The GmSTF1/2–GmBBX4 negative feedback loop acts downstream of blue-light photoreceptors to regulate isoflavonoid biosynthesis in soybean

Isoflavonoids, the secondary metabolites derived from the phenylalanine pathway, are predominantly biosynthesized in legumes, especially in soybean (Glycine max). They are not only essential for plants in response to biotic and abiotic stresses but also benefit human health. In this study, we report that light signaling controls isoflavonoid biosynthesis in soybean. Blue light photoreceptors GmCRY1s, GmCRY2s, GmPHOT1s, and GmPHOT2s and transcription factors GmSTF1 and GmSTF2 promote isoflavonoid accumulation, whereas E3 ubiquitin ligase GmCOP1b negatively regulates isoflavonoid biosynthesis in soybean. GmPHOT1s and GmPHOT2s stabilize GmSTF1/2, whereas GmCOP1b promotes the degradation of these two proteins in soybean. GmSTF1/2 regulate the expression of approximately 27.9% genes involved in isoflavonoid biosynthesis including GmPAL2.1, GmPAL2.3, and GmUGT2 in soybean. They also repress the expression of GmBBX4, a negative regulator of isoflavonoid biosynthesis in soybean. In addition, GmBBX4 physically interacts with GmSTF1 and GmSTF2 to inhibit their transcriptional activation activity towards target genes related to isoflavonoid biosynthesis. Thus, GmSTF1/2 and GmBBX4 form a negative feedback loop acting downstream of photoreceptors in the regulation of isoflavonoid biosynthesis. Our study provides novel insights into light signaling-controlled isoflavonoid biosynthesis in soybean and will contribute to breeding soybean cultivars with high isoflavonoid content using genetic and metabolic engineering.