Trends in Biosciences

An increasing number of resistance (R) genes have been isolated from rice, maize, wheat and barley that encode both structurally related and unique proteins. The R protein diversity may be attributable to the different fungus employed by pathogen in species, but it is also a consequence of multiple defence strategies being employed against phytopathogens. Mutational analysis of barley has identified additional genes required for activation of an R gene-mediated defence response upon pathogen infection. In some instances very closely related barley R proteins require different proteins for defence activation, demonstrating that, within a single plant species, multiple resistance signalling pathways and different resistance strategies have evolved to confer protection againsta single pathogen species. Although the apparent diversity ofcereal resistance mechanisms, some of the additional molecules required for R protein function are conserved amongst cereal and dicotyledonous species and even other eukar yotic species. Molecular analyses in cereals have further highlighted the complexity of plant–pathogen coevolution and have shown that numerous active and passive defence strategies are employed by plants against phytopathogens.