Lack of Galactose or Galacturonic Acid in Bradyrhizobium japonicum USDA 110 Exopolysaccharide Leads to Different Symbiotic Responses in Soybean

JUAN IGNACIO QUELAS; ELÍAS J. MONGIARDINI; ADRIANA CASABUONO; SILVINA L. LÓPEZ-GARCÍA; M. JULIA ALTHABEGOITI; JULIETA M. COVELLI; JULIETA PÉREZ-GIMÉNEZ; ALICIA COUTO; ANÍBAL R. LODEIRO

MOLECULAR PLANT-MICROBE INTERACTIONS

AMER PHYTOPATHOLOGICAL SOC

Lugar: Utah, EEUU; Año: 2010 vol. 23 p. 1 - 13

0894-0282

Exopolysaccharide (EPS) and lipopolysaccharide (LPS)from Bradyrhizobium japonicum are important forinfection and nodulation of soybean (Glycine max),although their roles are not completely understood. Tobetter understand this, we constructed mutants in B.japonicum USDA 110 impaired in galactose or galacturonicacid incorporation into the EPS without affecting the LPS.The derivative LP 3010 had a deletion of lspL-ugdH andproduced EPS without galacturonic acid whereas LP 3013,with an insertion in exoB, produced EPS without galactose.In addition, the strain LP 3017, with both mutations, hadEPS devoid of both galactosides. The missing galactosideswere not replaced by other sugars. The defects in EPS haddifferent consequences. LP 3010 formed biofilms andnodulated but was defective in competitiveness fornodulation; and, inside nodules, the peribacteroidmembranes tended to fuse, leading to the merging ofsymbiosomes. Meanwhile, LP 3013 and LP 3017 wereunable to form biofilms and produced emptypseudonodules but exoB suppressor mutants were obtainedwhen LP 3013 plant inoculation was supplemented withwild-type EPS. Similar phenotypes were observed with allthese mutants in G. soja. Therefore, the lack of eachgalactoside in the EPS has a different functional effect onthe B. japonicum–soybean symbiosis.