Research progress on regulation of root nodule formation and development of legume by light signals and photosynthetic products

Legumes coexist with rhizobia to form nodules to meet plant nitrogen requirements through symbiotic nitrogen fixation. Legume nitrogen fixation capacity is of great significance to reduce nitrogen fertilizer consumption and potential environmental pollution and achieve sustainable development of agricultural production. The symbiotic nitrogen fixation of legumes consumes a large amount of energy; therefore, the energy supply of plants regulates the process of symbiosis and nitrogen fixation. This study reviews the recent research progress on the control of root nodule formation and development by light through plant photomorphogenesis and photosynthesis. Plant leaves sense blue light signals through blue light receptors (CRY1b) and produce light receptor-dependent transcription factors (STFs/FTs) to move from ground to ground. In addition, they integrate root hair development and the rhizobia-induced symbiotic signal (NIN) pathway to form root nodules. The movement and attachment ability of rhizobia in the environment are stimulated by light. In the symbiosis between plants and rhizobia, plants exchange photosynthetic products to produce NH₄⁺, which is assimilated by rhizobia. In addition, photosynthetic products (sucrose) are transported to root nodules through the phloem for a long distance and are metabolized into organic acids through glycolysis and other pathways in root nodules, providing a carbon skeleton, energy, and reductant for nitrogen fixation of rhizobia. The intermediate metabolites of photosynthates, such as glucose, glucose-6-phosphate, and trehalose 6-phosphate, combine with the regulatory proteins of glucose signal metabolism proteins TOR and SnRK1 to initiate the complex growth and development pathway of nodule cortex cells. Under low-nitrogen conditions, photosynthates are also secreted into the rhizosphere in the form of flavonoids and organic acids, recruiting rhizobia to form symbiotic relationships with plants.