Interactions between Glomus species and Rhizobium strains affect the nutritional physiology of drought-stressed legume hosts

The growth of legume plants is usually enhanced by the dual symbiosis of arbuscular mycorrhizal (AM) fungi and Rhizobium bacteria. However, most reports on this topic have been carried out under optimal water regime conditions. Here, four Phaseolus vulgaris varieties were single or dual inoculated with two different AM fungus and/or two different Rhizobium strains. All plants were grown under moderate drought conditions. Surprisingly, most of the biological treatments involving one fungus and one Rhizobium together caused a deleterious effect on plant growth. However, these negative effects were dependent on the P. vulgaris variety used as well as on the symbionts implicated. The results showed that AM symbiosis inhibited nodule development and N2 fixation, causing diminution of plant growth. Therefore, under moderate drought conditions, the dual symbiosis formed by AM fungi and Rhizobium can be deleterious to P. vulgaris growth depending on the plant variety and the symbionts involved. Thus, under these common stress conditions, selection for the appropriated symbionts to each P. vulgaris variety is needed.