Exopolysaccharide production in response to medium acidification is correlated with an increase in competition for nodule occupancy

Mol Plant Microbe Interact. 2014 Dec;27(12):1307-17. doi: 10.1094/MPMI-06-14-0168-R.


Sinorhizobium meliloti strains unable to utilize galactose as a sole carbon source, due to mutations in the De-Ley Doudoroff pathway (dgoK), were previously shown to be more competitive for nodule occupancy. In this work, we show that strains carrying this mutation have galactose-dependent exopolysaccharide (EPS) phenotypes that were manifested as aberrant Calcofluor staining as well as decreased mucoidy when in an expR(+) genetic background. The aberrant Calcofluor staining was correlated with changes in the pH of the growth medium. Strains carrying dgoK mutations were subsequently demonstrated to show earlier acidification of their growth medium that was correlated with an increase expression of genes associated with succinoglycan biosynthesis as well as increased accumulation of high and low molecular weight EPS in the medium. In addition, it was shown that the acidification of the medium was dependent on the inability of S. meliloti strains to initiate the catabolism of galactose. To more fully understand why strains carrying the dgoK allele were more competitive for nodule occupancy, early nodulation phenotypes were investigated. It was found that strains carrying the dgoK allele had a faster rate of nodulation. In addition, nodule competition experiments using genetic backgrounds unable to synthesize either succinoglycan or EPSII were consistent with the hypothesis that the increased competition phenotype was dependent upon the synthesis of succinoglycan. Fluorescent microscopy experiments on infected root-hair cells, using the acidotropic dye Lysotracker Red DND-99, provide evidence that the colonized curled root hair is an acidic compartment.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amines
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Benzenesulfonates
  • Fluorescent Dyes
  • Galactose / genetics
  • Galactose / metabolism
  • Galactose Dehydrogenases / genetics
  • Galactose Dehydrogenases / metabolism
  • Genes, Reporter
  • Hydrogen-Ion Concentration
  • Medicago sativa / cytology
  • Medicago sativa / microbiology*
  • Mutation
  • Phenotype
  • Phosphotransferases (Alcohol Group Acceptor) / genetics*
  • Phosphotransferases (Alcohol Group Acceptor) / metabolism
  • Plant Roots / cytology
  • Plant Roots / microbiology
  • Polysaccharides, Bacterial / metabolism*
  • Root Nodules, Plant / cytology
  • Root Nodules, Plant / microbiology*
  • Seedlings / cytology
  • Seedlings / microbiology
  • Sinorhizobium meliloti / cytology
  • Sinorhizobium meliloti / genetics
  • Sinorhizobium meliloti / growth & development
  • Sinorhizobium meliloti / physiology*
  • Symbiosis
  • Time Factors


  • Amines
  • Bacterial Proteins
  • Benzenesulfonates
  • EPSI polysaccharide
  • Fluorescent Dyes
  • Polysaccharides, Bacterial
  • Red DND-99
  • succinoglycan
  • C.I. Fluorescent Brightening Agent 28
  • Galactose Dehydrogenases
  • Phosphotransferases (Alcohol Group Acceptor)
  • 2-dehydro-3-deoxygalactonokinase
  • Galactose