Integrated Analysis of Transcriptome and Metabolome Reveals the Accumulation of Anthocyanins in Black Soybean (Glycine max L.) Seed Coats Induced by Low Nitrogen Concentration in the Nutrient Solution

Anthocyanins are key antioxidants that play a significant role in plant responses to adverse stresses, including nitrogen deficiency. However, research on the metabolic and transcriptional regulation of anthocyanins in black soybean seed coats under low-nitrogen conditions remains limited. Here, we report that low-nitrogen treatment significantly alters the accumulation of anthocyanin metabolites and the gene expression profiles in black soybeans. Specifically, a greater number of differential anthocyanin metabolites are induced under low-nitrogen conditions, which contributes to the accumulation of anthocyanins in the seed coat. GO and KEGG enrichment analyses revealed that the differentially expressed genes (DEGs) are mainly enriched in multiple antioxidant pathways involved in responding to low-nitrogen stress; in flavonoid and phenylalanine metabolic pathways, as well as protein processing in endoplasmic reticulum, which are associated with anthocyanin biosynthesis; and in plant hormone signal transduction pathways involved in the regulation of anthocyanin accumulation. The expressions of genes encoding key enzymes in anthocyanin biosynthesis, such as dihydroflavonol 4-reductase (DFR) and O-methyltransferase (OMT), as well as genes encoding the blue light photoreceptor cryptochrome (CRY) and proteins related to cellular autophagy, were upregulated under low-nitrogen treatment. This suggests that these genes may play a key role in low-nitrogen-induced anthocyanin accumulation. This study provides a theoretical basis and novel perspective for understanding the regulatory mechanism underlying low-nitrogen-induced anthocyanin accumulation in black soybeans.