Pyrido[1,2‑a]pyrimidinone Mesoionic Compounds Containing a Hydrazone Moiety as Novel Plant Immune Inducer Against Viral Infections

To develop potential plant immune inducers, 35 novel pyrido[1,2-a]pyrimidinone mesoionic derivatives containing a hydrazone moiety were designed and synthesized. These target compounds were systematically evaluated for their antiviral activities against potato virus Y (PVY). Among them, compound L20 demonstrated remarkable protective efficacy, exhibiting an EC50 value of 124 mg/L, significantly lower than those of ningnanmycin (457 mg/L) and vanisulfane (271 mg/L). Transcriptomic and proteomic analyses revealed that L20 markedly up-regulated the expression of the 4-coumarate-CoA ligase gene and its corresponding protein in the phenylpropanoid biosynthetic pathway. Furthermore, defense enzyme activity assays confirmed that L20 robustly enhanced the key enzymatic activities of phenylalanine ammonia-lyase and peroxidase. Collectively, compound L20 activates plant systemic resistance against PVY, cucumber mosaic virus, and tomato yellow leaf curl virus, displaying broad-spectrum antiviral protection. These findings highlight L20 as a promising candidate for novel plant immune inducer development to combat viral pathogens in agriculture.