Crosstalk between the plant circadian clock and immunity

Abstract

Molecular circadian clocks are found across taxa to anticipate predictable daily changes in the external environment. In plants, the circadian clock is important to coordinate agriculturally significant processes that are vital to plant health, such as growth and metabolism, in response to rhythmic environmental cues and stressors. The plant circadian clock comprises of a circuitry of transcriptional translational feedback loops which coordinate the diel expression of approximately a third of the genome, including many genes involved in immunity. As such, the plant clock is important for governing the outcome of plant-pathogen interactions by priming the immune system towards the time of day when pathogen attack is most likely, resulting in temporal differences in susceptibility to pathogens over the course of the day. Plants are armed with a highly sophisticated and complex immune system that consists of a tiered cellular defence signalling network: pattern-triggered immunity (PTI) is rapidly initiated upon detection of pathogen-associated molecular patterns at the cell surface, which prompts the activation of a long-lasting layer throughout the rest of the plant, called systemic acquired resistance (SAR). Evidence of the role of the plant biological clock in orchestrating PTI and SAR signalling has grown in recent years, such as clock regulation of defence gene expression, the activity of key immunity proteins and the abundance of immune hormones. Further to this, there is the fascinating phenomenon that immune players, as well as plant pathogens themselves, can crosstalk to reciprocally control the activity circadian clock. The mechanisms underpinning clock-gated immune responses, however, are poorly understood, hampering the agricultural exploitation of this system to boost plant immunity. This thesis presents the findings produced from two independent projects. In the first project, a standardised resistance screening assay was developed to determine the extent of resistance to commercially important fungicides in the devastating fungal pathogen, Botrytis cinerea in the United Kingdom. The work in the second project explores the effect that bacteria-derived PTI elicitors, (flg22 and elf18) and the key SAR hormone (salicylic acid), have on the oscillating transcriptional activity of core clock genes. Conversely, the importance of the plant clock in gating plant defence is revealed in this study, as key immunity genes are differentially regulated between subjective dawn and dusk, and that the gating of these genes is deregulated in the arrhythmic line, CCA1ox. Taken together, the findings in this study show that the reciprocal interaction between the circadian clock and different layers of the immune response shapes the outcome of plant-pathogen interactions.