A “double agent” peptide found by molecular biologists at the University of Massachusetts Amherst may open doors to improving crop yields without increasing fertilizer use.  The peptide, found in alfalfa, was discovered by scientists who study nitrogen-fixing bacteria in plants.  Early reports indicate that alfalfa may be able to use the nitrogen-fixing bacteria, rhizobia, more efficiently.

Lead author and postdoctoral researcher Minsoo Kim and professor Dong Wang of UMass Amherst’s biochemistry and molecular biology department explain that legumes attract nitrogen-fixing bacteria to their roots from the surrounding soil. Once inside the host plant, rhizobia form nodules on its roots and the plant starts to transform the bacteria into their nitrogen-fixing state. In return for borrowing the rhizobia’s essential enzymes that turn nitrogen into useful ammonia, the plant gives the bacteria fixed carbon, the product of photosynthesis.

In alfalfa, this transformation of bacteria is called differentiation, which Wang likens to domestication, because it makes the bacteria reliant on their plant host. “They are no longer wild and able to live outside the plant,” he says. “I think of it as analogous to domestication of animals by humans.” He adds, “Bacteria that can no longer proliferate as free-living individuals are a bit like slaves at that point, living to serve the plant.”

At the molecular level, plant peptides found exclusively in the nodule, known as NCR peptides, act on the bacteria in the differentiation process. By studying this differentiation process in an alfalfa-clover, Medicago truncatula, the researchers discovered that one of these peptides, DNF4, also known as NCR211, can act as a sort of double agent, Wang says. DNF4 supports nitrogen-fixing bacteria when inside the plant, but its actions can kill free-living bacteria outside.

“At first sight, it may appear perplexing that DNF4/NCR211 supports the survival of differentiating bacteria in plants while also blocking free-living bacteria from forming colonies in culture,” Wang and Kim write.

The study is far from solved, but the discovery may be the beginning of improving legume crops without using more fertilizer, especially soybeans.  Next scientists will be taking a look at why the peptide helps bacteria inside the plant but kills what lives outside it.  Read more about their work here.