Crosstalk between the circadian clock and plant innate immunity in Arabidopsis thaliana

Abstract

Plant diseases caused by pathogens and pests result in billions of dollars of loss each year. It is crucial to understand how the defense systems work in order to mitigate disease. The circadian clock is an internal time measuring mechanism vital for many biological processes. Recent studies have implicated a key role of the circadian clock in defense. My research seeks to understand the molecular mechanisms underlying circadian regulation of plant defense. With my colleagues, I have developed a powerful and user-friendly method, using RStudio, to analyze large-scale data from time serial experiments for circadian clock assays. This critical method has aided in data analyses in the projects detailed here. Firstly, I investigated whether the key defense signaling molecule, salicylic acid (SA), regulates the circadian clock by using SA treatment and SA-related mutants expressing the circadian reporter CCA1:LUC. Our data clearly showed that a functional SA pathway is required for normal CCA1 expression, as too much or inhibition of SA biosynthesis/signaling pathways could lead to the suppression of CCA1 expression, perturbing the functioning of the circadian clock. This study suggests a feedback regulation of the circadian clock by SA-mediated defense. Secondly, I investigated the circadian control of the defense gene TGA3. TGA3 was identified as a potentially critical connection in defense-clock crosstalk via a study involving systems biology and machine learning approaches. Our data confirmed circadian regulation and SA induction of TGA3 expression, narrowing down the cis element responsible for this regulation within 543 bp of the TGA3 promoter. We identified transcription factors that bound to this TGA3 promoter in a high-throughput yeast-one hybrid screen. I primarily focused on DEWAX and HHO2 genes in my research. I found that dewax and hho2 mutants affected TGA3:LUC circadian expression only under SA treatment. Interestingly, both dewax and hho2 mutants exerted stronger effects on expression of the clock gene CCA1. Our data revealed novel roles of DEWAX and HHO2 in circadian regulation and TGA3 as a key node in mediating defense-clock crosstalk. Overall, my research shows strong avenues of crosstalk between the circadian clock and innate immunity.