The interaction of avirulence and resistance gene products in flax rust disease - providing advances in rust research

Pathogenic rust fungi constitute a major disease threat to agriculture, but their obligate parasitic lifestyle makes them difficult to study. Research on the model flax rust disease system has contributed greatly to our knowledge of rust infection and, in particular, the mechanisms of rust resistance and susceptibility controlled by resistance (R) genes in the host and avirulence (Avr) genes in the pathogen. Twenty flax R genes have been isolated and encode cytoplasmic proteins with nucleotide-binding domains and leucine-rich repeat regions. These proteins act as a surveillance system for the recognition of specific pathogen Avr proteins as signals of invasion and subsequently activate plant defences. Several of these rust Avr proteins have also been isolated and were found to be small secreted proteins that are expressed in haustoria (specialized structures that penetrate the host cell wall) and delivered into the host cytoplasm during infection. The identification of flax R genes and the corresponding fungal Avr genes has allowed more detailed analysis of the recognition events that trigger resistance, revealing a direct interaction between R and Avr proteins as the basis of resistance specificity in flax rust disease.