Over-expression of GmMYB39 leads to an inhibition of the isoflavonoid biosynthesis in soybean (Glycine max. L)

In this paper, we isolated and characterized a gene encoding the soybean MYB transcription factor, GmMYB39 (Accession No: XM_003538605). Analysis of the deduced amino acid sequence revealed that GmMYB39 contained N-terminal R2R3 repeats that corresponded to the DNA-binding domain of plant MYB-type proteins, which were highly conserved among the R2R3-MYB proteins. The detailed expression pattern of GmMYB39 in various tissues of soybean was investigated by quantitative RT-PCR. The transcript level was found to be higher in flowers than in other examined organs. In contrast, the GmMYB39 expression level was relatively weak in the pod. The GmMYB39–GFP fusion protein was found to localize in the nucleus of Arabidopsis mesophyll protoplasts. Over-expression of GmMYB39 in hairy roots resulted in a significant reduction of the transcript levels of PAL, C4H, CHS, 4CL, and CHR. Whereas, the IFS transcript level was slightly but not significantly increased, and no significant change in CHI expression was observed between over-expression and control roots. The repressing effect on expression of CHS was further supported by the results from co-transfection assays of CHS promoter (reporter) and GmMYB39 (effector) in soybean. Compared to the control (only with the reporter construct), a significant decrease in the GUS activity was observed in soybean hairy roots with both effector and reporter constructs. Furthermore, an ultimate decrease in isoflavonoids contents coincided with the decrease in the transcript levels of PAL, C4H, CHS, 4CL, and CHR. Overall, the results suggest that GmMYB39 plays an inhibiting role in regulating the isoflavonoid biosynthesis in soybean.