The Melampsora lini AvrL567 avirulence genes are expressed in haustoria and their products are recognized inside plant cells

The Linum usitatissimum (flax) L gene alleles, which encode nucleotide binding site-Leu rich repeat class intracellular receptor proteins, confer resistance against the Melampsora lini (flax rust) fungus. At least 11 different L resistance specificities are known, and the corresponding avirulence genes in M. lini map to eight independent loci, some of which are complex and encode multiple specificities. We identified an M. lini cDNA marker that cosegregates in an F2 rust family with a complex locus determining avirulence, on the L5, L6, and L7 resistance genes. Two related avirulence gene candidates, designated AvrL567-A and AvrL567-B, were identified in a genomic DNA contig from the avirulence allele, whereas the corresponding virulence allele contained a single copy of a related gene, AvrL567-C. Agrobacterium tumefaciens-mediated transient expression of the mature AvrL567-A or AvrL567-B (but not AvrL567-C) proteins as intracellular products in L. usitatissimum and Nicotiana tabacum (tobacco) induced a hypersensitive response-like necrosis that was dependent on coexpression of the L5, L6, or L7 resistance gene. An F1 seedling lethal or stunted growth phenotype also was observed when transgenic L. usitatissimum plants expressing AvrL567-A or AvrL567-B (but not AvrL567-C) were crossed to resistant lines containing L5, L6, or L7. The AvrL567 genes are expressed in rust haustoria and encode 127 amino acid secreted proteins. Intracellular recognition of these rust avirulence proteins implies that they are delivered into host cells across the plant membrane. Differences in the three AvrL567 protein sequences result from diversifying selection, which is consistent with a coevolutionary arms race.