Advances in the mechanism of cereal/legume intercropping promotion of symbiotic nitrogen fixation

Biological nitrogen fixation by rhizobia in symbiotic association with legumes is a valuable N source for agroecosystems, and thus it is crucial for increased sustainability of agricultural production. Previous studies found that legume and cereal intercropping can promote symbiotic nitrogen fixation mainly due to rhizosphere nitrogen depletion by the cereal and that soil mineral nitrogen is absorbed more by cereals than legumes, which forces the legume to increase its reliance on symbiotic nitrogen fixation. However, in addition to the nitrogen depletion theory in the rhizosphere, stoichiometry (such as nitrogen and phosphorus coupling) and molecular regulation mechanisms are also important in understanding legume nitrogen fixation. The legume usually maintains a stable stoichiometric relationship between nitrogen and phosphorus. Symbiotic nitrogen fixation of legumes tends to have a high demand for phosphorus, mainly because nitrogen fixation by legumes is a high energy-consuming process, and the required energy is directly provided by ATP. Therefore, when the phosphorus obtained by the intercropped legume is insufficient to meet the increase in symbiotic nitrogen fixation, legume growth is limited by phosphorus. Under phosphorus limitation, the phosphorus uptake strategy of the legume can be stimulated, i.e. by increasing the infection of arbuscular mycorrhizal fungi (AMF) and secretion of root exudates. The AMF mycelia of legumes can extend beyond the rhizosphere, linking the roots to the surrounding soil microhabitats and expanding the area of phosphorus absorbed by the roots. On the other hand, mobilizing rhizosphere insoluble phosphorus through root exudates is also an important pathway for improving phosphorus utilization. Increased soil phosphorus availability promotes plant growth and increased allocation of carbon (C) sources to roots and nodules, resulting in a larger root system or greater nodule formation, finally higher symbiotic nitrogen fixation. In addition, the formation of root nodules is influenced by rhizosphere talk between the cereal and legume, which are directly related to the level of nitrogen. A recent study found that under low N concentration conditions, cereal root exudates stimulated flavonoid biosynthesis and secretion partly through upregulating expression of the Chalcone-flavanone isomerase gene (CFI) in legumes. Meanwhile, an auxin-responsive GH3 family gene (GH3.1) and the nodulation gene ENODL2 displayed significant increases, suggesting that nodulation continues under the regulation of auxin signaling and the nodulin-like protein. At nodule maturity, cereal root exudates continue to up-regulate key nodulation genes, such as NODL4, ENOD93 and the N fixation gene FixI3, finally promoting symbiotic nitrogen fixation. Moreover, a high concentration of soil mineral N significantly suppressed expression of the early nodulin gene NIN in Lotus corniculatus, which inhibited nodulation and symbiotic nitrogen fixation. In the cereal/legume intercropping system, rhizosphere nitrogen depletion can relieve the inhibition of nitrate on the expression of key nodulation and N fixation genes, thus promoting nodulation and symbiotic nitrogen fixation. In conclusion, intercropping can promote nitrogen uptake and optimize nitrogen use efficiency through the coupling effects of rhizosphere nitrogen depletion, phosphorus mobilization and the regulation of nitrogen fixation genes. This review will help improve our understanding of the mechanism of symbiotic nitrogen fixation in the cereal/legume intercropping system.