Genome-Wide Identification of Mitochondrial Calcium Uniporter Family Genes in the Tomato Genus and Expression Profilings Under Salt Stress

The mitochondrial calcium uniporter (MCU) is a key channel controlling mitochondrial Ca2+ homeostasis, yet its role in plant stress responses remains unclear. Using the tomato pan-genome, this study identified 66 MCU genes across 12 tomato species and grouped them into two distinct evolutionary subfamilies. Phylogenetic, collinearity, and selection pressure analyses revealed that MCU genes are evolutionarily conserved and have undergone strong purifying selection. In addition, one MCU gene located on chromosome 6 appears to have originated before the divergence of monocots and dicots, indicating an ancient evolutionary trajectory. Gene structure and conserved motif analyses confirmed their structural conservation, while promoter cis-element analysis suggested that MCU genes are widely involved in light and hormone responsiveness. Expression profiling under salt stress showed that multiple MCU genes are differentially regulated in a time-dependent manner: SolycMCU1 and SolycMCU2 respond rapidly at early stages, whereas SolycMCU5 and SolycMCU6 are upregulated during middle and late phases. These results highlight the functional diversification of MCU genes in tomato under salt stress. This study provides the first comprehensive evolutionary and functional analysis of the tomato MCU gene family, offering insights into their stress-regulatory mechanisms and potential use in breeding salt-tolerant tomatoes.