Seaweed‐Derived Oligosaccharides Confer Broad‐Spectrum Antiviral Defenses in Tomato and Arabidopsis Plants

ABSTRACT Seaweed extracts (SWEs), particularly those derived from red algae, are increasingly recognized as sustainable, eco‐friendly alternatives to synthetic agrochemicals. AG Fort, a commercial formulation comprising derivatized low molecular weight oligosaccharides from Kappaphycus alvarezii , was previously reported to prime antibacterial immunity in Arabidopsis thaliana . In this study, we evaluated the antiviral efficacy of the seaweed‐based formulation AG Fort against Tomato leaf curl Palampur virus (ToLCPalV), a bipartite begomovirus that causes severe disease in tomato crops across northern India and parts of the Middle East and has a wide natural host range. Symptoms of ToLCPalV infection include extensive leaf curling, chlorosis, and stunted growth, leading to significant yield losses. With current control strategies relying heavily on hazardous pesticides targeting insect vectors, there is an urgent need for safer alternatives. We demonstrate that prophylactic application of AG Fort, either via foliar spray or transient root dipping, significantly reduces disease incidence and symptom severity in the virus‐challenged tomato plants. Treated plants exhibited upregulated expression of defense‐related genes, increased phytohormone levels, decreased accumulation of viral replicative intermediates, and reduced levels of viral transcripts. Notably, similar antiviral protection was also observed in AG Fort‐treated Arabidopsis plants challenged with Cucumber mosaic virus (CMV), suggesting a broad‐spectrum antiviral potential. Together, our findings highlight seaweed‐derived oligosaccharides as sustainable, seaweed‐based crop protection agents capable of mitigating viral infections in Arabidopsis thaliana and tomato, with broader crop applicability indicated in manufacturer data as part of integrated disease management against multiple plant viruses.