Glutaredoxins Play an Important Role in the Redox Homeostasis and Symbiotic Capacity of Azorhizobium caulinodans ORS571

Bacterial redox response factors in the management of environmental oxidative stress

Bacteria evolved to survive in the available environmental chemosphere via several cellular mechanisms. A rich pool of antioxidants and stress regulators plays a significant role in the survival of bacteria in unfavorable environmental conditions. Most of the microbes exhibit resistant phenomena in toxic environment niches. Naturally, bacteria possess efficient thioredoxin reductase, glutaredoxin, and peroxiredoxin redox systems to handle environmental oxidative stress. Further, an array of transcriptional regulators senses the oxidative stress conditions. Transcription regulators, such as OxyR, SoxRS, PerR, UspA, SsrB, MarA, OhrR, SarZ, etc., sense and transduce bacterial oxidative stress responses. The redox-sensitive transcription regulators continuously recycle the utilized antioxidant enzymes during oxidative stress. These regulators promote the expression of antioxidant enzymes such as superoxide dismutase, catalase, and peroxides that overcome oxidative insults. Therefore, the transcriptional regulations maintain steady-state activities of antioxidant enzymes representing the resistance against host cell/environmental oxidative insults. Further, the redox system provides reducing equivalents to synthesize biomolecules, thereby contributing to cellular repair mechanisms. The inactive transcriptional regulators in the undisturbed cells are activated by oxidative stress. The oxidized transcriptional regulators modulate the expression of antioxidant and cellular repair enzymes to survive in extreme environmental conditions. Therefore, targeting these antioxidant systems and response regulators could alter cellular redox homeostasis. This review presents the mechanisms of different redox systems that favor bacterial survival in extreme environmental oxidative stress conditions.

Effect of the Monothiol Glutaredoxin GrxD on 2,4-Diacetylphloroglucinol Biosynthesis and Biocontrol Activity of Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a plant root-associated bacterium that suppresses several soilborne plant diseases due to its production of the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG). The biosynthesis of 2,4-DAPG is controlled by many regulatory elements, including the global regulator of the Gac/Rsm regulon and the pathway-specific repressor PhlF. In this work, a novel genetic element grxD, which encodes the monothiol glutaredoxin GrxD, was identified and characterized in the production of 2,4-DAPG in P. fluorescens 2P24. Our data showed that the mutation of grxD remarkably decreased 2,4-DAPG production. GrxD lost its ability to alter the production of 2,4-DAPG when the active-site CGFS motif of GrxD was mutated by site-directed mutagenesis. Further studies showed that the RsmA and RsmE proteins were essential for the GrxD-mediated regulation of 2,4-DAPG and exoprotease production. In addition, our data revealed that the deletion of grxD increased the expression of phlF, which negatively regulated the production of 2,4-DAPG. In addition, the grxD mutant was severely impaired in the biocontrol effect against the bacterial wilt of tomato. Overall, our results indicated that the monothiol glutaredoxin GrxD is involved in the production of 2,4-DAPG of P. fluorescens by influencing the Gac/Rsm global signaling pathway and transcriptional regulator PhlF and is essential for the biocontrol properties.

Redox Regulation in Diazotrophic Bacteria in Interaction with Plants

Plants interact with a large number of microorganisms that greatly influence their growth and health. Among the beneficial microorganisms, rhizosphere bacteria known as Plant Growth Promoting Bacteria increase plant fitness by producing compounds such as phytohormones or by carrying out symbioses that enhance nutrient acquisition. Nitrogen-fixing bacteria, either as endophytes or as endosymbionts, specifically improve the growth and development of plants by supplying them with nitrogen, a key macro-element. Survival and proliferation of these bacteria require their adaptation to the rhizosphere and host plant, which are particular ecological environments. This adaptation highly depends on bacteria response to the Reactive Oxygen Species (ROS), associated to abiotic stresses or produced by host plants, which determine the outcome of the plant-bacteria interaction. This paper reviews the different antioxidant defense mechanisms identified in diazotrophic bacteria, focusing on their involvement in coping with the changing conditions encountered during interaction with plant partners.