I am building two large box gardens - 16 by 4 ft. and about 3 ft deep. I was wondering about how concerned I should be about slowing the wood rotting over time. I am using redwood. I was thinking of lining the interior with plastic, but I don't want it to leach anything toxic into the soil. Do I need to be consider a lining, and if so, is there a kind of plastic that is recommended?
Lining won't really help you much. Some, but not much. Plastics also deteriorate with UV exposure fairly quickly.
Do you have termites? If so, you might consider concrete block or similar instead. Heartwood is +/- more resistant than sapwood, but certainly not termite-proof. If you're in a termite zone, sooner or later, they'll find your wood in contact with soil. Likewise, sapwood is not nearly as resistant to decay as heartwood, and old growth is more resistant then second or third growth.
Here's the Forest Products Lab book on wood properties... I think you want Chapters 3 and 13, especially table 3-10 and the map in ch. 13. Depending on what you're buying, you may be paying premium prices for a product that won't do as well as something less expensive.
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