Responses of Metabolic Pathways in Soybean Nodules and Roots to Long-Term Indirect Nitrogen Supply by Dual-Root System

Background and aims While the effects of fertilizer nitrogen (N) on N fixation in soybean nodules lead to both local inhibition and systemic regulation, the specific mechanism is not clear. This study aimed to elucidate the mechanism by which systemic N regulation affects soybean root growth and N fixation. Methods Dual-root soybean plants containing unilateral nodulation were cultivated using grafting and sand culture with the addition of N-containing nutrient solution to the non-nodulated root portion and N-free nutrient solution to the nodulated portion during the VC-R1 period (28 d). The effects of N supplementation on changes in the expression of genes and proteins, as well as metabolite levels, in nodules and roots were examined, together with an evaluation of alterations in metabolic pathways in response to the indirect N supply. Results The results demonstrated that a 28-day supply of N to non-nodulated part of the root reduced nodulation while promoting the growth of the root system on the nodulation side. A comparative evaluation of the soybean plants cultivated with and without N supplementation revealed that N promoted the synthesis of signaling compounds, such as asparagine and trehalose, in nodules and inhibited flavonoid-associated metabolic pathways. Starch syntheis in nodules under long-term high-N was inhibited while the metabolism of organic acids was enhanced. Long-term indirect N supply also influenced pathways associated with amino acid metabolism and phenylpropanoid biosynthesis pathway in the roots. Conclusion The different response metabolic pathways of roots and nodules supported the different characteristics of soybean roots and nodules after indirect nitrogen supply. Additionally, the duration of nitrogen supply affects the regulatory pathway of nitrogen fixation of nodules.