Comparison of avrD alleles from Pseudomonas syringae pv. glycinea.

Avirulence gene D alleles resided on indigenous plasmids in races 0, 2, 3, 4, 5, and 6 of Pseudomonas syringae pv. glycinea (Psg), but the allele in race 1 appeared to be chromosomal. These were all nonfunctional avirulence genes because they neither induced the avirulence phenotype on Rpg4 soybean cultivars nor directed the production of syringolide elicitors when expressed in Escherichia coli cells. The predicted proteins encoded by the seven Psg avrD genes were very similar to that of a functional class II allele from P. syringae pv. phaseolicola G50 race 2, but contained mutations collectively affecting only nine amino acid positions. Despite these relatively small amino acid differences and the location of avrD from each isolate on a 5.6-kb HindIII restriction fragment, the flanking regions varied considerably among the Psg isolates. The presence of avrD alleles with few alterations but different locational contexts in all tested Psg races argues that they provide an important selected function in the bacteria but have been modified to escape defense surveillance in Rpg4 soybean plants.