Viral suppressor AC4 encoded by tomato leaf curl Palampur virus hijacks SA signaling to promote pathogenesis.

Systemic acquired resistance (SAR) is a vital long-distance defense mechanism in plants, orchestrated by salicylic acid (SA) and its mobile signaling derivatives, methyl salicylate (MeSA) and azelaic acid (AzA). While SAR-disabling viral suppressors have been extensively studied in monopartite begomoviruses, their role in bipartite viruses remains largely unexplored. This study provides the first experimental evidence that the AC4 protein encoded by Tomato leaf curl Palampur virus (ToLCPalV), a bipartite begomovirus, contributes to suppression of SAR responses. Using quantitative gene expression profiling and targeted metabolite assays, we observed a reduction in the transcription of SAR marker genes ICS1 and PR1, as well as significant reduction in SA, MeSA, and AzA levels at progressive infection stages. These alterations were substantially alleviated in plants infected with an AC4-deficient mutant virus, confirming that AC4 plays a functional role in diminishing SAR activation. Complementary in silico analyses predicted that both AC4 and another ToLCPalV-encoded suppressor AV2 may interact with key enzymes associated with SA biosynthesis and metabolism, and may bind SAR-related mobile signals, including MeSA, AzA, pipecolic acid, and glycerol-3-phosphate. Together, the experimental results supported by computational modeling indicate that AC4 likely interferes with specific steps of SA production and movement of SAR signals during systemic immunity. Our study suggests that the viral proteins AC4 and AV2 may suppress SAR in a bipartite begomovirus and provides a clear starting point for future experiments, including interaction studies, targeted mutations, or resistant plants, to test and confirm their roles in plant immunity.