UC San Diego

Many plant physiological and behavioral responses exhibit 24h rhythms that anticipate daily changes in the environment. This key adaptive response is regulated by the circadian clock, which provides an endogenous timekeeping mechanism that coordinates biological activities. Multiple environmental cues, including the infection of biotrophic plant pathogen Pseudomonas Syringae and pathogen induced phytohormone salicylic acid (SA) were shown to affect the clock function. However, the highly dynamic and variable nature of pathogen infection progression and related SA kinetics make it difficult to dissect the clock response to a practical infection. Here, we developed methods to perform local Pseudomonas Syringae infection and to apply transient SA treatment. These methods more closely resembled stimuli in a practical infection and allowed us to explore the circadian clock response to these stimuli. Our results indicated that a local infection lengthens the period and reduces the amplitude of circadian rhythms. Transient SA treatment recapitulated the amplitude phenotype and resulted in a phase delay with strongest phase effect at the beginning of the day. Apoplastic ROS production induced by transient SA treatment and Pseudomonas Syringae infection was shown to partly mediate these circadian clock responses. Moreover, NPR1, a master regulator of plant defense, counteracted the amplitude decrease by both infection and SA treatment. Furthermore, we discovered that localized Pseudomonas Syringae infection predominantly affected the circadian rhythms in the shoot apex and resulted in a delay in plant development. Some TCP transcription factors have been shown to regulate clock function and DNA binding activity of TCPs is regulated by ROS. We therefore characterized TCP transcription factors in order to explore the possible link between defense related ROS production and the regulation of circadian clock function. We performed yeast one hybrid analysis with a newly developed Y1H reporter system on class I TCPs and found that most of them interact with CCA1 promoter. Also, we developed a novel gene-centered Y2H screen system and discovered that class I TCP transcription factors were highly enriched for TCP21 interactors. These results uncovered the complex nature in which TCP transcription factors interact to regulate circadian clock function.