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.

The relationship between plants and associated soil microorganisms plays a major role in ecosystem functioning. Plant–bacteria interactions involve complex signaling pathways regulating various processes required by bacteria to adapt to their fluctuating environment. The establishment and maintenance of these interactions rely on the ability of the bacteria to sense and respond to biotic and abiotic environmental signals. In this context, MarR family transcriptional regulators can use these signals for transcriptional regulation, which is required to establish adapted responses. MarR-like transcriptional regulators are essential for the regulation of the specialized functions involved in plant–bacteria interactions in response to a wide range of molecules associated with the plant host. The conversion of environmental signals into changes in bacterial physiology and behavior allows the bacteria to colonize the plant and ensure a successful interaction. This review focuses on the mechanisms of plant-signal perception by MarR-like regulators, namely how they (i) allow bacteria to cope with the rhizosphere and plant endosphere, (ii) regulate the beneficial functions of Plant-Growth-Promoting Bacteria and (iii) regulate the virulence of phytopathogenic bacteria.

The climate crisis presents an urgent challenge for Earth’s living creatures and the habitats in which they have been adapted to thrive. Climate-related stress presents risks to microorganisms, the stability of the functions they provide, and their maintenance of beneficial interactions with their hosts and ecosystems. Microbes move across the continuum of anthropogenic influence on Earth’s ecosystems, from pristine to human-managed to fully urbanized environments. Because microbial feedback within and across this continuum exists at multiple, connected scales from molecules to ecosystem-level processes, predicting microbial responses to climate stress and their potentially wide-ranging consequences remains difficult. Here, we discuss the broad implications of microbial and microbiome responses to climate change as they interface with human, plant, and ecosystem health. For each section on human, plant and ecosystem health, we briefly discuss the state of knowledge for each and follow with proposed future research, including some directions that are promising but require more work to evaluate. We end by considering overarching microbial ecology research needs across these systems and microbial solutions under investigation as possible climate-resilient interventions to maintain human, plant, and ecosystem health. This work draws on diverse expertise to identify broad research directions across typically separated disciplines and builds a holistic framework for considering their interrelationships.

Extracellular vesicles (EVs) from phytobacteria are emerging key ecological actors of plant–bacteria interactions. They can promote colonization of the host by delivering virulence factors and cell wall-degrading enzymes. Extracellular vesicles also modulate the plant immunity by transporting compounds that either induce or suppress plant defense responses. Furthermore, they mediate interspecies and inter-kingdom communication, influencing microbial community dynamics. This review highlights the involvement of bacterial EVs in many parts of the plant–bacteria dialog. We emphasize the need for studying EVs from phytobacteria to better understand plant–bacteria interactions and the possible use of bacterial EVs in a One Health context.

Rhizobia associate with legumes and induce the formation of nitrogen-fixing nodules. The regulation of bacterial redox state plays a major role in symbiosis, and reactive oxygen species produced by the plant are known to activate signaling pathways. However, only a few redox-sensing transcriptional regulators (TRs) have been characterized in the microsymbiont. Here, we describe SydR, a novel redox-sensing TR of Sinorhizobium meliloti that is essential for the establishment of symbiosis with Medicago truncatula . SydR, a MarR-type TR, represses the expression of the adjacent gene SMa2023 in growing cultures, and this repression is alleviated by NaOCl, tert- butyl hydroperoxide, or H ₂ O ₂ treatment. Transcriptional psydR-gfp and p SMa2023- gfp fusions, as well as gel shift assays, showed that SydR binds two independent sites of the sydR -SMa2023 intergenic region. This binding is redox-dependent, and site-directed mutagenesis demonstrated that the conserved C16 is essential for SydR redox sensing. The inactivation of sydR did not alter the sensitivity of S. meliloti to NaOCl, tert -butyl hydroperoxide, or H ₂ O ₂ , nor did it affect the response to oxidants of the roGFP2-Orp1 redox biosensor expressed within bacteria. However, in planta , Δ sydR mutation impaired the formation of root nodules. Microscopic observations and analyses of plant marker gene expression showed that the Δ sydR mutant is defective at an early stage of the bacterial infection process. Altogether, these results demonstrated that SydR is a redox-sensing MarR-type TR that plays a key role in the regulation of nitrogen-fixing symbiosis with M. truncatula . IMPORTANCE: The nitrogen-fixing symbiosis between rhizobia and legumes has an important ecological role in the nitrogen cycle, contributes to nitrogen enrichment of soils, and can improve plant growth in agriculture. This interaction is initiated in the rhizosphere by a molecular dialog between the two partners, resulting in plant root infection and the formation of root nodules, where bacteria reduce the atmospheric nitrogen into ammonium. This symbiosis involves modifications of the bacterial redox state in response to reactive oxygen species produced by the plant partner. Here, we show that SydR, a transcriptional regulator of the Medicago symbiont Sinorhizobium meliloti , acts as a redox-responsive repressor that is crucial for the development of root nodules and contributes to the regulation of bacterial infection in S. meliloti/Medicago truncatula symbiotic interaction.

Bacterial extracellular vesicles (EVs) constitute a key driver of interspecies and inter-kingdom communication, and shape bacterial ecology, yet their role as a dynamic delivery system remains underexplored. Here, we show that the plant pathogen Agrobacterium fabrum C58 modulates its EVs in response to virulence-inducing conditions. Our multi-omics analysis revealed that these virulence-state EVs are significantly enriched in effectors from the Type IV secretion system and toxins from the Type VI secretion system, which were previously known to be delivered by conventional contact-dependent mechanisms. We demonstrate that these EVs can directly transfer virulence effectors into plant host cells, enhancing tumor formation. Furthermore, we show that these EVs can interact with and influence the development of several environmental bacteria. Finally, A. fabrum C58 EVs elicit distinct plant host metabolome responses compared to whole cells. Our findings establish EVs as a crucial and dynamic component of bacterial virulence and inter-kingdom communication, providing a new perspective on how bacteria adapt to and manipulate their environment.