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

Abstract

The leaf vascular bundle sheath cells (BSCs), which tightly envelop the leaf veins, are a selective dynamic barrier to water and solutes radially entering the mesophyll and play a major role in regulating the leaf radial hydraulic conductance (Kleaf). Blue light (BL) is known to increase Kleaf. Here we provide the mechanistic link for this phenomenon, based (a) on our recent demonstration that the BSCs' plasma membrane H + -ATPase, AHA2, increases Kleaf by acidifying the xylem sap, and (b) on the presence of the BL receptor genes, PHOT1and PHOT2, in the BSCs. The Kleaf of knockout mutant lines phot1-5, phot2-1, phot1-5phot2-1 and aha2-4 was lower than in WT and did not change under BL illumination. BSCs-directed (under a BSCs-specific promotor, SCR) respective complementation of phot1-5 and aha2-4 by PHOT1 and AHA2, restored the normal Kleaf. BSC-directed knockdown of PHOT1 or PHOT2 expression sufficed to abolish the BL sensitivity of Kleaf. Xylem-fed tyrosine kinase inhibitor, tyrphostine 9, abolished the BL-induced Kleaf increase but not the BLinduced stomatal conductance increase. In parallel, in phot1 mutants BL did not acidify the xylem sap, in contrast to WT and to leaves of phot1 complemented with SCR: PHOT1. Our results link the blue light control of water fluxes from the xylem to the mesophyll via the BSCs: BL →BSCs' PHOTs activation →tyrosine phosphorylation→BSCs' H +-ATPase activation →xylem acidification →Kleaf increase. Thus, a focus on the hydraulic valve in series with the stomata may provide new directions for crop manipulation for tolerance to the changing environment. Rev. Cell Dev. Biol. 15: 33-62. Brodribb, T.J. and Holbrook, N.M. (2004). Diurnal depression of leaf hydraulic conductance in a tropical tree species. Plant, Cell Environ. 27: 820-827. Clough, S.J. and Bent, A.F. (1998). Floral dip: A simplified method for Agrobacteriummediated transformation of Arabidopsis thaliana. Plant J. 16: 735-743.