Calcium spikes, waves and oscillations in plant development and biotic interactions.

The calcium ion (Ca2+) is a universal signal in all eukaryotic cells. A fundamental question is how Ca2+, a simple cation, encodes complex information with high specificity. Extensive research has established a two-step process (encoding and decoding) that governs the specificity of Ca2+ signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca2+ sensors and effectors that convert Ca2+ signals into cellular effects. Along this general paradigm, some signalling components may be highly conserved, but others are divergent among different organisms. In plant cells, Ca2+ participates in numerous signalling processes, and here we focus on the latest discoveries on Ca2+-encoding mechanisms in development and biotic interactions. In particular, we use examples such as polarized cell growth of pollen tube and root hair in which tip-focused Ca2+ oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca2+ spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca2+ oscillations. Herbivore attacks or mechanical wounding can trigger Ca2+ waves traveling a long distance to transmit and convert the local signal to a systemic defence program in the whole plant. What channels and transporters work together to carve out the spatial and temporal patterns of the Ca2+ fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca2+ channels that orchestrate specific Ca2+ signatures in each of these processes. Future work will further expand the toolkit for Ca2+-encoding mechanisms and place Ca2+ signalling steps into larger signalling networks.