Simultaneous Mutations in Multi-Viral Proteins Are Required for Soybean mosaic virus to Gain Virulence on Soybean Genotypes Carrying Different R Genes

BACKGROUND: Genetic resistance is the most effective and sustainable approach to the control of plant pathogens that are a major constraint to agriculture worldwide. In soybean, three dominant R genes, i.e., Rsv1, Rsv3 and Rsv4, have been identified and deployed against Soybean mosaic virus (SMV) with strain-specificities. Molecular identification of virulent determinants of SMV on these resistance genes will provide essential information for the proper utilization of these resistance genes to protect soybean against SMV, and advance knowledge of virus-host interactions in general. METHODOLOGY/PRINCIPAL FINDINGS: To study the gain and loss of SMV virulence on all the three resistance loci, SMV strains G7 and two G2 isolates L and LRB were used as parental viruses. SMV chimeras and mutants were created by partial genome swapping and point mutagenesis and then assessed for virulence on soybean cultivars PI96983 (Rsv1), L-29 (Rsv3), V94-5152 (Rsv4) and Williams 82 (rsv). It was found that P3 played an essential role in virulence determination on all three resistance loci and CI was required for virulence on Rsv1- and Rsv3-genotype soybeans. In addition, essential mutations in HC-Pro were also required for the gain of virulence on Rsv1-genotype soybean. To our best knowledge, this is the first report that CI and P3 are involved in virulence on Rsv1- and Rsv3-mediated resistance, respectively. CONCLUSIONS/SIGNIFICANCE: Multiple viral proteins, i.e., HC-Pro, P3 and CI, are involved in virulence on the three resistance loci and simultaneous mutations at essential positions of different viral proteins are required for an avirulent SMV strain to gain virulence on all three resistance loci. The likelihood of such mutations occurring naturally and concurrently on multiple viral proteins is low. Thus, incorporation of all three resistance genes in a soybean cultivar through gene pyramiding may provide durable resistance to SMV

Helper Component Proteinase of the Genus Potyvirus Is an Interaction Partner of Translation Initiation Factors eIF(iso)4E and eIF4E and Contains a 4E Binding Motif▿†

The multifunctional helper component proteinase (HCpro) of potyviruses (genus Potyvirus; Potyviridae) shows self-interaction and interacts with other potyviral and host plant proteins. Host proteins that are pivotal to potyvirus infection include the eukaryotic translation initiation factor eIF4E and the isoform eIF(iso)4E, which interact with viral genome-linked protein (VPg). Here we show that HCpro of Potato virus A (PVA) interacts with both eIF4E and eIF(iso)4E, with interactions with eIF(iso)4E being stronger, as judged by the data of a yeast two-hybrid system assay. A bimolecular fluorescence complementation assay on leaves of Nicotiana benthamiana showed that HCpro from three potyviruses (PVA, Potato virus Y, and Tobacco etch virus) interacted with the eIF(iso)4E and eIF4E of tobacco (Nicotiana tabacum); interactions with eIF(iso)4E and eIF4E of potato (Solanum tuberosum) were weaker. In PVA-infected cells, interactions between HCpro and tobacco eIF(iso)4E were confined to round structures that colocalized with 6K2-induced vesicles. Point mutations introduced to a 4E binding motif identified in the C-terminal region of HCpro debilitated interactions of HCpro with translation initiation factors and were detrimental to the virulence of PVA in plants. The 4E binding motif conserved in HCpro of potyviruses and HCpro-initiation factor interactions suggest new roles for HCpro and/or translation factors in the potyvirus infection cycle