Nonhost plant species as donors for resistance to pathogens with narrow host range. II. Concepts and evidence on the genetic basis of nonhost resistance

Summary It is commonly argued that wild plant species with a very high level of resistance to a micro-organism which is pathogenic on a crop species would be useful donors of resistance. Whether this principle is true if the wild species is a distinct nonhost, remains to be proved. It is assumed that nonhost resistance, if not based on avoidance, rests on a poor adaptation of the micro-organism to general defence mechanisms in the plant. No basic compatibility is achieved. Major-genic hypersensitive host resistance would, according to the concept, be superimposed on basic compatibility. The defence reaction is switched on if a resistance allele recognizes a specific avirulence factor produced by the pathogen. The frequency of the resistance allele would depend on the selection pressure exerted by the pathogen population.In contrast to these concepts, there is evidence that nonhost resistance, at least to formae speciales, rests on an extremely high allele frequency of effective major genes for resistance, rather than on a complex of genes that play part in a general defence system. Especially the high allele frequency and the durability of the nonhost resistance are in conflict to what would be expected if the concepts hold true. It is concluded that part of the theoretical concepts needs to be revised or extended to reconcile them with the evidence. A major-genic base for nonhost resistance would be very interesting from the breeder's point of view, because transfer of such a resistance to a crop would be less hard to accomplish than that of more complex inherited defence systems.