Exopolysaccharide-deficient mutants of Rhizobium meliloti that form ineffective nodules.

AUTOR(ES)

Leigh, J A

RESUMO

By screening with the fluorescent stain Calcofluor, we have isolated 26 independent transposon Tn5 insertion mutants of Rhizobium meliloti that are deficient in the production of a known extracellular polysaccharide (Exo-). The mutants belonged to six distinct genetic groups based on the ability of their Exo- phenotype to be complemented by different recombinant plasmids from a R. meliloti clone bank. With few exceptions, all of the mutants formed ineffective (non-nitrogen-fixing) nodules on alfalfa. For all but one group, the complementing plasmids restored effective nodulation. These results establish a firm and extensive correlation between the ability of Rhizobium to produce a particular polysaccharide and symbiotic proficiency. The ineffective nodules appeared to contain no bacteroids and to form without shepherds' crooks or infection threads; this symbiotic phenotype matches that described for a set of independently isolated mutants that belong phenotypically and genetically to the group B exopolysaccharide mutants described previously [Finan et al. (1985) Cell 40, 869-877]. Apparently the exopolysaccharide, although not required for nodule formation, is involved in wild-type nodule invasion.

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