Circadian clock shapes diurnal gene expression patterns linked to glucose metabolic processes in Chinese cabbage

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The plant circadian clock temporally drives gene expression through the day and coordinates various physiological process with diurnal environmental changes. It is essential to confer plant fitness and competitive advantage to survive and thrive under natural condition by circadian control of gene transcription. Chinese cabbage (Brassica rapa ssp. pekinensis) is an economically important vegetable crop worldwide; however, there is little information concerning its circadian clock system. Here we uncovered that gene expression patterns were affected by circadian oscillators at both transcriptional and post-transcriptional levels in Chinese cabbage. Time-course RNA-seq analyses were conducted on two short-period lines (SPcc-1 and SPcc-2) and two long-period lines (LPcc-1 and LPcc-2) under constant light. We showed that 32.7-50.5% of the genes were regulated by the circadian oscillator and the expression peak of cycling genes appeared even earlier in short-period lines compared to long-period lines. In addition, approximately 250 splicing events showed circadian regulation, of which intron retention (IR) accounted for a large proportion. Rhythmically spliced genes included the clock genes LATE ELONGATED HYOCOTYL (BrLHY), REVEILLE 2 (BrRVE2) and EARLY FLOWERING 3 (BrELF3). We also found that the circadian oscillator could notably influence the diurnal expression patterns of genes that are associated with glucose metabolism via photosynthesis, Calvin cycle and tricarboxylic acid (TCA) cycle at both transcriptional and post-transcriptional levels. Taken together, our results demonstrate that circadian-regulated physiological processes contribute to Chinese cabbage growth and development.