EVIDENCE THAT CHITINASE PRODUCED BY AEROMONAS-CAVIAE IS INVOLVED IN THE BIOLOGICAL-CONTROL OF SOIL-BORNE PLANT-PATHOGENS BY THIS BACTERIUM

A chitinolytic isolate of Aeromonas caviae was isolated from roots of healthy bean plants grown in soil artificially infested with Sclerotium rolfsii. Under greenhouse conditions, the bacterium controlled Rhizoctonia solani and Fusarium oxysporum fsp. vasinfectum in cotton (78 and 57% disease reduction, respectively) and S. rolfsii in beans (60% disease reduction). Seed coating was the most effective application method for controlling R. solani in cotton. There was no evidence of inhibition of the fungal pathogens by A. caviae. A caviae partially lysed live mycelium of R. solani, S. rolfsii and F. oxysporum f.sp. vasinfectum when their mycelium served as a sole carbon source in liquid medium. A high chitinolytic activity was found when colloidal chitin was used as a sole carbon source, with an optimum pH between 6.0-7.0. No beta-1-3-glucanase was produced by the bacterium. After partial purification of the enzyme by affinity adsorption to colloidal chitin, three bands appear in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). One strong band with a molecular weight of ca. 80 kDa, and two weak bands with molecular weights of 48 and 59 kDa. Using the chromogenic substrate pNp-chitobiose. the partially purified chitinase from A. caviae was shown to act in an exo-splitting manner.