Link to home

Roles for Riboflavin in the Sinorhizobium-Alfalfa Association

May 2002 , Volume 15 , Number  5
Pages  456 - 462

Guoping Yang , T. V. Bhuvaneswari , Cecillia M. Joseph , Maria D. King , and Donald A. Phillips

Department of Agronomy and Range Science, University of California, One Shields Avenue, Davis, CA 95616 U.S.A.

Go to article:
Accepted 25 January 2002.

Genes contributing to riboflavin production in Sinorhizobium meliloti were identified, and bacterial strains that overproduce this vitamin were constructed to characterize how additional riboflavin affects interactions between alfalfa (Medicago sativa) and S. meliloti. Riboflavin-synthesis genes in S. meliloti were found in three separate linkage groups and designated as ribBA, ribDribC, and ribH for their similarities to Escherichia coli genes. The ribBA and ribC loci complemented corresponding E. coli rib mutants. S. meliloti cells containing extra copies of ribBA released 10 to 20% more riboflavin than a control strain but grew at similar rates in a defined medium lacking riboflavin. Cells carrying extra copies of ribBA colonized roots to densities that were 55% higher than that of a control strain. No effect of extra rib genes was detected on alfalfa grown in the absence or presence of combined N. These results support the importance of extracellular riboflavin for alfalfa root colonization by S. meliloti and are consistent with the hypothesis that this molecule benefits bacteria indirectly through an effect on the plant.

Additional keywords: lumichrome , nitrogen fixation .

© 2002 The American Phytopathological Society