Elevation of cytosolic Ca²⁺ in response to energy deficiency in plants: the general mechanism of adaptation to low oxygen stress

Ca²⁺ can be released from cell compartments to the cytosol during stress conditions. We discuss here the causes of Ca²⁺ release under conditions of ATP concentration decline that result in the suppression of ATPases and activation of calcium ion channels. The main signaling and metabolic consequences of Ca²⁺ release are considered for stressed plant cells. The signaling function includes generation and spreading of calcium waves, while the metabolic function results in the activation of particular enzymes and genes. Ca²⁺ is involved in the activation of glutamate decarboxylase, initiating the γ-aminobutyric acid shunt and triggering the formation of alanine, processes which play a role, in particular, in pH regulation. Ca²⁺ activates the transcription of several genes, e.g. of plant hemoglobin (phytoglobin, Pgb) which scavenges nitric oxide and regulates redox and energy balance through the Pgb–nitric oxide cycle. This cycle involves NADH and NADPH oxidation from the cytosolic side of mitochondria, in which Ca²⁺- and low pH-activated external NADH and NADPH dehydrogenases participate. Ca²⁺ can also activate the genes of alcohol dehydrogenase and pyruvate decarboxylase stimulating hypoxic fermentation. It is concluded that calcium is a primary factor that causes the metabolic shift under conditions of oxygen deficiency.