Phenotypic, Physiological, and Transcriptomic Analyses Reveal Different Responses to Salt Stress in Cultivated Red Lettuce and Wild Lettuce Seedlings

Cultivated lettuce (<i>Lactuca sativa</i> L.) is considered one of the most important economic vegetables worldwide; however, it is subjected to different stresses (salt stress, etc.) during its growth and development, resulting in yield reductions. In this study, we selected cultivated red lettuce and wild lettuce species (<i>Lactuca serriola</i> L.) to investigate the phenotypic and physiological changes in these lettuce under different salt treatment conditions. Functional annotation and enrichment analysis of the differentially expressed genes (DEGs) in the lettuce leaves and roots between the control and salt treatments were performed, identifying the key genes responding to salt stress. The results showed that the growth of the two types of lettuce was limited by salt stress, with decreased leaf area, main root length, biomass, and photosynthesis parameters noted. The cultivated red lettuce and the wild lettuce exhibited similar trends in terms of the variation in their antioxidant enzymatic activity and the content of osmoregulatory compounds in their leaves. The results of our transcriptomic analysis revealed that the mitogen-activated protein kinase (MAPK) signaling pathway, transporters, cytochrome P450, phenylpropanoid biosynthesis, and isoflavonoid biosynthesis were involved in the response to salt stress in the lettuce seedlings. The red lettuce cultivar showed a greater abundance of DEGs related to secondary metabolite biosynthesis and aquaporins under the salt treatment, resulting in a salinity-tolerant capacity comparable to that of the wild lettuce species. These results reveal important biosynthesis pathways that may play a key role in the salt tolerance of lettuce seedlings and provide key candidate genes that could be functionally characterized further and utilized to genetically improve new salt-tolerant varieties.