Molecular Analysis of Phaseolotoxin Production in Pseudomonas syringae pv. phaseolicola

Abstract

Pseudomonas syringae pv. phaseolicola is the causal agent of halo blight of bean (Phaseolus vulgaris L.). The disease is manifested at cool temperatures when the bacteria enter the plant through stomata in the leaves. At the site of infection a green-yellow chlorotic zone or halo, ornithine accumulation and growth inhibition (Patil, 1974) occur. Systemic infection results in venetion of leaves and stunting of the growing point (Hoitink et al., 1966; Rudolph et al., 1966). The chlorosis, growth inhibion, and venetation are caused by an extracellular, nonspecific Phytotoxin known as phaseolotoxin (Fig. 1) (Mitchell, 1984; Moore et al., 1984). It is not required for pathogenicity of P.s. pv. phaseolicola since naturally occurring toxin-deficient strains as well as Tox^- mutants are still capable of producing water-soaking but not the other symptoms. (Rudolph et al., 1966; Patil, et al., 1974; Russell, 1975; Quigley et al., 1985; Peet et al., 1986). During the past 15 years, extensive studies on the physiology, biochemistry, chemical structure and mode of action of phaseolotoxin have been reported (Mitchell, 1984). However, little is known about the biochemistry or genetics of its production. Phaseolotoxin is a potent inhibitor of the enzyme, ornithine carbamoyltransferase (OCT) of bean in vitro (Patil et al., 1970; Kwok et al., 1979).