Genome-Wide Identification of the P-Type Ca²⁺-ATPase Gene Family in Maize and Its Expression Dynamics Under Abiotic and Biotic Stress Conditions

Calcium (Ca²⁺) functions as a second messenger in plants, coordinating development and stress responses through cytosolic Ca²⁺ dynamics. The P-type Ca²⁺-ATPases of the ECA (P-IIA) and ACA (P-IIB) subfamilies are central to Ca²⁺ homeostasis and signal termination by extruding Ca²⁺ from the cytosol. In this study, genome-wide identification was performed to identify the P-type Ca²⁺-ATPases according to the maize B73 v5 reference genome, followed by phylogenetic, structural, chromosomal, syntenic, network, and expression analyses. Nineteen genes were identified, comprising 4 ECAs and 15 ACAs. All 19 members retained the DKTGT phosphorylation site, while the CaATP_NAI (N- terminal autoinhibitory) extension distinguished all 15 ACAs from the 4 ECAs. Collinearity analysis revealed 11 maize-rice syntenic pairs, implicating segmental duplication. ECAs were preferentially expressed in reproductive tissues, whereas ACAs were broadly expressed across vegetative organs. RNA-seq-based profiling detected distinct stress-responsive expression patterns of Ca²⁺-ATPase genes. Under abiotic stress, ZmACA12-1 was consistently upregulated under drought, ZmACA6-2 dominated the heat response, and ZmACA4-2 showed the broadest cross-stress repression. Under biotic stress, ACA members again dominated, with ZmACA1-1 being the most broadly pathogen-responsive member, ZmECA1-3 the principal ECA-class responder, and ZmACA9 exhibiting consistent pathogen-associated repression. Additionally, ZmACA12-1 and ZmACA4-4 showed genotype-dependent regulation between resistant and susceptible lines. Collectively, these candidates represent priority targets for functional validation of the calcium efflux mechanisms that underlie maize adaptation to both abiotic and biotic stresses.