1. Resistance
Inherent capacity of a host plant to prevent or retard the development of an infectious disease
Complete resistance
vertical resistance
Highly specific (race specific)
Involves evolutionary genetic interaction (arms race)
between host and one species of pathogen
QUALITATIVE
Partial Resistance
horizontal resistance
Not specific- confers resistance to a range of pathogens
QUANTITATIVE

2. Gene-for-Gene theory of Complete Resistance
Pathogen has virulence (a) and avirulence (A) genes A a
Plant has resistance gene RR rr
If the pathogen has an Avirulence gene and the host a Resistance gene, then there is no infection

3. Gene-for-Gene theory of Complete Resistance
The Avirulence gene codes for an Elicitor molecule or protein controlling the synthesis of an elicitor
The Resistance gene codes for a receptor molecule which ‘recognises’ the Elicitor
A plant with the Resistance gene can detect the pathogen with the Avirulence gene
Once the pathogen has been detected, the plant responds to destroy the pathogen. .

4. Gene-for-Gene theory of Complete Resistance
What is an elicitor?
It is a molecule which induces any plant defence response.
It can be a polypeptide coded for by the pathogen avirulence gene, a cell wall breakdown product or low-molecular weight metabolites.
Not all elicitors are associated with gene-for-gene interactions.
What do the Avirulence genes (avr genes) code for?
They are very diverse!
In bacteria, they seem to code for cytoplasmic enzymes involved in the synthesis of secreted elicitor. In fungi, some code for secreted proteins, some for fungal toxins.

5. Gene-for-Gene theory
Specific resistance to a plant disease is based on what is called gene-for-gene recognition, because it depends on a precise match-up between a genetic allele in the plant and an allele in the pathogen.
This occurs when a plant with a specific dominant resistance alleles (R) recognizes those pathogens that possess complementary avirulence (Avr) alleles.
Specific recognition induces expression of certain plant genes, products of which defend against the pathogen.
If the plant host does not contain the appropriate R gene, the pathogen can invade and kill the plant.
There are many pathogens and plants have many R genes.

6. Gene-for-Gene theory
Resistance occurs if the plant has a particular dominant R allele that corresponds to a specific dominant Avr allele in the pathogen.
The product of an R gene is probably a specific receptor protein inside a plant cell or at its surface.
The Avr gene probably leads to production of some “signal” molecule from the pathogen, a ligand capable of binding specifically to the plant cell’s receptor.
The plant is able to “key” on this molecule as an announcement of the pathogen’s presence.
This triggers a signal-transduction pathway leading to a defense response in the infected plant tissue.

7. Gene-for-Gene theory
Disease occurs if there is no gene-for-gene recognition because (b) the pathogen has no Avr allele matching an R allele of the plant, (c) the plant R alleles do not match the Avr alleles on the pathogen, or (d) neither have recognition alleles.

9. Gene-for-Gene theory
Even if a plant is infected by a virulent strain of a pathogen - one for which that particular plant has no genetic resistance - the plant is able to mount a localized chemical attack in response to molecular signals released from cells damaged by infection.
Molecules called elicitors, often cellulose fragments called oligosaccharins released by cell-wall damage, induce the production of antimicrobial compounds called phytoalexins.

10. ELICITORS
Elicitors are proteins made by the pathogen avirulence genes, or the products of those proteins
Elicitors of Viruses
Coat proteins, replicases, transport proteins
Elicitors of Bacteria
40 cloned, kDa in size
Elicitors of Fungi
Several now cloned- diverse and many unknown function
Elicitors of Nematodes
Unknown number and function

11. Model for the action of Xa21 (rice blight resistance gene)
Leucine-rich receptor
Transmembrane domain
Kinase
Membrane
Elicitor
Signal transduction
([Ca2+], gene expression)
Cell Wall
Plant Cell

12. Plant Defense Response
Compatible interaction  disease
Incompatible interaction  resistance
Compatible interaction  disease
Incompatible interaction  resistance
3 aspects of response:
Hypersensitive
Local
Systemic

13. 4Before they die, infected cells release a chemical signal, probably salicylic acid.
5 The signal is distributed to the rest of the plant.
3 In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf.
6 In cells remote from the infection site, the chemical initiates a signal transduction pathway.
7 Systemic acquired
resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days.
Signal
Signal
transduction
pathway
Hypersensitive
response
2 This identification
step triggers a
signal transduction
pathway.
Signal transduction
pathway
Acquired
resistance
1 Specific resistance is
based on the binding of ligands from the pathogen to receptors in plant cells.
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance

14. HR (hypersensitive response)
Rapid localized necrosis of invaded tissue that accompanies containment of pathogen
Host membrane changes
oxygen radicals (H2O2, -OH) and nitrous oxide
excessive oxidation of polyphenols
signal transduction molecules activated
Lipoxygenases
Rapid accumulation of phytoalexins and pathogenesis-related proteins at infection site = decreased pathogen multiplication
Membrane collapse (releases antimicrobial substances and denies biotrophs the necessary living substrate)

15. Hypersensitive Response
The hypersensitive response
Causes cell and tissue death near the infection site
Induces production of phytoalexins and PR proteins, which attack the pathogen
Stimulates changes in the cell wall that confine the pathogen

16. Local responses Cessation of cell cycle
Induction of genes that promote resistance
Phenylpropanoid pathway induced: products include salicylic acid (secondary inducer: induces other pathogenesis-related proteins), lignins (cell wall), and flavonoids
Pathogenesis-related (PR) proteins
Phytoalexins increased
Fortification of cell walls with lignin, hydroxyproline-rich glycoproteins (HRGPs), etc.

17. Systemic Acquired Resistance
Inducer inoculation
Local acquired resistance
3 days to months,
then inoculate
Systemic acquired resistance
SAR- long-term resistance to a range of pathogens throughout plant caused by inoculation with inducer inoculum

19. Systemic Acquired Resistance (SAR)
It is a set of generalized defense responses in organs distant from the original site of infection
It is triggered by the signal molecule salicylic acid (which activates plant defenses throughout the plant before infection spreads)
Involves gene activation and a transmitted signal.
Genes induced:
chitinases
β 1,3- glucanases
other PR proteins