Transcriptomic and Metabolomic Analysis of Soybean Nodule Number Improvements with the Use of Water-Soluble Humic Materials

Water-soluble humic materials (WSHM) can enhance the nodule numbers of soybean plants. In this study, targeted metabolomics and transcriptomics were used to understand this mechanism. The results showed that 500 mg/L WSHM increased the adsorption and colonization of rhizobia in soybean roots. High-performance liquid chromatography (HPLC) and targeted metabolomics showed that WSHM could regulate the content and distribution of endogenous hormones in the roots, stems, and leaves of soybean plants at the initial stage of soybean nodulation. Transcriptomic analysis showed a total of 2406 differentially expressed genes (DEGs) by the 25th day under test conditions, accounting for 4.89 % of total annotation genes (49159). These DEGs were found to contribute primarily to the MAPK signaling pathway, glycolysis/gluconeogenesis, and plant hormone signal transduction, according to the -log10 (Padjust) value in the KEGG pathway. Subsequently, DEGs related to these hormones were selected for verification using Quantity-PCR. WSHM increased the number of nodules by regulating the expression of endogenous hormones in soybean plants. These findings provide new insights into the mechanisms by which WSHM benefits legume nodulation.