Soil salinization has been the major form of soil degradation under the dual influence of climate change and high-intensity human activities, threatening global agricultural sustainability and food security. High salt concentrations induce osmotic imbalance, ion stress, oxidative damage, and other hazards to plants, resulting in retarded growth, reduced biomass, and even total crop failure. Halo-tolerant plant growth promoting rhizobacteria (HT-PGPR), as a widely distributed group of beneficial soil microorganisms, are emerging as a valuable biological tool for mitigating the toxic effects of high salt concentrations and improve plant growth while remediating degraded saline soil. Here, the current status, harm, and treatment measures of global soil salinization are summarized. The mechanism of salt tolerance and growth promotion induced by HT-PGPR are reviewed. We highlight that advances in multiomics technologies are helpful for exploring the genetic and molecular mechanisms of microbiota centered on HT-PGPR to address the issue of plant losses in saline soil. Future research is urgently needed to comprehensively and robustly determine the interaction mechanism between the root microbiome centered on HT-PGPR and salt-stressed plants via advanced means to maximize the efficacy of HT-PGPR as a microbial agent. Halo-tolerant plant growth promoting rhizobacteria (HT-PGPR) are a valuable biological tool for mitigating the toxic effects of high salt concentrations. And the microbiome centered on HT-PGPR is solutions for sustainable agriculture in saline soils.

Background Oxidative stress mediated by reactive oxygen species (ROS) is a common denominator in arsenic toxicity. Arsenic stress in soil affects the water absorption, decrease stomatal conductance, reduction in osmotic, and leaf water potential, which restrict water uptake and osmotic stress in plants. Arsenic-induced osmotic stress triggers the overproduction of ROS, which causes a number of germination, physiological, biochemical, and antioxidant alterations. Antioxidants with potential to reduce ROS levels ameliorate the arsenic-induced lesions. Plant growth promoting rhizobacteria (PGPR) increase the total soluble sugars and proline, which scavenging OH radicals thereby prevent the oxidative damages cause by ROS. The main objective of this study was to evaluate the potential role of Arsenic resistant PGPR in growth of maize by mitigating arsenic stress. Methodology Arsenic tolerant PGPR strain MD3 (Pseudochrobactrum asaccharolyticum) was used to dismiss the 'As' induced oxidative stress in maize grown at concentrations of 50 and 100 mg/kg. Previously isolated arsenic tolerant bacterial strain MD3 Pseudochrobactrum asaccharolyticum was used for this experiment. Further, growth promoting potential of MD3 was done by germination and physio-biochemical analysis of maize seeds. Experimental units were arranged in Completely Randomized Design (CRD). A total of 6 sets of treatments viz., control, arsenic treated (50 & 100 mg/kg), bacterial inoculated (MD3), and arsenic stress plus bacterial inoculated with three replicates were used for Petri plates and pot experiments. After treating with this MD3 strain, seeds of corn were grown in pots filled with or without 50 mg/kg and 100 mg/kg sodium arsenate. Results The plants under arsenic stress (100 mg/kg) decreased the osmotic potential (0.8 MPa) as compared to control indicated the osmotic stress, which caused the reduction in growth, physiological parameters, proline accumulation, alteration in antioxidant enzymes (Superoxide dismutase-SOD, catalase-CAT, peroxidase-POD), increased MDA content, and H2O2 in maize plants. As-tolerant Pseudochrobactrum asaccharolyticum improved the plant growth by reducing the oxidation stress and antioxidant enzymes by proline accumulation. PCA analysis revealed that all six treatments scattered differently across the PC1 and PC2, having 85.51% and 9.72% data variance, respectively. This indicating the efficiency of As-tolerant strains. The heatmap supported the As-tolerant strains were positively correlated with growth parameters and physiological activities of the maize plants. Conclusion This study concluded that Pseudochrobactrum asaccharolyticum reduced the 'As' toxicity in maize plant through the augmentation of the antioxidant defense system. Thus, MD3 (Pseudochrobactrum asaccharolyticum) strain can be considered as bio-fertilizer.

Plants have limited resources to allocate to defences against infection and herbivory. While interactions between plant responses to microbial and herbivore attack are complex, it is often the case that the induction of one response will act antagonistically to the other. Recent studies have shown that plant growth promoting rhizobacteria, which improves overall plant health and general stress resistance, can enhance both anti-microbial and anti-herbivore defences. We investigated how soil application of the biofungicide Serenade ASO (Bacillus subtilis strain QST 713), which primes plant defences against fungal and bacterial infection and promotes plant growth, affects anti-herbivore defences by measuring both constitutive and induced defences. We applied Serenade one or two times to the soil of tomato plants and measured the numbers of type IV glandular trichomes on leaves, the weight gain of a generalist caterpillar (beet armyworms; BAW), and the activity of two enzymes associated with defence against insects (polyphenoloxidase and peroxidase). Serenade treated plants grew faster and foliage from treated plants had significantly higher numbers of glandular trichomes and higher polyphenoloxidase and peroxidase activities than untreated plants. However, Serenade treatment did not affect the degree of induction of plant defences when damaged by BAW feeding and did not slow the growth rate of BAW relative to plants not treated with Serenade. Therefore, the biofungicide Serenade increased plant growth and altered the densities of trichomes and the activities of two defensive enzymes in plants, but it did not affect overall susceptibility of the plants to a generalist herbivore.

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.

Drought is a major challenge for agriculture worldwide, being one of the main causes of losses in plant production. Various studies reported that some soil's bacteria can improve plant tolerance to environmental stresses by the enhancement of water and nutrient uptake by plants. The Atacama Desert in Chile, the driest place on earth, harbors a largely unexplored microbial richness. This study aimed to evaluate the ability of various Bacillus sp. from the hyper arid Atacama Desert in the improvement in tolerance to drought stress in lettuce (Lactuca sativa L. var. capitata, cv. Super Milanesa) plants. Seven strains of Bacillus spp. were isolated from the rhizosphere of the Chilean endemic plants Metharme lanata and Nolana jaffuelii, and then identified using the 16s rRNA gene. Indole acetic acid (IAA) production, phosphate solubilization, nitrogen fixation, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity were assessed. Lettuce plants were inoculated with Bacillus spp. strains and subjected to two different irrigation conditions (95% and 45% of field capacity) and their biomass, net photosynthesis, relative water content, photosynthetic pigments, nitrogen and phosphorus uptake, oxidative damage, proline production, and phenolic compounds were evaluated. The results indicated that plants inoculated with B. atrophaeus, B. ginsengihumi, and B. tequilensis demonstrated the highest growth under drought conditions compared to non-inoculated plants. Treatments increased biomass production and were strongly associated with enhanced N-uptake, water status, chlorophyll content, and photosynthetic activity. Our results show that specific Bacillus species from the Atacama Desert enhance drought stress tolerance in lettuce plants by promoting several beneficial plant traits that facilitate water absorption and nutrient uptake, which support the use of this unexplored and unexploited natural resource as potent bioinoculants to improve plant production under increasing drought conditions.