The Actin-Related Protein2/3 Complex Regulates Mitochondrial-Associated Calcium Signaling during Salt Stress in Arabidopsis

Microfilament and Ca²⁺ dynamics play important roles in stress signaling in plants. Through genetic screening of Arabidopsis thaliana mutants that are defective in stress-induced increases in cytosolic Ca²⁺ ([Ca2+]_cyt), we identified Actin-Related Protein2 (Arp2) as a regulator of [Ca2+]_cyt in response to salt stress. Plants lacking Arp2 or other proteins in the Arp2/3 complex exhibited enhanced salt-induced increases in [Ca2+]_cyt, decreased mitochondria movement, and hypersensitivity to salt. In addition, mitochondria aggregated, the mitochondrial permeability transition pore opened, and mitochondrial membrane potential Ψm was impaired in the arp2 mutant, and these changes were associated with salt-induced cell death. When opening of the enhanced mitochondrial permeability transition pore was blocked or increases in [Ca2+]_cyt were prevented, the salt-sensitive phenotype of the arp2 mutant was partially rescued. These results indicate that the Arp2/3 complex regulates mitochondrial-dependent Ca²⁺ signaling in response to salt stress.