Roles of Ion Channels in Initiation of Signal Transduction in Higher Plants.

All biological organisms perceive environmental and chemical signals via specific receptors or perception mechanisms. When stimulated, these receptors induce an intracellular cascade of events leading to modification of cellular activity or to regulation of specific genes, which in turn produces the biological response. Recently, progress has been made in identifying initial signal reception mechanisms and early events in signaling cascades in higher plants. lon channels, along with intracellular signaling proteins and second messengers, are critical components mediating early events in higher plant signal transduction. lon channel-mediated signal transduction in higher plants has notable differences from signaling mechanisms in animal systems. Of the many types of ion channels found in higher plants, recent findings have indicated that anion channels, along with Ca2+ channels, play critical and rate-limiting roles in the mediation of early events of signal transduction. We have now begun to obtain the first insights into the modes of regulation, membrane localization, and, in the case of K+ channels, molecular structure of higher plant ion channels. In this article, we focus mainly on nove1 findings concerning the function and regulation of anion and Caz+ channels and outline testable models of their involvement in signal transduction. Our objective is not only to summarize these findings but also to point out the many open questions involving early events in plant signal transduction. To illustrate the functions of higher plant ion channels in the initiation of signaling cascades, in the first section we discuss the molecular mechanisms of abscisic acid (ABA)-induced stomatal closing, with a special focus on new and emerging concepts. In the second section, we address Ca2+-dependent and Ca2+-independent signaling processes in plants and analyze certain putative parallels between initial guard cell signaling and both the initiation of defense responses and phytochrome-induced signaling.