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Boron sensing and intracellular trafficking of boron transporters
Plant roots take up minerals and translocate them toward tissues where they are utilized. Among the 17 essential elements of plants, here we focus on boron (B) . B is required for structure of pectin in plant cell wall, but is toxic when present in excess. B deficiency and toxicity are both major agricultural problems worldwide. Therefore, understanding the molecular mechanisms underlying B transport is important to develop technology to alleviate B deficiency and toxicity problems. Using the model plant Arabidopsis thaliana, 2 types of B transporters facilitating boron transport across the plasma membrane (PM) have been identified (Fig.1). Under low B conditions, a boric acid channel, NIP5;1, and a borate exporter, BOR1, are required for efficient B uptake into roots and subsequent translocation toward shoots (Takano et al. 2002 Nature; Takano et al. 2006 Plant Cell; Fig. 1). Under high B conditions, expression of NIP5;1 and BOR1 is reduced, and a borate exporter BOR4 (Bot1 in barley) is responsible for B exclusion from the root (Miwa et al. 2007 Science).

