The deposition of suberin lamellae determines the magnitude of cytosolic Ca2+ elevations in root endodermal cells subjected to cooling

A transient increase in cytosolic Ca2+ concentration (Ca2+]cyt) is thought to be a prerequisite for an appropriate physiological response to both chilling and salt stress. The Ca2+]cyt is raised by Ca2+ influx to the cytosol from the apoplast and/or intracellular stores. It has been speculated that different signals mobilise Ca2+ from different stores, but little is known about the origin(s) of the Ca2+ entering the cytosol in response to specific environmental challenges. We have utilised the developmentally regulated suberisation of endodermal cells, which is thought to prevent Ca2+ influx from the apoplast, to ascertain whether Ca2+ influx is required to increase Ca2+]cyt in response to chilling or salt stress. Perturbations in Ca2+]cyt were studied in transgenic Arabidopsis thaliana, expressing aequorin fused to a modified yellow fluorescent protein solely in root endodermal cells, during slow cooling of plants from 20 to 0.5 degrees C over 5 min and in response to an acute salt stress (0.333 m NaCl). Only in endodermal cells in the apical 4 mm of the Arabidopsis root did Ca2+]cyt increase significantly during cooling, and the magnitude of the Ca2+]cyt elevation elicited by cooling was inversely related to the extent of suberisation of the endodermal cell layer. No Ca2+]cyt elevations were elicited by cooling in suberised endodermal cells. This is consistent with the hypothesis that suberin lamellae isolate the endodermal cell protoplast from the apoplast and, thereby, prevent Ca2+ influx. By contrast, acute salt stress increased Ca2+]cyt in endodermal cells throughout the root. These results suggest that Ca2+]cyt elevations, upon slow cooling, depend absolutely on Ca2+ influx across the plasma membrane, but Ca2+]cyt elevations in response to acute salt stress do not. They also suggest that Ca2+ release from intracellular stores contributes significantly to increasing Ca2+]cyt upon acute salt stress.