GENE-FOR-GENE RECOGNITION IN PLANT-PATHOGEN INTERACTIONS

Mediated through specifically matching allele pairs in the host and pathogen (at resistance and avirulence loci respectively), plants have a refined and highly discriminating capability to recognize and differentiate among genetic variants of potential pathogens. Knowledge of pathogen recognition by plants has primarily resulted from research associated with the selective breeding of crop species for disease resistance. The phenomenon is well described at the cellular, whole plant and population levels in terms of genetics, histology and associated biochemistry. However, a full mechanistic understanding is only now becoming feasible following the isolation and sequencing of putatively interacting plant and pathogen genes. It is evident that the number of genes in plant genomes capable of specific pathogen recognition is large and that these genes are commonly clustered in complex loci sometimes comprising genes involved in the recognition of more than one taxonomically unrelated pathogen. For same genes, alleles with different recognition capabilities have been identified while there is also evidence for the existence of genes expressing identical recognition capability present at different loci in the same species as well as in different plant species. It seems likely that resistance genes are members of substantial multi-gene families and there is evidence that novel recognition capability may be created through interallelic recombination events. In natural populations, there can be a spatial mosaic of resistance genotypes with individuals carrying a range of resistance alleles at a varying number of loci, some of which will exert selection on the pathogen population only under certain conditions and at certain stages of development. There have been few studies of the costs and benefits of particular resistance genes in the presence or absence of pathogen variants against which a gene may be effective or ineffective. The stage of host development at which the pathogen exerts the greatest impact on survival and fecundity is of particular relevance in this respect, as is the influence of mother plant resistance on the characteristics and quantity of inoculum to which progeny are exposed.