Bean plants punish microbial partners.

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?

Toby Kiers of the University of California, Davis and her colleagues tested the idea that plants punish their less-productive microbial partners. In experiments on soybeans and a bacterial partner, the researchers exposed some parts of each plant's roots to a normal nitrogen concentration and other parts to a nitrogen-free atmosphere.

The no-nitrogen atmosphere forced the bacteria in those areas to cheat on their symbiotic deal with the soybean plant. The plants seemed to retaliate for this outrage by reducing the supply of oxygen in those sections. The forced-to-cheat bacteria reproduced only half as well as the productive bacteria did, the researchers report in the Sept. 4 Nature. This turn of events suggests that plants can impose sanctions on symbiotic partners that aren't doing their part.--S.M.