Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Published on August 11, 2006; 10.1104/pp.106.083519

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Received May 12, 2006
Accepted August 2, 2006

Down Regulation of 26S Proteasome Subunit RPN9 Inhibits Viral Systemic Transport and Alters Plant Vascular Development

Hailing Jin *, Songtao Li , and Andy Villegas Jr.

Center for Plant Cell Biology, Department of Plant Pathology, University of California, Riverside, CA 92521, USA

* Corresponding author; email: hailingj{at}ucr.edu.

Plant viruses utilize the vascular system for systemic movement. The plant vascular network also transports water, photosynthates, and signaling molecules and is essential for plant growth. However, the molecular mechanisms governing vascular development and patterning are still largely unknown. From a viral transport suppressor screening using virus-induced gene silencing (VIGS), we identified a 26S proteasome subunit, RPN9, which is required for broad-spectrum viral systemic transport. Silencing of RPN9 in N. benthamiana inhibits systemic spread of two taxonomically distinct viruses, tobacco mosaic virus (TMV) and turnip mosaic virus (TuMV). The 26S proteasome is a highly conserved eukaryotic protease complex controlling many fundamental biochemical processes, but the functions of many 26S proteasome regulatory subunits, especially in plants, are still poorly understood. We demonstrate that the inhibition of viral systemic transport after RPN9 silencing is largely due to alterations in the vascular tissue. RPN9-silenced plants display extra leaf vein formation with increased xylem and decreased phloem. We further illustrate that RPN9 functions at least in part through regulation of auxin transport and Brassinosteroid (BR) signaling, two processes that are crucial for vascular formation. We propose that RPN9 regulates vascular formation by targeting a subset of regulatory proteins for degradation. The BR signaling protein BZR1 is one of the targets.






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