A spectrum of PHAs has now been successfully synthesized in plants by using various metabolic pathways. These ranges from the stiff and brittle PHB to the more flexible P(HB-HV) plastic and MCL-PHA elastomers and glues. Experiments have shown that in some case very high amount of polymer can be produced at, however, a considerable metabolic cost. The challenge for the future is to succeed in the accumulation of adequate amounts of PHA (≥15% dwt) without affecting yield. For some agricultural production strategies, it will also be necessary to succeed in harvesting PHA without affecting the recovery of other plant products, such as oils, protein, or starch. This is important since in contrast to the production of PHA by bacterial fermentation, where the system is designed to produce mainly PHA with little residual waste, a large-scale agricultural production of PHA may be viable only through the recovery of not only PHA but also all other valuable components of the crop. For example, in the case of an oil crop such as B. napus, one must be able to recover PHA and the oil, as well as still being able to use the de-lipidized protein-rich meal for animal feed. In the case of a carbohydrate-producing crop such as either sugar beet or sugarcane, both sucrose and PHA would have to be recovered. An alternative strategy could be used whereby crop plants would be grown only for biomass and PHA production. An example would be the synthesis of PHA in switchgrass, where the residual biomass remaining after PHA extraction could be used for energy production. We know thus far that PHB can be produced in the seed of rape to 8% dwt without obvious deleterious effects on plant growth and germination (Houmiel et al., 1999). Thus, the goal of producing adequate level of PHA in crops without yield penalty appears realistic.
The success of using transgenic plants as a source of novel material will depend not only on the production levels achieved but also on whether the polymers can be extracted efficiently, economically, and ecologically from crops. Although a number of strategies have been described in the literature for the extraction of PHA, further work is required to validate these extraction processes in the context of large-scale production in plants (Poirier, 2001).
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