A single-repeat MYB transcription factor, GmMYB176, regulates CHS8 gene expression and affects isoflavonoid biosynthesis in soybean.

Here we demonstrate that GmMYB176 regulates CHS8 expression and affects isoflavonoid synthesis in soybean. We previously established that CHS8 expression determines the isoflavonoid level in soybean seeds by comparing the transcript profiles of cultivars with different isoflavonoid contents. In the present study, a functional genomic approach was used to identify the factor that regulates CHS8 expression and isoflavonoid synthesis. Candidate genes were cloned, and co-transfection assays were performed in Arabidopsis leaf protoplasts. The results showed that GmMYB176 can trans-activate the CHS8 promoter with maximum activity. Transient expression of GmMYB176 in soybean embryo protoplasts increased endogenous CHS8 transcript levels up to 169-fold after 48 h. GmMYB176 encodes an R1 MYB protein, and is expressed in soybean seed during maturation. Furthermore, GmMYB176 recognizes a 23 bp motif containing a TAGT(T/A)(A/T) sequence within the CHS8 promoter. A subcellular localization study confirmed nuclear localization of GmMYB176. A predicted pST binding site for 14-3-3 protein is required for subcellular localization of GmMYB176. RNAi silencing of GmMYB176 in hairy roots resulted in reduced levels of isoflavonoids, showing that GmMYB176 is necessary for isoflavonoid biosynthesis. However, over-expression of GmMYB176 was not sufficient to increase CHS8 transcript and isoflavonoid levels in hairy roots. We conclude that an R1 MYB transcription factor, GmMYB176, regulates CHS8 expression and isoflavonoid synthesis in soybean.