Effect of simulated latitudinal variation in daylength conditions on oscillations of circadian gene expression in populations of woodland strawberry (Fragaria vesca L.) from Northern Norway and Italy

Abstract

Almost all species on Earth possess circadian rhythms that align their physiological processes with suitable times of day and year based on predictable changes in the light cycle. While it is known that natural variations exist in photoperiodic responses regulated by the circadian clock, there are currently no studies assessing the effect of latitudinal variations in daylength on the properties of plant circadian genes. This thesis investigated the association of the properties of the circadian clock with daylength in two woodland strawberry (Fragaria vesca L.) clones originating from Italy (45°N) and Northern Norway (69°N). Plants were grown under two simulated daylengths (18h photosynthetic light/6h twilight and 15h photosynthetic light/1h twilight/8h dark) corresponding with the summer photoperiod at each latitude. Real-time qPCR was used to analyze the expression of eight circadian clock genes present in the leaves over a time-course of 48 hours. Six of the genes (FvLHY, FvRVE8, FvPRR9, FvPRR7, FvPRR5, and FvLUX) expressed clear circadian oscillations of transcription levels in both daylengths, with significantly longer phases of expression under the longer daylength. Interestingly, there were only differences in expression between the two clones 3.3% of the time, demonstrating a lack of specific latitudinal responses in circadian clock properties in this species. All genes maintained transcriptional rhythms in constant darkness except for two (FvPRR9 and FvPRDX) where transcription was completely abolished. FvPRDX also had no rhythm of transcription, though other studies have shown that the oxidation cycles of peroxiredoxin proteins do exhibit circadian rhythmicity. One gene (FvTOC1) had an irregular rhythm that warrants further testing due to its central role in the circadian pacemaker. This study shows that the oscillations of circadian clock transcription levels in F. vesca from a lower latitudinal origin are highly entrainable to the long daylength conditions that are characteristic of northern latitudes. While the circadian clock is able to acclimate its oscillation timing with the external daylength, photoperiodic outputs controlled by the clock, such as flowering time, do not exhibit the same level of plasticity to sudden changes in daylength. This provides an important resource for understanding the circadian clocks contribution to adaptability.