Growth promoting effect of two Sinorhizobium meliloti strains (a wild typeand 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 rhizosphe re 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, w hich had an enhanced biofertilizer capability. The PGP effect of these rhiz obial strains was tested in interaction with two arbuscular-mycorrhizal (AM ) fungi: G. mosseae or G. intraradices on lettuce (Lactuca sativa L.) plant s. 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 promotin g effect on lettuce. However, both rhizobial strains differed in mycorrhiza l 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 we re more effective on root growth and morphology at earlier developmental st ages (20 days of plant growth) but, in a later stage (after 40 days), the m icrobial effects were more relevant at increasing plant biomass. The intera ction between the GM rhizobial strain and G. mosseae produced the highest g rowth promoting effect (476% over control), in spite of the fact that G. in traradices showed a quicker and higher colonization ability than G. mosseae . Microbial interactions inducing PGP effects did not benefit AM colonizati on nor the succinate dehydrogenase activity in the AM fungal mycelium. Irre spective of the underlying mechanisms, which are being now investigated, th e interactions between rhizobial strains, as free-living saprophs, and AM f ungi are noteworthy, and depend on the microbial genotype involved. (C) 200 0 Elsevier Science Ireland Ltd. All rights reserved.