Boron transport and regulatory mechanisms in plants

Boron (B) is an essential micronutrient required for cross-linking rhamnogalacturonan II in plant cell walls. Plants must maintain B homeostasis to prevent deficiency and toxicity. This review summarizes recent advances in B sensing, transport, and regulatory mechanisms in Arabidopsis and other species. Under low-B conditions in Arabidopsis, the boric acid channel NIP5;1 facilitates B uptake into root cells, while clade I borate exporters BOR1 and BOR2 move B toward the stele. The opposing polar localizations of NIP5;1 and BOR1 in the plasma membrane of root cells toward the soil- and the stele-side, respectively, create a transcellular pathway that efficiently delivers B to the xylem. BOR1 is vital for root-to-shoot translocation, whereas BOR2 supports RG-II – B complex formation in meristematic cells. In shoots, NIP6;1 and BOR1 ensure B is preferentially allocated to young tissues and reproductive organs. During reproductive development, NIP7;1 and BOR1 in anther tapetum cells supply B to developing microspores; their absence leads to pollen defects. In contrast, under high-B conditions, plants utilize clade II borate transporters like BOR4 to export excess B. High-B tolerance also relies on the downregulation of NIP5;1 and BOR1, alongside physical barriers such as the Casparian strip. B availability triggers complex post-transcriptional regulation. For instance, NIP5;1 is downregulated via B-dependent ribosome stalling at the AUGUAA sequence in the 5’-untranslated region, causing translational suppression, mRNA degradation, and subsequent transcriptional inhibition. BOR1 is regulated through B-dependent ubiquitination, endocytosis, and vacuolar degradation, and a transport-coupled ubiquitination model has been proposed as a B sensing mechanism. Additionally, transcription factors modulate transporter expression in response to both low- and high-B conditions. Taken together, plant utilize sophisticated, multilayered systems to sense, transport, and maintain B homeostasis across different tissues and developmental stages. This review highlights recent progress in understanding these mechanisms and provides perspectives for research on B transport.