Suppression of bacterially-induced hypersensitive reaction and phytoalexin accumulation in bean by phaseotoxin

Abstract

PHASEOTOXIN is a trivial name given to an exotoxin produced by Pseudomonas phaseolicola, which causes halo blight of beans¹. Although the mechanism of chlorosis induction in susceptible hosts is unknown, evidence indicates that chlorosis is casually related to inhibition of ornithine carbamoyltransferase (OCT; EC 2.1.3.3) by phaseotoxin, a potent and specific inhibitor of the enzyme². Phaseotoxin may also be involved in the host specificity of the pathogen. We previously reported that in infected resistant bean plants (but not in infected susceptible ones) phaseotoxin production is suppressed in spite of substantial bacterial multiplication³. Further, in resistant beans treated with phaseotoxin, a larger number of bacteria are found than in non-treated plants⁴. These observations indicate that phaseotoxin may suppress the hypersensitive reaction (HR) in resistant hosts. The object of this study was to investigate whether pretreatment of resistant cultivars of bean with phaseotoxin suppresses the HR response of the host and phytoalexin accumulation on subsequent inoculation with P. phaseolicola, and our results seem to support this.