Abstract
Arabidopsis BOR1 and BOR2 are efflux transporters of B for xylem loading and required for root cell elongation under B limitation. We studied roles of BOR3, BOR4 and BOR5, BOR1 paralogs, in B transport. Transcripts of BOR3, BOR4 and BOR5 were all detected both in shoots and roots. Using transgenic A. thaliana lines carrying promoter-GUS constructs, we found that BOR3 is predominately expressed in cortex in roots, trichome and guard cells in shoots: BOR4 in endodermis in roots: BOR5 in epidermis at root tips and steles in roots. Expression of BOR3-5 decreased B concentration of yeast cells, suggesting that they encode functional efflux-type B transporters. T-DNA insertion mutants for BOR3-5 did not show apparent phenotype, but bor1-3/bor2-1/bor3-1, triple T-DNA insertion mutant showed more severe root growth retardation under B deficiency compared to the double mutant bor1-3/bor2-1, suggesting that BOR3 was supportive of BOR1-2 in roots.
