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Plant Defence & Nitrogen Fixation Flashcards

(49 cards)

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1
Q

What are R proteins?

A

Products of R genes, part of plant immunity. (R for resistance)

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2
Q

What are biotrophic pathogens?

A

Parasitise living cells, hide and stay secret from plant. Feed on living plant tissue

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3
Q

What are necrotrophic pathogens

A

Digest cells with proteases. Feed on dead cell matter

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4
Q

What is the general rule for plant immunity?

A

Resistance to pathogen is the norm. Susceptibility to the disease is the exception.

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5
Q

How do R genes interact with pathogens?

A

Microbial pathogens have effector proteins which are the virulence factors which help to colonise the host. Products of R genes scan cells for these effectors, recognition of an effector triggers defence. This pathogen is now avirulent.

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6
Q

What is an avirulence (AVR) protein?

A

An effector protein which can be recognised by an R protein in the plant. Rendering the pathogen avirulent.

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7
Q

What is race specific resistance?

A

R Gene resistance is very specific to the pathogen race (race specific effector) and the plant cultivar (does it have a specific R gene to counter it)
Can cross R genes from wild species into commercial cultivars

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8
Q

What is the hypersensitive response?

A

In HR recognition triggers cell suicide resulting in execution of challenged cells. Removing nutrients from invading pathogens.
This only works against biotrophs as they need living cells.
Also includes ROI production (no clue no context on slides :((() and callose deposition to limit pathogen entrance before cell death

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9
Q

What are the purpose of effector proteins?

A

They aim to inactivate other defences such as the unspecific Pattern Triggered Immunity (aims to recognise general traits eg flagella amino acids)

Some also help with plant entry as P. Syringae effectors induce opening of stomata

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10
Q

Requirements of infection?

A

Pathogen must overcome defences
Plant must be susceptible
Environment must be tipped in favour of pathogen

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11
Q

What is a Hemibiotroph?

A

Can switch between necro- and biotrophic

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12
Q

Methods of pathogens entering plant?

A

Enter through stomata
Brute force with high pressure to puncture wall
(No mentioned in notes but also cellulases?, Need to break down cuticle first though)

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13
Q

How does unspecific recognition work?

A

Plant has cytoplasmic pathogen recognition receptors to recognise general proteins from pathogens (flagella proteins eg). They induce signalling causing expression of anti pathogen defences

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14
Q

What is some general R protein structure?

A

Nucleotide binding site (as they induce expression of other proteins)
LRR (Leucine rich repeat)

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15
Q

What is an effect of R protein activation?

A

Production of Salicylate (stress hormone)

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16
Q

Two types of system immunity in plants?

A

LAR: Local acquired resistance (just leaf is resistant)
SAR: Systematic acquired resistance (entire plant is resistant)

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17
Q

Difference between virulent and avirulent?

A

A pathogen is virulent if the plant doesn’t have recognition for it - and so is susceptible
It is rendered avirulent once the plant recognises it and it induces defences
Preinoculation of a plant with an AVR protein that it recognises can protect from subsequent infection

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18
Q

What is the biochemical basis of SAR

A

Involves group of SAR-Pathogenisis related proteins (PR-Proteins)
PR proteins accumulate in local and systemic leaves during SAR development
PR-2 encodes Glucanase
PR-3 encodes Chitinase
Others encode proteins of unknown function
Induction of SAR involves motile signal as uninflected tissue has enhanced resistance to subsequent pathogens

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19
Q

What is the order of a resistance response?

A

Resistance response triggers release of signal
Signal establishes LAR in local leaf
Signal exits local leaf and establishes SAR in all leaves

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20
Q

What does salicylate signal induce?

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A

PR protein synthesis
SAR
It is a local signal for LAR , it is not the systemic mobile signal for SAR

21
Q

What does spray in with SA do?

Study These Flashcards
A

Activated SAR genes
PR-proteins accumulate
SAR develops

22
Q

What are the 2 methods for inducing SAR?

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A

Biological: inoculate with AVR protein
Chemical: spray with SA

23
Q

Downside of treating crops with SA?

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A

Decreases yield if done constantly as immune response diverts energy away from growth :(

24
Q

What sort of plants associate with rhizobia

25
What is rhizobia?
Common soil bacteria, gram negative, induces root nodules inlegumes Reduce N2 to ammonia which legume can use (triple bonded N2 too difficult for plant, Sad!)
26
How do plants attract rhizobia?
Flavonoids (phenolic compound derived from phenolpropanoid pathway) are released from roots, chemoattractants for rhizobia, attract them into rhizoshere
27
What are the 3 rhizobia genera
Rhizobium (grow fast) Bradyrhizobium (slow) Azorhizobium (free living) NOT close related!!!!
28
What are the functions of flavonoids?
Chemotaxis (attracting the bacteria) Nod gene activation Increased growth rate
29
Haemoglobin in nodule?
Nitrogenase enzyme v sensitive to oxygen Oxygen held away in haemoglobin to prevent inactivation
30
What is nod D
Regulatory nod gene global activator of nod genes Transcriptional activator Helix-turn-Helix binding motif Binds to conserved cis element NOD box present in promoters of all nod genes
31
Nod D mechanism?
1. Binds to flavonoid (early theory) 2. Confirmational change to DNA bind form 3. Binds to NOD boxes, nod gene expression activated (My notes say GroEl and (p)pGpp are needed for NodD function, no clue if bullshit or not)
32
What are the 2 host specific flavonoids for R. Meliloti
Luteolin and Alfalfa
33
What are the common nod genes
Nod A, B and C Easy peasy All others are specific to certain cases
34
What is a Nod factor?
Lipo Oligosaccharide produced by rhizobia which induce nodule formation in target
35
What is the process from flavonoid in plant to nodule?
Plant root secretes flavonoid Attracts rhizobia, other effects Binds to NodD in rhizobia NodD activated transcription of other Nod genes Nod A,B,C synthesise core nod factor structure Other Nod genes enable synthesis of specific nod factors Nod factors secretes by rhizobia Nodule formation induced in specific target legume
36
How do roots and bacteria test compatibility
Flavonoid signalling and nod factor signalling are specific and so work between compatible species
37
When does the symbiosis occur
Starvation events :,( Individuals would rather be independent
38
What is the nature of the symbiotic relationship?
Rhizobia induce symbiosis specific differentiation Establishes a niche for the rhizobia Rhizobia provided with photosynthate Provide fixed nitrogen under n limiting conditions
39
What is the first effect of nod factors on the plant?
Root hair curling Induced root tip growth, resulting in curling and deformation Rhizobia is trapped in the curl May produce hydrolytic enzymes to degrade cell wall, facilitating entry
40
Why do sym1 and sym5 mutants not form nodules in response to factor?
Encodes receptor like kinases Both required for nod-factor recognition (work synergistically) Designated NFR1 and NFR5 (Nod Factor Receptor)
41
How does nod factor signalling work in L. Japonicus?
Extracellular NFR1 and NFR5 domain (are together in membrane) interacts w nod factor Causes membrane depolarisation with ion flux SYMRK then causes signalling into nucleus via calcium signalling into nucleus through nuclear membrane Ca transporters, regulates gene expression) Root hair curling occurs Infection thread formation Nodule organogenesis
42
Infection thread formation?
Plant P.M. invaginates Rhizobia enters invagination Cell wall material deposited around invagination to form infection thread Infection moved through root hair and transverses root to reach root cortex Thransferred to root cortex
43
What are the two different nodule types?
Determinate nodules: infect outer cortex (L. Japonicus) Indeterminate nodules: infect inner cortex (M. Truncatula)
44
What does infection do to cortical cells (cortex)
Usually don’t divide, just transport water and such Undergo differentiation before penetrated by infection thread Nucleus moves to cell centre Cytoplasm and endonembranes move to cell periphery Cell forms a Phragmoblast Thread releases some rhizobia into cell via endocytosis Rhizobia become surrounded by a plant derived prebacterioid membrane Bacteria are now differentiated into bacterioids
45
How much ATP for nitrogenase to fix N2 to 2NH3?
16 MgATP (whatever that is lol) (Reduction need 8H+ and 8e-) Products are 2NH3, H2, 16MgADP, 16Pi
46
What is a reason for rhizobacteria to maintain symbiosis trait when they have to terminally differentiate?
Nodules don’t just contain terminally differentiated bacterioids, they also have undifferentiated bacteria, thus increasing bacterial population Differentiation of bacterioids is recent evolved trait imposed by legume to increase n fixing
47
How do rhizobium mitotically activate the cortex cells?
Probably modulate phytohormine balance of cells (auxin:cytokinin ratio)
48
What is the stele’s role in mitotic reactivation?
Nodule develops from cortex cells adjacent to protoxylem cells in the stele These protoxylem cells may produce factor which functions in combo with rhizobia in reactivation
49
How is the number of nodules regulated?
Tightly regulated Too many results in parasitism not symbiosis Systemic signal (unknown) moves through root system suppressing node formation Hypernodulation mutants: excess number of nodules Hyponodulation mutants: reduced number

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