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To be parasitic or symbiotic?

Rhizobial strains with HrrP degrade host-derived signaling peptides to become parasitic. Legumes obtain nitrogen through their symbiotic association with rhizobia, which are nitrogen-fixing bacteria. Conversion of free-living rhizobia to nitrogen-fixing bacteroids that are growth-arrested, symbiotic, and dwell in host plants involves cross-species communication through signaling molecules. Whether the interaction between the plant and the rhizobe is symbiotic or parasitic depends on these signals. Price et al. investigated the symbiotic-to-parasitic balance in the interaction between the rhizobe Sinorhizobium meliloti, with its host legume Medicago truncatula, by performing a screen for bacteria with mutations in the bacterial accessory plasmid that compromised nitrogen fixation of the host plant. The screen identified a single gene, hrrP, which encodes host range restriction peptidase (HrrP). Plants grown with bacteria that overexpressed HrrP had reduced shoot dry weight and increased yellowing of leaves, both of which are phenotypes associated with reduced nitrogen fixation. The effect of hrrP varied depending on the bacterial strain in which it was overexpressed and on the species of the host, with some plants exhibiting complete loss of nitrogen fixation to no effect. Structural modeling of HrrP enzyme based on crystal structures of M16A metallopeptidase family members identified a clamshell structure that is typical to peptidases. HrrP purified from bacteria cleaved previously identified host-encoded peptides called nodule-specific cysteine-rich (NCR) peptides. Bacterial growth and proliferation in nodules of host plants were increased in hrrP-deleted strains compared with control strains. Analysis of bacteroids with hrrP using transmission electron microscopy revealed that the middle section of individual root nodules had bacteroids with typical swollen shape, whereas sections closer to the root exhibited fragmentation, which indicated degeneration of the nodules. Thus, this study suggests that symbiotic rhizobial bacteria use HrrP to cleave host-encoded signaling peptides, thereby altering their own growth and development during nodulation. P. A. Price, H. R. Tanner, B. A. Dillon, M. Shabab, G. C. Walker, J. S. Griffitts, Rhizobial peptidase HrrP cleaves host-encoded signaling peptides and mediates symbiotic compatibility. Proc. Natl. Acad. Sci. U.S.A. 112, 15244–15249 (2015). [PubMed]