THE GENETICS OF PLANT-VIRUS INTERACTIONS - IMPLICATIONS FOR PLANT-BREEDING

Host resistance is the main means of control of plant virus diseases. This paper reviews the genetics of resistance and matching virulence. Theoretical models of basic compatibility between plant species and their viruses, and of resistance, are described and used to predict features of resistance genetics, and mechanisms. These predictions are compared with a survey of known examples of resistance. Resistance is mainly controlled at a single genetic locus, although more complex systems are known. About half of the resistance alleles studied were dominant, the remainder were either incompletely dominant or recessive. Doubt is cast on the reliability of assessing resistance genotypes (numbers of loci and dominance relationships) from 'distant' phenotypic measurements such as symptom severity or plant growth. A model is proposed to reconcile apparent inconsistencies between genotype and phenotype. Dominant resistance alleles are strongly associated with virus localising mechanisms normally involving local lesions. Incompletely dominant and recessive alleles allow spread of the virus, but inhibit multiplication or symptom development. Fully recessive alleles may be associated with complete immunity. Most resistance genes in the survey had been overcome by virulent virus isolates with dominant localising resistance alleles especially vulnerable. Comparatively few resistance genes have proved exceptionally durable. Acquisition of virulence can be associated with loss of general pathogenic fitness, but in some cases this can be restored by further selection of the virus in resistant hosts. Virulence/avirulence determinants have been mapped to individual base changes in different functional regions of the viral genome. A virus may contain several virulence determinants and may develop a stable gene-for-gene relationship with a host having several resistance genes. It may be possible to design robust, oligogenic resistance systems which will be difficult for the virus to overcome.