Plants with nitrogen-fixing bacteria in their roots make a good test bed for probing the give-and-take of biological partnerships. The bacteria take carbohydrates and oxygen from the plant; in return, the microbes snag inert atmospheric nitrogen and convert it into a form the plant's cells can use. Plants that harbor these so-called nitrogen-fixing bacteria often host several genetically distinct strains with differing outputs. Churning out a lot of fixed nitrogen takes a toll on bacteria, so making less of the nutrient for its host should help the bacteria thrive.
Why then haven't low-output strains of nitrogen-fixing bacteria taken over?