Sorry, but if they were biodegradable, they weren't plastic. Some plastics are partly photodegradable, but that takes ages and doesn't work if they're buried. Plastics presented as biodegradable just get broken down into smaller and smaller pieces, but the plastic itself remains the same kind of plastic it started out as. They end up small enough to become part of the soil and then absorbed by plants, which you then eat. Wouldn't surprise me if there are already plastic kidney stones.
I know that there has been work on creating bacteria specifically to convert plastics into their constituent elements/compounds, but they could mutate into God knows what and they'd probably have toxic by products anyway so I'm not in favour of that.
Interesting. Didnít know about the bacteria, I will have a read about that.