Calmodulin and calcium-mediated melatonin signaling mechanisms in plants

Modulation of melatonin signaling pathway occurs either by direct interaction with calmodulin (CaM) or through regulation of intracellular Ca²⁺ concentration. Intracellular CaM and CaM-dependent phosphodiesterase (PDE) activity are also possibly modulated by melatonin in vitro through melatonin-CaM interaction. Melatonin acts as a CaM antagonist and modulates CaM-dependent protein phosphorylation, thereby rhythmically regulating cellular functions. Melatonin also acts as a nitric oxide synthase (NOS) inhibitor by interfering with CaM binding to NOS peptide, thereby inhibiting NO biosynthesis. CaM also inhibits the effects of melatonin on cyclic GMP biosynthesis. Melatonin can cause increase in reactive oxygen species (ROS) generation through CaM–mediated activation of phospholipase A₂ (PLA₂). Thus, the prooxidant action of melatonin is mediated through CaM. Melatonin can directly scavenge ROS species and regulate the activities of various antioxidant enzymes though interaction with CaM. Melatonin causes microtubule depolymerization, thereby affecting cytoskeleton organization in the cells. CaM also inhibits tubulin polymerization. Melatonin is known to regulate photoperiodism and circadian rhythmicity in plants. Both salt stress and melatonin seem to modulate seedling growth by inhibiting CaM-mediated calcium signaling. Upregulation of genes encoding CaM-like protein, calcium-binding protein, calcium-dependent phosphodiesterase and CBL (calcineurin B-like proteins)-interacting protein kinase indicates possible calcium-mediated melatonin signaling in plants.