Salicylic Acid and Phytoalexin Induction by a Bacterium that Causes Halo Blight in Beans

pv. is a bacterium that causes halo blight in beans. Different varieties of beans have hypersensitive resistance to specific races of pv. . During hypersensitive resistance, also known as effector-triggered immunity, beans produce hormones that signal molecular processes to produce phytoalexins which are presumed to be antibiotic to bacteria. To shed light on hormone and phytoalexin production during immunity, we inoculated beans with virulent and avirulent races of pv. . We then used mass spectrometry to measure the accumulation of salicylic acid, the primary hormone that controls immunity in plants, and other hormones including jasmonate, methyljasmonate, indole-3-acetic acid, abscisic acid, cytokinin, gibberellic acid, and 1-aminocyclopropane-1-carboxylic acid. Salicylic acid, but no other examined hormone, consistently increased at sites of infection to greater levels in resistant beans compared to susceptible beans at 4 days after inoculation. We then monitored 10 candidate bean phytoalexins. Daidzein, genistein, kievitone, phaseollin, phaseollidin, coumestrol, and resveratrol increased alongside salicylic acid in resistant beans but not in susceptible beans. In vitro culture assays revealed that salicylic acid, daidzein, genistein, coumestrol, and resveratrol inhibited pv. race 5 culture growth. These results demonstrate that these phytoalexins may be regulated by salicylic acid and work with salicylic acid during effector-triggered immunity to restrict bacterial replication. This is the first report of antibiotic activity for daidzein, genistein, and resveratrol to pv. . These results improve our understanding of the mechanistic output of effector-triggered immunity toward this bacterial pathogen of beans.