Identification and Expression Pattern Analysis of the SOS Gene Family in Tomatoes

SOSs are key genes in the SOS (salt overly sensitive) signaling pathway, which plays an important role in maintaining ion homeostasis in plants under salt stress. Our aim was to clarify the biological function of the SOS gene family in tomato plants. We identified 14 SpeSOS genes, 10 SpiSOS genes, 11 SpmSOS genes, 9 SlmSOS genes, and 11 SlySOS genes from the genomes of “LA0716” (Spe), “LA2093” (Spi), “LA1589” (Spm), “M82” (Slm), and “Heinz 1706” (Sly) separately. The SOS protein family in tomatoes was divided into five subgroups (SOS1, SOS2, SOS3, SOS4, and SOS5) through phylogenetic analysis. The SOS proteins of the same subgroup in tomatoes contained similar conserved domains and motif structures. A subcellular localization prediction showed that the SOS1, SOS3, and SOS5 proteins in tomatoes were located on the cell membrane, while the SOS2 and SOS4 proteins in tomatoes were located on the cytoplasm and chloroplast, respectively. SlSOS1 contained the most exons and introns (23 and 22, respectively), while SlSOS5 contained only one exon. Via the analysis of the cis-elements in the promoters of those SOS genes in tomatoes, several hormone-, light-, and abiotic stress-related cis-elements were found. In addition, qRT-PCR revealed that the SpeSOS, SpiSOS, and SlySOS genes were induced by salt stress with similar expression patterns. Additionally, the expressions of SOS1-1, SOS1-2, SOS2-2, SOS3-3, SOS4-1, and SOS5-2 were higher in salt-tolerant tomatoes compared with salt-sensitive tomatoes under salt stress. In the salt-sensitive “LA1698” tomato and salt-tolerant “LA0516” tomato, most SOS genes had the highest expression in the roots. The expressions of SOS1-1, SOS1-2, SOS2-1, SOS2-2, SOS3-2, SOS3-3, and SOS5-1 in the leaves of salt-tolerant tomatoes were significantly higher than those in salt-sensitive tomatoes. Thereby, the SOS genes in tomatoes were induced by salt stress, indicating that they participated in the regulation mechanism of tomato salt tolerance. This study laid the foundation for further study on the function of the SOS gene family and revealed the molecular mechanism of tomato salt resistance.