Ca²⁺ Mobilization from Internal Stores in Electrical Membrane Excitation in Chara

Abstract

Stimulus-induced changes in the concentration of free Ca²⁺ in the cytoplasm (Ca²⁺ _cyt) have been recognized as key events in numerous physiological processes in plants. Recent reviews report how Ca²⁺ _cyt links environmental stimuli as well as hormonal signals to cellular processes such as the control of ion channels, cytoplasmic streaming, secretion and gene expression (Sanders et al. 1999; Blatt 2000; Reddy 2001). With increasingly elaborated methods it has now become clear that the signaling cascades that involve changes in Ca²⁺ _cyt are rather more complex than initially thought. Ca²⁺ _cyt can be found to increase in response to distinct stimuli to elevated steady-state levels of different amplitude. In other cases, Ca²⁺ _cyt was found to oscillate with characteristic frequencies and amplitudes (e.g., McAinsh et al. 1995; Allen et al. 2000, 2001; Plieth 2001). However, it is not only the patterns of Ca²⁺ _cyt elevation that appear to be complex, the mechanisms underlying such stimulus-induced changes also appear to be heterogeneous. In this context, it was found that Ca²⁺ can arrive in the cytoplasm from the external medium via channels in the plasma membrane (White 2000; Plieth 2001), via stimulus-coupled release from diverse internal stores (Sanders et al. 1999; Blatt 2000; Reddy 2001), or even from both (Grabov and Blatt 1998).