Low aquaporin content and low osmotic water permeability of the plasma andvacuolar membranes of a CAM plant Graptopetalum paraguayense: comparison with radish

Aquaporin facilitates the osmotic water transport across biomembranes and i s involved in the transcellular and intracellular water flow in plants. We immunochemically quantified the aquaporin level in leaf plasma membranes (P AI) and tonoplast of Graptopetalum paraguayense, a Crassulacean acid metabo lism (CAM) plant. The aquaporin content in the Graptopetalum tonoplast was approximately 1% of that of radish. The content was calculated to be about 3 mug mg(-1) of tonoplast protein. The level of PM aquaporin in Graptopetal um was determined to be less than 20% of that of radish, in which an aquapo rin was a major protein of the PM. The PM aquaporin was detected in the mes ophyll tissue of Graptopetalum leaf by tissue print immunoblotting. The osm otic water permeability of PM and tonoplast vesicles prepared from both pla nts was determined with a stopped-flow spectrophotometer. The water permeab ility of PM was lower than that of the tonoplast in both plants. The Grapto petalum PM vesicles hardly showed water permeability, although the tonoplas t showed a relatively high permeability. The water permeability changed dep ending on the assay temperature and was also partially inhibited by a sulfh ydryl reagent. Furthermore, measurement of the rate of swelling and shrinki ng in different mannitol concentrations revealed that the protoplasts of Gr aptopetalum showed low water permeability. These results suggest that the l ow content of aquaporins in PM and tonoplast is one of the causes of the lo w water permeability of Graptopetalum. The relationship between the water-s torage function of succulent leaves of CAM plants and the low aquaporin lev el is also discussed.