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

## TL;DR

This study shows that the plant protein NbSGT1 helps a virus infect plants by suppressing the plant's natural defense system called RNA silencing.

## Contribution

NbSGT1 is identified as a new host factor exploited by potyviruses to suppress RNA silencing and enhance infection.

## Key findings

- NbSGT1 interacts with the viral HC-Pro protein and enhances its RNA silencing suppression activity.
- NbSGT1 acts as an endogenous suppressor of RNA silencing, inhibiting local silencing in plants.
- NbSGT1 downregulates key RNA silencing pathway genes like AGOs, DCLs, RDRs, and SGS3.

## Abstract

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.

The molecular co‐chaperone NbSGT1 interacts with potyviral HC‐Pro and promotes viral infection in plants. Notably, (1) NbSGT1 enhances the RSS activity of HC‐Pro and (2) NbSGT1 acts as an endogenous suppressor of RNA silencing (ESR), suppressing RNA silencing in plants.