Creative Research Initiative Sousei, Hokkaido University, Kita-ku, Sapporo, Hokkaido 001-0021, Japan.
Ann Bot. 2010 Jun;105(7):1103-8. doi: 10.1093/aob/mcq044. Epub 2010 Mar 12.
The essentiality of boron (B) for plant growth was established > 85 years ago. In the last decade, it has been revealed that one of the physiological roles of B is cross-linking the pectic polysaccharide rhamnogalacturonan II in primary cell walls. Borate cross-linking of pectic networks serves both for physical strength of cell walls and for cell adhesion. On the other hand, high concentrations of B are toxic to plant growth. To avoid deficiency and toxicity problems, it is important for plants to maintain their tissue B concentrations within an optimum range by regulating transport processes. Boron transport was long believed to be a passive, unregulated process, but the identification of B transporters has suggested that plants sense and respond to the B conditions and regulate transporters to maintain B homeostasis.
Transporters responsible for efficient B uptake by roots, xylem loading and B distribution among leaves have been described. These transporters are required under B limitation for efficient acquisition and utilization of B. Transporters important for tolerating high B levels in the environment have also been identified, and these transporters export B from roots back to the soil. Two types of transporters are involved in these processes: NIPs (nodulin-26-like intrinsic proteins), boric acid channels, and BORs, B exporters. It is demonstrated that the expression of genes encoding these transporters is finely regulated in response to B availability in the environment to ensure tissue B homeostasis. Furthermore, plants tolerant to stress produced by low B or high B in the environment can be generated through altered expression of these transporters.
The identification of the first B transporter led to the discovery that B transport was a process mediated not only by passive diffusion but also by transporters whose activity was regulated in response to B conditions. Now it is evident that plants sense internal and external B conditions and regulate B transport by modulating the expression and/or accumulation of these transporters. Results obtained in model plants are applicable to other plant species, and such knowledge may be useful in designing plants or crops tolerant to soils containing low or high B.
硼（B）对植物生长的重要性在 85 年前就已确立。在过去的十年中，人们发现硼的一个生理作用是交联初生细胞壁中的果胶多糖鼠李半乳糖醛酸聚糖 II。硼酸盐交联果胶网络既为细胞壁提供物理强度，又为细胞黏附提供支持。另一方面，高浓度的硼对植物生长有毒。为了避免缺乏和毒性问题，植物通过调节运输过程将组织中的硼浓度维持在最佳范围内是很重要的。长期以来，硼的运输被认为是一个被动的、不受调节的过程，但 B 转运蛋白的鉴定表明，植物可以感知和响应 B 条件，并调节转运蛋白以维持 B 体内平衡。
负责通过根部有效吸收 B、木质部加载以及叶片间 B 分配的转运体已经被描述。在 B 限制下，这些转运体对于高效获取和利用 B 是必需的。也已经鉴定出对环境中高 B 水平具有耐受性的转运体，这些转运体将 B 从根部运回到土壤中。这两个过程都涉及到两种类型的转运体：NIPs（类豆球蛋白 26 内在蛋白）、硼酸通道和 BORs（B 外排蛋白）。研究表明，这些转运体基因的表达是精细调节的，以响应环境中 B 的可用性，从而确保组织内 B 的平衡。此外，通过改变这些转运体的表达，可以产生对环境中低 B 或高 B 产生的胁迫具有耐受性的植物。
第一个 B 转运体的鉴定导致发现 B 转运不仅是通过被动扩散介导的，而且是通过其活性受到 B 条件调节的转运体介导的。现在很明显，植物可以感知内部和外部的 B 条件，并通过调节这些转运体的表达和/或积累来调节 B 运输。在模式植物中获得的结果可应用于其他植物物种，这种知识可能有助于设计对含有低或高 B 的土壤具有耐受性的植物或作物。
