While most plant-derived recombinant proteins are currently intended for human pharmaceutical applications, plants have also been used to produce veterinary products and feed additives. One application is the use of feed plants to produce vaccines against animal diseases. For example, vaccine trials have demonstrated that pigs fed on transgenic corn expressing transmissible gastroenteritis virus glycoprotein S are protected from the disease (Streatfield et al., 2000).Plants engineered to express catabolic enzymes can be used to increase the nutritional value of feed. For example, transgenic canola expressing Aspergillus phytase is available as the feed additive PhytaSeed. A comparison of pigs fed on a diet substituted with Natuphos (a commercial phytase additive) and those fed on a diet supplemented with PhytaSeed showed similar levels of phosphate utilisation (Zhang et al., 2000). Several plant species have been manipulated to express amylases and cellulases to help break down starch and cellulose after harvesting, therefore, increasing their utilisable carbohydrate content (Biesgen et al., 2002; Fischer et al., in press).
In one of the most impressive examples, Zeigler et al. (2000) demonstrated that the thermostable endo-1,4-ββ-d-glucanase from Acidothermus cellulolyticus could be expressed at levels up to 26% total soluble protein in Arabidopsis thaliana, the highest level of recombinant protein ever achieved in a nuclear transgenic plant. The protein was secreted to the apoplast, but because of its high temperature optimum (>80°C) it did not cause any damage to the host plant, though it was in direct contact with the cell wall. It would be very useful if this level of expression could be repeated in commercial crop species.
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