Expression of Serratia marcescens Chitinase Gene in Rhizobium meliloti During Symbiosis on Alfalfa Roots. Yaron Sitrit. Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel. Zehava Barak(1), Yoram Kapulnik(2), Amos B. Oppenheim(3), and Ilan Chet(1). (1)Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel; (2)Department of Legume Inoculation, ARO, Bet Dagan, Israel; and (3)Department of Molecular Genetics, Hadassah Medical School, The Hebrew University of Jerusalem, Jerusalem 91010, Israel.. MPMI 6:293-298. Accepted 5 March 1993. Copyright 1993 The American Phytopathological Society.

Chitin is a major structural cell-wall component of many plant pathogenic fungi. The polymer is degraded by chitinase and the latter’s substantial role in biological control has been well established. We introduced the chitinase gene from Serratia marcescens into the plant symbiont Rhizobium meliloti, which colonizes the root nodules of alfalfa (Medicago sativa). Rhizobium colonies harboring the plasmid with the chitinase gene were identified by the clear halos of degraded chitin surrounding them. Functional analysis showed nitrogenase activity to be similar in the transconjugants and wild-type bacteria. Plants infected with either bacteria exhibited similar growth, with no symptoms of nitrogen deficiency. Western blot of proteins extracted from nodules infected with the transconjugants revealed a 58-kDa band corresponding to S. marcescens chitinase. The antifungal activity of R. meliloti during symbiosis on alfalfa roots was verified by lysis of Rhizoctonia solani hyphal tips treated with cell-free nodule extracts. The role of chitinase in plant defense mechanisms is discussed.