Background

1. Suppressors of a rhizobial lipopolysaccharide defect restore symbiosis on alfalfaHajeewaka C. Mendis*, Brian K. Washburn, and Kathryn M. Jones Background • The bacterium Sinorhizobiummeliloti forms symbiotic, nitrogen-fixing nodules on the roots of host plants alfalfa and Medicagotruncatula • S. meliloti and alfalfa communicate through species-specific signals throughout the invasion process • S. melilotipenetrates the root interior through invasion structures called infection threads that initiate in root hair cells • Individual S. meliloti cells are budded off of these infection threads within compartments of plant cell membrane origin • Within these compartments, the bacteria differentiate into bacteroids, the terminally-dfferentiated form that expresses the nitrogen-fixing enzyme nitrogenase • S. meliloti strains with a lesion in the lpsB gene, which encodes a mannosyltransferase, have a truncated lipopolysacchardie core and are unable to form an effective symbiosis1 • Summary • S. meliloti lpsB suppressor mutants were isolated from the rare successful nodules on alfalfa plants. • Isolated lpsB suppressor strains form a productive symbiosis with alfalfa, indicating that they have acquired the ability to compensate for the LPS defect and survive within the host. • The lpsB suppressor strains have the same level of sensitivity to SDS and the same LPS profile on SDS-PAGE gels as the original lpsB mutant. • Wild type S. meliloti maintain close contact with symbiosome membrane during the differentiation process and develop into bacteroid. • lpsB mutants lose close contact with symbiosome membrane during differentiation and degenerate rapidly. • lpsB suppressors can successfully infect alfalfa root and differentiate into bacteroids maintaining contact with symbiosome membrane while some bacteria degrade slowly. References 1) Campbell, et al. 2002, PNAS 99:3938-3943 Kathryn Jones, Department of Biological Science, Bio Unit I 230A, 645-8743, email: kmjones@bio.fsu.edu