Fungal avirulence genes: structure and possible functions.

Avirulence (Avr) genes exist in many fungi that share a gene-for-gene relationship with their host plant. They represent unique genetic determinants that prevent fungi from causing disease on plants that possess matching resistance (R) genes. Interaction between elicitors (primary or secondary products of Avr genes) and host receptors in resistant plants causes induction of various defense responses often involving a hypersensitive response. Avr genes have been successfully isolated by reverse genetics and positional cloning. Five cultivar-specific Avr genes (Avr4, Avr9, and Ecp2 from Cladosporium fulvum; nip1 from Rhynchosporium secalis; and Avr2-YAMO from Magnaporthe grisea) and three species-specific Avr genes (PWL1 and PWL2 from M. grisea and inf1 from Phytophthora infestans) have been cloned. Isolation of additional Avr genes from these fungi, but also from other fungi such as Uromyces vignae, Melampsora lini, Phytophthora sojae, and Leptosphaeria maculans, is in progress. Molecular analyses of nonfunctional Avr gene alleles show that these originate from deletions or mutations in the open reading frame or the promoter sequence of an Avr gene. Although intrinsic biological functions of most Avr gene products are still unknown, recent studies have shown that two Avr genes, nip1 and Ecp2, encode products that are important pathogenicity factors. All fungal Avr genes cloned so far have been demonstrated or predicted to encode extracellular proteins. Current studies focus on unraveling the mechanisms of perception of avirulence factors by plant receptors. The exploitation of Avr genes and the matching R genes in engineered resistance is also discussed.