Transgenic plants, plant material, plant cells, and genetic constructs for synthesis of biopolymers, for example polyhydroxyalkanoates (“PHA”) are provided. In one embodiment, the transgenic plants synthesize polyhydroxybutyrate (“PHB”). In one embodiment the transgenic plant encodes siRNA for one or more of the genes encoding enzymes for producing PHA. In a more preferred embodiment, the siRNA expression is under the control of an inducible regulatory element. In another embodiment, the transgenic plant contains transgenes that encode expression enzymes that will degrade the polymer. In a preferred embodiment, the expression of these enzymes is under the control of a germination specific, inducible, or minimal promoter. In another embodiment, the transgenic plant contains transgenes encoding enzymes that increase carbon flow for polymer synthesis. In a preferred embodiment, these transgenes encode enzymes that increase carbon flow in the Calvin Cycle.