Symbioses and Other Suggestions

I'm teaching Economically Important Plants this fall, and, needless to say, I want to make it the coolest class ever. Suggestions are welcome for activities, content, and readings that could help make it so.
I'm thinking of having one day (generally during the agriculture section) on soil and symbioses. I'll have them review nutrient cycles in advance (I think students really need to understand nitrogen in order to get fertilizer in order to understand any ag. issues) and we'll talk about nitrogen fixation (probably using either soybeans or peanuts for case study, possibly the cool bacteria (Anabaena?) that fixes nitrogen with the waterferns (azolla?) in rice paddies). I also want to talk about mycorrhizal associations and pollination.
At the moment, the very cool pollination associations I can think of (yucca and yucca moth, monarchs and milkweeds, bats and saguaros aren't heavy on economic importance. Honey bees are plenty important, but not as useful for cool co-evolution discussions.
Ideas? Vanilla orchids couldn't be cultivated elsewhere for lack of a very specific bee: buzz pollination and the solanaceae?
Any specific mycorrhizal associations that are interesting?
Other soil symbioses?