Abstract
Nitrogen (N) inhibits soybean (Glycine max L.) nodulation and N(2) fixation. Isoflavones secreted by soybean roots can stimulate signal transduction for symbiotic nodules, thus playing a key role in root nodule development and N(2) fixation. The relationship between the inhibition of soybean nodulation, N(2) fixation and isoflavones by N is still unclear. In this study, dual-root soybean plants were prepared by grafting, and N or isoflavones were supplied to unilateral roots. The number and dry weight of the soybean nodules, nitrogenase activity, isoflavone concentrations and relative changes in the level of expression of nodulation-related genes were measured to study the response relationship between the N systemic regulation the soybean nodule N(2) fixation and changes in the concentrations of isoflavones in its roots. The results showed that N supply to one side of the dual-root soybeans systematically affected the N(2) fixation of root nodules on both sides, and this effect began in the early stage of nodulation. Moreover, a unilateral supply of N systematically affected the concentrations of daidzein and genistein on both sides of the roots. The concentrations of isoflavones were consistent with the change trend of soybean root nodule and nodulation-related gene expression level. Treatment with unilateral N or isoflavones affected the soybean nodule N(2) fixation and its nodulation-related genes, which had the same response to the changes in concentrations of root isoflavones. N regulates soybean nodulation and N(2) fixation by systematically affecting the concentrations of isoflavones in the roots.