Ca²⁺talyzing Initial Responses to Environmental Stresses

Plants have evolved stress-sensing machineries that initiate rapid adaptive environmental stress responses. Cytosolic calcium ion (Ca²⁺) is the most prominent second messenger that couples extracellular signals with specific intracellular responses. Essential early events that generate a cytosolic Ca²⁺ spike in response to environmental stress are starting to emerge. We review sensory machineries, including ion channels and transporters, which perceive various stress stimuli and allow cytosolic Ca²⁺ influx. We highlight integrative roles of Ca²⁺ channels in plant responses to various environmental stresses, as well as possible interplay of Ca²⁺ with other early signaling components, which facilitates signal propagation for systemic spread and spatiotemporal variations in respect to external cues. The early Ca²⁺ signaling schemes inspire the identification of additional stress sensors.