A conserved rhizobial peptidase that interacts with host-derived symbiotic peptides

Abstract

In the Medicago truncatula-Sinorhizobium meliloti symbiosis, chemical signaling initiates rhizobial infection of root nodule tissue, where a large portion of the bacteria are endocytosed into root nodule cells to function in nitrogen-fixing organelles. These intracellular bacteria are subjected to an arsenal of plant-derived nodule-specific cysteine-rich (NCR) peptides, which induce the physiological changes that accompany nitrogen fixation. NCR peptides drive these intracellular bacteria toward terminal differentiation. The bacterial peptidase HrrP was previously shown to degrade host-derived NCR peptides and give the bacterial symbionts greater fitness at the expense of host fitness. The hrrP gene is found in roughly 10% of Sinorhizobium isolates, as it is carried on an accessory plasmid. The objective of the present study is to identify peptidase genes in the core genome of S. meliloti that modulate symbiotic outcome in a manner similar to the accessory hrrP gene. In an overexpression screen of annotated peptidase genes, we identified one such symbiosis-associated peptidase (sap) gene, sapA (SMc00451). When overexpressed, sapA leads to a significant decrease in plant fitness. Its promoter is active in root nodules, with only weak expression evident under free-living conditions. The SapA enzyme can degrade a broad range of NCR peptides in vitro.

Competitiveness and symbiotic efficiency in alfalfa of Rhizobium favelukesii ORY1 strain in which homologous genes of peptidases HrrP and SapA that negatively affect symbiosis were identified

“…M. truncatula KO-mutants for the NCR169 peptide, dnf7, are unable to induce bacteroid development, suggesting that the NCR169 peptide plays a role in the symbiotic communication (Horváth et al, 2015). Likewise, sapA (symbiotic-associated peptidase), a chromosomic gene of E. meliloti was shown to encode a HrrP-like M16 peptidase that modulates symbiosis in a similar way (Benedict et al, 2021). Here, we reported the presence of these genes in Oregon-type strains, which could contribute to the understanding of their poor symbiotic efficiency.…”

“…Additionally, these proteins could be implied in a similar host restriction mechanism as the one previously described in E. meliloti species. In particular, HrrP and SapA peptidases play a role in the digestion of NCR peptides produced by the host, for instance, the indispensable peptide for bacteroid differentiation NCR169 (Horváth et al, 2015;Price et al, 2015;Benedict et al, 2021). These peptides are necessary for the correct establishment of the symbiosis E. meliloti -M. truncatula (Price et al, 2015;Benedict et al, 2021).…”

Scent of a Symbiont: The Personalized Genetic Relationships of Rhizobium—Plant Interaction

“…These genes are involved in a plethora of functions, including metabolic pathways, transporters, chemotaxis and motility, from systems spanning from the Bradyrhizobium diazoefficiens host range to S. meliloti [ 48 , 72 , 73 ]. In addition to simple gene presence/absence on the rhizobial symbiont, recent genomic and transcriptomic studies have stirred the attention on genotype-by-genotype (G × G) interactions in symbiotic outcomes [ 41 , 49 , 52 , 74 , 75 ]. Experiments carried out on M. truncatula and S. meliloti strains demonstrated that the ability to enter plant nodules increased rhizobium fitness, and plants reward more cooperative rhizobium strains by preferentially forming nodules with them, but the rewarded strains differ among plant genotypes [ 45 ].…”

Mechanisms underlying legume–rhizobium symbioses

“…As rhizobia are the recipients of host defense molecules that block their reproduction, these bacteria have evolved tolerance or resistance to these molecules. The genome of Sinorhizobium meliloti , the symbiont of M. truncatula , encodes the transporter BacA and the peptidase SapA, which can ameliorate the toxicity of NCR peptides (Haag et al, 2011; Benedict et al, 2021). Some S. meliloti strains have evolved the additional peptidase HrrP, which degrades NCR peptides (Price et al, 2015).…”