One protein holds the door open for nitrogen-fixing bacteria in legume roots

Why can peas, beans, and clover pull nitrogen straight from the air while wheat, rice, and tomatoes cannot? The answer involves a partnership between legumes and soil bacteria called rhizobia, an alliance so effective that it lets these plants essentially fertilize themselves. But the molecular details of how legumes open their cellular doors to these bacterial partners have remained surprisingly murky. A study published in Science has now identified the doorman.

SYFO2: the formin that rearranges the welcome mat

The protein is called SYFO2, a type of formin found in the roots of legumes and certain other plants. Formins are proteins that help organize the actin cytoskeleton, the network of protein filaments that gives cells their shape and enables internal transport. The research team, led by Professor Thomas Ott at the University of Freiburg, demonstrated that SYFO2 is essential for the critical transition between trapping rhizobia at the root hair surface and actually letting them inside the cell.

Here is how the process works. When a legume root hair encounters compatible rhizobia in the soil, it curls around the bacteria, physically ensnaring them. But entrapment alone is not enough. The bacteria need to enter the root cell interior, travel through an infection thread, and eventually reach newly formed root nodules where nitrogen fixation occurs. SYFO2 initiates the reorganization of the actin cytoskeleton that makes this intracellular entry possible.

Without SYFO2, the bacteria get caught but cannot get in. The door stays shut.

From catching to opening

The international team used a combination of live-cell imaging, molecular biology, and genetic knockout experiments to trace SYFO2's role. They found that the protein localizes to specific nanodomains on the cell membrane, small patches of membrane with distinct protein compositions. When rhizobia trigger signaling cascades at these sites, SYFO2 activates and begins remodeling the actin network, creating the structural conditions for bacterial entry.

The study's title captures the finding precisely: SYFO2 is a nanodomain-localized formin that gates symbiotic microbial entry. It sits at the molecular boundary between capture and invasion, and its activity determines whether the symbiosis proceeds or stalls.

The tomato experiment

Perhaps the most intriguing finding came from work with tomatoes, which are not legumes and do not naturally form nitrogen-fixing symbioses with rhizobia. Tomatoes do, however, carry their own version of the SYFO2 gene. The researchers found that by introducing NIN, a transcription factor involved in nodule formation in legumes, they could activate the tomato's dormant SYFO2.

This result matters because it suggests the genetic machinery for bacterial entry may already exist, in a latent form, in non-legume crop plants. The long-term goal of engineering nitrogen fixation into crops like wheat or rice has been a major target in agricultural biotechnology for decades. If the entry mechanism can be activated by flipping existing genetic switches rather than building one from scratch, the engineering challenge becomes considerably more tractable.

An older role in fungal symbiosis

The researchers also discovered that SYFO2 plays a role in an older, more widespread form of plant symbiosis: mycorrhizal partnerships with fungi. Most land plants, including non-legumes, form mycorrhizal associations in which fungi colonize root cells and exchange soil nutrients for plant sugars. In some of these plants, SYFO2 is required for the initial fungal entry step.

This finding suggests that legumes may have co-opted an ancient fungal-entry mechanism and repurposed it for bacterial symbiosis. The evolutionary implication is significant: the molecular toolkit for letting microbes inside plant cells was not invented from nothing for nitrogen fixation. It was borrowed and adapted.

What this does not yet solve

Activating SYFO2 in tomato is a proof of concept, not a finished product. The protein enables microbial entry, but nitrogen fixation requires an entire cascade of downstream processes: nodule formation, oxygen regulation within the nodule, and the metabolic exchange between bacterium and plant. None of these are addressed by SYFO2 activation alone.

The study was conducted in controlled laboratory conditions using model legume species and transgenic tomato lines. Whether the findings translate to field crops grown in variable soil conditions, with diverse microbial communities, remains to be tested. The genetic tools used in the study, including constitutive promoters and model organisms, would need substantial adaptation for agricultural applications.

The work was funded by Gates Agricultural Innovations through the Enabling Nutrient Symbiosis in Agriculture (ENSA) project, reflecting the scale of investment required to move from molecular discovery to crop engineering. That pipeline typically takes a decade or more.