Out of the blue: Phototropins of the leaf vascular bundle sheath mediate the regulation of leaf hydraulic conductance by blue light

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 (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. 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.