We could soon be harvesting anti-viral antibodies from tobacco plants

It's a classic plant model system, and is particularly amenable to transformation. You can do transient assays by loading your construct into agrobacterium, and then simply squirting that onto some leaves. Generally you include some markers that shows you both what tissue is transfected, and also something that indicates whatever activity you're looking for.

With most plants, you have to do tissue culture and transform that material, and then coax that into a viable plant. That gives you a (potentially) stable line, but it's a lot of work. Benthy is quick.

In other words, there is lots of experience with transformation, and being a non-food crop, has fewer regulatory issues.