Ca²⁺: Calmodulin Phytohormone-Linked Plant Senescence Control

Abstract

Further insight into the understanding of the regulatory role of Ca was the discovery some 2 decades ago of the calcium-binding protein calmodulin (CM) and the subsequent demonstration that several key enzymes, including some phospholipases [22] are dependent on Ca or a Ca:CM complex for activation. In both mammals and plants, Ca concentration on the cell’s interior is several orders of magnitude greater than that on its exterior [4], while CM gradients manifest an opposite distribution - Ca being primarily in the cystosol, very sparse in the membrane and virtually absent from the cell’s exterior [15]. Moreover, it is claimed [17, 18] that peroxidized polyunsaturated fatty acids (PUFA) and phosphatidic acid may serve as endogenous Ca ionophores, i.e. without lysing membranes they may transfer Ca from the cell’s exterior to the interior, cause breakdown of the transmembrane Ca gradient and equilibrate concentration on either side.