Differential Expression of Isoflavone Biosynthetic Genes in Soybean During Germination

Soybean isoflavones are essential secondary metabolites synthesized through the phenylpropanoid pathway, and they play vital roles in human health. Isoflavone content is a complex quantitative trait controlled by multiple genes, and the genetic mechanisms underlying isoflavone biosynthesis remain largely unknown. Therefore, the present study analyzed the content of isoflavone and expression of six key genes involved in its biosynthesis (i.e., CHS6, HID, IF7GT, IF7MaT, GmIMaT1, and GmIMaT3) during soybean seed germination. Isoflavone content was quantified using high-performance liquid chromatography, and isoflavone biosynthetic gene expression was analyzed using quantitative real-time PCR. Two cultivars, namely ‘Daepung2ho’ and ‘Pungsannamulkong’, which are high- and low-isoflavone cultivars, respectively, were used. Isoflavone accumulation gradually increased with the progression of the germination period. As such, malonyl glucosides accounted for over 80% of the total content, whereas acetyl glucosides were present at trace amounts. Transcriptional analysis of isoflavone biosynthetic genes demonstrated expression patterns parallel to isoflavone content; however, there was no clear correlation between isoflavone content and gene expression. Moreover, most isoflavone biosynthetic genes showed different expression patterns depending on the individual gene or genotypes. Among the tested genes, HID showed consistently higher expression, except at 3 days after germination, and its expression was upregulated in ‘Daepung2ho’ but downregulated in ‘Pungsannamulkong’. In addition, all tested genes exhibited different expression patterns between cotyledons and hypocotyls and responded differently to the germination period. These findings suggest that the expression levels of isoflavone biosynthetic genes are not consistent with the germination period and appear to be genotype-dependent.