Diurnal Dynamics Shape Plant Defense Responses: Temporal Variations in Disease Progression and Metabolic Reprogramming in Brassica oleracea

Plants synchronize their immune responses with the circadian cycle, yet the impact of diurnal timing on disease resistance remains poorly understood in crop species. In this study, we investigated how diurnal variation modulates defense responses in Brassica oleracea against Xanthomonas campestris pv. campestris (Xcc). Using a resistant cabbage variety (Badger Inbred-16) and a susceptible broccoli variety (Early Big), we found that plants inoculated in the evening exhibited significantly reduced disease severity compared to morning infections. Metabolomic and gene expression analyses revealed that the circadian clock orchestrates metabolic reprogramming during infection, influencing key defense-related compounds. Lipid metabolism, particularly the accumulation of fatty acid amides such as oleamide, was strongly associated with evening resistance. Additionally, genes involved in jasmonic acid biosynthesis and energy metabolism were upregulated at night, suggesting a coordinated defense response. Exogenous application of oleamide mimicked the evening resistance phenotype, reducing lesion size and inhibiting bacterial growth in vitro. These findings indicate that diurnal immune regulation is a critical factor in plant-pathogen interactions and that leveraging circadian-driven metabolic shifts could enhance crop disease management strategies.