INTRODUCTIONPlant virus vectors have been developed as a result of 20 years of advances in molecular biology, which produced infectious plasmid-based clones of RNA and DNA plant viruses and viroids that could be routinely amplified in E. coli. This work built on a century of progress in virology, much of which was based on understanding the biology of Tobacco mosaic virus (TMV) (1). With the availability of the rice and Arabidopis draft genome sequences, expressed sequence tags (ESTs) and microarrays, crystal structures of many plant viruses, and public interest in non-transgenic strategies for crop improvement, the technology of plant virus gene vectors holds much promise for the coming decade (2-4). The intent of this review is to define virus-vector strategies, describe uses for improving crop health, and point out possible beneficial overlaps with veterinary and human medicine.*To whom correspondence should be addressed.(1) Texas Agricultural Experiment Station, Weslaco, TX 78596.
DEFINING THE SYSTEMSMethods to regenerate transgenic plants are widely used in plant biology and biotechnology. This involves the use of either Agrobacterium tumefaciens or biolistics (e.g., gene guns) to deliver genetically customized gene expression cassettes into the nucleus of a plant cell. In this case, the DNA becomes integrated into the chromosome and with the use of selectable markers and tissue culture procedures, the cells containing the foreign insert serve as the progenitors for regeneration of whole transgenic plants. This process usually takes at least several months, and mostly longer, to obtain plants with a stable genetic trait.Plant viruses cause harmful diseases that can result in significant reductions in crop yield and economic losses to growers. However, from a biotechnology point of view, viruses collectively can also be regarded as 'molecular tools' (2). One intriguing applicable feature of plant viruses is their capacity to serve as gene vectors. With rare exception (5, 6), plant viruses do not integrate into the plant chromosome but instead transiently express their genes upon infection (7). Because plant viruses replicate to a very high copy number in infected cells, substantial levels of gene expression can be achieved. By standard molecular biology techniques, plant viruses can be supplied with foreign genes and within a few days after inoculation, plant virus gene vectors can provide high levels of transient foreign gene expression (2,4,(8)(9)(10)(11)(12).Many proteins used for biotechnology applications are not biologically active unless they undergo specific post-translational modifications, such as glycosylation or phosphorylation, in eukaryotic cells. Plants are attractive substrates for the synthesis and purification of proteins because they can carry out these modifications and, through the ages of agricultural development, it has become fairly straightforward to grow crop plants that yield a large, uniform biomass. In many cases it is desirable to have the foreign gene expressed in transgenic p...