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

The calcium ion (Ca) is a universal signal in all eukaryotic cells. A fundamental question is how Ca, 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 Ca signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca sensors and effectors that convert Ca 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, Ca participates in numerous signalling processes, and here we focus on the latest discoveries on Ca-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 Ca oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca oscillations. Herbivore attacks or mechanical wounding can trigger Ca 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 Ca fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca channels that orchestrate specific Ca signatures in each of these processes. Future work will further expand the toolkit for Ca-encoding mechanisms and place Ca signalling steps into larger signalling networks.