Chitosan oligosaccharide regulates host defense in pepper plants against cucumber mosaic virus

The pepper plant, a member of the Solanaceae family, is widely cultivated and valued as an important spice in many dishes worldwide. Viruses cause significant damage to both the quality and quantity of pepper production, with cucumber mosaic virus (CMV) being particularly detrimental. Chitosan oligosaccharide (COS), a natural oligosaccharide derived from chitosan degradation, act as a plant immunity inducer. The present study aimed to verify the role of COS in inducing CMV resistance in capsicum and to explore its potential defense-related signaling pathways. The optimal condition for inducing resistance to CMV is 100 mg/L of COS pretreatment on day 5 of CMV inoculation. COS enhances the removal of harmful free radicals by increasing the activity of antioxidant enzymes including peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and phenylalanine ammonia-lyase (PAL). Concurrently, COS treatment can increase the increases chlorophyll content. Additionally, COS upregulates the expression of defense-related genes (such as NPR1, PR1, PR5, PR10, CAT, POD, SOD, PAL, and LOX), which contribute to pepper resistance against CMV. Proteomic analysis revealed that COS induces resistance in capsicum by activating the calcium signaling pathway. Taken together, these data indicate that COS enhances capsicum's innate immunity against CMV through the salicylic acid pathway in combination with calcium signaling. This study sheds light on the effects of COS and its signaling mechanisms in plant viral immunity and offers a foundation for the discovery of novel antiviral agents and further mechanistic studies.