About How Nitrate Controls Nodulation: Will Soybean Spill the Bean?

Legumes have the beneficial capacity to establish symbiotic interactions with rhizobia, which provide their host plants with fixed nitrogen. However, in the presence of nitrogen, this process is rapidly repressed to avoid unnecessary investments of carbon in the symbiosis. Several players involved in regulating nodulation in response to nitrate availability have been identified, including peptide hormones, microRNAs and transcription factors. Nevertheless, how these molecular players are linked to each other and what underlying molecular mechanisms are at play to inhibit nodulation remain unresolved. Nitrate-mediated control of nodulation seems to differ between model legumes, such as Medicago and Lotus, compared to legume crops such as soybean. A deeper understanding of these regulatory processes, particularly in soybean, is expected to contribute to establishing increased nodulation efficiency in modern agricultural systems, hence improving sustainability by reducing the need for environmentally hazardous nitrogen fertilizers. This review describes the state of the art of nitrate-regulated nodulation in soybean, while drawing parallels with molecular mechanisms described in other legumes and addressing knowledge gaps that require future study.