Abstract
In Arabidopsis, trichome formation is regulated by the interplay of R3 MYBs and several others transcription factors including the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1), the R2R3 MYB transcription factor GLABRA1 (GL1), the bHLH transcription factor GLABRA3 (GL3) or ENHANCER OF GLABRA3 (EGL3), and the homeodomain protein GLABRA2 (GL2). R3 MYBs including TRICHOMELESS1 (TCL1), TCL2, TRYPTICHON (TRY), CAPRICE (CPC), ENHANCER OF TRY AND CPC1 (ETC1), ETC2 and ETC3 negatively regulate trichome formation by competing with GL1 for binding GL3 or EGL3, thus blocking the formation of TTG1–GL3/EGL3–GL1, an activator complex required for the activation of the trichome positive regulator gene GL2. However, it is largely unknown if R3 MYBs in other plant species especially woody plants have similar functions. By BLASTing the Populus trichocarpa protein database using the entire amino acid sequence of TCL1, an Arabidopsis R3 MYB transcription factor, we identified a total of eight R3 MYB transcription factor genes in poplar, namely P. trichocarpa TRICHOMELESS1 through 8 (PtrTCL1–PtrTCL8). The amino acid signature required for interacting with bHLH transcription factors and the amino acids required for cell-to-cell movement of R3 MYBs are not fully conserved in all PtrTCLs. When tested in Arabidopsis protoplasts, however, all PtrTCLs interacted with GL3. Expressing each of the eight PtrTCL genes in Arabidopsis resulted in either glabrous phenotypes or plants with reduced trichome numbers, and expression levels of GL2 in all transgenic plants tested were greatly reduced. Expression of PtrTCL1 under the control of TCL1 native promoter almost completely complemented the mutant phenotype of tcl. In contrast, expression of PtrTCL1 under the control of TRY native promoter in the try mutant, or under the control of CPC native promoter in the cpc mutant resulted in glabrous phenotypes, suggesting that PtrTCL1 functions similarly to TCL1, but not TRY and CPC.
Highlights
Single-repeat R3 MYB transcription factors (R3 MYBs) are small proteins that typically contain ∼100 amino acids, largely consist of a single MYB DNA-binding repeat, and are best characterized for their regulatory roles in trichome and root hair development (Wang and Chen, 2014)
The amino acid signature [D/E]L×2[R/K]×3L×6L×3R, that is required for the interaction with R/B-like bHLH transcription factors (Zimmermann et al, 2004), is fully conserved in all seven R3 MYB transcription factors in Arabidopsis (Wang et al, 2007, 2008; Gan et al, 2011), but is found in only three poplar R3 MYBs including PtrTCL1, PtrTCL2, and PtrTCL3 (Figure 1A)
In this study we identified poplar homologs of Arabidopsis R3 MYB transcription factors, and analyzed their function in trichome formation in Arabidopsis
Summary
Single-repeat R3 MYB transcription factors (R3 MYBs) are small proteins that typically contain ∼100 amino acids, largely consist of a single MYB DNA-binding repeat, and are best characterized for their regulatory roles in trichome and root hair development (Wang and Chen, 2014). All seven R3 MYBs contain the residues [D/E]L×2[R/K]×3L×6L×3R, a conserved amino acid signature required for interaction of MYBs with R/B-like bHLH transcription factors (Zimmermann et al, 2004), and W×M, a sequence motif that has been shown to be required for cell-to-cell movement of CPC (Kurata et al, 2005). TTG1, GL1, and GL3 or EGL3 form an activator complex to induce the expression of GL2, a positive regulatory gene of trichome formation (Rerie et al, 1994). R3 MYBs can move from a trichome precursor cell to its neighboring cells, and compete with GL1 for binding GL3 or EGL3, limiting the formation of the activator complex (Hülskamp et al, 1994; Schellmann et al, 2002; Esch et al, 2003; Schiefelbein, 2003; Pesch and Hülskamp, 2004; Ishida et al, 2008), resulting in inhibition of trichome initiation
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