Biotic Elicitors and Nitric Oxide Synergistically Enhance Innate Immunity and Secondary Metabolite Production in Catharanthus roseus

Medicinal and aromatic plants are increasingly important in modern society, as approximately 70% of global populations rely on plant‐based natural products for their primary healthcare, according to the World Health Organisation (WHO). Catharanthus roseus is a medicinal plant that produces two key terpenoid indole alkaloids (TIAs), namely vinblastine and vincristine, which are used in anticancer therapies. C. roseus has shown susceptibility towards pathogenic fungi such as Alternaria alternata , which can significantly reduce plant growth. In this study, 17 different biotic elicitors were evaluated to determine alterations in innate immunity and production of TIAs in C. roseus . Among tested elicitors, LMW_CS‐0.01% showed immunity‐boosting capacity, significantly increasing peroxidase (2.73‐fold), polyphenol oxidase (3.93‐fold), phenylalanine ammonia‐lyase (1.57‐fold), β ‐1,3 glucanase (1.51‐fold), phenolic compounds (2.13‐fold) and flavonoid (1.67‐fold) accumulations on the ninth day after treatment compared to the control. Increases in vinblastine (3.15‐fold) and vincristine (2.76‐fold) were also quantified through LC–MS analysis. These biochemical responses show the activation of effector‐triggered immunity, which strengthens the plant cell against pathogenic microbes and improves resistance against leaf blight disease caused by A. alternata . Our results showed that chitosan not only induced activity of defence‐related enzymes, but also modified NO‐mediated signalling pathways. In addition, it reduced pathogen‐induced oxidative stress by lowering reactive oxygen species and minimising cellular damage during infection.