Genome-Wide Identification and Expression Profiling of the Laccase Gene Family in Tomato (Solanum lycopersicum) under Abiotic Stress and Hormonal Treatments

Laccase (LAC) is a class of multi-copper oxidases that plays an important role in plant lignin synthesis, cell wall formation, growth and development, and stress responses. However, systematic research on the tomato LAC gene family remains relatively limited. This study identified 24 members of the SlLAC gene family in the tomato genome and comprehensively analyzed their physicochemical properties, phylogenetic relationships, conserved domains, chromosome distribution, expression patterns, and miRNA regulation. Results showed that SlLAC proteins ranged in length from 484 to 614 aa and in molecular weight from 53.78 to 68.72 kDa; most members were basic and hydrophilic. The 24 SlLAC genes were unevenly distributed across 10 chromosomes, with chromosomes 5 and 6 containing the most members. Phylogenetic analysis divided the SlLAC family into several subgroups and showed high homology with Arabidopsis thaliana AtLAC proteins. Multiple sequence alignment results showed that all SlLAC proteins contained typical copper ion-binding conserved domains (L1–L4), indicating that they belong to a typical multi-copper oxidase family. Tissue expression analysis revealed that SlLAC genes exhibited significant spatiotemporal expression specificity in different tissues and fruit development stages. Among them, SlLAC23 was highly expressed in leaves, roots, and young fruits, while SlLAC22 was most significantly expressed in flowers. Stress treatment results showed that most SlLAC genes responded to salt, drought, and low-temperature stress to varying degrees, with some genes, such as SlLAC10, SlLAC11, and SlLAC22, showing significant induced expression under multiple stress conditions. Hormone treatment analysis further indicated that SlLAC genes exhibited complex dynamic expression patterns in response to plant hormones such as Eth, GA₃, IAA, SA, and JA. miRNA targeting prediction results showed that eight SlLAC genes could be targeted and regulated by sly-miR397, suggesting that the miR397-SlLAC module may be involved in the regulation of tomato lignin metabolism and stress response. In summary, this study systematically revealed the structural characteristics, evolutionary relationships, and expression patterns of the SlLAC gene family, providing an important theoretical basis for further research on the functions of SlLAC genes in tomato growth and development, fruit ripening, and stress adaptation.