Functional diversity of R3 single-repeat genes in trichome development.

Trichome and root hair patterning are governed by a conserved cassette of bHLH and MYB factors, the WD40 protein TTG1, and six single-repeat MYB R3 factors that are thought to counteract them. In this work we focus on the single-repeat R3 factor ETC3 and show that its major role is in the regulation of trichome density in a redundant manner. Diversification of the ETC3 gene has occurred at the promoter level, as etc3 mutants can be rescued by expressing ETC3 under the control of the TRY or CPC promoter. ETC3 movement was detected between epidermal cells as well as between the epidermis and underlying tissues. Finally, we found marked differences in the ability of the single-repeat R3 factors to interfere with the dimerisation of GL1 and GL3 in a yeast three-hybrid system, with CPC being the most potent inhibitor followed by ETC1, TRY, ETC3 and ETC2. Mathematical analysis predicts that this behaviour has a major impact on protein mobility, suggesting a tight reverse correlation between inhibitory function and the diffusion/transport range of the inhibitors. This prediction is supported by a comparison of CPC and ETC3 mobility in egl3 gl3 double mutants and 35S:GL3 lines.