Modulation of disease severity by plant positive-strand RNA viruses: The complex interplay of multifunctional viral proteins, subviral RNAs and virus-associated RNAs with plant signaling pathways and defense responses.

Plant viruses induce a range of symptoms of varying intensity, ranging from severe systemic necrosis to mild or asymptomatic infection. Several evolutionary constraints drive virus virulence, including the dependence of viruses on host factors to complete their infection cycle, the requirement to counteract or evade plant antiviral defense responses and the mode of virus transmission. Viruses have developed an array of strategies to modulate disease severity. Accumulating evidence has highlighted not only the multifunctional role that viral proteins play in disrupting or highjacking plant factors, hormone signaling pathways and intracellular organelles, but also the interaction networks between viral proteins, subviral RNAs and/or other viral-associated RNAs that regulate disease severity. This review focusses on positive-strand RNA viruses, which constitute the majority of characterized plant viruses. Using well-characterized viruses with different genome types as examples, recent advances are discussed as well as knowledge gaps and opportunities for further research.