UC Riverside

As we consider the negative impacts of climate change-fueled crop loss with an increasing human population, it is critical that we understand how plants respond to environmental stresses. The circadian clock, which is a key regulatory unit that controls plant growth, metabolism, and physiology in concert with external stimuli, presents a compelling target to study plant stress response mechanisms. Here, I investigate how the circadian clock regulates the plant temperature stress response in Arabidopsis. First, a transcriptomic approach using clock mutants, cca1lhy and ppr7prr9, temperature stress, and two time points as variables revealed that the magnitude and occurrence of the Arabidopsis transcriptional response to heat stress is dependent on the time of day that the stress is applied. The transcriptome analysis culminated in the identification of ~200 clock regulated or time of day dependent genes including the cold-responsive, CDF6, and heat-responsive, PLATZ2, transcription factors. Second, characterization of CDF6 demonstrated that CDF6 is gated by the clock during cold stress. Vasculature expressed CDF6 impacts germination during ambient temperature and photoperiodic flowering through downregulation of FT, CO, and BFT at ambient temperature and to a greater extent during cold stress. Third, examination of the PLATZ family of transcription factors determined that about half of the PLATZ family members exhibit rhythmic oscillation and respond to heat stress. PLATZ2 heat induction is dependent on the time of day the heat stress occurs, and it may contribute to the Arabidopsis response to high temperature. PLATZ orthologs in sorghum (Sorghum bicolor) and rice (Oryza sativa) display cyclic pattern of expression and one sorghum ortholog is also heat-responsive suggesting that the clock regulation of temperature stress response through the PLATZ family may be conserved among other species. To conclude, this work provides both an overview of clock regulation of the temperature stress response and a directed investigation of clock controlled genes during temperature stress. Overall, this work highlights the importance of considering the effect of both time of day and the circadian clock to understand how the clock adjusts to changing environmental conditions.