Gene for gene concept Hem raj pant

Msc. Ag
Department of plant pathology
AFU- Agriculture and Forestry University
Rampur, chitwan
www.afu.edu.np

1. Welcome & Namaste

2. Gene for gene hypothesis for developing resistant variety against obligate pathogens Presenter Hem Raj Pant Department of pathology

3. DISEASE DEVELOPMENT Pathogen Host environment Disease Fig: Disease triangle Conditions for disease: Host should be susceptible Pathogen should be virulent Environment should be favourable for the disease

4. Resistance The ability of an organism to exclude or overcome, completely or in some degree, the effect of a pathogen or other damaging factor. 2 types of resistance 1. Vertical resistance/major gene resistance/qualitative resistance 2. Horizontal resistance/minor gene resistance/quantitative resistance

5. Basis of disease of resistance Components a) R genes: Present in host plant Control a major step in the recognition of the pathogen and play a major role in expression of resistance Control Gene-for-Gene interaction R gene product inactivate toxin

6. b) Avr genes: Avr genes were first identified by H.H. Flor in 1950 Mild genes of pathogen Responsible for activation of certain defense response in host Lead to resistance including hypersensitive response c) Interaction (host pathogen interaction)

7. GENE FOR GENE RELATIONSHIP Flor (1942-1956) conducted series of experiments on inheritance of resistance in the host and virulence in the parasite in relation to each other He concluded his results in the gene for gene hypothesis This hypothesis states that for each resistance gene in the host there is a corresponding gene for avirulence in the pathogen confering resistance and viceversa

8. Table: Host reaction to Gene-for-Gene combination in M. lini - Flax system Host resistance genes at locus L Pathogen virulence genes conditioned by host genes at locus L Type of rust reaction L L AL AL or AL aL Resistant L L aL aL Susceptible l l AL AL or AL aL Susceptible l l aL aL Susceptible

9. This indicates that a plant is resistant only when it is homozygous for resistance (L L ) and is attacked by a pathogen that is homozygous (AL AL ) or heterozygous (AL aL ) for avirulence When, however, the plant lacks gene for resistance (l l ) at locus L, or the pathogen is homozygous for virulence ( aL aL ) for that locus, then the plant is susceptible This interaction has been proved by inoculation of crosses of two varieties with 2 races of the pathogen One variety was resistant to one race and the other to the second race All host pathogen combinations giving dominant complimentary genes in both the host and the pathogen resulted in resistant reaction while the rest resulted in susceptibility

10. The gene for gene hypothesis proved for flax- M. lini system can explain and predict all host-pathogen systems found applicable to a large number of host-parasite relationships including the following: Apple- Venturia inaequalis (scab) Coffee- Hemileia vastatrix (coffee rust) Potato- Phytophthora infestans (late blight) Rice- Magnaporthe grisea (blast of rice) Wheat- Ustilago segetum tritici (Loose smut of wheat)

11. CRITERIA FOR GENE-FOR-GENE TYPE OF RELATIONSHIP Gene-for-gene type of relationship has not been found in all host-parasite systems Most common in specialized or obligate parasites and totally absent in non-specialized (facultative) pathogens According to Van der Plank (1973) gene-for-gene relationship can exist only in those host parasite systems where both the components are living for a certain critical period for time in order to let their nuclei transcribe, translate and subsequently exchange genetic information This criterion is, however, not applicable to some host-pathogen systems which are necrotrophs producing host-specific toxin such as oat- Helminthosporium victoriae and necrosis-inducing peptides such as barely- Rhychosporium secalis .

12. MOLECULAR BASIS FOR GENE-FOR-GENE RELATIONSHIP On the basis of molecular interactions involved in producing resistant/susceptible responses in the host, the gene-for-gene relationship may be classified into two general groups: a) Incompatible reaction b)compatible reaction

13. Incompatible reaction Found in biotrophic pathogens (obligate parasites) and is associated with hypersensitive response of the host Only one of the 4 combinations would lead to the resistant response since the products of R and A would recognize and interact with each other The product of alleles a and r are unable to recognize each other, and there is no interaction between them hence reaction of host becomes susceptible   Allele A of the virulence gene specifies avirulence Allele a of the virulence gene governs virulence Plant Resistance/ Susceptiblity genes Pathogen Avirulence /virulence genes A a R Resistance Susceptible r Susceptible Susceptible

14. Compatible reaction Found in heterotrophic pathogens (facultative parasites) The allele for susceptibility of the host (r) and those for virulence in the pathogen produce specific compound, which interacts with each other to produce susceptible response One of the 4 combinations would lead to susceptibility and rest lead to resistant Allele A of the virulence gene specifies avirulence Allele a of the virulence gene governs virulence Plant Resistance/ Susceptiblity genes Pathogen Avirulence /virulence genes A a R Resistance Resistance r Resistance Susceptible

15. Biotrophy and gene-for-gene systems All the parasites in which gene for gene relationship has been proved are essentially biotrophic or biotrophs at least for some time after start of infection e.g. Xanthomonas campestris pv . Malvacearum , Phytophthora infestans , Venturia inaequalis , etc. The genes-for-gene systems thus involve biotrophy But the converse is not necessarily true. For example, Plamodiophora brassicae , the cause of club root of crucifers, is biotrophic but no evidence has yet been presented in the literature to suggest that host-pathogen interaction in them is based on a gene-for –gene systems

16. Applications of Gene-for-gene hypothesis For developing resistant variety against obligate pathogens Source of pathogenic variability in pathogens Mutability of resistance and virulence genes Why host resistance is expressed under one set of conditions and not others Prediction for putative genotypes Cataloguing and storing of R genes in the form of plant seeds or cuttings and V genes in the form of pathogen strains