The Arabidopsis leaf veins bundle sheath cells (BSCs) – a selective barrier to water and solutes entering the mesophyll – increase the leaf radial hydraulic conductance (K_leaf) by acidifying the xylem sap by their plasma membrane H⁺-ATPase, AHA2. Based on this and on the BSCs’ expression of PHOT1 and PHOT2, and the known blue-light (BL)-induced K_leaf increase, we hypothesized that, resembling the guard cells, BL perception by the BSCs’ phots activates its H⁺-ATPase, which, consequently, upregulates K_leaf. Indeed, under BL, the K_leaf of the knockout mutant lines phot1-5, phot2-1, phot1-5phot2-1, and aha2-4 was lower than that of the WT. BSC-only-directed complementation of phot1-5 or aha2-4 by PHOT1 or AHA2, respectively, restored the BL-induced K_leaf increase. BSC-specific silencing of PHOT1 or PHOT2 prevented such K_leaf 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.

中文翻译：

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出乎意料：叶片维管束鞘的向光素介导蓝光对叶片水力传导的调节

拟南芥叶脉束鞘细胞 (BSC) - 水和溶质进入叶肉的选择性屏障 -通过其质膜 H ⁺ -ATP 酶 AHA2酸化木质部汁液来增加叶片径向水力传导 (K_叶)。基于此以及 BSCs 的PHOT1和PHOT2 表达，以及已知的蓝光 (BL) 诱导的 K_叶增加，我们假设，类似于保卫细胞，BSCs 的 BL 感知激活其 H ⁺ - ATPase，因此，上调 K_叶。事实上，在 BL 下，敲除突变系phot1-5、_phot2-1、phot1-5phot2-1和aha2-4低于 WT。分别由PHOT1或AHA2对phot1-5或aha2-4进行BSC-only 定向互补，恢复了 BL 诱导的 K_叶增加。PHOT1或PHOT2的 BSC 特异性沉默阻止了这种 K_叶增加。木质部喂养的激酶抑制剂（tyrphostin 9）也在 WT 植物中复制了这一点。白光——在phot1-5突变体中无效——酸化 WT 和PHOT1互补的phot1-5中的木质部汁液（相对于黑暗）. 这些结果得到了 BL 增加 BSC 原生质体的水渗透性和细胞溶质 pH 值以及它们被 BL 超极化的支持，将 BSC 鉴定为叶片中的第二个光控水导元件，与气孔导度串联。通过两者，BL 调节叶片水分平衡。