Circadian Gating as a Regulatory Mechanism in Wheat

Daily rhythms in gene expression are thought to allow organisms to co-ordinate internal processes with external cycles of environmental conditions. In part, these rhythms are driven by the circadian clock, a molecular timing mechanism capable of generating and sustaining rhythms of approximately 24 h. Whilst diel cycles of light and temperature entrain the circadian clock to local conditions, the circadian clock can also regulate the magnitude of molecular and physiological responses to environmental stimuli. Temporal modulation of response pathways is known as circadian gating.As the climate crisis develops, there has been an increased focus on improving the resilience of crops to adverse environmental conditions. However, little is known about the significance of circadian gating in modulating environmental responses at a genome wide level, particularly in complex polyploid crops. Using socioeconomically important bread wheat Triticum aestivum as a model, I demonstrate that circadian gating of the wheat transcriptome in response to cold is extensive. However, circadian gating is frequently unbalanced between gene homoeologs. Furthermore, independent sequential cold treatments pause the oscillation of certain circadian clock components, while others are unaffected, suggesting a bifurcated cold response within the wheat circadian clock.I show that robust circadian gating of chloroplast targeted but nuclear encoded SIGMA FACTOR 5 can persist over 72 hours. Cold responsive circadian gating of the entire TaSIG5 triad is morning phased, permitting increases in transcript when cold and light are perceived together in natural conditions. Despite similarities in transcript regulation, homology modelling indicates there are structural differences between TaSIG5 homoeologs, a sign of unequal functionality across the triad. I also discover that TtSIG5 contributes to maintaining photosynthesis under photoinhibition-inducing conditions. Using CRISPR-Cas9 technology, I was able to edit TaSIG5 homoeologs, demonstrating how gene-editing could be used in future to manipulate TaSIG5 regulation and improve wheat environmental resilience