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Plants need nitrogen to grow. Many legumes meet this need through a symbiotic relationship: they harbor bacteria that fix atmospheric nitrogen and make it available to the plant. Until now, it was largely unclear how a perennial plant regulates this symbiosis without destroying its bacterial partners. An international team led by TU Braunschweig, has now described a previously unknown mechanism: The black locust (Robinia pseudoacacia) employs a newly discovered family of small proteins that specifically “reprogram” its symbiotic bacteria for nitrogen fixation while keeping them alive. The findings are published in the prestigious journal “Science Advances.”
Nitrogen is one of the most important nutrients for plants and is therefore essential for high agricultural yields as well as for healthy forests. Although nitrogen gas (N₂) makes up about 78 percent of the air, plants cannot use it directly. Legumes such as peas, clover, and black locust rely on nodule bacteria (rhizobia): In small swellings on the roots, known as root nodules, these bacteria convert atmospheric nitrogen into ammonium that plants can use. In return, the plant supplies the bacteria with energy.
“Nitrogen fixation is one of the most important biological processes. It is central to natural ecosystems and, at the same time, reduces the need for energy-intensive fertilizers,” explains Prof. Dr. Robert Hänsch from the Institute of Plant Biology at Technische Universität Braunschweig.
In these nodules, researchers from the Technical University of Braunschweig and Southwest University in Chongqing (China) discovered an unusual group of short proteins—so-called peptides—that are particularly rich in the amino acids proline and glycine. The team calls them NPG peptides (short for “nodule-specific proline-glycine-rich peptides”).
They are produced exclusively in the nodules as soon as the bacteria colonize the root and accumulate specifically in the infected cells. When the team exposed the bacterium Mesorhizobium robiniae to one of these peptides in the laboratory, its gene activity shifted significantly toward nitrogen fixation.
“What is particularly remarkable about the newly discovered peptides is that they do not impair the nodule bacteria to the extent that they lose their ability to reproduce. This distinguishes them from all previously known peptides used by other legumes to control their symbiosis,” says Robert Hänsch.
This highlights a remarkable contrast between two strategies. Many annual legumes, such as the pea, rely on coercion: Their NCR peptides drive the bacteria into “terminal differentiation,” during which they swell, fix as much nitrogen as possible, but irrevocably lose their ability to divide. This also serves as a form of quality control, since bacteria that fail to supply nitrogen cannot exploit the nodules, which are maintained at great cost. However, it remains a one-way strategy. The black locust takes the opposite approach: its NPG peptides encourage the rhizobia to settle and fix nitrogen without forcing them into this final stage. The bacteria remain viable and capable of division, becoming permanent cohabitants. For a long-lived tree that must maintain a stable symbiosis over many growing seasons, this gentle form of cooperation is a plausible advantage.
“Until now, it was assumed that plants control their symbiotic partners primarily through highly damaging or terminally differentiating peptides. Our results now reveal an alternative pathway,” adds the corresponding author Dr. Kevin Oliphant, who designed the study. He was previously a postdoctoral researcher at the Technical University of Braunschweig and is now conducting research at the University of Oxford (United Kingdom).
Furthermore, NPG peptides have so far only been detected in black locust trees and appear to be unique among the other legumes studied. In the long term, the researchers hope that such findings will open up new approaches for more sustainable agriculture and help reduce the use of energy-intensive synthetic fertilizers.
The study involved researchers from a wide range of disciplines—from plant biology to microbiology, physical chemistry, and geomechanics—who used high-resolution computed tomography to visualize the development of the root nodules. In addition to the Technical University of Braunschweig and Southwest University in Chongqing, the Helmholtz Center for Infection Research (HZI) in Braunschweig, Helmholtz Munich, and the Leibniz HKI in Jena also contributed to the study.
Prof. Dr. Robert Hänsch
Technische Universität Braunschweig
Institut für Pflanzenbiologie
Phone: +49 531 391-5867
Email: r.haensch@tu-braunschweig.de
Bin Hu, Robert Hänsch, Kyra Grunau … Kevin D. Oliphant: “Symbiotic peptides modulate rhizobial physiology without terminal differentiation.” Science Advances (2026). DOI: 10.1126/sciadv.aed281
https://magazin.tu-braunschweig.de/pi-post/neue-peptidfamilie-steuert-stickstoff...
https://www.science.org/doi/10.1126/sciadv.aed2816
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