Soybeans solve the free-riders problem:

“A new study shows that soybean plants can apply sanctions against symbiotic bacteria when the bugs don’t deliver their fair share of nitrogen.”

“The work… portrays symbiotic relationships “not as a simple, friendly interaction where every party happily gains, but as trade with a dark side: ‘Provide the resource I require and I will reciprocate; do not, and suffer dire consequences.'”

BRIC: The Dark Side of Cooperation
[Thanks Fiona…]