Lack of galactose or galacturonic acid in Bradyrhizobium japonicum USDA 110

J.I. QUELAS; E.J. MONGIARDINI; A.CASABUONO; S.L. LÓPEZ-GARCÍA; M.J. ALTHABEGOITI; J. M. COVELLI; J. PÉREZ-GIMÉNEZ; A.S. COUTO; A. R.LODEIRO

MOLECULAR PLANT-MICROBE INTERACTIONS

AMER PHYTOPATHOLOGICAL SOC

Año: 2010 vol. 23 p. 1592 - 1604

0894-0282

Exopolysaccharide (EPS) and lipopolysaccharide (LPS) from Bradyrhizobium japonicum are important for infection and nodulation of soybean (Glycine max) roots, although their roles are not completely understood. To advance in this knowledge, we constructed mutants in B. japonicum USDA 110 specifically impaired in galactose or galacturonic acid incorporation into the EPS without affecting the LPS. The derivative LP 3010 had a deletion of lspL-ugdH and produced EPS without galacturonic acid, while LP 3013, with an insertion in exoB, produced EPS without galactose. In addition, the strain LP 3017, with both mutations, had EPS devoid of both galactosides. The missing galactosides were not replaced by other sugars. The defects in EPS had different consequences. LP 3010 formed biofilms and nodulated, but it was defective in competitiveness for nodulation, and inside nodules, the peribacteroid membranes tended to fuse, leading to the merging of symbiosomes. However, plant growth in the absence of combined N was unaltered. Meanwhile, LP 3013 and LP 3017 were unable to form biofilms and produced empty pseudonodules, but exoB suppressor mutants were obtained when LP 3013 plant inoculation was supplemented with wild type EPS. Similar phenotypes were observed with all these mutants in G. soja. Therefore, the lack of each galactoside in the EPS has a different functional effect on the B. japonicum-soybean symbiosis.