Abstract
Boron (B) is essential for plant growth but toxic when present in excess. Arabidopsis thaliana BOR1 is a plasma membrane borate exporter and is essential for efficient B translocation from roots to shoots under B limitation. In transgenic plants expressing BOR1-GFP under control of the BOR1 promoter, BOR1-GFP was localized preferentially in the plasma membrane of proximal side of various root cells under B limitation, and was transferred to vacuole for degradation upon high-B supply. In contrast, GFP tagged BOR4, a BOR1 homolog suggested to be involved in B exclusion from root cells, was accumulated stably in the plasma membrane of distal side in epidermal cells irrespective of B conditions. Analysis of BOR1/BOR4 chimera and mutant proteins revealed that sorting motifs embedded in a probable cytoplasmic-loop region of BOR1 but not of BOR4 are involved in the endocytic and polar trafficking.
