Molecular and functional characterization of Arabidopsis thaliana VPNB1 gene involved in plant vascular development

•thaliana AtVPNB1 gene encodes for a novel Armadillo repeat protein.•AtVPNB1 is a plant specific protein localized in nuclear bodies.•vpnb1 loss-of-function mutants display delayed germination and growth rate.•Mutants show deregulation of several master genes of secondary wall biosynthesis.•Overexpression of AtAPRF1 inverts the mutant phenotype. Armadillo (ARM) repeat containing proteins constitute a large family in plants and are involved in diverse cellular functions, like signal transduction, proliferation and differentiation. In animals, ARM repeat proteins have been implicated in cancer development. In this study, we aimed in characterizing the VPNB1 gene from Arabidopsis thaliana and its role in plant development, by implementing a number of genetic and molecular approaches. AtVPNB1 encodes for an ARM repeat protein of unknown function, exclusively expressed in the cambium as well as in the differentiating xylem and phloem cells of the vascular system. Subcellular localization experiments showed that VPNB is confined in nucleoplasmic speckle-like structures unrelated to cajal bodies. Transgenic VPNB-impaired plants exhibit a slower growing phenotype and a non-canonical pattern of xylem tissue. On the contrary, VPNB overexpression lines display an inverted phenotype of increased growth, accompanied by an increased deposition of phloem and xylem cell layers. In line with the above data, qPCR analysis revealed a deregulation of several key master genes of secondary wall biosynthesis, underlining the involvement of VPNB1 in the regulation and differentiation of the root and shoot vascular tissue.