EVOLUTION OF RESISTANCE TO FUNGAL PARASITISM IN NATURAL ECOSYSTEMS

It is argued that the resistance of non-host plants to plant parasitic fungi is the result of parasite-non-specific basic resistance rather than the consequence of specific recognition of each fungal parasite. In crop plants, and probably wild plant species, this basic resistance in maintained by a mixture of passive and active defenses that exhibit inter-, and possibly intra-, specific differences. Many, if not all, of the components of basic resistance not only defend the plant against stresses in addition to those exerted by plant parasitic fungi, but also have other roles in plant structure and function. Therefore, these components may be subject to random effects and multiple selection pressures of which fungal parasitism may be a minor component. Nevertheless, the coupling of the recognition of fungal products or activities with the inducible components of basic resistance seems to have occurred early in plant evolution, although not necessarily in response to selective pressures exerted by ancient fungi. It is suggested that in natural ecosystems, parasite-specific resistance should have no selective advantage until the fungus has established sufficient basic compatibility (by means of specific pathogenicity factors) with its host species that the plant's fitness is reduced. This resistance may not necessarily be the result of 'gene-for-gene' interactions, and may be difficult to distinguish from pathovar-specific, 'residual' basic resistance resulting from fungal genotypes differing in their complement of pathogenicity factors such that each one is able to completely overcome the basic resistance of only some genotypes of the host. Superimposed on any co-evolutionary changes in plant and fungus are longer-term changes in basic resistance that may eventually change a susceptible host into a non-host if the fungus cannot produce the necessary new or modified pathogenicity factors. Major determinants of the evolutionary paths followed by any interacting plant and fungus are the actual mechanisms of plant resistance and fungal pathogenicity or virulence. Therefore, the complex web of interactions that exists today reflects the evolutionary path that each organism has followed as well as more recent selective pressures on the interactions themselves.