Institute of Plant Science and Resources, Okayama University, Kurashiki 710-0046, Japan.
State Key Laboratory of Subtropical Silviculture, Zhejiang A&F University, Zhejiang 311300, China.
Plant Physiol. 2022 Mar 4;188(3):1649-1664. doi: 10.1093/plphys/kiab575.
Uptake of boron (B) in rice (Oryza sativa) is mediated by the Low silicon rice 1 (OsLsi1) channel, belonging to the NOD26-like intrinsic protein III subgroup, and the efflux transporter B transporter 1 (OsBOR1). However, it is unknown how these transporters cooperate for B uptake and how they are regulated in response to B fluctuations. Here, we examined the response of these two transporters to environmental B changes at the transcriptional and posttranslational level. OsBOR1 showed polar localization at the proximal side of both the exodermis and endodermis of mature root region, forming an efficient uptake system with OsLsi1 polarly localized at the distal side of the same cell layers. Expression of OsBOR1 and OsLsi1 was unaffected by B deficiency and excess. However, although OsLsi1 protein did not respond to high B at the protein level, OsBOR1 was degraded in response to high B within hours, which was accompanied with a significant decrease of total B uptake. The high B-induced degradation of OsBOR1 was inhibited in the presence of MG-132, a proteasome inhibitor, without disturbance of the polar localization. In contrast, neither the high B-induced degradation of OsBOR1 nor its polarity was affected by induced expression of dominant-negative mutated dynamin-related protein 1A (OsDRP1AK47A) or knockout of the mu subunit (AP2M) of adaptor protein-2 complex, suggesting that clathrin-mediated endocytosis is not involved in OsBOR1 degradation and polar localization. These results indicate that, in contrast to Arabidopsis thaliana, rice has a distinct regulatory mechanism for B uptake through clathrin-independent degradation of OsBOR1 in response to high B.
水稻（Oryza sativa）对硼（B）的吸收是由低硅水稻 1（OsLsi1）通道介导的，该通道属于 NOD26 样内在蛋白 III 亚群，以及外排转运蛋白 B 转运蛋白 1（OsBOR1）。然而，目前尚不清楚这些转运蛋白如何合作进行 B 吸收，以及它们如何响应 B 波动进行调节。在这里，我们研究了这两种转运蛋白在转录和翻译后水平上对环境 B 变化的反应。OsBOR1 在成熟根区近侧的内皮层和外皮层均表现出极性定位，与在同一细胞层远侧极性定位的 OsLsi1 形成有效的吸收系统。OsBOR1 和 OsLsi1 的表达不受 B 缺乏和过量的影响。然而，尽管 OsLsi1 蛋白在蛋白质水平上对高 B 没有反应，但 OsBOR1 在数小时内响应高 B 被降解，这伴随着总 B 吸收的显著减少。高 B 诱导的 OsBOR1 降解在蛋白酶体抑制剂 MG-132 的存在下被抑制，而不会干扰其极性定位。相比之下，高 B 诱导的 OsBOR1 降解及其极性都不受显性负突变的动力蛋白相关蛋白 1A（OsDRP1AK47A）的诱导表达或衔接蛋白-2 复合物的 μ 亚基（AP2M）的敲除的影响，表明网格蛋白介导的内吞作用不参与 OsBOR1 降解和极性定位。这些结果表明，与拟南芥不同，水稻具有通过高 B 诱导的 OsBOR1 非网格蛋白依赖性降解来调节 B 吸收的独特调节机制。
