Genome-wide identification and expression analysis of <i>STP</i> gene family in <i>Malus domestica</i>

The sugar transporter protein (<italic>STP</italic>) gene family serves a pivotal role in monosaccharide translocation, plant growth, and developmental processes. Utilizing the <italic>Arabidopsis thaliana STP</italic> gene family as a reference, comprehensive analyses were conducted on apple (<italic>Malus domestica</italic>) <italic>STP</italic> genes, encompassing sequence alignment, physicochemical characterization, gene structure organization, conserved motif identification, and tissue-specific expression profiling. Quantitative real-time PCR (qRT-PCR) was employed to investigate the hormone-responsive expression patterns of the apple <italic>STP</italic> gene family. A total of 30 <italic>STP</italic> genes were identified, encoding polypeptides ranging from 235 to 655 amino acids in length. Gene structure analysis revealed variations in exon number and positional distribution among different <italic>STP</italic> genes, indicative of functional diversification. Cis-regulatory element analysis demonstrated the presence of core promoter elements, enhancer sequences, and regulatory motifs associated with hormone signaling, abiotic stress responses, organ-specific expression, and light regulation. Microarray-based expression profiling indicated that the majority of <italic>STP</italic> genes exhibited preferential expression in seeds, seedlings, roots, and stems. Notably, <italic>MdSTP3</italic>, <italic>MdSTP13</italic>, and <italic>MdSTP23</italic> were identified as the predominant isoforms expressed in apple fruits, with their transcriptional activity being positively modulated by <italic>MdERF</italic> transcription factors. qRT-PCR validation further elucidated the differential responsiveness of the 30 <italic>MdSTP</italic> genes to indole-3-acetic acid (IAA), gibberellic acid (GA), and abscisic acid (ABA) treatments, with IAA exerting a pronounced upregulatory effect on <italic>STP</italic> gene expression. Collectively, these findings underscore the functional heterogeneity of the apple <italic>STP</italic> gene family in terms of tissue-specific expression and hormonal regulation, thereby providing a robust foundation for elucidating the molecular mechanisms underlying STP-mediated growth and developmental processes in plants.