An aromatic amino acid and associated helix in the C-terminus of the potato leafroll virus minor capsid protein regulate systemic infection and symptom expression

Authors: XU, YI - Cornell University; DASILVA, WASHINGTON - Cornell University; QIAN, YAJUAN - Zhejiang University; Gray, Stewart

Submitted to: PLoS Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2018
Publication Date: 11/15/2018
Citation: Xu, Y., Dasilva, W., Qian, Y., Gray, S.M. 2018. An aromatic amino acid and associated helix in the C-terminus of the potato leafroll virus minor capsid protein regulate systemic infection and symptom expression. PLoS Pathogens. 14(11):e1007451. https://doi.org/10.1371/journal.ppat.1007451.
DOI: https://doi.org/10.1371/journal.ppat.1007451

Interpretive Summary: Potato leafroll virus and related viruses are responsible for economically impactful disease in many staple food crops. Resistance is incomplete at best and nonexistent for most affected crops. Disease management relies upon control of the aphid vectors and although insecticide application can prevent disease spread within the crop they do not prevent disease introduction to the crop which with heavy aphid flights can be devastating. This work examines the regulation of production of a multifunctional virus protein that is needed for both aphid transmission and for virus to spread in a plant host. We found that a very small segment of the protein controls its function in the spread of virus in the plant host. This segment controls how well the virus can move out of the main vasculature of the plant into smaller veins in leaves and cause a systemic infection and also make the virus available for acquisition by its aphid vectors. Minor disruption of this segment can be corrected by the virus as it replicates its genome and the repairs will allow the virus to return to its normal function. Therefore we need to target a larger piece of the virus for modification so that altered virus cannot fix itself. Understanding the fine details of these interactions allows for the development of sustainable strategies to disrupt stages in the virus life cycle and alters its ability to cause disease.

Technical Abstract: The C-terminal region of the minor structural protein of potato leafroll virus (PLRV), known as the readthrough protein (RTP), is involved in efficient virus movement, tissue tropism and symptom development. Analysis of numerous C-terminal deletions identified a five-amino acid motif that is required for RTP function. A PLRV mutant expressing RTP with these five amino acids deleted (Delta 5aa-RTP) was compromised in systemic infection and symptom expression. Although the Delta 5aa-RTP mutant was able to move long distance, limited infection foci were observed in systemically infected leaves suggesting that these five amino acids regulate virus phloem loading in the inoculated leaves and/or unloading into the systemically infected tissues. The 5aa deletion did not alter the efficiency of RTP translation, nor impair RTP self-interaction or its interaction with P17, the virus movement protein. However, the deletion did alter the subcellular localization of RTP. When co-expressed with a PLRV infectious clone, a GFP tagged wild-type RTP was localized to discontinuous punctate spots along the cell periphery and was associated with plasmodesmata, although localization was dependent upon the developmental stage of the plant tissue. In contrast, the Delta 5aa-RTP-GFP aggregated in the cytoplasm. Structural modeling indicated that the 5aa deletion would be expected to perturb an Alpha-helix motif. Two of 30 plants infected with Delta 5aa-RTP developed a wild-type virus infection phenotype ten weeks post-inoculation. Analysis of the virus population in these plants by deep sequencing identified a duplication of sequences adjacent to the deletion that were predicted to restore the Alpha-helix motif. The subcellular distribution of the RTP is regulated by the 5-aa motif which is under strong selection pressure and in turn contributes to the efficient long distance movement of the virus and the induction of systemic symptoms.