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Phloem Unloading of Potato virus X Movement Proteins Is Regulated by Virus and Host Factors

August 2008 , Volume 21 , Number  8
Pages  1,106 - 1,117

Tefera Mekuria,1 Devinka Bamunusinghe,1 Mark Payton,1,2 and Jeanmarie Verchot-Lubicz1

1Department of Entomology and Plant Pathology and 2Department of Statistics, Oklahoma State University, Stillwater 74078, U.S.A.

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Accepted 4 April 2008.

To determine the requirements for viral proteins exiting the phloem, transgenic plants expressing green fluorescent protein (GFP) fused to the Potato virus X (PVX) triple gene block (TGB)p1 and coat protein (CP) genes were prepared. The fused genes were transgenically expressed from the companion cell (CC)-specific Commelina yellow mottle virus (CoYMV) promoter. Transgenic plants were selected for evidence of GFP fluorescence in CC and sieve elements (SE) and proteins were determined to be phloem mobile based on their ability to translocate across a graft union into nontransgenic scions. Petioles and leaves were analyzed to determine the requirements for phloem unloading of the fluorescence proteins. In petioles, fluorescence spread throughout the photosynthetic vascular cells (chlorenchyma) but did not move into the cortex, indicating a specific barrier to proteins exiting the vasculature. In leaves, fluorescence was mainly restricted to the veins. However, in virus-infected plants or leaves treated with a cocktail of proteasome inhibitors, fluorescence spread into leaf mesophyll cells. These data indicate that PVX contributes factors which enable specific unloading of cognate viral proteins and that proteolysis may play a role in limiting proteins in the phloem and surrounding chlorenchyma.

Additional keywords:phloem transport, proteasome degradation, virus movement.

© 2008 The American Phytopathological Society