Phytohormone regulation of Medicago truncatula–rhizobia interactions. A review

The successful symbiosis between legumes and rhizobia depends on multiple mechanisms that orchestrate infection, nodule development, and later senescence. Plant hormones play roles in all aspects of nodulation to optimize both infection and developmental processes, as well as the control of nodule numbers. This review discusses the involvement of the plant hormones abscisic acid (ABA), auxin, brassinosteroids, cytokinin, ethylene, gibberellin, jasmonic acid (JA), and strigolactones in nodulation of Medicago truncatula. There is evidence that Nod factor signaling influences hormone synthesis, response, transport, and degradation. The role of hormones is complicated by multiple interactions between different hormones, by their concentration dependence as well as often their specific roles in different cell types and during different stages of nodulation. In addition, rhizobia synthesize plant hormones, and while these do not appear to be essential for the symbiosis, they might improve nodulation. There are many remaining questions in this field, including how plant hormones integrate plant development with the external abiotic environment.