Potential mode of action of multispecies inoculums on wheat growth under water stress.

Manipulating microbial communities could increase crop resistance to environmental stressors such as drought. It is, however, not clear what would be the best approach to do so and what microbial traits are important. Here, we first compare multispecies inoculums created using different approaches. The only inoculum that increased wheat fresh biomass under drought was the one created from 25 isolates that had showed a capacity to grow under high osmolarity. We then looked at two potential mechanisms of action of this inoculum: (i) direct action, by sequencing and screening the genomes of the inoculated bacteria, (ii) indirect action, by sequencing the 16S ribosomal ribonucleic acid gene and internal transcribed spacer region of rhizosphere, root, and leaves microbial communities. The microbes in the inoculum harbored many traits related to plant growth promotion, competition, and water stress resistance. The inoculation also resulted in significant shifts in the microbial communities associated with wheat, including some microorganisms (e.g. Rhizobium, Shinella, and Klebsiella) previously reported to improve plant drought resistance. We conclude that the inoculum studied here increased wheat growth because it potentially acted on two fronts: directly, through the traits it was selected for, and indirectly, through inducing shifts in the resident plant microbial communities.