Right time, right place: The dynamic role of hormones in rhizobial infection and nodulation of legumes

Many legume plants form beneficial associations with rhizobial bacteria that are hosted in new plant root organs, nodules, in which atmospheric nitrogen is fixed. This association requires the precise coordination of two separate programs, infection in the epidermis and nodule organogenesis in the cortex. There is extensive literature indicating key roles for plant hormones during nodulation, but a detailed analysis of the spatial and temporal roles of plant hormones during the different stages of nodulation is required. This review analyses the current literature on hormone regulation of infection and organogenesis to reveal the differential roles and interactions of auxin, cytokinin, brassinosteroids, ethylene, and gibberellins dur-ing epidermal infection and cortical nodule initiation, development, and function. With the exception of auxin, all of these hormones suppress infection events. By contrast, there is evidence that all of these hor-mones promote nodule organogenesis, except ethylene, which suppresses nodule initiation. This differen-tial role for many of the hormones between the epidermal and cortical programs is striking. Future work is required to fully examine hormone interactions and create a robust model that integrates this knowledge into our understanding of nodulation pathways.

Automated summary

This review analyzes the role of plant hormones, including auxin, cytokinin, brassinosteroids, ethylene, and gibberellins, during the process of infection and organogenesis in legume plants. The findings suggest that these hormones have differential roles in the epidermal infection and cortical nodule initiation, development, and function. While most hormones suppress infection events, they promote nodule organogenesis, except for ethylene which suppresses nodule initiation.