Dynamic physiological and transcriptome changes reveal a po-tential relationship between the circadian clock and salt stress response in Ulmus pumila

Despite the important role the circadian clock plays in numerous critical physiological responses in plants, such as hypocotyl elongation, leaf movement, stomatal opening, flowering, and stress responses, there have been no investigations into the effect of the circadian clock on physiological and transcriptional networks under salt stress. Ulmus pumila L.—a major tree species used for timber, shelter, food, medicine, fodder, and ecological protection—has been reported to tolerate 100~150 mM NaCl treatment. We performed a time-course physiological and transcriptome analysis of 2-years-old U. pumila seedlings under salt treatment to dissect the physiological regulation and potential relationship between the circadian clock and the salt stress response. Seedlings in 150 mM NaCl treatment exhibited salt-induced physiological enhancement compared to the control group. A total of 7,009 unigenes were identified under salt stress, of which 283 unigenes were identified as circadian rhythm genes and 16 unigenes were differentially expressed (circadian rhythm-related DEGs). Further analysis of dynamic expression changes revealed that DEGs involved in four crucial pathways—photosynthesis, thiamine metabolism, abscisic acid synthesis and metabolism, and the hormone-MAPK signal crosstalk pathway—are closely related to the circadian clock. Finally, we constructed a co-expression network between the circadian clock and these four crucial pathways. Our results help shed light on the molecular link between the circadian clock and salt stress tolerance in U. pumila.