GLABRA2, a Common Regulator for Epidermal Cell Fate Determination and Anthocyanin Biosynthesis in Arabidopsis

Epidermal cell fate determination—including trichome initiation, root hair formation, and flavonoid and mucilage biosynthesis in Arabidopsis (Arabidopsis thaliana)—are controlled by a similar transcriptional regulatory network. In the network, it has been proposed that the MYB-bHLH-WD40 (MBW) activator complexes formed by an R2R3 MYB transcription factor, a bHLH transcription factor and the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) regulate the expression of downstream genes required for cell fate determination, flavonoid or mucilage biosynthesis, respectively. In epidermal cell fate determination and mucilage biosynthesis, the MBW activator complexes activate the expression of GLABRA2 (GL2). GL2 is a homeodomain transcription factor that promotes trichome initiation in shoots, mucilage biosynthesis in seeds, and inhibits root hair formation in roots. The MBW activator complexes also activate several R3 MYB genes. The R3 MYB proteins, in turn, competing with the R2R3 MYBs for binding bHLH transcription factors, therefore inhibiting the formation of the MBW activator complexes, lead to the inhibition of trichome initiation in shoots, and promotion of root hair formation in roots. In flavonoid biosynthesis, the MBW activator complexes activate the expression of the late biosynthesis genes in the flavonoid pathway, resulting in the production of anthocyanins or proanthocyanidins. Research progress in recent years suggests that the transcriptional regulatory network that controls epidermal cell fate determination and anthocyanin biosynthesis in Arabidopsis is far more complicated than previously thought. In particular, more regulators of GL2 have been identified, and GL2 has been shown to be involved in the regulation of anthocyanin biosynthesis. This review focuses on the research progress on the regulation of GL2 expression, and the roles of GL2 in the regulation of epidermal cell fate determination and anthocyanin biosynthesis in Arabidopsis.