Boron‐sensing mechanisms involved in boron transport regulation in plants

Abstract

AbstractBoron (B) is an essential micronutrient for plants. B deficiency and toxicity are widespread agricultural problems worldwide. To maintain appropriate B levels in tissues, plants require B‐sensing mechanisms to regulate its transport. In Arabidopsis thaliana, the boric channel NIP5;1, the borate transporter BOR1, and their homologs in the plasma membrane are responsible for efficient uptake and translocation of B when its supply is limited. Importantly, these transport systems are downregulated under high‐B conditions at multiple post‐transcriptional levels. NIP5;1 is downregulated by mRNA degradation following the B‐dependent ribosome stalling at the minimum upstream open reading frame (uORFs, AUGUAA) in the 5′ untranslated region (5′ UTR). Upon high B supply, BOR1 is ubiquitinated and transported to the vacuole via the endocytic pathway for degradation. Under higher B supply, BOR1 is further downregulated by translational suppression dependent on multiple uORFs in the 5′ UTR. These responses are important for preventing B toxicity. High‐B‐induced ubiquitination of BOR1 was shown to be dependent on its transport activity, suggesting that conformation transition during the borate transport cycle in the plasma membrane controls the rate of ubiquitination. The ribosome stalling that leads to mRNA degradation of NIP5;1 and translational suppression of BOR1 is presumably dependent on B binding to factors involved in translation in the cytosol. This review summarizes recent studies on B sensing and proposes future directions for research.