Polyhydroxyalkanoate synthesis in transgenic plants as a new tool to studycarbon flow through beta-oxidation

Transgenic plants producing peroxisomal polyhydroxyalkanoate (PHA) from int ermediates of fatty acid degradation were used to study carbon Row through the beta-oxidation cycle. Growth of transgenic plants in media containing f atty acids conjugated to Tween detergents resulted in an increased accumula tion of PHA and incorporation into the polyester of monomers derived from t he beta-oxidation of these fatty acids. Tween-laurate was a stronger induce r of beta-oxidation, as measured by acyl-CoA oxidase activity, and a more p otent 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 com pared to plants expressing only the PHA synthase. PHA produced in double tr ansgenic plants contained mainly saturated monomers ranging from 6 to 10 ca rbons, indicating an enhanced flow of capric acid towards beta-oxidation. T ogether, these results support the hypothesis that plant cells have mechani sms which sense levels of free or esterified unusual fatty acids, resulting in changes in the activity of the beta-oxidation cycle as well as removal and degradation of these unusual fatty acids through beta-oxidation. Such e nhanced flow of fatty acids through beta-oxidation can be utilized to modul ate the amount and composition of PHA produced in transgenic plants. Furthe rmore, synthesis of PHAs in plants can be used as a new tool to study the q uality and relative quantity of the carbon flow through beta-oxidation as w ell as to analyse the degradation pathway of unusual fatty acids.