Solstice Party: Contrasting Transcriptional Velocity and Transcriptome Composition at Winter and Summer Solstices in Douglas-fir

Perennial growth in plants is the product of interdependent cycles of daily and annual environmental stimuli that induce periods of growth and dormancy. In conifers, leaves (“needles”) are a perennial organ that integrates seasonal signals from light, temperature, and water availability. Our previous work implicated photoperiod as the dominant driver of annual and diurnal transcription patterns. To further explore the dynamics of transcriptome responses to seasonal cues in Douglas-Fir (Pseudotsuga menziesii) needles we characterized 26,298 annually cycling transcripts. Their model-defined amplitude is a useful measure of the transcriptome "velocity" that varies predictably throughout the year-- a fact which has implications for sampling strategies. The timing of maximum gene expression across annual periods shows a bimodal response, with ~50% of transcripts reaching maximum expression +/- 20 days from the winter and summer solstices. Categorization into summer, winter, and equinox transcripts is associated with significant differences in expression level, gene length, and transcriptional velocity. Transcripts peaking near the winter solstice are more abundant, more concentrated in their peak activity, show more variation in length while still tending to be longer than summer transcripts. They also show lower maximum velocities than those that peak near the summer solstice. This peak of activity during dormancy may represent transcriptional “anticipation” for the onset of spring growth or, it may suggest that mRNA transcription or maturation occurs more slowly during dormancy making the capture of longer transcripts more likely during winter months; Putative spliceosome regulating transcripts show the same basic bimodal pattern as the other transcripts.