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Journal of Virology, June 2009, p. 5419-5429, Vol. 83, No. 11
0022-538X/09/$08.00+0     doi:10.1128/JVI.02312-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

The C Terminus of the Polerovirus P5 Readthrough Domain Limits Virus Infection to the Phloem{triangledown}

Kari A. Peter,1,2 Frederick Gildow,3 Peter Palukaitis,4 and Stewart M. Gray1,2*

Biological Integrated Pest Management Research Unit, USDA/ARS, Ithaca, New York 14853,1 Department of Plant Pathology and Plant Microbe Biology, Cornell University, Ithaca, New York 14853,2 Department of Plant Pathology, The Pennsylvania State University, University Park, Pennsylvania 16802,3 Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom4

Received 4 November 2008/ Accepted 10 March 2009

Poleroviruses are restricted to vascular phloem tissues from which they are transmitted by their aphid vectors and are not transmissible mechanically. Phloem limitation has been attributed to the absence of virus proteins either facilitating movement or counteracting plant defense. The polerovirus capsid is composed of two forms of coat protein, the major P3 protein and the minor P3/P5 protein, a translational readthrough of P3. P3/P5 is required for insect transmission and acts in trans to facilitate long-distance virus movement in phloem tissue. Specific potato leafroll virus mutants lacking part or all of the P5 domain moved into and infected nonvascular mesophyll tissue when the source-sink relationship of the plant (Solanum sarrachoides) was altered by pruning, with the progeny virus now being transmissible mechanically. However, in a period of months, a phloem-specific distribution of the virus was reestablished in the absence of aphid transmission. Virus from the new phloem-limited infection showed compensatory mutations that would be expected to restore the production of full-length P3/P5 as well as the loss of mechanical transmissibility. The data support our hypothesis that phloem limitation in poleroviruses presumably does not result from a deficiency in the repertoire of virus genes but rather results from P3/P5 accumulation under selection in the infected plant, with the colateral effect of facilitating transmission by phloem-feeding aphid vectors.

* Corresponding author. Mailing address: Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phone: (607) 255-7844. Fax: (607) 255-2459. E-mail: smg3{at}cornell.edu

{triangledown} Published ahead of print on 18 March 2009.

Journal of Virology, June 2009, p. 5419-5429, Vol. 83, No. 11
0022-538X/09/$08.00+0     doi:10.1128/JVI.02312-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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