ON THE COVER: The opening and closing of tulip petals is reproduced temperature-dependently in the dark. Petal movement is concomitant with water transport regulated by reversible phosphorylation of plasma membrane intrinsic protein (PIP), one member of aquaporin family. In this issue, Azad et al. (pp. 1196–1208) identified and cloned four full-length cDNAs of PIPs from tulip petals. Of these homologs, only TgPIP2;2 displayed significant water channel activity, which was abolished by mercury and affected by inhibitors of protein kinase and protein phosphatase. They showed that Ser35, Ser116 and Ser274 are the putative phosphorylation sites of TgPIP2;2. This homolog is ubiquitously expressed in all organs, petals, stems, leaves, bulbs and roots, but the transcript levels of others were relatively low or negligible. TgPIP2;2 might be modulated by phosphorylation and dephosphorylation for regulating water channel activity, and may play a role in transcellular water transport in all tulip organs.
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