Cultivar-specific avirulence and virulence functions assigned to avrPphF in Pseudomonas syringae pv. phaseolicola, the cause of bean halo-blight disease

The avrPphF gene was cloned from Pseudomonas syringae pathovar phaseolicola (Pph) races 5 and 7, based on its ability to confer avirulence towards bea n cultivars carrying the R1 gene for halo-blight resistance, such as Red Me xican. avrPphF comprised two open reading frames, which were both required for function, and was located on a 154 kb plasmid (pAV511) in Pph, Strain R W60 of Pph, lacking pAV511, displayed a loss in virulence to a range of pre viously susceptible cultivars such as Tendergreen and Canadian Wonder. In T endergreen virulence was restored to RW60 by avrPphF alone, whereas subclon ed avrPphF in the absence of pAV511 greatly accelerated the hypersensitive resistance reaction caused by RW60 in Canadian Wonder. A second gene from p AV511, avrPphC, which controls avirulence to soybean, was found to block th e activity of avrPphF in Canadian Wonder, but not in Red Mexican. avrPphF a lso conferred virulence in soybean. The multiple functions of avrPphF illus trate how effector proteins from plant pathogens have evolved to be recogni zed by R gene products and, therefore, be classified as encoded by avirulen ce genes.