Legume tasters: symbiotic rhizobia host preference and smart inoculant formulations

Mutualistic interactions have great importance in ecology, with genetic infor-mation that takes shape through interactions within the symbiotic partners and between the partners and the environment. It is known that variation of the host-associated microbiome contributes to buffer adaptation challenges of the host’s physiology when facing varying environmental conditions. In agriculture, pivotal examples are symbiotic nitrogen-fixing rhizobia, known to contribute greatly to host (legume plants) adaptation and host productivity. A holistic view of increasing crop yield and resistance to biotic and abiotic stresses is that of microbiome engineering, the exploitation of a host-associated microbiome through its rationally designed manipulation with synthetic microbial commu-nities. However, several studies highlighted that the expression of the desired phenotype in the host resides in species-specific, even genotype-specific in-teractions between the symbiotic partners. Consequently, there is a need to dissect such an intimate level of interaction, aiming to identify the main ge-netic components in both partners playing a role in symbiotic differences/host preferences. In the present paper, while briefly reviewing the knowledge and the challenges in plant–microbe interaction and rhizobial studies, we aim to promote research on genotype x genotype interaction between rhizobia and host plants for a rational design of synthetic symbiotic nitrogen-fixing microbial communities to be used for sustainably improving leguminous plants yield.