EXPRESSION OF SERRATIA-MARCESCENS CHITINASE GENE IN RHIZOBIUM-MELILOTI DURING SYMBIOSIS ON ALFALFA ROOTS

Chitin is a major structural cell-wall component of many plant pathoge nic fungi. The polymer is degraded by chitinase and the latter's subst antial role in biological control has been well established. We introd uced the chitinase gene from Serratia marcescens into the plant symbio nt Rhizobium meliloti, which colonizes the root nodules of alfalfa (Me dicago sativa). Rhizobium colonies harboring the plasmid with the chit inase gene were identified by the clear halos of degraded chitin surro unding them. Functional analysis showed nitrogenase activity to be sim ilar in the transconjugants and wild-type bacteria. Plants infected wi th either bacteria exhibited similar growth, with no symptoms of nitro gen deficiency. Western blot of proteins extracted from nodules infect ed with the transconjugants revealed a 58-kDa band corresponding to S. marcescens chitinase. The antifungal activity of R. meliloti during s ymbiosis on alfalfa roots was verified by lysis of Rhizoctonia solani hyphal tips treated with cell-free nodule extracts. The role of chitin ase in plant defense mechanisms is discussed.