Novel epidermal-cortical pattern and root apical silencing of VcCPC promoter activity in Arabidopsis might suggest a joint contribution to hairless roots in blueberry.

backgroundRoot hairs are specialized tubular extensions that play a pivotal role in nutrient absorption and plant-soil interactions. Typically, they originate from epidermal cells, which differentiate into either hair cells or non-hair cells. In blueberry (Vaccinium spp.), the roots are devoid of root hairs. To examine the mechanisms responsible for the hairless root phenotype, this study conducted anatomical and molecular biological investigations on the hairless roots of blueberry.

resultsThe epidermal cells of blueberry roots exhibit uniformity in size and cross-sectional dimensions, with each cell interfacing with two neighboring cortical cells. This distinctive epidermal cell pattern, which differs from the three known patterns observed in other plant species, is classified as type IV. The CAPRICE (CPC) gene, which encodes an R3-type MYB transcription factor, is a positive regulator of hair cell differentiation in Arabidopsis. Functional analyses utilizing Arabidopsis demonstrated that the blueberry VcCPC homolog retained its capacity to promote root hair formation in wild-type and cpc mutant Arabidopsis plants. GUS histochemical analysis revealed that the VcCPC promoter is active in multiple organs of transgenic Arabidopsis, including leaves, flowers, fruit, and the root maturation zone, but not in regions associated with root hair cell fate determination, such as the root elongation zone and meristematic zone. Complementation of the cpc mutant with a chimeric VcCPCpro::VcCPC construct did not restore the root hair-deficient phenotype. Substitution of the VcCPC promoter sequence (nt - 709 to - 1) with the AtCPC promoter sequence (nt - 683 to - 1) effectively restored the root hair phenotype in the cpc mutant.

conclusionsBlueberry roots exhibit a distinctive type IV epidermal cell patterning, which differs from previously identified epidermis-cortex cell patterns. Although VcCPC maintains its conserved role in promoting root hair differentiation in Arabidopsis, root apical silencing of VcCPC, in conjunction with the novel epidermal-cortical pattern, may contribute to the root hair-deficient phenotype in blueberry. Substituting the native VcCPC promoter with the AtCPC promoter restored root hair development in the Arabidopsis cpc mutant, indicating that regulatory elements present in the AtCPC promoter, but absent in the VcCPC promoter, might mediate the root phenotypic divergence.