Chitosan nanoparticles enriched with brown algae extract enhance the defense response against cucumber mosaic virus (CMV) in tomato plants

Abstract Cucumber mosaic virus (CMV) is widely distributed throughout the world. It severely reduces tomato crop productivity and threatens commercial production. Affordable, mass-produced antivirals are desperately needed. Natural sources, including brown algae, which produce bioactive compounds, are promising. Sargassum denticulatum extract (SE) and chitosan (CS) along with their combined form, are utilized to enhance plant antiviral resistance. Combined SE -CSNPs at 50 mg.L⁻ 1 reduced disease at 16 dpi and inhibited CMV accumulation by 50.8% compared to a CMV-positive reference. SE-CSNPS (20 and 30 mg.L⁻ 1 ) foliar spray 48 h before CMV inoculation reduced virus infectivity rate by 100% at 16 days post CMV inoculation (dpi). Loaded SE-CSNPs increased antioxidant enzymes associated with plant defense, polyphenol oxidase (PPO), peroxidase (POD), and catalase (CAT) enzymes, at 4dpi, which subsequently levelled off by 10 and 16 dpi. The treatment increased the expression of numerous plant defense genes ( PR-1 and PR-5 ) involved in the salicylic acid (SA)-mediated signaling pathway and Thaumatin-like Protein compared to untreated infected plants. The SE-CSNPs treatments (20, 30, and 50 mg.L⁻ 1 ) at 45 days post-CMV-inoculation reduce CMV-CP expression and viral movement more than the CMV alone. CMV-infected tissues showed chloroplast misshaping and severe degeneration on transmission electron microscopy (TEM). Plant growth parameters increased significantly. The SE-CSNPs-treated tissues had ellipsoid chloroplasts with stroma-adsorbed SE-CSNPs. There was a healthy nucleus and united mitochondrial surface and structure. The potential of SE-CSNP nanoparticles to stimulate plant development and systemic resistance to CMV infection will encourage their application as a bio-control agent in crop protection, and sustainable agriculture.