Two classes of Rhizobium meliloti infection mutants differ in exopolysaccharide production and in coinoculation properties with nodulation mutants

Symbiotic mutants of Rhizobium meliloti were isolated following Tn5 mutagenesis. Besides four nodulation mutants (Nod-) unable to induce nodule formation on alfalfa, five infection mutants (Inf-), which induce the formation of root nodules without detectable infection threads or bacteroids, were obtained. The Inf- mutants were subdivided into two classes. One class contains mutants which fail to synthesize acidic exopolysaccharide (EPS-). The other class is comprised of mutants which produce excess amounts of acidic exopolysaccharide (EPS*). 13C nuclear magnetic resonance spectroscopy of the exopolysaccharide isolated from one of the latter type of Inf- mutant, 101.45, revealed that the side chain of the repeating octosaccharide unit lacks the terminal pyruvate residue. Complementing cosmids were isolated for all Inf- mutants. In the case of the Inf- EPS- mutants the complementing cosmids contain DNA segments which overlap and are part of megaplasmid 2. For two mutants the mutations were found to map on a 7.8 kb EcoRI fragment. In the case of the Inf- EPS* mutants the complementing cosmids carry chromosomal DNA. The mutations of two Inf- EPS* mutants were localized on a 6.4 kb EcoRI fragment. Coinoculation of alfalfa plants with Nod- and Inf- EPS- mutants resulted in effective symbiosis. The nodules appeared wild type and fixed nitrogen. In constrast, coinoculations with Nod- mutants and the Inf- EPS* mutant 101.45 did not result in the formation of effective nodules.