The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot 76100, Israel.
Plant Cell. 2022 May 24;34(6):2328-2342. doi: 10.1093/plcell/koac089.
The Arabidopsis (Arabidopsis thaliana) leaf veins bundle-sheath cells (BSCs)-a selective barrier to water and solutes entering the mesophyll-increase the leaf radial hydraulic conductance (Kleaf) by acidifying the xylem sap by their plasma membrane H+-ATPase, AHA2. Based on this and on the BSCs' expression of phototropins PHOT1 and PHOT2, and the known blue light (BL)-induced Kleaf increase, we hypothesized that, resembling the guard cells, BL perception by the BSCs' phots activates its H+-ATPase, which, consequently, upregulates Kleaf. Indeed, under BL, the Kleaf of the knockout mutant lines phot1-5, phot2-1, phot1-5 phot2-1, and aha2-4 was lower than that of the wild-type (WT). BSC-only-directed complementation of phot1-5 or aha2-4 by PHOT1 or AHA2, respectively, restored the BL-induced Kleaf increase. BSC-specific silencing of PHOT1 or PHOT2 prevented such Kleaf increase. A xylem-fed kinase inhibitor (tyrphostin 9) replicated this also in WT plants. White light-ineffective in the phot1-5 mutant-acidified the xylem sap (relative to darkness) in WT and in the PHOT1-complemented phot1-5. These results, supported by BL increase of BSC protoplasts' water permeability and cytosolic pH and their hyperpolarization by BL, identify the BSCs as a second phot-controlled water conductance element in leaves, in series with stomatal conductance. Through both, BL regulates the leaf water balance.
拟南芥（Arabidopsis thaliana）叶片叶脉束鞘细胞（BSCs）——一种选择性阻止水分和溶质进入叶肉的屏障——通过其质膜 H+-ATPase AHA2 酸化木质部汁液，增加叶片径向水力传导度（Kleaf）。基于这一点，以及 BSCs 表达的向光素 PHOT1 和 PHOT2，以及已知的蓝光（BL）诱导的 Kleaf 增加，我们假设，类似于保卫细胞，BSCs 的光受体感知 BL 会激活其 H+-ATPase，从而上调 Kleaf。事实上，在 BL 下，phot1-5、phot2-1、phot1-5 phot2-1 和 aha2-4 缺失突变体系的 Kleaf 低于野生型（WT）。通过 PHOT1 或 AHA2 分别对 phot1-5 或 aha2-4 进行 BSC 特异性互补，恢复了 BL 诱导的 Kleaf 增加。BSC 特异性沉默 PHOT1 或 PHOT2 阻止了这种 Kleaf 增加。木质部喂养激酶抑制剂（tyrphostin 9）在 WT 植物中也复制了这一现象。在 phot1-5 突变体中无效的白光——相对于黑暗——酸化了木质部汁液（相对于黑暗），在 WT 和 PHOT1 互补的 phot1-5 中也是如此。这些结果得到了 BL 增加 BSC 原生质体水通透性和细胞质 pH 值以及 BL 使它们超极化的支持，将 BSCs 鉴定为叶片中第二个光控水传导元件，与气孔导度串联。通过这两种方式，BL 调节叶片的水分平衡。
