The Legume Nodule Microbiome: A Source of Plant Growth-Promoting Bacteria

Legume nodules harbour two types of bacteria, the rhizobia, responsible for their formation, and other endophytic bacteria whose role in the nodule is still poorly known. These bacteria constitute the nodule microbiome from which the rhizobia have been widely studied for decades, whereas the nodule endophytes have been started to be studied in the last years. These studies showed a more complex bacterial composition than previously thought, including bacteria from very different phylogenetic groups. Unlike other plant microbiomes, which have been widely studied by metagenomic techniques, the nodule microbiomes have been basically studied by culture-dependent methods because the main objective of the legume nodule studies is the selection of plant growth-promoting bacteria to be used in agronomic practices in a sustainable agriculture context. In this chapter we revise the groups of bacteria found to date in legume nodules that present in vitro mechanisms of plant growth promotion, with special emphasis in those that are able to promote the plant growth in plant assays.