Gene for gene hypothesis

The concept of gene for gene hypothesis was first developed by Flor in 1956 based on his studies of host pathogen interaction in flax, for rust caused by Melampsora lini. The gene for gene hypothesis states that for each gene controlling resistance in the host, there is corresponding gene controlling pathogenicity in the pathogen. The resistance of host is governed by dominant genes and virulence of pathogen by recessive genes. The genotype of host and pathogen determine the disease reaction. When genes in host and pathogen match for all loci, then only the host will show susceptible reaction. If some gene loci remain unmatched, the host will show resistant reaction. Now gene – for –gene relationship has been reported in several other crops like potato, sorghum, wheat, etc. The gene for gene hypothesis is also known as “Flor Hypothesis.”

1. Gene for Gene Hypothesis Speaker: B.Rachana RAD/2018-18 Ph.D 1 st year (GPBR) GP-510 Breeding for Biotic and Abiotic stress Resistance

2. Disease is an abnormal condition in an organism produced by an organism or an environmental factor. Host is the plant affected by a disease. Pathogen is the organism that produces the disease on the host. Environment refers to sum total of conditions influencing the disease development.

3. Disease Development Conditions for disease : Host should be susceptible Pathogen should be virulent Environment should be favourable for the disease Disease Triangle

4. What is Resistance…..? The ability of an organism to exclude or overcome, completely or in some degree, the effect of a pathogen or other damaging factor . Vertical resistance Horizontal Resistance Two types of Resistance

5. Basis of Disease resistance R genes AVR genes Interaction

6. R genes Present in host plant . Control s a major step in the recognition of the pathogen and play s a major role in expression of resistance . Control Gene-for-Gene interaction . R gene product inactivate s the toxin .

7. Avr Genes Avr genes were first identified by H. H. Flor in 1950 . Mild genes of pathogen . Res p on si b le f or a c t i v a ti o n of c ert ain defense response in host . Le a d s to res i st a nce in c lu di n g hypersensitive response .

8. (Gururani et al . 2012)

9. P a th o gen Avr Gene Pl a nt R Gene Resi s tanc e Resp o nses incl. the HR EL I C I TOR

10. Examples of Avr genes and corresponding R- genes Plants Avirulent gene Pathogen Matching R genes References Rice AvrPITA Magnaporthe grisea Pi-ta Valent (1998) Tomato AvrPto Pseudomonas syringae pv. tomato Prf Salmeron et. al . (1996) Tomato AvrRpp8 Meloidogyne incognita and Macrosiphum euphorbia Mi Milligan et. al . (1998) Rossi et. al . (1998) Potato Coat protein Potato virus X (PVX ) Rx Bendahmane et. al . (1999) Potato Elicitin or AvrD Phytophthora infestans Pto Cai et. al. (2001) Tobacco Replicase Tobacco mosaic virus(TMV ) N Whitham et. al . (1994)

11. Gene-for-Gene Concept ‘ For each resistance gene in the host there is a corresponding gene for avirulence in the pathogen conferring resistance and viceversa ’ H.H . Flor ( 1955)

12. Linseed Rust caused by Melampsora lini

13. H . H. Flor conducted studies with flax ( Linum usitatissimum) and the flax rust pathogen ( Melampsora lini) ------ to understand the genetic basis of the interaction between resistance and virulence. Flor proposed gene ‐ for ‐ gene theory based on observations from his experiments ------ making crosses between both plants and pathogens to determine the inheritance of resistance and avirulence .

14. 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: Incompatible reaction Compatible reaction

15. Incompatible reaction Found in biotrophic pathogens (obligate parasites) and is associated with hypersensitive response of the host . Only one of the four combinations would lead to the resistant response since the products of R & A vr would recognize & interact with each other. The product of alleles a & r are unable to recognize each other, & there is no interaction between them , hence reaction of host becomes susceptible.

16. Plant Resistance /susceptibility genes Pathogen Avirulence /virulence genes A a R Resistance Susceptible r Susceptible Susceptible Allele A of the virulence gene specifies avirulence . Allele a of the virulence gene governs virulence.

17. Found in heterotrophic pathogens (facultative parasites ) . The allele for susceptibility of the host ( r) and those for virulence in the pathogen produce s specific compound, which interact s with each other to produce susceptible response. one of the four combinations would lead to susceptibility and rest lead s to resistant . Compatible reaction

18. Plant Resistance /susceptibility genes Pathogen Avirulence /virulence genes A a R Resistance Resistance r Resistance Susceptible Allele A of the virulence gene specifies avirulence . Allele a of the virulence gene governs virulence.

19. Gene-for-gene hypothesis Multifactor Interactions (Agrios 2007) Avirulenc e/vi r ulenc e Resistance/susceptibility R1 R2 r1 R2 R1r2 r1r2 A1A2 - - - + a1A2 - - + + A1a2 - + - + a1a2 + + + + w h e r e , – = Resistance + = Susceptible

20. Flor’s gene –for- gene hypothesis is purely a hypothesis of identities . T he r e si s tance g e ne i n the h ost a n d the corresponding virulence gene can be identified by this hypothesis. But it does not tell us about the gene quality . A second gene for gene hypothesis proposed by Vanderplank , which is an extension of Flor’s hypothesis, tells us about the quality of genes . Second Gene for Gene Hypothesis

21. The quality of resistance gene in the host determines the fitness of matching gene in the pathogen to survive, when this gene for virulence is unnecessary. Unnecessary gene means- a gene for virulence in t he path o gen popula ti on a ga i nst w h i ch m a t ch i ng resistance gene in the host is not present. Reciprocally, the fitness of the virulence gene in the parasite to survive when it is unnecessary d e t er m i nes t he q ua li t y o f m a t ch i ng r e s i s t ance gene in the host. For instance, there are ten or more genes in the host for resistance to late blight of potato , R1, R2, R3------------R10 .

22. Of these, the first four R1---R4 have been well studied. These genes have not been found of equal importance and strength. From the reports available in the literature, R4 has not been successfully used on its own by breeders although it has occasionally been used in combination with other genes. The R1 gene has often been used alone and it has given protection to the varieties against blight.

23. The difference between these R genes is that virulences on R4 preexisted in population of Phytophthora infestans whereas virulences on R1 didn’t (Van der Plank, 1975 ). The ratio for virulence between R1 and R4 genes has been found to differ significantly. Thus there is difference in the quality of resistance genes R1 and R4.

24. The source of pathogenic variability in pathogens . The mutability of resistance and virulence genes . Why host resistance is expressed under one set of conditions and not others . Cataloguing and storing of R genes in the form of plant seeds or cuttings . M a na g e m ent and d e p l o y m ent o f r e s i s t a n ce ge n es in space and t i me . Geographic distribution of R and V genes . Synthesis of multilines and multigene cultivars. Gene for Gene hypothesis is used to study the following

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26. Genetic studies of the flax-flax rust interaction led to the formulation of the gene-for-gene hypothesis and identified resistance genes (R) and pathogenicity genes, including avirulence (Avr). R genes have been cloned from four of the five loci in flax and all encode proteins of the Toll, Interleukin-1receptor, R gene-nucleotide binding site- leucine -rich repeat (TIR-NBS-LRR) class. Avr genes have been cloned from four loci in flax rust and encode small secreted proteins with no close homologs.

27. The AvrL567 gene product is recognized in plant cells by L5. (a) The flax rust gene variant AvrL567-A was modified for plant expression using the CaMV 35S promoter and nopaline synthase ( Nos ) and cloned for Agrobacterium-mediated transient plant transformation. ( b) Transient expression of AvrL567-A induced a necrotic response in flax leaves of the L5 genotype but not in NIL leaves lacking L5, L6 or L7 . (c ) The AvrL567-A protein would be secreted in to the space between the haustorial cell wall and invaginated host cell plasmamembrane ( extrahaustorial matrix, EHM). The secreted protein would then be transported across the plant membrane by an unidentified mechanism and recognized by direct interaction with the cytoplasmically localized L5 protein.

28. Avr proteins enter the host cell, have virulence effector functions and in resistant host genotypes, are recognized by direct and specific interaction with host R proteins, leading to activation of rust resistance defense responses. Direct interaction between R and Avr proteins is the basis of gene for-gene specificity in the flax-flax rust system . R and Avr genes have the signatures of diversifying selection, suggesting the existence of a co-evolutionary race between the host plant and its obligate rust pathogen.

29. In maize, the leaf blight disease caused by the fungus Cochliobolus carbonum race 1 (CCR1) affects net yield potential and its asexual form (i.e., Helminthosporium carbonum (HC)) is the most destructive biotic fungal pathogen at any stage of development . In maize the R gene Hm1 provides complete protection against southern leaf blight caused by CCR1. Hm1 was the first DR gene to be cloned, which disarms the pathogen directly instead of participating in the plant recognition and response system as most DR genes do. Apart from Hm1 gene, certain lines of maize contain a second DR gene named Hm2, encodes a structurally truncated duplicate of Hm1 . Both Hm1 and Hm2 encode nitrate reductases that detoxify the HC-toxin of CCR1 . Both these genes are different in two aspects; 1. Hm1 is completely dominant conferring absolute resistance to plants, whereas Hm2 exhibits incomplete dominance. 2. Hm1 - absolute protection in all parts of the plant at all stages of development, while Hm2 confers effective resistance only at maturity.

30. The NADPH-dependent HC-toxin reductases (HCTR1 and 2) encoded by enzymatic class of disease resistance homologous genes ( Hm1 and Hm2 ) protect maize by detoxifying a cyclic tetrapeptide , HC-toxin, secreted by the fungus  Cochliobolus carbonum  race 1(CCR1). Maize gene Hm 1 and HM2 Invasion by pathogen NADPH-Dependent HC toxin reductase Inactivates/Detoxify HC - toxin Prev e nts E n co d e s

31. F u ncti o nal g e n o m i c tools to dise a se r esist a nce – intera c ti o ns b e tween defense signaling and other plant processes. Str u ctural basis o f recognition will e n able u s - d e sign R prot e ins that recognize essential virulence factors. New trans g enic r e sistant p lants by exploiting both avirulence and resistance genes in molecular resistance breeding. Using avir u l ence ge n e pro d ucts/r a c e -specif i c signal transduction pathways can be studied.