Heterologous viral suppressor of RNA silencing breaks protein-based viral immunity in mixed viral infection

Dear Editor:RNA silencing confers immunity against most viruses and is initiated by host Dicer processing of viral doublestranded RNA into virus-derived small interfering RNA(siRNA)(Guo et al.,2019).Viruses that cause plant disease usually evolve viral suppressor of RNA silencing(VSR)to inhibit antiviral silencing(Li and Ding,2006).Some plant nucleotide-binding leucine-rich repeat proteins(NLRs)emerge as immune receptors to recognize specific virus and trigger rapid hypersensitive response(HR)at infection site to protect plants from devastating viruses(Cui et al.,2015;Deng et al.,2018).Recent studies showed that NLR immune receptors are subjected to negative regulation by plant miRNAs(Deng et al.,2018).It is well known that in some cases,mixed virus infections lead to synergism in susceptible plants due to cooperative inhibition of antiviral silencing by multiple VSRs(Pruss et al.,1997).In case of mixed virus infection in a plant with NLR targeting a single virus,heterologous VSR might play dual roles for the NLR-targeted virus.On one hand,the VSR may promote virulence by cooperative inhibition of antiviral silencing.