Gating of aquaporins by cytosolic pH regulate Arabidopsis root water transport during anoxic stress

COLETTE TOURNAIRE-ROUX, MOIRA SUTKA, HÉLÈNE JAVOT, ELISABETH GOUT, DOAN-TRUNG LUU, OLIVIER POSTAIRE, RICHARD BLIGNY AND CHRISTOPHE MAUREL

Montpellier, Francia

Workshop; 13th International Workshop on Plant Membrane Biology; 2004

Comite organizador ad hoc

Flooding of soils results in acute oxygen deprivation (anoxia) of plant roots and is a major problem for agriculture. A fall in cytosolic pH is one early response of plants to anoxia. Anoxia also induces a down-regulation of water uptake due to inhibition of the hydraulic conductivity of roots (Lpr). Root water uptake is in large part-mediated by aquaporins of the Plasma Membrane Intrinsic Protein (PIP) subgroup. These aquaporins may mediate stress-induced inhibition of Lpr but the mechanisms involved are largely unknown. Here we delineate the whole root and cell bases for inhibition of water uptake by anoxia [1]. Cytosolic pH was measured by in vivo proton-decoupled 31P-NMR and root water permeability was investigated using pressure chamber measurements. A parallel between changes in cytosolic pH and changes in Lpr was established under conditions of anoxia, treatments by respiratory inhibitors or weak acids (acid loads). These results were consistent with the early observation that the water channel activity of purified plasma membrane vesicles can be blocked by protons [2]. To investigate this process further, the effects of extra- and intra-cellular acidification on the activity of specific PIP isoforms were studied using heterologous expression in Xenopus oocytes. The finding that all PIPs investigated were specifically blocked by intracellular acidification implies that these aquaporins have a conserved structural basis for cytosolic pH sensing. A structurefunction study allowed to identify a conserved His residue in cytoplasmic loop D (H197 in PIP2;2) as a major site for pH sensing by PIP aquaporins [1]. Because it is conserved in all PIPs, this mechanism provides a basis to explain the inhibition of Lpr by anoxia and possibly other stresses. The preparation of transgenic Arabidopsis overexpressing pH-insensitive PIP2 aquaporin mutants is in progress. These transgenic plants will provide a valuable material to study the physiological significance of aquaporin regulation by cytosolic pH. [1] Tournaire-Roux C et al. 2003 Nature 425:393-397 [2] Gerbeau P et al. 2002 Plant J. 30:71-81