Diurnal timing influences metabolic reprogramming and resistance of Brassica oleracea to Xanthomonas campestris infection

• Time of infection affects disease severity in Brassica • Morning infection increases Xanthomonas susceptibility • Evening inoculation enhances metabolic defenses • Oleamide lowers symptoms via lipid remodeling • Daily plant state modulates stress resistance Plants integrate circadian cues to coordinate metabolism and defense, yet the impact of infection timing on crop immunity remains largely unexplored. Here, we demonstrate that Brassica oleracea exhibits strong diurnal variation in resistance to Xanthomonas campestris pv. campestris ( Xcc ): inoculation at the end of the photoperiod (PM) consistently reduced disease severity compared with morning (AM) infections. Integrating physiological measurements, targeted primary-metabolite profiling, and untargeted UHPLC-QToF metabolomics, we show that PM-infected plants display enhanced metabolic flexibility, characterized by increased TCA-cycle intermediates, branched-chain amino acids, jasmonate-related lipids, and fatty acid primary amides. Among these, oleamide emerged as a key time-dependent metabolite, accumulating specifically in PM-infected plants. Exogenous oleamide application partially reproduced the PM-resistant metabolic state, attenuating disease symptoms and inducing stable lipid and phenylpropanoid remodeling. Metabolomics also indicated increased endogenous oleamide after treatment, while in vitro assays revealed direct growth inhibition of Xcc , suggesting complementary antimicrobial activity. Together, our results uncover a temporal dimension in B. oleracea immunity and identify oleamide as a promising lipid-derived modulator of diurnal defense reprogramming with potential for time-optimized disease management.