Fig. 1.

A schematic model of B transport in A. thaliana roots under B limitation. Cell layers of a cross-section of A. thaliana roots where the Casparian strip is developed in the endodermis (State I; White, 2001) are illustrated. It is likely that endodermal cells subsequently become suberized and unable to take up solutes directly from the root apoplast (Moore et al., 2002). Under B limitation, NIP5;1 increases the permeability of boric acid to cell membranes, and facilitates influx of B into root cells from the soil. BOR1 exports B out of the cells toward the xylem against the concentration gradient. It is likely that the co-ordinated expression patterns of BOR1 and NIP5;1 are essential for efficient transcellular transport of B as NIP5;1 possibly facilitates B influx, following the B concentration gradient that BOR1 generates. Under high levels of B supply, expression of both NIP5;1 and BOR1 is decreased by transcriptional and post-transcriptional regulation, respectively. The downregulation of NIP5;1 and BOR1 might be beneficial for avoidance of overloading of high concentrations of B to shoots.