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Title: The Capsid Protein of Turnip Crinkle Virus Overcomes two Separate Defense Barriers to Facilitate Viral Systemic Movement in Arabidopsis

item CAO, MINGXIA - The Ohio State University
item YE, XIAOHONG - University Of Nebraska
item LIN, JUNYAN - The Ohio State University
item ZHANG, XIUCHUN - The Ohio State University
item Redinbaugh, Margaret
item SIMON, ANNE - University Of Maryland
item MORRIS, T - University Of Nebraska
item QU, FENG - The Ohio State University

Submitted to: Journal of Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2010
Publication Date: 8/1/2010
Citation: Cao, M., Ye, X., Lin, J., Zhang, X., Redinbaugh, M.G., Simon, A.E., Morris, T.J., Qu, F. 2010. The Capsid Protein of Turnip Crinkle Virus Overcomes two Separate Defense Barriers to Facilitate Viral Systemic Movement in Arabidopsis. Journal of Virology. 84(15):7793-7802.

Interpretive Summary: Plant viruses, like all pathogens, are in an 'arms race' with their hosts. Plants develop defense mechanisms to prevent virus invasion, then the virus evolves a mechanism to counter that defense. One major example of plant defense against viruses is 'RNA silencing', which is a potent antiviral defense that allows the plant host to degrade the viral genome. Viruses counter this defense with 'VSRs' (virus-encoded suppressors of RNA silencing), which are present in all successful plant viruses. Plant viruses are very small, with their genomes encoding only a few proteins. These proteins frequently have multiple functions. For example, the turnip crinkle virus (TCV) coat protein is important for virus movement and assembly in plants, and also acts as a VSR. In this study, we investigated the roles the virus coat protein plays in carrying out these different functions. By using Arabidopsis plants that lacked 'RNA silencing' we could investigate the properties of the coat protein that are important for virus assembly and long-distance movement. We found that the assembled virus particles were not necessary for the virus to replicate and move through leaf cells to the vascular system. TCV could even enter the vascular system without being assembled into virus particles. However, only viruses that were assembled into particles could the vascular system to infect a new leaf. Since earlier studies had indicated that the VSR function of the coat protein was required for long-distance movement, our results show that efficient movement of the virus from leaf to leaf in the plant requires both the VSR and the assembly functions of the coat protein.

Technical Abstract: The capsid protein (CP) of Turnip crinkle virus (TCV) is a multi-functional protein needed for virus assembly, suppression of RNA silencing-based antiviral defense, and long distance movement in infected plants. In this report, we have examined genetic requirements for the different functions of TCV CP, and evaluated the inter-dependence of these functions. A series of TCV mutants containing alterations in the CP coding region were generated. These alterations range from single amino acid substitutions, domain truncations, to knockouts of CP translation. The latter category also contained two constructs in which the CP coding region was replaced by either the cDNA of a silencing suppressor of a different virus, or that of green fluorescence protein . These mutants were used to infect Arabidopsis plants with diminished antiviral silencing capability (dcl2 dcl3 dcl4 plants). There was a strong correlation between the ability of mutants to reach systemic leaves and the silencing suppressor activity of mutant CP. Virus particles were not essential for entry of the viral genome into vascular bundles in the inoculated leaves in the absence of antiviral silencing. However, virus particles were necessary for egress of the viral genome from the vasculature of systemic leaves. Our experiments demonstrate that TCV CP not only allows the viral genome to access the systemic movement channel through silencing suppression, but also ensures its smooth egress by way of assembled virus particles. These results illustrate that efficient long-distance movement of TCV requires both functions afforded by the CP.