Well although it hasnt been done, it is totally possible.
a friend of mine who is a biology professor at UC riverside specializes in spiders and their silks. Because of the amazing properties of spider silks (tensile strength, flexiblity, elasticity, etc.) some people want to find ways to utilize the silk. unfortunatley its really hard to extract the silk from the individual spiders.
to solve this problem, she put spider silk producing DNA into a tomato plant and that tomato plant produces spider silk proteins at from every cell of the plant. the same could be done with THC.
having a THC producing tomato would be easy to do in comparison to other trans-species DNA transplanting that has been done (ie. spider silk-tomatoes, or glow in the dark mice)
just fyi: My 9th grade brother's bio class did a DNA substitution to make this bacteria glow. (granted this is much easier than the aforementioned examples due to the single celled nature of the bacteria)
[FONT=Arial, Helvetica, sans-serif][FONT=arial,helvetica,sans-serif]
This is a simplified explenation of DNA substitution[/FONT]
[FONT=Arial, Helvetica, sans-serif]Biochemically isolate all the required enzymes for the production of THC.[/FONT]
[FONT=arial,helvetica,sans-serif]
Step Two:[/FONT]
[FONT=arial,helvetica,sans-serif]Perform N-terminal sequencing on isolated enzymes, design degenerate PCR (polymerase chain reaction) primers and amplify the genes. [/FONT]
[FONT=arial,helvetica,sans-serif]
Step Three:[/FONT]
[FONT=arial,helvetica,sans-serif]Clone genes into an agrobacterial vector by introducing the desired piece of DNA into a plasmid containing a transfer or T-DNA. The mixture is transformed into Agrobacterium tumefaciens, a gram negative bacterium. [/FONT]
[FONT=arial,helvetica,sans-serif]
Step Four:[/FONT]
[FONT=arial,helvetica,sans-serif]Use the Agrobacterium tumefaciens to infect citrus plants after wounding. The transfer DNA will proceed to host cells by a mechanism similar to conjugation. The DNA is randomly integrated into the host genome and will be inherited. [/FONT]
[/FONT]