We all know beans are a magical fruit, but most of their wonder comes from a hidden source! The truth is that beans couldn’t do it without bacteria. Leguminous plants, like beans, lentils, and peas, form a mutually beneficial symbiosis with rhizobial bacteria. Rhizobia are a phylogenetically diverse group of soil bacteria that are able to establish a nitrogen fertilizer factory in their roots. This process is called nitrogen fixation, and it involves the plant providing carbohydrates to the bacteria, and the bacteria converting atmospheric nitrogen into ammonium that the plant can use for protein synthesis. This process reduces the need for synthetic nitrogen fertilizers. One example of rhizobial bacteria that engages symbiotically with legumes is the bacterium Sinorhizobium meliloti. S. meliloti is a soil bacterium that engages in a nitrogen-fixing symbiosis with leguminous plants of the genera Medicago, Melilotus, and Trigonella. In this way, S. meliloti holds an essential role in the nitrogen cycle.
Figure 1. Images of Sinorhizobium meliloti & Medicago truncatula.
The symbiotic relationship between S. meliloti and Medicago species relies on the development of root nodules. For the bacterium, the nodules provide a perfect home, and for the plant, the nodules allow them to produce nitrogen, enhancing their growth. What are these nodules and how are they built?
There is extreme specificity in legume symbiosis, for both host and symbiont. This means that there are typically a limited number of different species that either the host or symbiont associates with. This is important because plant roots exude specific phenolic compounds (secondary metabolites that engage in plant defense systems) for which the relevant rhizobia has receptors. The rhizobia then expresses its nodulation or nod factors, for which the plant has specific receptors. It is the binding of the nod factor that induces root hair modification and the subsequent formation of an infection thread into the cortex of the root itself (Fig. 2). Once the rhizobia enter the nodule cells, they physiologically differentiate (the process of unspecialized cells becoming specialized to conduct distinct functions) due to a lack of oxygen. This is called the bacteroid state, and it allows for the rhizobia to begin the process of nitrogen fixation.
Figure 2. Depiction of the formation of bacteroids (Atherton & Breiding, 2026).
Nicoud et al., 2021 dives into the complex world of these underground rhizobial dynamics. This study explores S. meliloti’s ability to survive within the root nodules of a species of legume within the Medicago family, M. truncatula (barrelclover), despite the antimicrobial peptides, called nodule-specific cysteine-rich peptides (NCRs), that the host plant produces. NCRs are what differentiate the bacteria into the bacteroid nitrogen-fixing state, but they can also kill the bacteria by damaging their cell membranes. This study shows that S. meliloti utilizes several protective mechanisms to resist NCR destruction, including peptide transporters, modifications to its bacterial membrane structures, and stress response regulators.
To test this, S. meliloti mutants lacking specific genes that were thought to be tied to peptide transporters (yejABEF), outer membrane structure (lpsB and lpxXL), and stress response (rpoH1) were created. Both wild-type and mutant M. truncatula plants were infected with bacterial mutants and wild-type bacteria. Specifically, they looked at the M. truncatula dnf1 mutant, which, according to previous studies, is reported to block NCR transport to the infecting rhizobia. Therefore, comparing dnf1 mutant and wild-type M. truncatula shows that NCRs are involved in this symbiosis, whereas comparing the S. meliloti wild-type and mutants shows what protective mechanisms are responsible for guarding against NCRs.
S. meliloti mutants were exposed to NCR peptides in vitro, and showed increased sensitivity compared to the wild-type (or S. meliloti individuals with intact hypothesized protective mechanisms). Researchers then infected both dnf1 mutant and wild-type M. truncatula plants with these bacterial mutants. Here, they examined differentiation in nodule formation, nitrogen fixation capacity, and bacterial differentiation using confocal microscopy and flow cytometry (laser-based techniques that examine the structural characteristics of cells or particles). Examining the morphological structure of nodules as well as the amount of membrane permeabilization allows for the assessment of how mutant and wild-type S. meliloti bacteria with different genetic protective capacities respond to NCRs. We can also prove that NCRs are at work here, with different levels of cell membrane damage under dnf1 mutant and wild-type M. truncatula plants.
To investigate bacteroid membrane permeability between S. meliloti wild-type and mutants, the researchers conducted live-dead staining of S. meliloti nodule sections in symbiosis with both wild-type (Fig. 3A) and dnf1 mutant (Fig. 3B) M. truncatula.
Figure 3. Symbiotic phenotypes of Sinorhizobium meliloti mutants during symbiosis with (A) wild-type or (B) dnf1 mutant Medicago truncatula (Nicoud et al., 2021).
For both Fig. 3A and 3B, the top row shows full nodule sections, while the bottom row shows enlarged images of symbiotic cells. Nodule bacteria with a green fluorescence signal suggest a well preserved membrane, while nodule bacteria with a red fluorescence signal suggest a highly permeable membrane (Fig. 3).
As previously reported, we can see that the nodule bacteria of all the S. meliloti mutants have high membrane permeability in comparison to the nodule bacteria of the wild-type, which formed symbiotic cells infected with green-stained elongated bacteroids (Fig. 3). Also to be expected was that nodules infected with the bacA mutant formed cells with small undifferentiated red-stained bacteria. BacA is a bacterial broad-specificity peptide uptake transporter that helps protect the endosymbionts against the toxic activity of NCRs. The removal of this transporter therefore resulted in damage to the cell membrane.
An unexpected piece of the puzzle was the yejA, yejE, and yejF mutants, which were able to form nitrogen-fixing bacteroids despite being substantially altered compared to wild-type bacteroids. Most of the bacteroids that formed after infection with these three mutants were stained red, pointing to a high membrane permeability, and therefore greater destruction by NCRs (Fig. 3A). In terms of the other mutants, the lpxXL and rpoH1 mutants formed a handful of fully differentiated bacteroids although most of the nodule bacteria were undifferentiated. The lpsB mutant nodule bacteria did not differentiate.
Overall, we can observe in this figure a strongly enhanced membrane permeability in the nodule bacteria of the mutants. This is dependent on the transport of NCRs to the endosymbionts.
The research done in this paper allowed for the linking of specific bacterial genes to NCR resistance, membrane protection, and successful symbiotic function. S. meliloti relies on multiple coordinated protective systems to survive NCR antimicrobial peptides and successfully differentiate into nitrogen-fixing bacteroids within Medicago nodules, allowing for a mutualistic symbiotic relationship.
Having an understanding of the inner-workings of nitrogen fixation is important because it aids in our effective utilization of this symbiosis for agriculture. Nitrogen-fixing bacteria are essential for plant growth and are widely recognized as a vital biotechnological tool with a high potential to improve agricultural productivity.
About the Authors:
Madigan Breiding ‘26 is a Biology and Art Studio double major. She is interested in molecular biology and genetics, and worked in the Drummond lab at Mount Holyoke College. She is very interested in the overlap between science and visual arts, specifically in the role of science communication. She loves painting, pranking, whale watching, and blasting music in her car.
Charlotte Atherton ‘26 is a Biology and Environmental Studies double major. They are interested in plant ecology and phenology in the context of climate change, which they were able to explore through their time in the Drummond lab at Mount Holyoke College. In their free time, they enjoy hiking, playing guitar, and watching bad movies.
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