Rhizobial peptidase HrrP cleaves host-encoded signaling peptides and mediates symbiotic compatibility

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In the nitrogen-fixing symbiosis between legume hosts and rhizobia, the bacteria are engulfed by a plant cell membrane to become intracellular organelles. In the model legume Medicago truncatula, internalization and differentiation of Sinorhizobium (also known as Ensifer) meliloti is a prerequisite for nitrogen fixation. The host mechanisms that ensure the long-term survival of differentiating intracellular bacteria (bacteroids) in this unusual association are unclear. The M. truncatula defective nitrogen fixation4 (dnf4) mutant is unable to form a productive symbiosis, even though late symbiotic marker genes are expressed in mutant nodules. We discovered that in the dnf4 mutant, bacteroids can apparently differentiate, but they fail to persist within host cells in the process. We found that the DNF4 gene encodes NCR211, a member of the family of nodule-specific cysteine-rich (NCR) peptides. The phenotype of dnf4 suggests that NCR211 acts to promote the intracellular survival of differentiating bacteroids. The greatest expression of DNF4 was observed in the nodule interzone II-III, where bacteroids undergo differentiation. A translational fusion of DNF4 with GFP localizes to the peribacteroid space, and synthetic NCR211 prevents free-living S. meliloti from forming colonies, in contrast to mock controls, suggesting that DNF4 may interact with bacteroids directly or indirectly for its function. Our findings indicate that a successful symbiosis requires host effectors that not only induce bacterial differentiation, but also that maintain intracellular bacteroids during the host-symbiont interaction. The discovery of NCR211 peptides that maintain bacterial survival inside host cells has important implications for improving legume crops.nitrogen-fixing symbiosis | Sinorhizobium meliloti | Medicago truncatula | legume | NCR antimicrobial peptides

Section: Discussionmentioning

confidence: 99%

An antimicrobial peptide essential for bacterial survival in the nitrogen-fixing symbiosis

Kim

Chen

et al. 2015

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124

“…Rhizobial factors such as NCR-degrading host-range restriction peptidases (HrrPs) play roles in late-stage compatibility (21). We suspected that the symbiotic effects of the NFS1 alleles might be altered by expressing in Rm41 the HrrP peptidase from S. meliloti B800 (21).…”

Section: Significancementioning

confidence: 99%

Microsymbiont discrimination mediated by a host-secreted peptide in Medicago truncatula

Yang

Wang

Fedorova

et al. 2017

Self Cite

112

100

The legume-rhizobial symbiosis results in the formation of root nodules that provide an ecological niche for nitrogen-fixing bacteria. However, plant-bacteria genotypic interactions can lead to wide variation in nitrogen fixation efficiency, and it is not uncommon that a bacterial strain forms functional (Fix + ) nodules on one plant genotype but nonfunctional (Fix − ) nodules on another. Host genetic control of this specificity is unknown. We herein report the cloning of the Medicago truncatula NFS1 gene that regulates the fixation-level incompatibility with the microsymbiont Sinorhizobium meliloti Rm41. We show that NFS1 encodes a nodulespecific cysteine-rich (NCR) peptide. In contrast to the known role of NCR peptides as effectors of endosymbionts' differentiation to nitrogen-fixing bacteroids, we demonstrate that specific NCRs control discrimination against incompatible microsymbionts. NFS1 provokes bacterial cell death and early nodule senescence in an allele-specific and rhizobial strain-specific manner, and its function is dependent on host genetic background.legumes | nodulation | nitrogen fixation specificity | symbiosis persistence | NCR peptides P lants of the legume family can supply their own nitrogen needs through symbioses with nitrogen-fixing soil bacteria called rhizobia. This symbiotic interaction commences when the host perceives rhizobial lipo-chitooligosaccharides known as nodulation (Nod) factors and initiates development of nodule primordia that become infected by the rhizobia (1). Infection of most legumes, including the model legume Medicago truncatula, starts in root hairs and involves formation of plant-made tubular structures known as infection threads (2). Infection threads direct bacteria to these primordia, where the rhizobia are released into the cytoplasm of host cells. During this process, the bacteria become surrounded by a host membrane, and these membrane compartments containing rhizobium are named symbiosomes. Subsequently, the rhizobia differentiate into nitrogen-fixing bacteroids (3).The legume-rhizobial symbiosis shows a high level of specificity, occurring at both species and genotypic levels (4, 5). Incompatible interactions at initial stages of the association can block bacterial infection and nodule organogenesis. This incompatibility can be caused by failed Nod factor or exopolysaccharide recognition (6-9) or by induced plant immune responses (9-11). Symbiotic incompatibility also takes place at later stages of nodule development, resulting in the formation of infected but nonfunctional nodules (12,13). This latter situation is well-documented in the Medicago-Sinorhizobium symbiosis, in which the bacteria undergo terminal differentiation (14). We previously screened a core collection of Medicago accessions using multiple Sinorhizobium meliloti strains, evaluating many host-strain combinations (13). In that experiment, ∼40% of the plant-strain combinations produced small, white infected nodules that were defective in nitrogen fixation (Fix − ) whereas only ∼2% resulte...

“…This region encompasses, among other genes, Medtr8g465280, which encodes an NCR peptide (17). Because NCRs have been shown to be host effectors that mediate terminal bacteroid differentiation required for nitrogen fixation in the Medicago-Sinorhizobium symbiosis (18,19,(22)(23)(24), we postulated that Medtr8g465280 was a candidate gene of NFS2.…”

mentioning

confidence: 99%

Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula

Wang

Yang

Liu

et al. 2017

119

Legumes engage in root nodule symbioses with nitrogen-fixing soil bacteria known as rhizobia. In nodule cells, bacteria are enclosed in membrane-bound vesicles called symbiosomes and differentiate into bacteroids that are capable of converting atmospheric nitrogen into ammonia. Bacteroid differentiation and prolonged intracellular survival are essential for development of functional nodules. However, in the Medicago truncatula-Sinorhizobium meliloti symbiosis, incompatibility between symbiotic partners frequently occurs, leading to the formation of infected nodules defective in nitrogen fixation (Fix − ). Here, we report the identification and cloning of the M. truncatula NFS2 gene that regulates this type of specificity pertaining to S. meliloti strain Rm41. We demonstrate that NFS2 encodes a nodule-specific cysteine-rich (NCR) peptide that acts to promote bacterial lysis after differentiation. The negative role of NFS2 in symbiosis is contingent on host genetic background and can be counteracted by other genes encoded by the host. This work extends the paradigm of NCR function to include the negative regulation of symbiotic persistence in host-strain interactions. Our data suggest that NCR peptides are host determinants of symbiotic specificity in M. truncatula and possibly in closely related legumes that form indeterminate nodules in which bacterial symbionts undergo terminal differentiation.legumes | rhizobial symbiosis | nitrogen fixation | symbiotic specificity | NCR peptides