Glucomannan-mediated attachment of Rhizobium leguminosarum to pea

WILLIAMS A; WILKINSON A; KREHENBRINK M; RUSSO, D. M.; ZORREGUIETA A; DOWNIE JA

JOURNAL OF BACTERIOLOGY

American Society for Microbiology

Año: 2008 vol. 190 p. 4706 - 4715

0021-9193

SUMMARY The Rhizobium leguminosarum biovar viciae genome contains several genes predicted to determine surface polysaccharides. Mutants predicted to affect the initial steps of polysaccharide synthesis were identified and characterised. In addition to the known cellulose (cel) and acidic exopolysaccharide (EPS) (pss) genes, we mutated three other loci; one (gmsA) determines glucomannan synthesis, one (gelA) determines a gel-forming polysaccharide, but the role of the other (an exoY-like gene) was not identified. Mutants were tested for attachment and biofilm formation in vitro and on root hairs; the mutant lacking the EPS was defective for both, but mutation of gelA or the exoY-like gene had no effect on either type of attachment. The cellulose (celA) mutant attached and formed normal biofilms in vitro, but did not form a biofilm on root hairs, although attachment did occur. The cellulose-dependent biofilm on root hairs appears not to be critical for nodulation, because the celA mutant competed similarly with the wild-type for nodule infection. The glucomannan (gmsA) mutant attached and formed normal biofilms in vitro, but was defective for attachment and biofilm formation on root hairs. Although this mutant formed nodules on peas, it was very strongly outcompeted by the wild-type in mixed inoculations, showing that glucomannan is critical for competitive nodulation. The polysaccharide synthesis genes around gmsA are highly conserved among other rhizobia and agrobacteria, but absent from closely-related bacteria (such as Brucella spp.) that are not normally plant associated, suggesting that these genes may play a wide role in bacterial-plant interactions.