VvBOR1, the Grapevine Ortholog of AtBOR1, Encodes an Efflux Boron Transporter That is Differentially Expressed Throughout Reproductive Development of Vitis vinifera L.

Boron (B) is an essential micronutrient for normal development of roots, shoots and reproductive tissues in plants. Due to its role in the structure of rhamnogalacturonan II, a polysaccharide required for pollen tube growth, B deficiency has been associated with the occurrence of parthenocarpic seedless grapes in some varieties of Vitis vinifera L. Despite that, it is unclear how B is mobilized and accumulated in reproductive tissues. Here we describe the characterization of an efflux B transporter, VvBOR1, homolog to AtBOR1, which is involved in B xylem loading in Arabidopsis thaliana roots. VvBOR1-green fluorescent protein (GFP) fusion protein expressed in A. thaliana localizes in the proximal plasma membrane domain in root pericycle cells, and VvBOR1 overexpression restores the wild-type phenotype in A. thaliana bor1-3 mutant plants exposed to B deficiency. Complementation of a mutant yeast strain indicates that VvBOR1 corresponds to a B efflux transporter. Transcriptional analyses during grapevine reproductive development show that the VvBOR1 gene is preferentially expressed in flowers at anthesis and a direct correlation between the expression pattern and B content in grapes was established, suggesting the involvement of this transporter in B accumulation in grapevine berries.