SummaryTransgenic plants producing peroxisomal polyhydroxyalkanoate (PHA) from intermediates of fatty acid degradation were used to study carbon¯ow through the b-oxidation cycle. Growth of transgenic plants in media containing fatty acids conjugated to Tween detergents resulted in an increased accumulation of PHA and incorporation into the polyester of monomers derived from the b-oxidation of these fatty acids. Tween±laurate was a stronger inducer of b-oxidation, as measured by acyl-CoA oxidase activity, and a more potent modulator of PHA quantity and monomer composition than Tween±oleate. Plants co-expressing a peroxisomal PHA synthase with a capryl-acyl carrier protein thioesterase from Cuphea lanceolata produced eightfold more PHA compared to plants expressing only the PHA synthase. PHA produced in double transgenic plants contained mainly saturated monomers ranging from 6 to 10 carbons, indicating an enhanced¯ow of capric acid towards b-oxidation. Together, these results support the hypothesis that plant cells have mechanisms which sense levels of free or esteri®ed unusual fatty acids, resulting in changes in the activity of the b-oxidation cycle as well as removal and degradation of these unusual fatty acids through b-oxidation. Such enhanced ow of fatty acids through b-oxidation can be utilized to modulate the amount and composition of PHA produced in transgenic plants. Furthermore, synthesis of PHAs in plants can be used as a new tool to study the quality and relative quantity of the carbon¯ow through boxidation as well as to analyse the degradation pathway of unusual fatty acids.

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