Plasma Membrane Ca2+-Permeable Channels are Differentially Regulated by Ethylene and Hydrogen Peroxide to Generate Persistent Plumes of Elevated Cytosolic Ca2+ during Transfer Cell Trans-Differentiation

Abstract

The enhanced transport capability of transfer cells (TCs) arises from their ingrowth wall architecture comprised of a uniform wall on which wall ingrowths are deposited. The wall ingrowth papillae provide scaffolds to amplify plasma membranes that are enriched in nutrient transporters. Using Vicia faba cotyledons, whose adaxial epidermal cells spontaneously and rapidly (hours) undergo a synchronous TC trans-differentiation upon transfer to culture, has led to the discovery of a cascade of inductive signals orchestrating deposition of ingrowth wall papillae. Auxin-induced ethylene biosynthesis initiates the cascade. This in turn drives a burst in extracellular H2O2 production that triggers uniform wall deposition. Thereafter, a persistent and elevated cytosolic Ca2+ concentration, resulting from Ca2+ influx through plasma membrane Ca2+-permeable channels, generates a Ca2+ signal that directs formation of wall ingrowth papillae to specific loci. We now report how these Ca2+-permeable channels are regulated using the proportionate responses in cytosolic Ca2+ concentration as a proxy measure of their transport activity. Culturing cotyledons on various combinations of pharmacological agents allowed the regulatory influence of each upstream signal on Ca2+ channel activity to be evaluated. The findings demonstrated that Ca2+-permeable channel activity was insensitive to auxin, but up-regulated by ethylene through two independent routes. In one route ethylene acts directly on Ca2+-permeable channel activity at the transcriptional and post-translational levels, through an ethylene receptor-dependent pathway. The other route is mediated by an ethylene-induced production of extracellular H2O2 which then acts translationally and post-translationally to up-regulate Ca2+-permeable channel activity. A model describing the differential regulation of Ca2+-permeable channel activity is presented.