Protoplasmic pH regulates an aquaporin water pathway in plant vacuoles

GABRIELA AMODEO, MOIRA SUTKA, RICARDO DORR AND MARIO PARISI

Göteborg, Suecia

Congreso; Third International Conference on Molecular Biology and Physiology of Water and Solute Transport; 2000

Comité organizador ad hoc

Water is essential for plant growth and development. Therefore, the study of the mechanisms that regulate water permeability in plants is basic to understand plant responses under limiting factors like water stress or drought. The discovery of water channels (aquaporins) have requestioned the role of the cell to cell pathway in water transport and the progressive understanding of their possible regulatory properties is giving important clues to comprehend their ubiquitous presence [1]. In a previous work we focused our interest in understanding water movements in Beta vulgaris storage roots, and we were able to describe a transcellular water movement sensitive to mercurial compounds [2]. In this species a tonoplast intrinsic protein (TIP) has been described in the root parenchyma [3]. In general, it has been reported that water permeability at the tonoplast level is very high, which strongly supports the role of TIPs in water transfers across vacuole membranes.  We therefore focused our goal in studying the properties of water pathway transfer in sugar beet storage root vacuoles and its possible regulation.  For this puropose, vacuoles were mechanically isolated from parenchyma of Beta vulgaris L.  storage roots to study volume changes using videomicroscopy. After a hypertonic challenge employing a non permeant solute (PEG-3350), the vacuole volume stabilized in a new value, corresponding to a perfect osmometer response. When the vacuoles were exposed to anisosmotic conditions by modifying mannitol concentration and when urea partial and isosmotically replaced mannitol, a more complex response was observed, probably reflecting both water and solute movements. If vacuoles were pre-incubated in the presence of HgCl2 (isosmotic conditions) and then a hyposmotic challenge was applied, a reduction in the equilibrium volume was observed. This reduction was concentration-dependent. When HgCl2 concentration arrived to 300 mM the volume change was completely abolished. The volume changes induced by a hyposmotic challenge were also strongly dependent on medium pH. Surprisingly, the response to medium hyposmolarity was completely abolished at pH 6.6, being this effect fully reversible. The pH-dependent volumetric change induced by a hyposmotic shock was a classical dose-response curve. This evidences are consistent with recently findings  that the AQP3 transport of water and glycerol is gated by H+ [4].We concluded that mercury sensitive water pathway present in the studied are strongly regulated by the pH of the medium, showing that plants also probably developed proton regulation mechanisms of water pathways. [1] Maurel C (1997) Ann Rev Plant Physiol Plant Mol Biol 48: 399-429 [2] Amodeo G, Dorr R, Vallejo A, Sutka M, Parisi M (1999) J Exp Botany 50: 509-516 [3] Marty-Mazars D, Clémencet MC, Dozolme P, Marty F (1995) Eur J Cell Biol 66: 106-118 [4] Zeuthen T, Klaerke DA (1999)  J Biol Chem 274: 21631-21636