Effects of salicylic acid and proline on lettuce resistance to salt stress based on transcriptome sequencing

Background: Salicylic acid (SA) and proline is a regulatory factor that mediates plants responses to microbial pathogens and saline conditions. The mechanism of salicylic acid and proline induced lettuce resistance to salt stress is still unknown.  Materials and methods: Firstly, the lettuces were cultured by hydroponics under salt stress (100 mmol/L NaCl) with application of SA (1.0 mmol/L) or proline (0.5 mmol/L), and the biological parameters and physiological properties of lettuce were detected. Secondly, we collected total RNA from lettuce cultured in nutrient solution with 1.0 mmol/L salicylic acid or 0.5 mmol/L proline for transcriptome sequencing. Results: SA and proline all could alleviate the growth inhibition effect induced by salt stress in lettuce. And after de novo transcriptome assembly and gene annotation, differentially expressed genes were enriched in several GO terms such as stimulus, catalytic activity, metabolic process, and bonding, and KEGG pathways including the biosynthesis of secondary metabolites, plant-pathogen interactions, limonene and pinene degradation and flavonoid biosynthesis. We propose that salicylic acid and proline may regulate secondary metabolites such as limonene and flavonoid to increase plant resistance and delay the accumulation of oxyradicals.  Conclusion: We suggest that salicylic acid and proline could be applied exogenously to promote plant resistance to salt stress.