Study Reveals How Legume Plants Select Their Microbial Partners

Scientists have identified how legume plants choose specific soil bacteria, a discovery that could lessen agriculture's reliance on chemical fertilizers.

The findings explain how soybeans and alfalfa allow only compatible nitrogen-fixing bacteria, known as rhizobia, to form root nodules, which convert air nitrogen into plant-usable nitrogen.

The study, published in the journal Science, was led by Jeremy Murray and Zhang Yu of the Center for Excellence in Molecular Plant Sciences at the Chinese Academy of Sciences.

Legume roots release chemical compounds known as flavonoids into the surrounding soil. The researchers found that NodD, a bacterial protein, detects these flavonoid signals and triggers genetic processes related to plant-microbe symbiosis.

The team found a novel two-pocket arrangement in which NodD binds flavonoids using high-resolution structural analysis. Minor variations in key protein regions determine which flavonoids different rhizobial species respond to, explaining why some bacteria are limited to certain plant hosts.

By modifying these regions, the researchers were able to reconfigure the NodD from an incompatible rhizobia so that it could be used to form an effective nitrogen-fixing partnership.

The authors suggested that this highly specific recognition system evolved over millions of years to prevent mismatches among multiple plant species coexisting in the same environment.

The findings may help develop nitrogen-fixing bacteria for agriculture, reducing fertilizer use and eventually extending natural nitrogen fixation to non-legume crops like rice and maize.