A Hydrophobic Mutant of Rhizobium etli Altered in Nodulation Competitiveness and Growth in the Rhizosphere.

We isolated and characterized CE3003, a Tn5-induced mutant with altered colony morphology derived from Rhizobium etli CE3. CE3003 produced domed colonies and was highly hydrophobic as indicated by its ability to partition into hexadecane, whereas its parent produced flat colonies and was hydrophilic. On bean plants, CE3003 induced nodules and reduced acetylene. CE3003 and CE3 grew at similar rates when they were grown separately or together in culture medium or inoculated singly onto bean seeds. However, when they were mixed at a 1:1 ratio and applied to seeds, CE3003 achieved significantly lower populations than CE3 in the rhizosphere. Five days after coinoculation of CE3 and CE3003, the population of the mutant was less than 10% of the population of CE3 in the bean rhizosphere. To determine the nodulation competitiveness of the mutant, it was coinoculated with CE3 at various ratios at planting, and the ratio of the nodules occupied by each strain was determined 21 days later. A 17,000-fold excess of CE3003 in mixed inocula was required to obtain equal nodule occupancy by the two strains. A genomic library of strain CE3 was mobilized into CE3003, and we identified a cosmid, pRA3003, that restored the parental colony morphology and hydrophilicity to the mutant. Restoration of the parental colony morphology was accompanied by recovery of the ability to grow competitively in the rhizosphere and to compete for nodulation of beans. The data show an association between cell surface hydrophobicity, nodulation competitiveness, and competitive growth in the rhizosphere in mutant CE3003.