Light-stimulated inositolphospholipid turnover in Samanea saman leaf pulvini

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Abstract
Leaflets of Samanea saman open and close rhythmically, driven by an endogenous circadian clock. Light has a rapid, direct effect on the movements and also rephases the rhythm. We investigated whether light signals might be mediated by increased inositolphospholipid turnover, a mechanism for signal transduction that is widely utilized in animal systems. Samanea motor organs (pulvini) labeled with [3H]inositol were irradiated briefly (5-30 sec) with white light, and membrane-localized phosphatidylinositol phosphates and their aqueous breakdown products, the inositol phosphates, were examined. After a 15-sec or longer light pulse, labeled phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate decreased and their labeled metabolic products inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate increased, changes characteristic of inositolphospholipid turnover. We conclude that inositolphospholipid turnover may act as a phototransduction mechanism in Samanea pulvini in a manner that is similar to that reported in animal systems.