Transcriptional regulation of protein trafficking machinery in the legume-rhizobia symbiosis

Authors: Arther, Christina; SUN, JULIE - University Of Massachusetts; WANG, DONG - University Of Massachusetts

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2026
Publication Date: 3/12/2026
Citation: Arther, C.M., Sun, J., Wang, D. 2026. Transcriptional regulation of protein trafficking machinery in the legume-rhizobia symbiosis. Molecular Plant-Microbe Interactions. https://doi.org/10.1094/MPMI-07-25-0092-R.
DOI: https://doi.org/10.1094/MPMI-07-25-0092-R

Interpretive Summary: Nitrogen is often a limiting nutrient for plant growth. The air that we breathe is about 78% nitrogen, but this source of nitrogen is mostly unavailable for plants to use. However, Medicago truncatula and other legume plants such as alfalfa and soybean can access this source of nitrogen. These types of plants associate with soil bacteria that supply the plant with nitrogen from the air. The bacteria are contained within the cells of a special organ on the plant root. This relationship, called the legume-rhizobia symbiosis, relies on a very accurate exchange of products and information from the plant to the bacteria and vice versa. The plant delivers many proteins to these bacteria; the accuracy of this delivery is important for the success of the relationship, and the amount of nitrogen that the bacteria provide the plant. Despite the importance of accurate protein delivery from the plant to the bacteria, how the plants accomplish this has been largely unexplored. In this study, we investigated how the plant coordinates the accurate delivery of proteins to their bacteria partners. We found that the plant uses the same methods for protein delivery to the bacteria as they use in other cells, but the protein delivery machinery used in the symbiosis is controlled in a very specific and unique way. Without this tight regulation of the protein delivery machinery in the plant cell, the bacteria do not supply the plant with nitrogen. This study adds to the body of research that will be important for the future of agriculture, namely, engineering crops such as corn to utilize nitrogen from the air like legumes can.

Technical Abstract: The model legume Medicago truncatula delivers nodule-specific cysteine-rich peptides to the intracellular bacteria within nodules to coerce the microbe into terminal differentiation, which coincides with nitrogen fixation in this species. Inside the host cell, the anterograde protein trafficking pathway is repurposed toward a new compartment, the symbiosome. Precise protein delivery within the nodule is critical to the success of the symbiosis in M. truncatula; without it, nodules form but do not fix nitrogen. For example, when the plant lacks a functional DNF1, the nodule-specific 22 kDa subunit of the signal peptidase complex (SPC), the intracellular bacteria are under-differentiated, leading to defective nitrogen fixation. This study shows that DNF1 became specialized in symbiosis through its nodule-specific expression, and we identified cis-elements that are crucial for that transcriptional control. Furthermore, we identified the nodule-specific SPC catalytic subunit and demonstrated that CRISPR/Cas9-induced mutation of this gene causes a symbiosis defect phenocopying the dnf1 mutant. These results suggest that a dedicated signal peptidase complex in the nodule is co-opted for symbiosis through transcriptional regulation.