ABSTRACT
Polyhydroxyalkanoates (PHAs) are a family of polyesters produced in bacteria as a carbon and energy reserve. Depending on their chemical structure, PHAs can have properties ranging from stiff and brittle plastics to elastomers and rubbers. These polymers are completely biodegradable in the environment, being metabolised by microorganisms to carbon dioxide and water. There is a growing interest in using PHAs to replace synthetic plastics in consumer products. Although bacterial fermentation has been used to produce PHAs on a small industrial scale, costs associated with fermentation are too high, making the biological polymer too expensive in comparison to synthetic plastics. In view of producing PHAs on a large scale and at low cost, the possibility of producing these polymers in plants was explored. The poly-hydroxybutyrate (PHB) biosynthetic pathway has been created in the plant Arabidopsis thaliana using genes from the bacterium Alcaligenes eutrophus. Expression of the PHB pathway in the cytoplasm of plant cells led to a low level of PHB accumulation and was deleterious to plant growth. In contrast, expression of the PHB pathway in the plastid led to PHB accumulation of up to 14% of leaf dry weight, with no significant effects on plant growth. Synthesis of PHB in Arabidopsis opened the way for production of PHAs in crop plants. The range of PHAs synthesised in plants has recently been expanded to include small-chain-length and medium-chain-length PHA copolymers. Further progress on the production of PHAs in crops, representing a variety of plastic properties, will require synergism between knowledge of the enzymes and genes involved in PHA synthesis in bacteria, and engineering of plant metabolic pathways.
