Signaling in cells and organisms — calcium holds the line

• Ca2+ and ROS are important cellular second messengers in plants.
• Ca2+, ROS and changes in membrane potential (WASPs) contribute to cell-to-cell communication.
• These signals appear to be interconnected in long-distance signaling in plants.

Previous research has established calcium (Ca2+) and reactive oxygen species (ROS) as important cellular second messengers in eukaryotes. Recently, the occurrence of cell-to-cell moving Ca2+ and ROS waves was reported in plants. This was paralleled by the discovery of long-distance wound-activated surface potential changes (WASPs) that require the function of putatively Ca2+-releasing glutamate receptor-like channels (GLRs) in Arabidopsis. Although the functional interconnection of Ca2+-dependent phosphorylation and ROS waves via NADPH oxidase activation has been clearly established, potential further interconnections between these long-distance signaling processes are less clear. In this review we cover emerging concepts and existing open questions that interconnect cellular and global signaling via Ca2+, ROS and WASPs.