Phaseotoxin suppresses bacterially induced hypersensitive reaction and phytoalexin synthesis in bean cultivars

Abstract

The hypersensitive reaction (HR) of leaves of bean seedlings of cultivars GN Nebraska 27 (GN), P.I. 150414 (PI) and U.I. 34 (UI) to incompatible isolates of Pseudomonas phaseolicola was accompanied by accumulation of antibacterial phytoalexins, phaseollin, phaseollinisoflavan, coumestrol and kievitone. When GN and PI plants were treated with phaseotoxin prior to inoculation with the incompatible isolate HB-36 of the pathogen, HR was suppressed and typical susceptible symptoms were produced in inoculated leaves. Concomitant with the suppression of HR a significant reduction in the accumulation of phytoalexins was observed. Furthermore, in toxin-treated tissues bacterial multiplication was several orders of magnitude greater than in hypersensitively responding controls. A similar pattern of HR suppression, phytoalexin accumulation and increase in bacterial multiplication was observed in UI plants inoculated with the toxigenic strain HB-36 (which elaborated phaseotoxin “ in vivo”). UI plants inoculated with the non-toxigenic isolate HB-33 of P. phaseolicola which served as controls showed HR, accumulation of phytoalexins and restriction of bacterial growth. These studies indicate that phaseotoxin-induced susceptibility of bean cultivars to P. phaseolicola may result from the suppression of synthesis of antibacterial compounds.