Nickel enhances daidzein biosynthesis in roots increasing nodulation, biological nitrogen fixation and seed yield of soybean plants

Nickel (Ni) plays an important role in nitrogen metabolism and biological nitrogen fixation in legume plants. Recent studies have demonstrated the beneficial effects of Ni fertilization on soybean nodulation and crop yield; however, the physiological mechanism regulating Rhizobia chemotaxis is not understood. This study aimed to investigate the effects of Ni fertilization on urease activity, flavonoid profile, nodulation, ureide metabolism and its impact on plant growth and yield. We tested fertilization with three Ni doses (0.0, 1.5, and 3.0 mg kg-1) and two soybean genotypes (commercial soybean (TMG 2158) and urease-null (eu3-a), which is a near-isogenic line (NIL), cultivated under greenhouse conditions. Ni fertilization decreased nitrate reductase activity of the NIL and increased the activity for TMG 2158 plants. Ni increased urease activity, breaking down urea into ammonia in the leaves. Ni enhanced daidzein and genistein in roots and nodulation mainly for TMG 2158 plants. There was a positive correlation between daidzein, nodulation, ureides and seed yield in response to Ni fertilization, only to the genotype TMG 2158. These results suggest that Ni plays a role in flavonoids metabolism (mainly daidzein) inducing chemotaxis between soybean plants and rhizobia boosting nodulation and ureide metabolism. Enhanced biological nitrogen fixation induced by Ni fertilization promoted higher nodulation generating more organic nitrogen available to soybean plants resulting in a higher number of pods per plant and seed yield. This study showed important information on how Ni modulates nodulation to increase the yield of commercial soybean plants.