Gene-for-gene interaction results from the genetic relationship between a pathogen and a host. The expression of susceptibility or resistance of the host toward the presence of a specific pathogen is dependent on the genotype of both of them; similarly, the virulence of the pathogen is expressed on the basis of the host genotype.
Gene-for-gene hypotesis states that such an interaction is determined by the presence of a single plant dominant R-gene which allows direct or indirect recognition of pathogens producing specific proteins (codified by avirulence genes). (Staskawicz BJ. Genetics of plant-pathogen interactions specifying plant disease resistance.Plant Physiol. 2001 Jan;125(1):73-6. PMID: 11154300)
This interaction leads to a compatibility or an incompatibility relation. The first is achieved when the pathogen can develop and reproduce in absence of an effective resistance response of the host. If the host develops a resistence allele towards the presence of the pathogen, a condition of incompatibility is achieved and the pathogen isn't able to develop effectively because of the combination between R-genes and Avr genes (an epistatic relation). (Crute IR, Pink D. Genetics and Utilization of Pathogen Resistance in Plants.Plant Cell. 1996 Oct;8(10):1747-1755. PMID: 12239360)
Plants have evolved defense mechanisms capable of both recognizing and responding to several bacterial, fungal, oomycete, and viral pathogens, other than resistance against nematodes and insects. Despite the broad spectrum of resistance promoted by R-proteins, these gene products can be classed into two main groups, based on protein domain organization.
(Chisholm ST, Coaker G, Day B, Staskawicz BJ. Host-microbe interactions: shaping the evolution of the plant immune response. Cell. 2006 Feb 24;124(4):803-14. PMID: 16497589
Plant resistance (R) proteins recognise pathogen avirulence (Avr) determinants and in turn trigger signal transduction cascades that lead to rapid defence activation. Direct association between R–Avr proteins has been found in "in vitro" studies, suggesting a receptor–ligand model, data confirming indirect R–Avr protein interactions (involving perception of pathogen-derived proteins within a complex). (Hammond-Kosack KE, Parker JE. Deciphering plant-pathogen communication: fresh perspectives for molecular resistance breeding. Curr Opin Biotechnol. 2003 Apr;14(2):177-93. PMID:12732319)
The possible genetic interactions between palnts and pathogens are represented in the following figure. Pathogen TOX gene codify for a toxic product required by the pathogen to induce virulence, while tox gene is the corrispective recessive allele. Rx gene allows the effective response of the plant to the pathogen attack, while rr is the recessive allele. An incompatibility (I) interaction is established when the plant can respond to pathogen attack or when pathogen can not produce effective toxin o virulence product. Conversely a compatible (C) interaction is allowed when pathogen produce the virulence factor and the plant can not develop a resistence response. Similarly, if two resistance genes (R1 and R2 the dominant effective resistance allele, r1 and r2 represent the recessive alleles) are recognized in the host plant genome and two corresponding avirulence genes are present in pathogen genome, compatibility (C) is achieved when the host is unable to produce a resistance response or the pathogen to encode a functional avirulence factor, and convesely for incompatibility (I). (Hammond-Kosack KE, Jones JD. PLANT DISEASE RESISTANCE GENES.Annu Rev Plant Physiol Plant Mol Biol. 1997 Jun;48:575-607. PMID: 15012275).
Compatibility between a pathogen and a host occurs when plant resistance genes or pathogen avirulence genes are lost or modified.