Boron-Dependent Translational Suppression of the Borate Exporter Contributes to the Avoidance of Boron Toxicity.

Boron (B) is an essential element for plants; however, as high B concentrations are toxic, B transport must be tightly regulated. BOR1 is a borate exporter in Arabidopsis () that facilitates B translocation into shoots under B deficiency conditions. When the B supply is sufficient, expression is down-regulated by selective degradation of BOR1 protein, while additional regulatory mechanisms are proposed to exist. In this study, we identified a novel B-dependent translational suppression mechanism. In vivo and in vitro reporter assays demonstrated that translation was reduced in a B-dependent manner and that the 5'-untranslated region was both necessary and sufficient for this process. Mutational analysis revealed that multiple upstream open reading frames in the 5'-untranslated region were required for translational suppression, and this process depended on the efficiency of translational reinitiation at the open reading frame after translation of the upstream open reading frames. To understand the physiological significance of regulation, we characterized transgenic plants defective in either one or both of the regulation mechanisms. translational suppression was induced at higher B concentrations than those triggering BOR1 degradation. Plants lacking both regulation mechanisms exhibited more severe shoot growth reduction under high-B conditions than did plants lacking BOR1 degradation alone, thus demonstrating the importance of translational suppression. This study demonstrates that two mechanisms of posttranscriptional regulation, each induced under different B concentrations, contribute to the avoidance of B toxicity in plants.