Identification of the ACBP Family Genes and Their Response to Cold and Salt Stress in Citrus

Acyl-CoA-binding proteins (ACBPs) are essential lipid carrier proteins involved in plant lipid metabolism. However, the systematic identification and expression profiles of the ACBP gene family in citrus species remain poorly understood. Here, Citrus sinensis and Poncirus trifoliata were chosen as model species to examine the biological properties of citrus ACBPs. Using bioinformatics methods, five ACBP gene members were found in each species and named CsACBPs and PtrACBPs, respectively. All obtained ACBP members were divided into four subfamilies based on conserved domains and amino acid sequences. CsACBP and PtrACBP genes exhibited structural variation in motifs and exons. The predicted protein structures of CsACBPs and PtrACBPs exhibited conservation between the two species while displaying distinct variation within each species. Collinearity analysis showed one intraspecific pairing relationship in each of the two citrus species. Furthermore, there were more collinear couplings between citrus species and Arabidopsis thaliana but none between citrus species and Oryza sativa (rice). Notably, the analysis of cis-acting elements in ACBP gene promoters identified a number of motifs associated with light, abiotic stresses, and phytohormones. Expression profiling confirmed tissue-specific expression patterns of CsACBP1~5 and PtrACBP1~5. RT-qPCR analysis revealed that all CsACBP and PtrACBP genes responded to cold and salt stresses, though the magnitude of their responses varied significantly. Specially, although PtrACBP5 did not respond to low temperatures as rapidly as other members, its expression level increased significantly after 24 h of low-temperature treatment. Protein–protein interaction (PPI) network predictions indicated tight associations among four of the five CsACBPs, with CsACBP5 excluded from these interactions. Moreover, CsACBP1, CsACBP2, and CsACBP3 were predicted to be potential targets of csi-miR3952, csi-miR396a, and csi-miR477b, respectively. Overall, our research provides a solid foundation for further investigations into the biological functions and regulatory mechanisms of ACBP genes in citrus growth, development, and stress adaptation.