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First published online August 13, 2004; 10.1104/pp.104.043000 Plant Physiology 135:2301-2317 (2004) © 2004 American Society of Plant Biologists Dynamic Changes in the Osmotic Water Permeability of Protoplast Plasma Membrane1,[w]Unité de Biochimie Physiologique, Institut des Science de la Vie, Université Catholique de Louvain, Goix du Sud 2–20, B–1348 Louvain-la-Neuve, Belgium (M.M., F.C.); and The Robert H. Smith Institute for Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel (N.M.)
The osmotic water permeability coefficient (Pf) of plasma membrane of maize (Zea mays) Black Mexican Sweet protoplasts changed dynamically during a hypoosmotic challenge, as revealed using a model-based computational approach. The best-fitting model had three free parameters: initial Pf, Pf rate-of-change (slopePf), and a delay, which were hypothesized to reflect changes in the number and/or activity of aquaporins in the plasma membrane. Remarkably, the swelling response was delayed 2 to 11 s after start of the noninstantaneous (but accounted for) bath flush. The Pf during the delay was
1 This work was supported by the Belgian Fund for Scientific Research and the Interuniversity Attraction Poles Programme—Belgian Science Policy (grants to F.C.), by an individual Marie Curie European fellowship (to M.M.), by the Israel Science Foundation (grant no. 550/01–1), and by the U.S.-Israel Binational Science Foundation (grant no. 2000191 to N.M.). 2 These authors contributed equally to the paper. [w] The online version of this article contains Web-only data. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.043000. * Corresponding author; e-mail nava.moran{at}huji.ac.il; fax 972-8-9467763. Received March 18, 2004; returned for revision May 7, 2004; accepted May 26, 2004. This article has been cited by other articles:
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1 µm s–1. During the swelling period following the delay, Pf changed dynamically: within the first 15 s Pf either (1) increased gradually to approximately 8 µm s–1 (in the majority population of low-initial-Pf cells) or (2) increased abruptly to 10 to 20 µm s–1 and then decreased gradually to 3 to 6 µm s–1 (in the minority population of high-initial-Pf cells). We affirmed the validity of our computational approach by the ability to reproduce previously reported initial Pf values (including the absence of delay) in control experiments on Xenopus oocytes expressing the maize aquaporin ZmPIP2;5. Although mercury did not affect the Pf in swelling Black Mexican Sweet cells, phloretin, another aquaporin inhibitor, inhibited swelling in a predicted manner, prolonging the delay and slowing Pf increase, thereby confirming the hypothesis that Pf dynamics, delay included, reflected the varying activity of aquaporins. 
