Arabidopsis CAPRICE-LIKE MYB 3 (CPL3) controls endoreduplication and flowering development in addition to trichome and root hair formation

CAPRICE (CPC) encodes a small protein with an R3 MYB motif and promotes root hair cell differentiation in Arabidopsis thaliana. Three additional CPC-like MYB genes, TRY (TRIPTYCHON), ETC1 (ENHANCER OF TRY AND CPC 1) and ETC2 (ENHANCER OF TRY AND CPC 2) act in a redundant manner with CPC in trichome and root hair patterning. In this study, we identified an additional homolog, CPC-LIKE MYB 3 (CPL3), which has high sequence similarity to CPC, TRY, ETC1 and ETC2. Overexpression of CPL3 results in the suppression of trichomes and overproduction of root hairs, as has been observed for CPC, TRY, ETC1 and ETC2. Morphological studies with double, triple and quadruple homolog mutants indicate that the CPL3 gene cooperatively regulates epidermal cell differentiation with other CPC homologs. Promoter-GUS analyses indicate that CPL3 is specifically expressed in leaf epidermal cells, including stomate guard cells. Notably, the CPL3 gene has pleiotropic effects on flowering development, epidermal cell size and trichome branching through the regulation of endoreduplication.     

Root hair formation at the root-hypocotyl junction in CPC-LIKE MYB double and triple mutants of Arabidopsis

In Arabidopsis thaliana, R3-type MYB genes, CAPRICE (CPC) and its family of genes including TRIPTYCHON (TRY), ENHANCER OF TRY AND CPC1 (ETC1), ETC2 and CPC-LIKE MYB3 cooperatively regulate epidermal cell differentiation. Root hair formation is greatly reduced by a mutation in CPC, and try and etc1 enhance this phenotype. In this study, we demonstrate that CPC, TRY and ETC1 are also involved in root hair formation at the root-hypocotyl junction. The cpc try and cpc etc1 double mutants showed a reduced number of root hairs in that area. Additionally, the expression of ETC1::GUS was higher near this area. These results suggest that CPC family of genes also cooperatively regulates root hair formation at the root-hypocotyl junction in unique ways.     

Epidermal cell patterning and differentiation throughout the apical-basal axis of the seedling

The idea of common pathways guiding different fates is an emerging concept in plant development, and epidermal cell-fate specification in Arabidopsis thaliana is an excellent example to illustrate it. In the root epidermis, both hair patterning and differentiation depend on a complex interaction between both negative (WER, TTG, GL3, EGL3, and GL2) and positive (CPC, TRY, and ETC1) regulators of hair cell fate. These regulators pattern and differentiate hairs through a bi-directional signalling mechanism. The same molecular components (WER, TTG, GL3, EGL3, and GL2) seem to be involved in the patterning of stomata in the embryonic stem. However, the possible role of CPC, TRY, and ETC1 on stomatal patterning and/or differentiation has not been studied, questioning whether they, and the underlying bi-directional mechanism, guide patterning formation and differentiation in the hypocotyl.     

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.