Interactions between Red Light, Abscisic Acid, and Calcium in Gravitropism

Abstract

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.

Stimulation of Root Elongation and Curvature by Calcium

“…The light effect on gravitropism of light-requiring orthogravitropic roots is considered to be mediated by phytochrome in the root cap (8), and agents such as the Ca2+ ionophore, A23187 (23,25), and ABA (19) (Table IV). Ca2+-stimulated curvature of pretreated roots was 33.2 ± 1.8°in 4 h, whereas roots without pretreatment showed no curvature.…”

“…In Merit corn roots which require red light for orthogravitropism, a Ca" ionophore, ABA, and osmotic shock, each proposed to elevate cytoplasmic Ca2+ levels in roots, do substitute for the light effect and induce an orthogravitropic response in the dark (19,23,25). The cytoplasmic Ca2+/ calmodulin system is also reported to play a role in such signal transduction of gravity (2,27).…”

“…The agravitropic roots of the pea mutant, ageotropum, are also highly sensitive to the stimulatory action ofCa>, which may provide further information concerning the mechanism ofCa> action in root growth and root tropisms. (16,19). Accordingly, excess water or swollen mucilage on the root surface, of particular importance in corn (20,29), was carefully removed with filter paper before treatment with agar blocks, while using care to avoid drying during experimental manipulations.…”

Light Moderates the Induction of Phosphoenolpyruvate Carboxylase by NaCl and Abscisic Acid in Mesembryanthemum crystallinum

“…Both (4,17). Inducers such as light and osmotic stress may recruit the plant growth regulator ABA to instigate and/or moderate these changes in gene expression (10,14).…”

Inhibition by White Light of 86Rb+ Absorption in the Root Apex of Corn

“…Specifically, white light has been shown to reduce the rate of root elongation of several species (23,27,30) including corn. Also, there is evidence that this response may be associated with the mechanism of gravity response (10,15,18).…”