Hormonal interactions in the regulation of the nitrogen-fixing legume-Rhizobium symbiosis

Plant hormones have long been associated with the ability of legumes to form nitrogen-fixing symbioses. During the nodulation program, two events are required: The formation of a new root organ, the nodule, and the successful infection of rhizobia into the root, and later the nodule. Cell-type specific hormone synthesis, perception and downstream responses are required in the epidermis, the root cortex and in vascular tissues to coordinate infection and nodule development. Recent progress in this area has demonstrated the crucial role of plant hormones in feedback regulation of nodule development and infection. Hormonal communication between shoot and root has also been linked to the systemic autoregulation of nodule numbers. Future studies will need to focus on how legumes regulate nodule development separately from other organogenesis processes like lateral root formation, even though the same hormones are involved in both. Similarly, balancing nodulation with restraints of abiotic and biotic stresses will require integration of hormone studies in plants facing several simultaneous challenges. While most studies have focused on the effects of single hormones, it will also be necessary to develop new tools for single cell hormone analyses and concurrent analysis of multiple hormones.