ATTENUATION OF SYMBIOTIC EFFECTIVENESS BY RHIZOBIUM-MELILOTI SAF22 RELATED TO THE PRESENCE OF A CRYPTIC PLASMID

Several wild-type strains of Rhizobium meliloti isolated from alfalfa nodules exhibited different plasmid profiles, yet did not differ in gr owth rate in yeast mannitol medium, utilization of 43 different carbon sources, intrinsic resistance to 14 antibiotics, or detection of 16 e nzyme activities. In contrast, three measures of effectiveness in symb iotic nitrogen fixation with alfalfa (shoot length, dry, weight, and n itrogen content) indicated that R. meliloti SAF22, whose plasmid profi le differs from those of the other strains tested, is significantly le ss effective than other wild-type strains in symbiotic nitrogen fixati on. Light microscopy of nodules infected with strain SAF22 showed an a bnormal center of nitrogen fixation zone III, with bacteria occupying a smaller portion of the infected host cells and vacuoles occupying a significantly larger portion of adjacent uninfected host cells. In con trast, the effective nodules infected with other wild types or plasmid pRmSAF22c-cured segregants of SAF22 did not display this cytological abnormality.