Journal Article

Low Aquaporin Content and Low Osmotic Water Permeability of the Plasma and Vacuolar Membranes of a CAM Plant <i>Graptopetalum paraguayense</i>: Comparison with Radish

Yuko Ohshima, Ikuko Iwasaki, Shinobu Suga, Masanori Murakami, Keiko Inoue and Masayoshi Maeshima

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 42, issue 10, pages 1119-1129
Published in print October 2001 | ISSN: 0032-0781
Published online October 2001 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pce141
Low Aquaporin Content and Low Osmotic Water Permeability of the Plasma and Vacuolar Membranes of a CAM Plant Graptopetalum paraguayense: Comparison with Radish

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Aquaporin facilitates the osmotic water transport across biomembranes and is involved in the transcellular and intracellular water flow in plants. We immunochemically quantified the aquaporin level in leaf plasma membranes (PM) and tonoplast of Graptopetalum paraguayense, a Crassulacean acid metabolism (CAM) plant. The aquaporin content in the Graptopetalum tonoplast was approximately 1% of that of radish. The content was calculated to be about 3 µg mg–1 of tonoplast protein. The level of PM aquaporin in Graptopetalum was determined to be less than 20% of that of radish, in which an aquaporin was a major protein of the PM. The PM aquaporin was detected in the mesophyll tissue of Graptopetalum leaf by tissue print immunoblotting. The osmotic water permeability of PM and tonoplast vesicles prepared from both plants was determined with a stopped-flow spectrophotometer. The water permeability of PM was lower than that of the tonoplast in both plants. The Graptopetalum PM vesicles hardly showed water permeability, although the tonoplast showed a relatively high permeability. The water permeability changed depending on the assay temperature and was also partially inhibited by a sulfhydryl reagent. Furthermore, measurement of the rate of swelling and shrinking in different mannitol concentrations revealed that the protoplasts of Graptopetalum showed low water permeability. These results suggest that the low content of aquaporins in PM and tonoplast is one of the causes of the low water permeability of Graptopetalum. The relationship between the water-storage function of succulent leaves of CAM plants and the low aquaporin level is also discussed.

Keywords: Key words: Aquaporin — CAM plant — Graptopetalum paraguayense — Plasma membrane — Radish — Tonoplast.; Abbreviations: CAM, Crassulacean acid metabolism; EGTA, ethyleneglycol bis(2-amino-ethyl)tetraacetic acid; PM, plasma membrane; Pos, osmotic water permeability; VM23, radish vacuolar membrane aquaporin of 23 kDa.

Journal Article.  7187 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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