Exopolysaccharide-Deficient Mutants of Rhizobium sp. Strain CIAT899 Induce Chlorosis in Common Bean (Phaseolus vulgaris). Kevin P. O’Connell. Department of Plant Pathology and Center for the Study of Nitrogen Fixation, University of Wisconsin, Madison 53706 U.S.A. Ricardo S. Araujo, and Jo Handelsman. Department of Plant Pathology and Center for the Study of Nitrogen Fixation, University of Wisconsin, Madison 53706 U.S.A. MPMI 3:424-428. Accepted 3 August 1990. Copyright 1990 The American Phytopathological Society.

Mutants of Rhizobium sp. strain CIAT899 deficient in exopolysaccharide (EPS) were generated with Tn5 and ethyl methanesulfonate. The EPS-deficient mutants produced nonmucoid colonies on solid media and released 8.1-41.0% of the amount of EPS produced by CIAT899 in broth culture. All eight mutants tested induced an interveinal chlorosis in the first trifoliolate leaf of their symbiotic host, common bean (Phaseolus vulgaris), whereas CIAT899 caused slight chlorosis on a small proportion of inoculated plants. Two other bean-nodulating strains, R. leguminosarum bv. phaseoli CE3 and KIM5s, did not induce chlorosis, and neither did EPS-deficient mutants derived from these two strains. CIAT899 and its EPS-deficient mutants all induced pink, effective nodules, indicating that the chlorosis was not due to a lack of nitrogen resulting from an ineffective symbiosis. One EPS-deficient mutant, CT9005, was applied to Leucaena leucocephala, and it was found to induce foliar chlorosis and retained the ability to induce nodules on that host. Cultures of chlorosis-inducing strains lost the ability to induce chlorosis upon dilution, while retaining sufficient cells to nodulate abundantly, suggesting that nodulation is not sufficient for chlorosis induction. These data suggest that EPS-deficient mutants of CIAT899, and CIAT899 to a lesser extent, produce a chlorosis-inducing factor either in broth culture or in the rhizosphere.