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 0.5degreesC 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.