Plant virus movement proteins

Abstract

C. Michael Deem,” Moshe Lapidot,t and Roger N. Beachyt *Department of Plant Pathology University of Georgia Athens, Georgia 30602 tDivision of Plant Biology The Scripps Research Institute La Jolla, California 92037 The ease with which plant viruses move throughout their hosts appears quite remarkable, considering that viral progeny must cross the plant cell wall to move from cell to cell. The cell wall acts as a barrier to the extracellular release and subsequent uptake of virus progeny by either surface fusion or receptor-mediated endocytosis, the two general pathways for entry used by animal viruses. Conse- quently, plant viruses have evolved a distinct mechanism for cell-to-cell movement. With few exceptions, virus particles are initially intro- duced into host cells during mechanical damage or biologi- cal damage (via vectors) to the integrity of the cell wall and the plasma membrane. If, following entry and replication, progeny viruses are not capable of moving into adjacent healthy cells, a subliminal infection occurs, and the host appears to be resistant to thevirus. If, however, virus prog- eny are capable of spreading into adjacent cells, a progres- sive virus infection results. While the specific virus-host interaction will ultimately define the nature of the infection, virus movement represents an important component in determining pathogenicity and virulence. The movement of virus progeny throughout a host is an active process that can be divided into two broadly defined phases. The first phase is characterized by cell-to-cell (short-distance) spread of the virus. The second phase is long-distance movement from an infected tissue, such as a leaf, to other tissues via the vascular tissue. The exit of the virus from the vascular tissue subsequently results in short-distance movement of virus progeny from cell to cell. In some virus-host interactions, virus spread may be lim- ited to short-distance movement, for example, in the inocu- lated leaf or in vascular or vascular-associated tissue. It is generally accepted that plant viruses circumvent the cell wail by exploiting plasmodesmata. Plasmodesmata, the plant functional analog of gap junctions in animals, are channels that transverse cell walls and provide cyto- plasmic continuity between adjacent cells. These intercel- lular connections play an important role in cell-cell com- munication and the dissemination of water and nutrients throughout the plant. Plasmodesmata have not been iso- lated or biochemically characterized, and structural mod- els of plasmodesmata are based on interpretations of elec- tron micrographs. (A prevailing model is seen in Figure 1.) Extending through the plasma membrane-lined chan- nel is the desmotubule, a modified extension of the endo- plasmic reticulum. The area between the desmotubule and the plasma membrane, the cytoplasmic sleeve, likely