Distinct and overlapping roles of single-repeat MYB genes in root epidermal patterning

Cell specification in the root epidermis of Arabidopsis generates a position-dependent pattern of root-hair cells and non-hair cells. Here we conduct a comprehensive analysis of the five members of a single-repeat R3 MYB gene family, including CAPRICE (CPC), TRIPTYCHON (TRY), ENHANCER of TRY and CPC 1, 2, and 3 (ETC1, ETC2, and ETC3), and study their role and functional relationship in root epidermal cell specification. Based on genetic and expression analyses, CPC, TRY and ETC1, but not ETC2 or ETC3, promote the hair cell fate by inhibiting non-hair specification. Further, we find that single-repeat MYB activity is required for epidermal patterning throughout root development, beginning during embryogenesis. We also identify a novel regulatory interaction whereby GLABRA2 (GL2) promotes TRY (but not CPC or ETC1) expression in the root epidermis, which generates a second lateral inhibition feedback loop. Gene fusion experiments combining CPC regulatory elements with protein-coding regions of each single-repeat MYB gene suggest that all five proteins are functionally similar, although TRY and ETC2 exhibit distinctions from CPC/ETC1/ETC3. These results provide new insight into the function of these single-repeat MYBs and suggest that divergence of their regulatory sequences is largely responsible for their distinct roles in epidermal cell patterning.