direct.
Many microarray studies - hundreds of genes up-regulated in the plant in response to pathogen invasion. In most cases, we don’t yet know their biochemical functions.
Genes are also turned off. They may include negative regulators of defence.
What antimicrobial compounds are produced by the plant in PTI?
Some PR proteins directly attack pathogen cell walls (e.g. chitinases, glucanases and proteases).
Plants also produce reactive oxygen species (ROS) such as H2O2. This is also toxic to pathogens.
H2O2 also acts as a signalling molecule to neighbouring, uninfected cells.
What are phytoalexins?
Phytoalexins – products of the phenolpropenoid pathway.
Induced locally in cells near to a site of infection.
Several hundred have been isolated and characterised from at least 15 plant families.
Not usually active pathway- active in localised infection.
What structural defences does the plant have in PTI?
Lignin synthesis thickens cell walls to provide a barrier.
Callose can be synthesised- carbohydrate similar to starch- a component of papillae formed around the site of pathogen penetration.
What are systemic induced defences?
A systemic signal travels to new leaves where enhanced resistance responses to pathogen are observed.
Localised cell death- hypersensitive response.
How is SA signalling involved in systemic resistance?
Salicylic acids- plants make aspirin.
One of the pathways activated in the hypersensitive response.
Reprogramming happens- faster and more effective.
Allows other, uninfected cells to use their resources more carefully.
Use of resources is localised- not whole plant using energy and resources.
What happens in systemic acquired resistance (SAR), and systemic wound response?
SAR- Necrotic lesions caused by virus, fungus or bacterium (necrotrophs).
Phloem mobile signal.
Expression of defense-related genes in distant tissues results in enhanced resistance.
Increase in salicylic acid.
SWR- Chewing insects or mechanical wounds.
Phloem mobile signal.
Expression of defense-related genes in distant tissue results in enhanced resistance.
Increase in ethylene and jasmonic acid.
Multiple signals involved.
