Phytoalexin production by wounded white bean (Phaseolus vulgaris) cotyledons and hypocotyls in response to inoculation with rhizobacteria

Abstract

Fifteen rhizobacteria were tested for their ability to induce a hypersensitive response and phytoalexin production by wounded white bean (Phaseolus vulgaris) cotyledons and hypocotyls. Of these, nine strains of the genus Pseudomonas and two Serratia strains induced a hypersensitive response and the production of three antifungal compounds by cotyledons. Two of the antifungal compounds shared uv spectra, retention time in high-performance liquid chromatography, and Rf by thin-layer chromatography that were identical to those of thephytoalexins kievitone and phaseollinisoflavan. The hypersensitive response was rarely observed and phytoalexins were not detected in extracts from cotyledons treated with water, bacterial culture fluids, Bacillus sp., Bacillus subtilis, Corynebacterium sp., Azospirillum brasilense sp. strain 7, or the pathogen Pseudomonas syringae pv. phaseolicola. The white bean root rot pathogen Fusarium solani f.sp. phaseoli induced production of five phytoalexins by cotyledons. Three were identical in their chemical properties to phytoalexins induced by the rhizobacteria, but only F. solani induced phaseollin production. Kievitone levels from cotyledons treated with various rhizobacteria varied from 83 to 333 μg/g fresh mass, whereas in cotyledons treated with F. solani the kievitone levels averaged 850 μg/g fresh mass. In similar studies with wounded hypocotyls, only the two Serratia spp. and F. solani induced production of phytoalexins other than phaseollin.Key words: rhizobacteria, plant defense mechanisms, phytoalexins, hypersensitive response, Phaseolus vulgaris.