Avirulence proteins of rust fungi: penetrating the host-haustorium barrier

The continued deployment of resistance genes in Australian wheat varieties plays a crucial role in the prevention of crop losses as a result of rust infection. Resistance genes in plants act as an immune system by recognising specific components of the rust pathogen, encoded by avirulence (Avr) genes, and initiating defence responses. Recent advances in the model rust species, Melampsora lini, which infects cultivated. flax (Linum usitatissimum), has led to the identification of several avirulence genes. These encode secreted proteins that are expressed in haustoria, specialised rust structures that penetrate the host cell wall. The AvrL567, AvrM, AvrP4, and AvrP123 proteins are recognised when expressed intracellularly in resistant plants, and the AvrL567 proteins interact directly with the corresponding L5 and L6 cytoplasmic resistance proteins. These observations imply that rusts deliver a range of small effector proteins directly into the plant cytoplasm, which are likely to perform key functions in facilitating pathogen infection. The plants' resistance protein system has evolved to recognise the presence of these effectors during infection. Selection imposed by host resistance genes has led to the diversification of flax rust Avr genes to escape recognition. Nevertheless, virulent strains of flax rust retain variant forms of the Avr genes, suggesting that they have a positive fitness value to the pathogen. This knowledge is now being applied to develop screens for avirulence and pathogenicity-related proteins from important rust pathogens of wheat.