PHOTOTROPISM and gravitropism in the shoots and roots of higher plants are the result of asymmetric growth. This is explained by the redistribution of growth regulators following exposure to gravity or unilateral light (the Cholodny-Went hypothesis1). The positive phototropism2 and the negative geotropism of grass seedling coleoptiles are believed to result from lateral movement of auxin from the irradiated to the shaded side3-5 and from the upper to the lower side6-8, respectively. Many physiological processes in plants, including auxin-induced cell elongation, are reported to be under the control of calcium9,10. Added auxin triggers oscillations in cytosolic free calcium ([Ca2+]cyt) and cytosolic pH (pHcyt) in epidermal cells of maize coleoptiles11-13. Until recently14, it has not been possible to visualize these changes spatially with the commonly used fluorescent cation indicators. Using a scanning laser confocal microscope, a new visible wavelength Ca2+ probe fluo-3 and the fluorescent pH indicator BCECF, we have recorded rapid light-induced increases in [Ca2+]cyt and a lowering of pHcyt of cells on the shaded side of maize coleoptiles. In horizontally orientated coleoptiles, [Ca2+]cyt increases and pHcyt decreases in the more rapidly elongating cells on the lower side. For the first time, rapid changes in [Ca2+]cyt and pHcyt are correlated directly with increases in cell elongation stimulated by light and gravity.
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