The Molecular Cochaperone <scp>NbSGT1</scp> May Function as an Endogenous Suppressor of <scp>RNA</scp> Silencing That Is Recruited by a Potyvirus in Infection of Plants

Recent evidence indicates that plant cells contain specific endogenous suppressors of RNA silencing (ESRs) that plant viruses can exploit to counteract host defences. However, the underlying mechanisms are not yet fully understood. The helper component-proteinase (HC-Pro) of potyviruses is known to suppress RNA silencing and facilitate viral infection. Here, we used affinity purification followed by mass spectrometry to identify potential host proteins that interact with HC-Pro during telosma mosaic virus (TelMV, genus Potyvirus) infection in plants. We found that the molecular co-chaperone SGT1 (suppressor of the G2 allele of Skp1) interacts with HC-Pro via its SGS domain. Virus-induced gene silencing and RNAi-mediated knockdown of NbSGT1 resulted in decreased viral accumulation in Nicotiana benthamiana plants. Conversely, transient overexpression of NbSGT1 promoted TelMV multiplication. Through alanine-scanning mutagenesis, we identified three residues (K217, I227 and E332) in HC-Pro that are essential for its interaction with NbSGT1. Mutant viruses carrying these mutations exhibited reduced viral accumulation without affecting the RNA suppression of silencing (RSS) activity of HC-Pro. NbSGT1 enhanced the RSS activity of HC-Pro and increased the expression of foreign genes at both the protein and mRNA levels. Additionally, we demonstrated, through an Agrobacterium infiltratrion assay, that NbSGT1 may act as an ESR, inhibiting local but not systemic RNA silencing in plants. Finally, NbSGT1 significantly downregulated the expression of AGOs, DCLs, RDRs and SGS3, key genes of the RNA silencing pathway. Collectively, our findings provide evidence that the molecular co-chaperone SGT1 may act as a host ESR and is recruited by a potyvirus to facilitate viral infection.