Rhizobium leguminosarum as a plant growth-promoting rhizobacterium: Direct growth promotion of canola and lettuce

Early seedling root growth of the nonlegumes canola (Brassica campestris is cv. Tobin, Brassica napus cv. Westar) and lettuce (Lactuca sativa cv. Grand Rapids) was significantly promoted by inoculation of seeds with certain strains of Rhizobium leguminosarum, including nitrogen- and nonnitrogen-fixing derivatives under gnotobiotic conditions. The growth-promotive effect appears to be direct, with possible involvement of the plant growth regulators indole-3-acetic acid and cytokinin. Auxotrophic Rhizobium mutants requiring tryptophan or adenosine (precursors for indole-3-acetic acid and cytokinin synthesis, respectively) did not promote growth to the extent of the parent strain. The findings of this study demonstrate a new facet of the Rhizobium-plant relationship and that Rhizobium leguminosarum? can be considered a plant growth-promoting rhizobacterium (PGPR).