MAKING ANALLY FROM ANENEMY: Plant Virology and the New Agriculture

Abstract

Historically, the study of plant viruses has contributed greatly to the elucidation of eukaryotic biology. Recently, concurrent with the development of viruses into expression vectors, the biotechnology industry has developed an increasing number of disease therapies utilizing recombinant proteins. Plant virus vectors are viewed as a viable option for recombinant protein production. Employing pathogens in the process of creating added value to agriculture is, in effect, making an ally from an enemy. This review discusses the development and use of viruses as expression vectors, with special emphasis on (+) strand RNA virus systems. Further, the use of virus expression vectors in large-scale agricultural settings to produce recombinant proteins is described, and the technical challenges that need to be addressed by agriculturists and molecular virologists to fully realize the potential of this latest evolution of plant science are outlined.

Transient Virus Expression Systems for Recombinant Protein Expression in Dicot- and Monocotyledonous Plants

“…Virus vector expression systems offer significant advantages at considerably reduced costs to current cell-based manufacturing systems, such as employed by Protalix and Dow AgroSciences, while avoiding concerns associated with stable plant transformation [29,30]. Virus based expression systems have been extensively tested and shown safe and environmentally-friendly in both indoor and outdoor tests since 1991 [12] and multiple products completing early stage human clinical investigations [31,32]. Additional advantages also exist compared with traditional cell-based fermentation approaches include: 1) speed and low cost of genetic manipulation; 2) rapid manufacturing cycles; 3) no mammalian pathogen contamination; 4) minimal endotoxin concentrations and 5) economical production [12-15, 33, 34].…”

“…The apparent pausing of the ribosome, and the discontinuity of the peptide bond that results, allows proteins upstream and downstream of the 2A sequence to be differentially targeted, such as a single chain antibody accumulating in the plant apoplast while the CP was sequestered in its normal cytosolic localization [43]. Using these strategies, independent-virus systems have been derived from the genomes of potexviruses (including potato virus X; PVX), tobamoviruses (including TMV), comoviruses (including cowpea mosaic virus), potyviruses, tobraviruses, closteroviruses and others [12,13,15].…”

“…These viruses have a (+) sense single stranded RNA genome of ~6400 nucleotides helically encapsidated by many ~2,100 copies of a 17.5 kDa CP in rigid rod shaped particles [12]. The viral replication-associated proteins are directly transcribed from the genomic RNA directly, whereas expression of internal genes is through the production of subgenomic RNAs.…”

“…This movement produces the systematic infection and the foreign gene is expressed in all cells that express other virus protein products. Within the cell, the foreign protein is deposited in the site dictated by its protein sequence, either naturally or purposely engineered [12,46].…”

“…The rapid replication cycle of the virus systems provided amplification of messenger RNA and the resulting proteins providing for a "burst" of recombinant expression that can provide impressive yields (reviewed in [12][13][14][15]). While these early vectors were useful in plant cell systems to produce recombinant protein products with potential market value [16], these early systems could not support large scale manufacturing nor did they exploit the advantages of agriculture to provide cost-effective products.…”