Sinorhizobium meliloti 1021 Loss-of-Function Deletion Mutation in chvI and Its Phenotypic Characteristics

Abstract

Bacterial two-component regulatory systems (TCS) are common components of complex regulatory networks and cascades. In Sinorhizobium meliloti, the TCS ExoS/ChvI controls exopolysaccharide succinoglycan production and flagellum biosynthesis. Although this system plays a crucial role in establishing the symbiosis between S. meliloti and its host plant, it is not well characterized. Attempts to generate complete loss-of-function mutations in either exoS or chvI in S. meliloti have been unsuccessful; thus, it was previously suggested that exoS or chvI are essential genes for bacterial cell growth. We constructed a chvI mutant by completely deleting the open reading frame encoding this gene. The mutant strain failed to grow on complex medium, exhibited lower tolerance to acidic condition, produced significantly less poly-3-hydroxybutyrate than the wild type, was hypermotile, and exhibited an altered lipopolysaccharide profile. In addition, this mutant was defective in symbiosis with Medicago truncatula and M. sativa (alfalfa), although it induced root hair deformation as efficiently as the wild type. Together, our results demonstrate that ChvI is intimately involved in regulatory networks involving the cell envelope and metabolism; however, its precise role within the regulatory network remains to be determined.

Mutation of the Sensor Kinase chvG in Rhizobium leguminosarum Negatively Impacts Cellular Metabolism, Outer Membrane Stability, and Symbiosis

“…In addition to confirming the role of ExoS and ChvI in symbiosis and exopolysaccharide production, they reported significant metabolic defects in the mutants, which had a 75% reduction in the number of carbon sources able to support growth compared to the wild type (2). A separate chvI deletion mutant was found to be acid sensitive and hypermotile, have an altered lipopolysaccharide (LPS) profile, and produce less poly-ß-hydroxybutyrate (PHB) (43). Furthermore, a total of 59 genes encoding members of a number of diverse functional groups, including EPS synthesis proteins, outer membrane proteins, catabolism-related proteins, and transporters, have been identified as putative targets for direct regulation by ChvI (6).…”

“…These bacteria are uniquely adapted to survive in the rhizosphere as well as intracellularly within legume plant cells (12). During symbiosis, the cell envelope of rhizobia undergoes significant changes, and several cell envelope components are critical for a successful transition from a free living cell to an intracellular bacteroid (2,4,9,12,43). While significant attention has been focused on characterizing individual cell envelope components in free-living cells and during plant infection, a major challenge moving forward is to elucidate how rhizobia integrate multiple signals from their environment to coordinate an appropriate adaptive response to their cell envelope.…”

“…The most extensively studied TCS in the Rhizobiaceae are the ChvG/ChvI system from Agrobacterium tumefaciens and the homologous ExoS/ChvI system from Sinorhizobium meliloti, where ChvG (ExoS) is the sensor kinase and ChvI is the response regulator (5,7). These TCSs play important roles in both pathogenic and beneficial host-microbe interactions (2,5,7,8,19,43). Mutation of either chvG or chvI in A. tumefaciens results in avirulence toward several different host plants, including infection of stems of Zinnia elegans and Lycopersicon esculentum and of Bryophyllum daigremontianum leaves (5).…”

“…The ChvG/ChvI TCS in A. tumefaciens positively regulates a number of acid-inducible genes, including aopB, katA, virB, and virE, leading to the hypothesis that the Agrobacterium ChvG/ChvI acts as a pH sensor that regulates virulence genes in response to the acidic conditions encountered during the early stages of infection (19). In S. meliloti, a number of studies have shown that ExoS and ChvI are required for successful symbiosis with alfalfa, the host legume (2,43,45). The ExoS/ChvI TCS in S. meliloti is required for production of succinoglycan and galactoglucan, the two symbiotically active exopolysaccharides (EPSs) (2,7,43,45).…”

“…In S. meliloti, a number of studies have shown that ExoS and ChvI are required for successful symbiosis with alfalfa, the host legume (2,43,45). The ExoS/ChvI TCS in S. meliloti is required for production of succinoglycan and galactoglucan, the two symbiotically active exopolysaccharides (EPSs) (2,7,43,45). These EPSs are required for proper infection thread initiation and elongation (7,45).…”