There are a total of six genes in the Arabidopsis genome that encode a unique subfamily of MYB transcription factors, namely single-repeat R3 MYB transcription factors. These transcription factors, including TRIPTYCHON (TRY) [1, 2], CAPRICE (CPC) , TRICHOMELESS1 (TCL1) , ENHANCER of TRY and CPC 1, 2, and 3 (ETC1, ETC2 and ETC3 (CPL3)) [5–9], are characterized by their short sequence (75–112 amino acids) and consist largely of the single MYB domain (e.g. without other predicted motifs). It is generally believed that these single-repeat R3 MYB transcription factors mediate lateral inhibition during epidermal patterning. In general, these single-repeat R3 MYB transcription factors act as negative regulators for trichome formation in shoots, but as positive regulators for root hair formation in roots.
T-DNA insertion mutants are available for each of these six single-repeat R3 MYB genes. Among them, only single loss-of-function mutants for TRY, CPC and TCL1 display major defects in trichome and/or root hair cell specification [1–4], whereas loss-of-function alleles of ETC1, ETC2 or ETC3 cause little or no phenotypic effect [5–9]. The analysis of double and triple mutants indicated that ETC1 and ETC3 can function redundantly with TRY and CPC to control leaf trichome and root hair formation [5, 6, 9], and that ETC2 functions redundantly with TRY and CPC to control trichome formation on petioles . Further, CPC functions redundantly with TCL1 to control trichome formation on the inflorescence stems and pedicels . However, a role of TCL1 in leaf trichome and root hair formation has not been established.
Available evidence suggests that single-repeat R3 MYB transcription factors, a WD40-repeat protein, TRANSPARENT TESTA GLABRA1 (TTG1) [10, 11], an R2R3 MYB-type transcription factor, GLABRA1 (GL1)  or WEREWOLF (WER) [13–15], a bHLH transcription factor, GLABRA3 (GL3) or ENHANCER OF GLABRA3 (EGL3) [16, 17], and a homeodomain protein, GLABRA2 (GL2) [18, 19], regulate trichome and/or root hair formation (reviewed in [20–22]). Based on the results of yeast-two-hybrid interaction assays, it has been proposed that TTG1, GL1 or WER, and GL3 or EGL3 form an activator complex to induce GL2 expression . The single-repeat R3 MYB transcription factors are proposed to move from a trichome precursor cell to its neighboring cell (in the shoot epidermis) or from an N cell to an H cell (in the root epidermis) to compete with GL1 or WER for binding GL3 or EGL3, thus limiting the activity of the activator complex [2, 3, 20–24]. Recently, we showed that one of the single-repeat R3 MYB transcription factors, TCL1, can directly suppress the transcription of GL1 , providing an additional loop of regulation of the activity of the proposed activator complex by single-repeat R3 MYB transcription factors.
It has been proposed that the same activator complex that activates GL2 can also activate the expression of single-repeat R3 MYB genes (reviewed in [20–22]), but so far only CPC has been identified as a direct target gene for WER [25, 26], and GL3 has been shown to be recruited to the promoter region of CPC and ETC1 .
To gain new insight into the role of single-repeat R3 MYB transcription factors in controlling epidermal development, we conduct a comprehensive analysis of the single-repeat R3 MYB gene family. By generating and analyzing higher order mutants among six single-repeat R3 MYB genes, we identified previously unrecognized roles of single-repeat R3 MYB transcription factors in the regulation of trichome and root hair formation. We demonstrate that TCL1 participates in the control of leaf trichome and root hair formation, and that ETC1 and ETC3 have a role in regulating trichome formation on the inflorescence stems and pedicles. We also discover that single-repeat R3 MYBs normally suppress trichome formation on siliques and cotyledons. By using an Arabidopsis protoplast transfection system, we show that cotransfection of GL1 or WER, with GL3 or EGL3, is sufficient to activate the transcription of TRY, CPC, ETC1 and ETC3, but not ETC2 and TCL1. Our results suggest that although the six single-repeat R3 MYB genes have largely overlapping functions in controlling epidermal development, the transcriptional regulation of these single-repeat R3 MYB genes involves distinct mechanisms.