Understanding the regulatory mechanisms of B transport to develop crop plants with B efficiency and excess B tolerance

Boron (B) is a micronutrient required for optimal plant growth and development. It is primarily involved in pectin crosslinking in the rhamnogalacturonan II region to ensure cell wall integrity. In plants grown under limited soil B availability, seed yield and quality are often compromised. However, excessive B accumulation can also be toxic to plants. Therefore, plants must maintain optimal tissue concentrations of B by regulating transport processes. This article is a critical review of the molecular mechanisms involved in B transport under limited and excess B conditions. We focus on various B transport proteins and their B-dependent regulation, mainly as studied in the model plant Arabidopsis thaliana, and summarize research progress on B transport and its regulation in Brassica napus, Oryza sativa, Hordeum vulgare, Triticum aestivum, and Zea mays. Under limited B conditions, the nodulin 26-like intrinsic protein (NIP) AtNIP5;1 and the borate transporter (BOR) AtBOR1 mediate the radial transport of boric acid or borate from soil to the xylem. AtBOR2 also plays a role in root cell elongation by increasing the B-RGII dimerization rate. Under excess B conditions, AtBOR4 is involved in the directional export of B from roots to soil. In crop plants, NIPs and BORs responsible for B efficiency and excess B tolerance are becoming key targets of plant breeding. These B transport proteins are regulated at the transcription, mRNA degradation, translational repression, and endocytic degradation levels to maintain B homeostasis. Understanding the complex regulatory mechanisms of B transport will allow us to develop high-yielding plants under fluctuating soil B conditions.