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nodD1 of Sinorhizobium fredii HH103, which is identical to that of S. fredii USDA257 and USDA191, repressed its own expression. Spontaneous flavonoid-independent transcription activation (FITA) mutants of S. fredii HH103 M (=HH103 RifR pSym::Tn 5-Mob) showing constitutive expression of nod genes were isolated. No differences were found among soybean cultivar Williams plants inoculated with FITA mutants SVQ250 or SVQ253 or with the parental strain HH103M. Soybean plants inoculated with mutant SVQ255 formed more nodules, and those inoculated with mutant SVQ251 had symptoms of nitrogen starvation. Sequence analyses showed that all of the FITA mutants carried a point mutation in their nodD1 coding region. Mutants SVQ251 and SVQ253 carried the same mutation, but only the former was symbiotically impaired, which indicated the presence of an additional mutation elsewhere in the genome of mutant SVQ251. Mutants SVQ251 and SVQ255 were outcompeted by the parental strain for nodulation of soybean cultivar Williams. The symbiotic plasmids of mutants SVQ251 and SVQ255 (pSym251 and pSym255, respectively) and that (pSymHH103M) of the parental strain were transferred to pSym-cured derivatives of S. fredii USDA192 and USDA193 (USDA192C and USDA193C, respectively). Soybean responses to inoculation with S. fredii USDA192C and USDA193C transconjugants carrying pSym251 and pSymHH103M were not significantly different, whereas more nodules were formed after inoculation with transconjugants carrying pSym255. Only transconjugant USDA192C(pSym255) produced a significant increase in soybean dry weight.

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The plant rhizosphere is an important soil ecological environment for plant-microorganism interactions, which include colonization by a variety of microorganisms in and around the roots that may result in symbiotic, endophytic, associative, or parasitic relationships within the plant, depending on the type of microorganisms, soil nutrient status, and soil environment. Rhizosphere competence may be attributable to the differences in the extent of bacterial attachment to the root surface. We present results of the effect of various factors on the attachment to bean (Phaseolus vulgaris) and soybean (Glycine max) roots of some bacterial species of agronomic importance, such as Rhizobium tropici, Rhizobium etli, Ensifer fredii (homotypic synonym Sinorhizobium fredii), and Azospirillum brasilense; as well as the attachment capability of the plant growth promoting rhizobacteria Pseudomonas fluorescens and Chryseobacterium balustinum. Additionally, we have studied various bacterial traits, such as autoaggregation and flagella movements, which have been postulated to be important properties for bacterial adhesion to surfaces. The lack of mutual incompatibility between rhizobial strains and C. balustinum has been demonstrated in coinoculation assays.