The application of rhizobia-legume symbioses is a sustainable approach to alleviate water stress and restore damaged areas. In this context, three strains Bradyrhizobium sp. BA2, RDI18 and RDT46 previously isolated from root nodules of Retama dasycarpa grown in the Moroccan High Atlas Mountains, were selected to investigate their prominent drought-tolerance capacity and significant plant growth-promoting (PGP) traits under drought stress. Subsequently, we analyzed the impact of individual or combined inoculations by the three strains on R. dasycarpa responses to three water regimes (40, 70, and 100 field capacity). The three strains tolerate different concentrations of PEG 6000 and possess different PGP activities, including phosphate solubilization, production of siderophore, exopolysaccharides, and auxin, under osmotic stress. The inoculation had a positive impact on plant response under all applied water regimes as it improved shoot and root length biomass, and chlorophyll content. The water stress reduced shoot length and dry weight of all plants. However, the inoculated plants maintained the highest values. The water stress reduced the infectivity of strains BA2 and RDI18, but not strain RDT46, which is not competitive at any water regime. Furthermore, water stress had no effect on the three strains' symbiotic efficiency, whereas it increased considerably the efficiency index of strains BA2 and RDI18. Proline and protein content increased in non-inoculated plants; whereas the inoculation significantly increased the catalase activity in plants under 40 % FC. These results show that the inoculation with appropriate strains such as BA2 and RDI18, enhance plant resilience to drought season.

The continuous release of glucosinolates into the soil by Brassicaceae root exudation is a prerequisite to maintaining toxic levels of breakdown products such as isothiocyanates (ITCs). ITCs influence plant and microbial diversity in ecosystems, while fungi and Rhizobiaceae are particularly injured. Studies explaining the molecular mechanisms of the negative effects are presently limited. Therefore, we investigated the early effects of cyclic ITC goitrin on proteomes of the host and symbiotic Mesorhizobium loti in the nodules of Lotus japonicus and of free-living bacteria. In the nodules, many host proteins had a higher abundance, among them, peroxidases and pathogenesis-related PR-10 proteins functioning in the abscisic-acid-activated signaling pathway. In the microsymbiont, transporter proteins as a prominent group are enhanced; some proteins involved in N-fixation decreased. The proteomes give a report about the loss of immunity suppression resulting in the termination of symbiosis, which initiates nodule senescence. Free-living M. loti are severely damaged, indicated, i.a., by a decrease in transporter proteins, the assumed candidates for goitrin protein complex formation, and high proteolysis. The production of chicoric acid by the accompanying bacteria is inhibitory for M. loti but connected to goitrin elimination, as confirmed by mass spectrometric (MS) analysis. In summary, the nodulation process is severely affected by goitrin, causing nodule dysfunction and failed nodule development. N deficiency conditions leads to yellowish leaves and leaf abscission.

A new sustainable approach was aimed to explore the damage caused to legume grown in cadmium (Cd) polluted soil. Owing to the importance of chickpea (Cicer arietinum L.) as a source of protein which is exposed to Cd that imposes severe health hazards. A greenhouse pot experiment was designed to evaluate the potential of Rhizobium application in the amelioration of cadmium stress (Cd; 50 and 100 mg kg(- 1) soil) on chickpea cultivar namely Pusa-BG372 on growth (plant length; plant dry biomass; leaf area; and nodule number), photosynthetic pigments (total chlorophyll and carotenoids), stress biomarkers (malondialdehyde, MDA; superoxide radicles; cell viability), defense (proline, superoxide dismutase, SOD; peroxidase, POD; catalase, CAT; stomatal behaviour), and the major enzymes involved in nitrate assimilation (nitrate reductase; NR) and Calvin Cycle (carbonic anhydrase; CA). Among the different tested concentrations, 100 mg kg(-1) of Cd reduced the growth, photosynthetic variables, biochemical enzymes activity and increased oxidative stress under Cd stress. However, chickpea plants supplemented with Rhizobium-inoculation under the Cd toxicity revealed significantly increased chlorophyll, carotenoid, and proline contents, activity of CA, NR, and antioxidant enzymes. Aside from improved antioxidant enzyme performance and lower lipid peroxidation, cell viability and stomatal functioning were also improved in Rhizobium-inoculated plants. These observations depicted that application of Rhizobium inoculation to seeds could be useful approach to assist stress tolerance against Cd in crop plants grown in Cd contaminated sites.

Soil is an environment with numerous niches, where bacteria are exposed to diverse conditions. Some bacteria are exposed earlier than others to pressure, and the emission of signals that other bacteria can receive and perceive may allow a better response to an eminent stimulus. To shed light on how bacteria trigger their response and adapt to changes in the environment, the intra- and interspecific influences of volatiles on bacterial strains growing under non-stressed and cadmium-stressed conditions were assessed. Each strain was exposed to its volatiles emitted by cells growing under different conditions to test whether the environment in which a cell grows influences neighboring cells. The five genera tested showed different responses, with Rhizobium displaying the greatest influence. In a second experiment, 13 strains from different genera were grown under control conditions but exposed to volatiles released by Cd-stressed Rhizobium cells to ascertain whether Rhizobium's observed influence was strain-specific or broader. Our results showed that the volatiles emitted by some bacteria under stress are differentially perceived and translated into biochemical changes (growth, alteration of the antioxidant response, and oxidative damage) by other bacteria, which may increase the adaptability and resilience of bacterial communities to environmental changes, especially those with a prooxidant nature. Cadmium (Cd) contamination of soils constitutes a risk to the environment and human health. Here, we showed the effects of Cd exposure on bacteria and how volatile communication influences the biochemistry related to coping with oxidative stress. This knowledge can be important for remediation and risk assessment and highlights that new biological features, such as volatile communication, should be considered when studying and assessing the impact of contaminants on soil ecosystems.

Titanium dioxide nanoparticles (TiO2 NPs) are among the most commonly used nanomaterials and are most likely to end up in soil. Therefore, it is pertinent to study the interaction of TiO2 NPs with soil microorganisms. The present in vitro broth study evaluates the impacts of low-dose treatments (0, 1.0, 5.0, 10.0, 20.0, and 40.0 mg L-1) of TiO2 NPs on cell viability, morphology, and plant growth promoting (PGP) traits of rhizobia isolated from mung bean root nodule. Two types of TiO2 NPs, that is, mixture of anatase and rutile, and anatase alone were used in the study. These TiO2 NPs were supplemented in broth along with a multifunctional isolate (Bradyrhizobium sp.) and two reference cultures. The exposure of TiO2 (anatase+rutile) NPs at low concentrations (less than 20.0 mg L-1) enhanced the cell growth, and total soluble protein content, besides improving the phosphate solubilization, Indole-3-acetic acid (IAA) production, siderophore, and gibberellic acid production. The TiO2 (anatase) NPs enhanced exopolysaccharide (EPS) production by the test rhizobial cultures. The radical scavenging assay was performed to reveal the mode of action of the nano-TiO2 particles. The study revealed higher reactive oxygen species (ROS) generation by the TiO2 (anatase) NPs as compared with TiO2 (anatase+rutile) NPs. Exposure to TiO2 NPs also altered the morphology of rhizobial cells. The findings suggest that TiO2 NPs could act as promoters of PGP traits of PGP bacteria when applied at appropriate lower doses.

The rhizosphere consists of a plethora of microbes, interacting with each other as well as with the plants present in proximity. The root exudates consist of a variety of secondary metabolites such as strigolactones and other phenolic compounds such as coumarin that helps in facilitating communication and forming associations with beneficial microbes in the rhizosphere. Among different secondary metabolites flavonoids (natural polyphenolic compounds) continuously increasing attention in scientific fields for showing several slews of biological activities. Flavonoids possess a benzo-gamma-pyrone skeleton and several classes of flavonoids have been reported on the basis of their basic structure such as flavanones, flavonols, anthocyanins, etc. The mutualistic association between plant growth-promoting rhizobacteria (PGPR) and plants have been reported to help the host plants in surviving various biotic and abiotic stresses such as low nitrogen and phosphorus, drought and salinity stress, pathogen attack, and herbivory. This review sheds light upon one such component of root exudate known as flavonoids, which is well known for nodulation in legume plants. Apart from the well-known role in inducing nodulation in legumes, this group of compounds has anti-microbial and antifungal properties helping in establishing defensive mechanisms and playing a major role in forming mycorrhizal associations for the enhanced acquisition of nutrients such as iron and phosphorus. Further, this review highlights the role of flavonoids in plants for recruiting non-mutualistic microbes under stress and other important aspects regarding recent findings on the functions of this secondary metabolite in guiding the plant-microbe interaction and how organic matter affects its functionality in soil.

Soil nitrogen is crucial for agriculture, but it is often limited, affecting crop yields. Deficiency requires synthetic fertilizers, but their improper use results in environmental damage and high costs. Bacteria of the genus Rhizobium , symbionts of legumes, offer a sustainable solution by fixing nitrogen, thus reducing dependence on fertilizers. This research determined the most probable number (MPN) of cells of Rhizobium spp. from two commercial biofertilizers of Ecuadorian and Mexican origin under greenhouse conditions. For this, direct inoculation with serial dilutions (10(-1) to 10(-10) ) was performed in pots with steam -sterilized pumice where Blue Lake variety snap bean ( Phaseolus vulgaris L.) plants were germinated. The following morphological indicators were evaluated at 45 days after sowing (DAS): leaf area, plant wet weight, plant height, and number of flowers, determining statistical differences between the type of biofertilizer and the concentration of each dilution. The experiment followed a randomized complete block design with a split -plot arrangement, with three replicates per dilution, considering temperature fluctuations in the study area. The MPN at 95% confidence was 4.45x10(7) rhizobia g -1 of pumice at a 10(-5) dilution for the Mexican biofertilizer, and 1.48x10(5) rhizobia g(-1) of pumice at a 10(-4) dilution for the Ecuadorian biofertilizer. The estimated optimal dilution for both products was 10(-8).