Requeriments of an intact LPS-inner core for the establishment of a functional symbiosis between Bradyrhizobium japonicum and Glicine max

FERRARI W; ROSSI F; PRINCIPE A; CASTRO M; MEDEOT D; RIVERO MR; CENDOYA E; LAGARES A; JOFRE E

Piriapolis

Congreso; XXV Reunión Latinoamericana de Rizobiología. I Congreso Nacional de Microorganismos promotores de crecimiento vegetal.; 2011

The soil bacteria known as rhizobia can establish a nitrogen-fixing symbiosis with their host legumes plant, resulting in the formation of nodules where atmospheric nitrogen is fixed into ammonia. Nodule development is a complex process where bacterial cell surface components play an important role in this specific interaction. One of these rhizobial components is the Lipopolysaccharide (LPS) which consist of lipid A, core oligosaccharide and distal polysaccharide (O antigen). It has been shown in several rhizobia that alterations in the LPS structure result in a defective nodulation. In Rhizobium leguminosarum the lpcC gene encodes a glycosyltransferase (LpcC) involved in the inner core biosynthesis of the LPS. Previous report demonstrated that lpcC mutants affected in the LPS-inner core from Rhizobium leguminosarum, are unable to form functional nodules on Pisum sativum. Moreover, mutants in the ortolog gene (lpsB) from Sinorhizobium meliloti produce nodules unable to fix nitrogen on Medicago truncatula. Complementation studies show that lpsB gene from S.meliloti was able to complement the lpcC mutation of R. leguminosarum. However, the lpcC gene from R leguminosarum did not restore the symbiotic deficiency of S. meliloti lpsB mutant. Interestingly, the LpcC protein is widely conserved in others á- proteobacteria symbionts such as S. meliloti, B. japonucum, Mesorhizobium loti among others, as well as in the plant and human pathogens Agrobacterium tumefaciens, Brucella melitensis and Bartonella hensellae. In this study we evaluated the role of LpcC form B. japonicum during the establishment of the symbiosis with soybean plants. The B. japonicum lpcC mutant was generated by site-directed plasmid integration, this mutant showed an altered LPS pattern as compared with the wild-type strain, suggesting that LpcC is required for LPS synthesis. Heterologous complementation with lpcC from R. leguminosarum, A. tumefaciens and rfaC from M. loti restored the wild-type LPS profile. Nevertheless, lpsB from S. meliloti was unable to restore the wild-type LPS profile. Nodulation assays on Glycine max plants showed that the B. japonicum lpcC mutants as well as the lpsB-complemented strain were unable to form nodules on Glycine max roots; whereas, the lpcC mutant complemented with lpcC from R. leguminosarum, A. tumefaciens, and rfaC from M. loti were able of restoring the nodulating phenotype. The results confirm the requirement of an intact LPS for an effective nodulation of Glycine max by B. japonicum, and suggest the existen ce of a functional diversity despite the high degree of conservation among these proteins.