C3-P-10Studies on intra- and extracellular calcium translocation during the gravitropic response of soybean roots by X-ray microanalysis

Abstract

It is well-known that Ca plays a significant role in regulating the physiological phenomena of living organisms. The gravitropic response of plant roots is a typical tropism, and Ca ions are believed to be one of the influential messengers involved in graviperception in columella cells. However, the details of this linking process remain unknown. To examine the role of Ca in gravitropic response, the Ca distribution change in columella cell of soybean roots under the simulated microgravity was studied by Ca cytochemistry and a quantitative X-ray microanalysis of cryosections. Results showed that the release and uptake of Ca ions by vacuolar electron-dense structures is closely related to signal transmission in the gravitropic response and that Ca movement may occur in an opposite manner to that of K. Furthermore, since the extremely high Ca was detected in the vacuolar electron-dense structure under simulated microgravity, it has also been assumed that intracellular Ca ions are always supplied from the cell wall region (apoplast) in soybean roots. Then, furthermore experiments concerning the movement of Ca through the apoplast as a gravitropic response were studied by the same electron microscopic methods. The intra- and extracellular distribution and translocation of Ca in the elongation zone of soybean roots reoriented horizontally from their original vertical orientation and in columella cells of soybean roots after incubation with high-Ca medium were examined. Results showed that Ca moves apoplastically in the root cap region and the cortex region of soybean root, and that the vacuole plays a significant role in the regulation of Ca concentration in cytoplasmic matrices to avoid cytotoxic damage. In conclusion, Ca ions function as the intracellular regulator for the gravitropic response.