Transcriptional Regulation of Protein Trafficking Machinery in the Legume-Rhizobia Symbiosis

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 DNF1, the nodule-specific 22 kDa subunit of the signal peptidase complex (SPC), the intracellular bacteria fail to fully differentiate, leading to defective nitrogen fixation. The present study shows that DNF1 became specialized in symbiosis through its nodule-specific expression, and we identified nodule-specific 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 which phenocopies the dnf1 mutant. These results suggest a dedicated signal peptidase complex in the nodule is co-opted for symbiosis through transcriptional regulation.