Analysis of TTG1 and CPC-like MYB genes during Arabidopsis epidermal cell differentiation

The development of Arabidopsis thaliana epidermal cells includes the differentiation of trichomes and root hairs. The TRANSPARENT TESTA GLABRA 1 (TTG1) gene encodes a WD40 protein that induces trichome differentiation and reduces root hair formation in Arabidopsis. The CAPRICE (CPC) gene family includes CPC, ENHANCER OF TRY AND CPC1 (ETC1), ENHANCER OF TRY AND CPC2 (ETC2), and CPC LIKE MYB3 (CPL3), which encode R3-type MYB transcription factors that inhibit trichome differentiation and promote root hair formation. CPC expression is positively regulated by a transcriptional complex that includes TTG1. To determine whether ETC1, ETC2, and CPL3 are also regulated by the TTG1 complex, we examined the functional relationship between TTG1 and CPC-like MYB genes. Double mutant analysis showed that the ttg1 mutant is epistatic to the cpc, etc1, etc2, and cpl3 mutants in trichome cell fate determination but not in root hair development. In roots, the cpc mutant is epistatic to the ttg1 mutant in root epidermal cell fate determination. Promoter-GUS analysis indicated that TTG1 is necessary for the expression of ETC1 and CPL3, but not for ETC2 expression. These results indicate that TTG1 had a stronger effect on trichome formation than CPC-like MYBs. By contrast, CPC had a stronger effect on root hair formation than TTG1. Our results suggest that ETC1 and CPL3 are also regulated by the TTG1 complex as is the case for CPC; however, ETC2 is not regulated by this complex. We concluded that ETC2 does not have a role in trichome and root hair formation.