Growth promoting effect of two Sinorhizobium meliloti strains (a wild type and its genetically modified derivative) on a non-legume plant species in specific interaction with two arbuscular mycorrhizal fungi.

In the present study, we have investigated whether the ubiquitous rhizosphere soil organism Sinorhizobium meliloti has a plant growth promoting (PGP) effect on non-leguminous plant species. Such PGP activity was investigated for both a wild type strain and its genetically modified (GM) derivative, which had an enhanced biofertilizer capability. The PGP effect of these rhizobial strains was tested in interaction with two arbuscular-mycorrhizal (AM) fungi: G. mosseae or G. intraradices on lettuce (Lactuca sativa L.) plants. Both rhizobial strains were efficient in increasing lettuce biomass and also induced modifications on root morphology, particularly in mycorrhizal plants; thus these strains behave as plant growth promoting rhizobacteria. In non-mycorrhizal plants, both strains exhibited a similar growth promoting effect on lettuce. However, both rhizobial strains differed in mycorrhizal plants with regard to (i) biomass production, (ii) the length of axis and lateral roots, and (iii) the number of lateral roots formed; effects which were, in turn, affected by the AM fungus involved. Microbial treatments were more effective on root growth and morphology at earlier developmental stages (20 days of plant growth) but, in a later stage (after 40 days), the microbial effects were more relevant at increasing plant biomass. The interaction between the GM rhizobial strain and G. mosseae produced the highest growth promoting effect (476% over control), in spite of the fact that G. intraradices showed a quicker and higher colonization ability than G. mosseae. Microbial interactions inducing PGP effects did not benefit AM colonization nor the succinate dehydrogenase activity in the AM fungal mycelium. Irrespective of the underlying mechanisms, which are being now investigated, the interactions between rhizobial strains, as free-living saprophs, and AM fungi are noteworthy, and depend on the microbial genotype involved.