Identification and Functional Characterization of R3 MYB Transcription Factor Genes in Soybean

In Arabidopsis, trichome formation is regulated by a MYB-bHLH-WD40 (MBW) transcriptional activator complex, which can activate the expression of GLABRA2 (GL2) and R3 MYB genes. GL2 is required for trichome formation, whereas R3 MYBs inhibit trichome formation by blocking the formation of the MBW complex, thus inhibiting the expression of GL2. By using the amino acid sequence of the Arabidopsis R3 MYB transcription factor TRICHOMELESS1 (TCL1) to BLAST the soybean (Glycine max) protein database, we found that there are a total of six R3 MYB genes in soybean, namely Glycine max TRICHOMELESS1 through 6 (GmTCL1-GmTCL6). By generating transgenic plants, we found that trichome formation in soybean plants overexpressing each of the GmTCLs remained largely unchanged, and the expression of putative GL1 and GL2 genes in the transgenic plants was unaffected. However, all the GmTCLs interacted with GLABRA3 (GL3) in transfected Arabidopsis protoplasts, expression each of the GmTCLs in Arabidopsis inhibited trichome formation, and the expression levels of GL1 and GL2 were greatly reduced in the Arabidopsis transgenic plants. Moreover, phenotypic complementary analysis showed that GmTCL1 is functionally equivalent to TCL1. Taken together, these results suggest that GmTCLs are functional R3 MYBs, however, they do not regulate trichome formation in soybean.