Perception of a wide range of developmental and stress signals by plants results in rapid elevation of cytosolic free calcium ([Ca2+](c): Bush, 1995). The change in [Ca2+](c) is widely accepted to comprise an early step in signal transduction, with downstream targets of the Ca2+ signal including activation of calmodulin-dependent enzymes, calmodulin-domain protein kinases (CDPKs: Roberts & Harmon, 1992), or activation of ion channels (Schroeder & Hagiwara, 1989). Yet this simple notion gives rise to a number of equally elementary questions:

From which compartment is Ca2+ mobilised?

Which membrane pathways (ion channels) facilitate passive Ca2+ flow into the cytosol in response to the primary signal?

How do these Ca2+-permeable channels interact with other cellular response elements (including other ion channels, ligands and phosphorylation cascades) to evoke the Ca2+ signal?

How is stimulus specificity encoded in the Ca2+ signal?

Answers to all four questions require as a first step an understanding of the properties of Ca2+-permeable channels in plant cells: which membranes the channels are located in, what activates (gates) them, and how their activities might be integrated with other signalling pathways in the cell. The remainder of this chapter highlights areas of achievement and of ignorance in our attempts to address these questions.