Plasma Membrane Aquaporins in the Motor Cells of Samanea saman

Leaf-moving organs, remarkable for the rhythmic volume changes of their motor cells, served as a model system in which to study the regulation of membrane water fluxes. Two plasma membrane intrinsic protein homolog genes, SsAQP1 and SsAQP2, were cloned from these organs and characterized as aquaporins in Xenopus laevis oocytes. Osmotic water permeability (Pf) was 10 times higher in SsAQP2-expressing oocytes than in SsAQP1-expressing oocytes. SsAQP1 was found to be glycerol permeable, and SsAQP2 was inhibited by 0.5 mM HgCl2 and by 1 mM phloretin. The aquaporin mRNA levels differed in their spatial distribution in the leaf and were regulated diurnally in phase with leaflet movements. Additionally, SsAQP2 transcription was under circadian control. The Pf of motor cell protoplasts was regulated diurnally as well: the morning and/or evening Pf increases were inhibited by 50 μM HgCl2, by 2 mM cycloheximide, and by 250 μM phloretin to the noon Pf level. Our results link SsAQP2 to the physiological function of rhythmic cell volume changes.

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