Cytokinin signal transduction through Ca2+ in mosses

Abstract

The importance of Ca2+ in a wide variety of physiological processes in both animal and plant cells has been emphasized for many years. For instance Ca2+ has been proposed as a major regulator of signal transduction in plant cells in response to various external stimuli such as hormones, light, or gravity (see Hepler and Wayne, 1985; Poovaiah and Reddy, 1987; Boss and Morre, 1989; Saunders, 1990a). Although cAMP, a well documented second messenger in animal systems, has also been detected in plants, the significance of cAMP in any cellular processes in higher plants has yet to be elucidated. Thus Ca2+ alone has been proposed to be the major mediator of various physiological responses in plant cells, such as polarized growth, mitosis, cytoplasmic streaming, cell elongation, and cell differentiation. Plant cells contain all the elements for a Ca2+-based messenger system that could couple the external stimuli of hormones to their physiological response. These include a highly regulated and low cytoplasmic [Ca2+], plasma membrane and endomembrane Ca2+ pumps and channels, and spatially-controlled Ca2+-dependent regulatory proteins and kinases (Boss and Morre, 1989). However much of the evidence for Ca2+ as a second messenger in plants is fragmentary and circumstantial. The use of Ca2+ sensitive fluorescent indicators such as indo-1 and the Ca2+-selective vibrating probe allows plant biologists to directly measure changes in intracellular Ca2+ ([Ca2+]) and Ca2+ fluxes at the plasma membrane. Detection of an increase in [Ca2+];is an essential first step in establishing that an environmental stimulus is directly correlated with activation of a Ca2+ second messenger system.