Salinity is an abiotic constraint that limits the growth and productivity of tomato (Solanum lycopersicum L.). However, the role of small ubiquitin-like modifier (SUMO) E3 ligases (SIZs) in the salt stress response in tomato remains unknown. Here, a genome-wide analysis identified four SIZ-type genes in tomato, designated SlSIZ1–SlSIZ4. Gene structure and conserved motif analyses revealed the canonical SAP (SAF-A/B, Acinus, and PIAS) domain and the SP-RING (Siz/PIAS RING finger) domain in all SlSIZ proteins. Phylogenetic analysis grouped SIZ proteins from tomato and representative plant species into five major groups (A–E), with SlSIZ members distributed across distinct clades, and members within each clade shared conserved gene structures. Online transcriptome data and our qRT-PCR analyses showed that SlSIZ4 was highly expressed in leaves, flowers, and fruits, whereas SlSIZ1 showed broad expression, with relatively higher levels in roots, flowers, and multiple fruit stages. Under salt stress, abscisic acid (ABA), and methyl jasmonate (MeJA) treatments, the expression of SlSIZ1 and SlSIZ4 was strongly induced, whereas the expression of SlSIZ2 and SlSIZ3 was rarely promoted. In addition, virus-induced gene silencing (VIGS) of SlSIZ1 and SlSIZ4 markedly reduced salt tolerance of tomato seedlings, as reflected by growth inhibition, chlorophyll loss, increased reactive oxygen species (ROS), reduced proline accumulation, and diminished antioxidant gene expressions. Collectively, these findings identify SlSIZ1 and SlSIZ4 as positive regulators of salt stress tolerance in tomato and provide a framework for further dissection of the SlSIZ gene family.

Tomato (Solanum lycopersicum L.) is an important horticultural crop. Carotenoid cyclase dioxygenase (CCD) is an enzyme responsible for cleaving carotenoids, which is involved in regulating plant growth and response to abiotic stresses. However, the role of SlCCDs in tomato stress resistance remains unclear. This study used the tomato variety ‘Micro-Tom’ as the material to investigate the function of SlCCDs in stress responses. Through whole-genome analysis, a total of 12 SlCCDs members (SlCCD1–SlCCD12) were identified. Systematic evolutionary analysis classified them into four branches, and members within the same branch maintained a conserved structure. The promoter analysis revealed that SlCCDs contain multiple hormones and stress response elements. The qRT-PCR analysis revealed that SlCCD11 was the most highly expressed gene in the leaves. In addition, multiple SlCCDs showed significant responses to abscisic acid (ABA), methyl jasmonate (MeJA), light, and sodium chloride (NaCl) treatments. Among them, the expression of SlCCD11 significantly increased under salt stress. By silencing SlCCD11 using virus-induced gene silencing (VIGS) technology, it was found that the chlorophyll content, antioxidant enzyme activity, and ABA-related gene expression in the TRV:SlCCD11 plants under salt stress were all lower than the control samples, while the carotenoid content and ROS accumulation were higher. This indicates that SlCCD11 is a positive regulatory factor for salt stress. In conclusion, this study systematically analyzed the SlCCD gene family and revealed the positive role of SlCCD11 in tomato response to salt stress, providing a candidate gene for genetic improvement of crop stress resistance.