There has been some recent success in this area. Still, they may not be useful as vaccines against cholera. The reason is that cholera and E. coli LT are not only good antigens, they are also good adjuvants, increasing the immune response to other antigens that either are given at the same time or are physically connected to the toxins. This may be because the volunteers were already immunologically tolerant to the potato antigens. For this reason, work is now being done on transgenic bananas.
Bananas have their own problems.
The researchers also don't think local villagers will grow these transgenic vaccine bananas in the garden outside their hut. They would be grown in greenhouses in each country where trained workers monitor the genetics and growth of the plants and other trained people administer controlled amounts of the banana vaccine. The most likely success story for edible vaccines will be with viral capsid proteins that aggregate into particulate structures. These particles are much better immunogens than soluble proteins and may not require an adjuvant like cholera toxin. Encouragingly, research has shown that some plant viruses cause the production of antibodies when they are fed to mice.