Trichome patterning in Arabidopsis is controlled by several different types of transcription factors. Based on available evidence, it was proposed that an R2R3 MYB-type transcription factor GLABRA1 (GL1) , a bHLH transcription factors GLABRA3 (GL3)  or ENHANCER OF GLABRA3 (EGL3) , and a WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) [4, 5], form an activator complex TTG1-GL3/EGL3-GL1. This activator complex activates the expression of GLABRA2 (GL2), which encodes a homeodomain protein that promotes trichome formation in shoots [6–8]. The same activator complex also induces the expression of some single-repeat R3 MYB genes.
So far a total of six genes in the Arabidopsis genome have been reported encoding single-repeat R3 MYB transcription factors, including TRIPTYCHON (TRY) [9, 10], CAPRICE (CPC) [11, 12], TRICHOMELESS1(TCL1) , and ENHANCER OF TRY AND CPC1, 2 and 3 (ETC1, ETC2 and ETC3/CPL3) [14–18]. Single-repeat MYB transcription factors are characterized by their short sequence (75-112 amino acids) and consist largely of the single MYB domain . Single-repeat R3 MYBs can move from a trichome precursor cell to its neighboring cells to block the formation of the activator complex by competing with GL1 in binding GL3 or EGL3, thus inhibiting trichome formation in shoots [8, 13, 18, 19]. These single-repeat R3 MYB transcription factors differ in their binding strength to GL3 and their capacity to compete with GL1 for binding GL3. A yeast three-hybrid assays suggested that CPC is the most potent inhibitor followed by ETC1, TRY, ETC3 and ETC2 . A protoplast transfection assay showed that TCL1 is stronger than CPC in their ability to bind GL3 .
Although over expressing any of these six single-repeat R3 transcription factors in Arabidopsis causes a glabrous phenotype, single mutants of these genes have different phenotypes. Mutation in TRY results in trichome cluster phenotype [9, 10], mutation in CPC increases trichome number [11, 12], and mutation in TCL1 causes ectopic trichome formation on inflorescence stems and pedicels . Mutation in ETC1, ETC2 or ETC3 does not dramatically affect trichome formation. However, analysis of double, triple and higher order mutants between these single mutants indicated that all six single-repeat MYB transcription factors function in a highly redundant manner to control trichome formation in Arabidopsis [13–18, 20].
In addition to competing with GL1 for binding GL3, our previous results showed that TCL1 could also directly suppress the expression of GL1 . Interestingly, unlike many other single-repeat MYBs, the expression of TCL1 is not controlled by the activator complex formed by GL1 and GL3/EGL3. Recently, it has been found that microRNA156 (MIR156)-targeted SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) 9 can activate TCL1 and TRY expression through binding to their promoters .
Here we analyzed the role of TRICHOMELESS2 (TCL2), a previously uncharacterized single-repeat MYB transcription factor in trichome patterning in Arabidopsis. We demonstrated that TCL2, like other single-repeat MYBs, negatively regulates trichome formation. Overexpression of TCL2 conferred glabrous phenotype while knockdown of TCL2 via RNAi induced ectopic trichome formation on inflorescence stems and pedicels, a phenotype that was previously observed in tcl1 mutants. Furthermore we provide genetic evidence that TCL2 and TCL1 may not be fully functional equivalent. We also showed that MIR156 is involved in the regulation of TCL2, and SPLs may require co-activators to regulate the expression of their target genes, including TCL1, TRY, and possibly, TCL2.