Interaction Between Nitric Oxide and Silicon on Leghaemoglobin and S-Nitrosothiol Levels in Soybean Nodules

Nitrogen fixation in legume nodules is crucial for plant growth and development. Therefore, this study aims to investigate the effects of nitric oxide [S-nitrosoglutathione (GSNO)] and silicon [sodium metasilicate (Si)], both individually and in combination, on soybean growth, nodule formation, leghaemoglobin (Lb) synthesis, and potential post-translational modifications. At the V1 stage, soybean plants were treated for 2 weeks with 150 µM GSNO, and Si at concentrations of 1 mM, 2 mM, and 4 mM. The results showed that NO and Si enhance the nodulation process by increasing phenylalanine ammonia-lyase activity and Nod factors (<i>NIP2-1</i>), attracting rhizobia and accelerating nodule formation. This leads to a greater number and larger diameter of nodules. Individually, NO and Si support the synthesis of Lb and leghaemoglobin protein (<i>Lba</i>) expression, ferric leghaemoglobin reductases (<i>FLbRs</i>), and S-nitrosoglutathione reductase (<i>GSNOR</i>). However, when used in combination, NO and Si inhibit these processes, leading to elevated levels of S-nitrosothiols in the roots and nodules. This combined inhibition may potentially induce post-translational modifications in <i>FLbRs</i>, pivotal for the reduction of Lb³⁺ to Lb²⁺. These findings underscore the critical role of NO and Si in the nodulation process and provide insight into their combined effects on this essential plant function.