Plant Defence

Question 1

Passive defences

Answer 1

• Basic structure of plant
• Cuticular waxes
• Stomata in higher densities on lower side of leaf
• Leaf architecture
• Shedding of leaves
• Seasonality of growth
• Plant-derived germination inhibitors on leaf surface
• Nutrient-poor environment on leaf surface
• Apoplast nutrient-poor
• Commensal microbes to protect against pathogens

Question 2

How can commensal microbes protect against plant pathogens?

Answer 2

Sphingomonas suppressed disease symptoms and repressed the growth of the pathogen Pseudomonas syringae pv. tomato when infecting Arabidopsis thaliana
Innerebner et al. 2011

Question 3

Active defences: changes to plant cell wall

Answer 3

• Thickening of cell wall with cellulose and phenolic compounds
• Rapid decomposition of material below penetration sites
• Papillae deposited to surround and block out pathogen
• Swelling of wall fibres
• Corky layers (rapid cell division followed by wall thickening and deposition of antimicrobial phenolics)
• Abscission (excision of plant material leaving hole)
• Tyloses block xylem vessels ahead of pathogen
• Gums and resins isolate pathogen from host

Question 4

Examples of chemical defences used by plants

Answer 4

• Alkaloids
• Lectins
• Saponins - bind to steriles in fungal cell walls and puncture membrane
• Tannins
• Phytoalexins - antimicrobial, accumulate around infection site, include alkaloids and terpenoids

Question 5

Protein-based defence

Answer 5

• Antimicrobial proteins
• Enzyme inhibitors - block activity of cell wall degrading enzymes
• Enzymes - chitinases, lysozymes, etc.
• Pathogenesis-related (PR) proteins - secreted in response to infection into the apoplast, includes chitinases

Question 6

Oxidative burst (active defence)

Answer 6

• Rapid production of hydrogen peroxide (attacks pathogen and modifies plant lipids to warn other cells)
• Triggers hypersensitive response
• Membrane lipids oxidised by H2O2, and triggers jasminate production

Question 7

Hypersensitive response

Answer 7

• Controlled suicide of a few plant cells (apoptosis)
• Restricts the growth and spread of biotrophic fungi and viruses
• Accompanied by toxins, phenolics, etc
• For most plant species, HR is dominantly expressed and controlled by one gene, which recognises a specific pathogen and turns on a series of host defence genes

Question 8

Systemic acquired response

Answer 8

• Diseased/wounded cells send signals throughout plant
• Signals switch on or prepare the defence pathways in neighbouring tissues
• Signal molecules include systemin, salicylic acid, methyl jasmonate and ethylene

Question 9

NBS-LRRs

Answer 9

Nucleotide bonding site - leucine rich repeats
LRR forms a receptor
Protein contains NBS activity
Signal transduction cascade when activated which switches on resistance pathways
E.g. tobacco N gene
NBS-LRRs can be used for extracellular detection and intracellular detection including within the nucleus

Question 10

N gene

Answer 10

Example of single gene which controls HR
Isolated from N. glutinosa - hosts resistant to TMV
N gene confers resistance to all known Tobamoviruses except Ob
Single nucleotide mutation within replicase gene responsible for Ob resistance to N gene
-Expressed by alternative splicing: N(S) and N(L)
-N(S) more prevalent before and 3 hours after TMV infection
-N(L) truncated and more prevalent 4-8 hours after infection