The red light requirement for positive gravitropism in roots of corn (Zea mays. cv "Merit") provides an entry for examining the participation of calcium in gravitropism. Applcations of calcium chelators inhibit the light response. Calcium channel blockers (verapamil, lanthanum) can also inhibit the light response, and a calcium ionophore, A23187, can substitute for light. One can substitute for red light by treatments which have elsewhere been shown to trigger Ca2l influx into the cytosol, e.g. heat or cold shock. Agents which are known to be agonists of the phosphatidylinositol second messenger system (serotonin, 2,4dichlorophenoxyacetic acid, deoxycholate) can each pardally substitute for the red light, and Li+ can inhibit-the light effect. These experiments suggest that the induction of positive gravitropism by red light involves a rise in cytoplasmic Ca2l concentration, and that a contribution to this end may be made by the phosphatidylinositol second messenger system.
The effect of red light on orthogravitropism of Merit com (Zea mays L.) roots has been attributed to its effects on the transduction phase of gravitropism (AC Leopold, SH Wettlaufer [1988] Plant Physiol 87:803-805). In an effort to characterize the orthogravitropic transduction system, comparative experiments have been carried out on the effects of red light, calcium, and abscisic acid (ABA). The red light effect can be completely satisfied with added ABA (100 micromolar) or with osmotic shock, which is presumed to increase endogenous ABA. The decay of the red light effect is closely paralleled by the decay of the ABA effect. ABA and exogenous calcium show strong additive effects when applied to either Merit or a line of com which does not require red light for orthogravitropism. Measurements of the ABA content show marked increases in endogenous ABA in the growing region of the roots after red light. The interpretation is offered that red light or ABA may serve to increase the cytoplasmic concentrations of calcium, and that this may be an integral part of orthogravitropic transduction.
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