ABSTRACT
The Arabidopsis 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 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, phot1-5phot2-1, 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 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.
One-Sentence summary Blue light regulates the leaf hydraulic conductance via the bundle-sheath cells’ blue light PHOT receptors which, via an autonomous signaling pathway, activate the BSCs’ AHA2 H+-pump.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* Both authors contributed equally to this work.
The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantcell.org) are Yael Grunwald (Yael.grunwald{at}huji.ac.il), Sanbon Gosa (Sanbon.Gosa{at}mail.huji.ac.il) Tanmayee Torne Srivastava (Tanmayee.torne{at}mail.huji.ac.il) , Nava Moran (Nava.moran{at}mail.huji.ac.il), Menachem Moshelion (menachem.moshelion{at}mail.huji.ac.il).
All figures were converted from "bar graphs" to "box and whiskers"