Phototropism and geotropism in maize coleoptiles are spatially correlated with increases in cytosolic free calcium

Abstract

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 side^3–5 and from the upper to the lower side^6–8, respectively. Many physiological processes in plants, including auxin-induced cell elongation, are reported to be under the control of calcium^9,10. Added auxin triggers oscillations in cytosolic free calcium ([Ca²⁺]_cyt) and cytosolic pH (pH_cyt) in epidermal cells of maize coleoptiles^11–13. 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 pH_cyt of cells on the shaded side of maize coleoptiles. In horizontally orientated coleoptiles, [Ca²⁺]_cyt increases and pH_cyt decreases in the more rapidly elongating cells on the lower side. For the first time, rapid changes in [Ca²⁺]_cyt and pH_cyt are correlated directly with increases in cell elongation stimulated by light and gravity.