Genetics of Plant Resistance to Viruses

Summary

This paper concerns the genetics of resistance used by the plant breeder to produce cultivars resistant to viruses. Non-host immunity, and resistance induced in normally susceptible individuals, are discussed only where they may share mechanisms with cultivar resistance. Conclusions about the genetics of resistance and of virulence (the ability of a virus isolate to overcome a specific resistance gene) are drawn from a survey of 63 combinations of hosts and viruses, and from comparisons with the predictions made from various theoretical models of host-virus interactions. Most resistance mechanisms that result in virus localization appear to involve an inducible, positive inhibitor of virus replication or spread, which tends to be temperature sensitive. Resistance mechanisms which permit some systemic spread of virus tend to be incompletely dominant (gene-dosage dependent) and are determined by quantitative interactions between host- and virus-specified functions. Completely recessive resistance is rare, and may involve a negative mechanism where the resistant plant lacks a susceptibility function. Most of the resistance genes considered have been overcome by virulent isolates of virus; extreme durability is rare. It appears easier for viruses to mutate to overcome dominant localizing resistance than recessive immunity mechanisms.