Halotolerant Rhizobacteria Promote Growth and Enhance Salinity Tolerance in Peanut

Microbial diversity and functions in saline soils: A review from a biogeochemical perspective

BACKGROUND

Soil salinization threatens food security and ecosystem health, and is one of the important drivers to the degradation of many ecosystems around the world. Soil microorganisms have extremely high diversity and participate in a variety of key ecological processes. They are important guarantees for soil health and sustainable ecosystem development. However, our understanding of the diversity and function of soil microorganisms under the change of increased soil salinization is fragmented.

AIM OF REVIEW

Here, we summarize the changes in soil microbial diversity and function under the influence of soil salinization in diverse natural ecosystems. We particularly focus on the diversity of soil bacteria and fungi under salt stress and the changes in their emerging functions (such as their mediated biogeochemical processes). This study also discusses how to use the soil microbiome in saline soils to deal with soil salinization for supporting sustainable ecosystems, and puts forward the knowledge gaps and the research directions that need to be strengthened in the future.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Due to the rapid development of molecular-based biotechnology (especially high-throughput sequencing technology), the diversity and community composition and functional genes of soil microorganisms have been extensively characterized in different habitats. Clarifying the responding pattern of microbial-mediated nutrient cycling under salt stress and developing and utilizing microorganisms to weaken the adverse effects of salt stress on plants and soil, which are of guiding significance for agricultural production and ecosystem management in saline lands.

Physiological and biochemical mechanisms underlying the role of anthocyanin in acquired tolerance to salt stress in peanut (Arachis hypogaea L.)

Anthocyanin is an important pigment that prevents oxidative stress and mediates adaptation of plants to salt stress. Peanuts with dark red and black testa are rich in anthocyanin. However, correlation between salt tolerance and anthocyanin content in black and dark red testa peanuts is unknown. In this study, three peanut cultivars namely YZ9102 (pink testa), JHR1 (red testa) and JHB1 (black testa) were subjected to sodium chloride (NaCl) stress. The plant growth, ion uptake, anthocyanin accumulation, oxidation resistance and photosynthetic traits were comparatively analyzed. We observed that the plant height, leaf area and biomass under salt stress was highly inhibited in pink color testa (YZ9102) as compare to black color testa (JHB1). JHB1, a black testa colored peanut was identified as the most salt-tolerance cultivar, followed by red (JHR1) and pink(YZ9102). During salt stress, JHB1 exhibited significantly higher levels of anthocyanin and flavonoid accumulation compared to JHR1 and YZ9102, along with increased relative activities of antioxidant protection and photosynthetic efficiency. However, the K+/Na+ and Ca2+/Na+ were consistently decreased among three cultivars under salt stress, suggesting that the salt tolerance of black testa peanut may not be related to ion absorption. Therefore, we predicted that salt tolerance of JHB1 may be attributed to the accumulation of the anthocyanin and flavonoids, which activated antioxidant protection against the oxidative damage to maintain the higher photosynthetic efficiency and plant growth. These findings will be useful for improving salt tolerance of peanuts.

Impact of two Erwinia sp. on the response of diverse Pisum sativum genotypes under salt stress

Currently, salinization is impacting more than 50% of arable land, posing a significant challenge to agriculture globally. Salt causes osmotic and ionic stress, determining cell dehydration, ion homeostasis, and metabolic process alteration, thus negatively influencing plant development. A promising sustainable approach to improve plant tolerance to salinity is the use of plant growth-promoting bacteria (PGPB). This work aimed to characterize two bacterial strains, that have been isolated from pea root nodules, initially called PG1 and PG2, and assess their impact on growth, physiological, biochemical, and molecular parameters in three pea genotypes (Merveille de Kelvedon, Lincoln, Meraviglia d'Italia) under salinity. Bacterial strains were molecularly identified, and characterized by in vitro assays to evaluate the plant growth promoting abilities. Both strains were identified as Erwinia sp., demonstrating in vitro biosynthesis of IAA, ACC deaminase activity, as well as the capacity to grow in presence of NaCl and PEG. Considering the inoculation of plants, pea biometric parameters were unaffected by the presence of the bacteria, independently by the considered genotype. Conversely, the three pea genotypes differed in the regulation of antioxidant genes coding for catalase (PsCAT) and superoxide dismutase (PsSOD). The highest proline levels (212.88 μmol g-1) were detected in salt-stressed Lincoln plants inoculated with PG1, along with the up-regulation of PsSOD and PsCAT. Conversely, PG2 inoculation resulted in the lowest proline levels that were observed in Lincoln and Meraviglia d'Italia (35.39 and 23.67 μmol g-1, respectively). Overall, this study highlights the potential of these two strains as beneficial plant growth-promoting bacteria in saline environments, showing that their inoculation modulates responses in pea plants, affecting antioxidant gene expression and proline accumulation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01419-8.

A novel PGPR strain homologous to Beijerinckia fluminensis induces biochemical and molecular changes involved in Arabidopsis thaliana salt tolerance

The use of PGPR is widely accepted as a promising tool for a more sustainable agricultural production and improved plant abiotic stress resistance. This study tested the ability of PVr_9, a novel bacterial strain, homologous to Beijerinckia fluminensis, to increase salt stress tolerance in A. thaliana. In vitro plantlets inoculated with PVr_9 and treated with 150 mM NaCl showed a reduction in primary root growth inhibition compared to uninoculated ones, and a leaf area significantly less affected by salt. Furthermore, salt-stressed PVr_9-inoculated plants had low ROS and 8-oxo-dG, osmolytes, and ABA content along with a modulation in antioxidant enzymatic activities. A significant decrease in Na+ in the leaves and a corresponding increase in the roots were also observed in salt-stressed inoculated plants. SOS1, NHX1 genes involved in plant salt tolerance, were up-regulated in PVr_9-inoculated plants, while different MYB genes involved in salt stress signal response were down-regulated in both roots and shoots. Thus, PVr_9 was able to increase salt tolerance in A. thaliana, thereby suggesting a role in ion homeostasis by reducing salt stress rather than inhibiting total Na+ uptake. These results showed a possible molecular mechanism of crosstalk between PVr_9 and plant roots to enhance salt tolerance, and highlighted this bacterium as a promising PGPR for field applications on agronomical crops.

Unraveling lipid peroxidation-mediated regulation of redox homeostasis for sustaining plant health

Lipid peroxidation (LPO) is a complex process that, depending on the context, can either result in oxidative injury or promote redox homeostasis. LPO is a series of reactions in which polyunsaturated fatty acids are attacked by free radicals that result in the synthesis of lipid peroxides. LPO can alter membrane fluidity and operation and produce secondary products that amplify oxidative stress. LPO can activate cellular signaling pathways that promote antioxidant defense mechanisms that provide oxidative stress protection by elevating antioxidant enzyme action potentials. Enzymatic and nonenzymatic mechanisms tightly regulate LPO to prevent excessive LPO and its adverse consequences. This article emphasizes the dual nature of LPO as a mechanism that can both damage cells and regulate redox homeostasis. In addition, it also highlights the major enzymatic and nonenzymatic mechanisms that tightly regulate LPO to prevent excessive oxidative damage. More importantly, it emphasizes the importance of understanding the cellular and biochemical complexity of LPO for developing strategies targeting this process for efficient management of plant stress.

Enterobacter sp. DBA51 produces ACC deaminase and promotes the growth of tomato (Solanum lycopersicum L.) and tobacco (Nicotiana tabacum L.) plants under greenhouse condition

Bacterial isolated from rhizospheric soil associated with the semi-desertic plant Coronilla juncea L. were screened for 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity, a common trait for plant-growth-promoting rhizobacteria (PGPR). Among bacterial isolates, strain DBA51 showed phosphate solubilizing index (PSI), producing indole acetic acid (IAA), and with the hemolysis-negative test. Sequencing and analysis of the 16S rDNA gene identified DBA51 as Enterobacter. DBA51 did not show antagonistic activity in vitro against bacterial (Clavibacter michiganensis, Pseudomonas syringae pv. tomato DC3000 and Pectobacterium cacticidum FHLGJ22) and fungal phytopathogens (Alternaria sp., Fusarium oxysporum fsp. lycopersici, Fusarium oxysporum fsp. cubense M5, and Rhizoctonia sp.). Root inoculations with DBA51 in tomato (Solanum lycopersicum L.) and tobacco (Nicotiana tabacum L.) plants were performed under greenhouse conditions. Plant height (20 %) and root biomass (40 %) were significantly enhanced in tomato plants inoculated with DBA51 compared to non-inoculated plants, although for tobacco plants, only root biomass (27 %) showed significant differences with DBA51. In addition, physiological parameters such as photosynthetic rate (µmol CO2 m-2 s-1), stomatal conductance (mol H2O m-2 s-1), and transpiration rate (mmol H2O m-2 s-1) were also evaluated, and no differences were detected between DBA51-inoculated and control treatment in tomato and tobacco leaves. The observed results indicate that the DBA51 strain could be used as a biofertilizer to improve yields of horticultural crops.

Halophilic Plant-Associated Bacteria with Plant-Growth-Promoting Potential

The salinization of soils is a growing agricultural concern worldwide. Irrigation practices, drought, and climate change are leading to elevated salinity levels in many regions, resulting in reduced crop yields. However, there is potential for a solution in the microbiome of halophytes, which are naturally salt-tolerant plants. These plants harbor a salt-tolerant microbiome in their rhizosphere (around roots) and endosphere (within plant tissue). These bacteria may play a significant role in conferring salt tolerance to the host plants. This leads to the possibility of transferring these beneficial bacteria, known as salt-tolerant plant-growth-promoting bacteria (ST-PGPB), to salt-sensitive plants, enabling them to grow in salt-affected areas to improve crop productivity. In this review, the background of salt-tolerant microbiomes is discussed and their potential use as ST-PGPB inocula is explored. We focus on two Gram-negative bacterial genera, Halomonas and Kushneria, which are commonly found in highly saline environments. These genera have been found to be associated with some halophytes, suggesting their potential for facilitating ST-PGPB activity. The study of salt-tolerant microbiomes and their use as PGPB holds promise for addressing the challenges posed by soil salinity in the context of efforts to improve crop growth in salt-affected areas.

Harnessing Rhizospheric Microbes for Eco-friendly and Sustainable Crop Production in Saline Environments

Soil salinization is a global issue that negatively impacts crop yield and has become a prime concern for researchers worldwide. Many important crop plants are susceptible to salinity-induced stresses, including ionic and osmotic stress. Approximately, 20% of the world's cultivated and 33% of irrigated land is affected by salt. While various agricultural practices have been successful in alleviating salinity stress, they can be costly and not environment-friendly. Therefore, there is a need for cost-effective and eco-friendly practices to improve soil health. One promising approach involves utilizing microbes found in the vicinity of plant roots to mitigate the effects of salinity stress and enhance plant growth as well as crop yield. By exploiting the salinity tolerance of plants and their associated rhizospheric microorganisms, which have plant growth-promoting properties, it is possible to reduce the adverse effects of salt stress on crop plants. The soil salinization is a common problem in the world, due to which we are unable to use the saline land. To make proper use of this land for different crops, microorganisms can play an important role. Looking at the increasing population of the world, this will be an appreciated effort to make the best use of the wasted land for food security. The updated information on this issue is needed. In this context, this article provides a concise review of the latest research on the use of salt-tolerant rhizospheric microorganisms to mitigate salinity stress in crop plants.

Synergistic action of Pseudomonas fluorescens with melatonin attenuates salt toxicity in mustard by regulating antioxidant system and flavonoid profile

Salt stress is an alarming abiotic stress that reduces mustard growth and yield. To attenuate salt toxicity effects, plant growth-promoting rhizobacteria (PGPR) offers a sustainable approach. Among the various PGPR, Pseudomonas fluorescens (P. fluorescens NAIMCC-B-00340) was chosen for its salt tolerance (at 100 mM NaCl) and for exhibiting various growth-promoting activities. Notably, P. fluorescens can produce auxin, which plays a role in melatonin (MT) synthesis. Melatonin is a pleiotropic molecule that acts as an antioxidant to scavenge reactive oxygen species (ROS), resulting in stress reduction. Owing to the individual role of PGPR and MT in salt tolerance, and their casual nexus, their domino effect was investigated in Indian mustard under salt stress. The synergistic action of P. fluorescens and MT under salt stress conditions was found to enhance the activity of antioxidative enzymes and proline content as well as  promote the production of secondary metabolites. This led to reduced oxidative stress following effective ROS scavenging, maintained photosynthesis, and improved growth. In mustard plants treated with MT and P. fluorescens under salt stress, eight flavonoids showed significant increase. Kaempferol and cyanidin showed the highest concentrations and are reported to act as antioxidants with protective functions under stress. Thus, we can anticipate that strategies involved in their enhancement could provide a better adaptive solution to salt toxicity in mustard plants. In conclusion, the combination of P. fluorescens and MT affected antioxidant metabolism and flavonoid profile that could be used to mitigate salt-induced stress and bolster plant resilience.

Reduction in Salt Stress Due to the Action of Halophilic Bacteria That Promote Plant Growth in Solanum lycopersicum

Soil salinity is one of the most important factors reducing agricultural productivity worldwide. Halophilic plant growth-promoting bacteria (H-PGPB) represent an alternative method of alleviating saline stress in crops of agricultural interest. In this study, the following halophilic bacteria were evaluated: Bacillus sp. SVHM1.1, Halomonas sp. SVCN6, Halomonas sp. SVHM8, and a consortium. They were grown under greenhouse conditions in Solanum lycopersicum at different salinity concentrations in irrigation water (0, 20, 60, and 100 mM NaCl) to determine the effects on germination, fruit quality, yield, and concentration of osmoprotectors in plant tissue. Our results demonstrate the influence of halophilic bacteria with the capacity to promote plant growth on the germination and development of Solanum lycopersicum at higher salinity levels. The germination percentage was improved at the highest concentration by the inoculated treatments (from 37 to 47%), as were the length of the radicle (30% at 20 mM) and plumule of the germinated seed, this bacterium also increased the weight of the plumule (97% at 100 mM). They also improved the yield. The dry weight of the plant, in addition to having an influence on the quality of the fruit and the concentration of osmoprotectors (Bacillus sp. SVHM 1.1) had the greatest effect on fruit yield (1.5 kg/plant at 20 mM), by the otherhand, Halomonas sp. SVHM8 provided the best fruit quality characteristics at 100 mM. According to the above results, the efficiency of halophilic PGPB in the attenuation of salt stress in Solanum lycopersicum has been proven.

The halotolerant rizhobacterium Glutamicibacter sp. alleviates salt impact on Phragmites australis by producing exopolysaccharides and limiting plant sodium uptake

Salinity is a widespread abiotic stress, which has strong adverse effects on plant growth and crop productivity. Exopolysaccharides (EPS) play a crucial role in plant growth-promoting rhizobacteria (PGPR)-mediated improvement of plant stress tolerance. This study aimed to assess whether Glutamicibacter sp. strain producing large amounts of EPS may promote tolerance of common reed, Phragmites australis (Cav.) Trin. ex Steud., towards salt stress. This halotolerant rizhobacterium showed tolerance to salinity (up to 1 M NaCl) when cultivated on Luria-Bertani (LB) medium. Exposure to high salinity (300 mM NaCl) significantly impacted the plant growth parameters, but this adverse effect was mitigated following inoculation with Glutamicibacter sp., which triggered higher number of leaves and tillers, shoot fresh weight/dry weight, and root fresh weight as compared to non-inoculated plants. Salt stress increased the accumulation of malondialdehyde (MDA), polyphenols, total soluble sugars (TSSs), and free proline in shoots. In comparison, the inoculation with Glutamicibacter sp. further increased shoot polyphenol content, while decreasing MDA and free proline contents. Besides, this bacterial strain increased tissue Ca+ and K+ content concomitant to lower shoot Na+ and root Cl- accumulation, thus further highlighting the beneficial effect of Glutamicibacter sp. strain on the plant behavior under salinity. As a whole, our study provides strong arguments for a potential utilization of EPS-producing bacteria as a useful microbial inoculant to alleviate the deleterious effects of salinity on plants.

Manipulating rhizosphere microorganisms to improve crop yield in saline-alkali soil: a study on soybean growth and development

Introduction

Rhizosphere microorganisms can effectively promote the stress resistance of plants, and some beneficial rhizosphere microorganisms can significantly promote the growth of crops under salt stress, which has the potential to develop special microbial fertilizers for increasing the yield of saline-alkali land and provides a low-cost and environmentally friendly new strategy for improving the crop yield of saline-alkali cultivated land by using agricultural microbial technology.

Methods

In May 2022, a field study in a completely randomized block design was conducted at the Heilongjiang Academy of Agricultural Sciences to explore the correlation between plant rhizosphere microorganisms and soybean growth in saline-alkali soil. Two soybean cultivars (Hening 531, a salt-tolerant variety, and 20_1846, a salt-sensitive variety) were planted at two experimental sites [Daqing (normal condition) and Harbin (saline-alkali conditions)], aiming to investigate the performance of soybean in saline-alkali environments.

Results

Soybeans grown in saline-alkali soil showed substantial reductions in key traits: plant height (25%), pod number (26.6%), seed yield (33%), and 100 seed weight (13%). This underscores the unsuitability of this soil type for soybean cultivation. Additionally, microbial analysis revealed 43 depleted and 56 enriched operational taxonomic units (OTUs) in the saline-alkali soil compared to normal soil. Furthermore, an analysis of ion-associated microbes identified 85 mOTUs with significant correlations with various ions. A co-occurrence network analysis revealed strong relationships between specific mOTUs and ions, such as Proteobacteria with multiple ions. In addition, the study investigated the differences in rhizosphere species between salt-tolerant and salt-sensitive soybean varieties under saline-alkali soil conditions. Redundancy analysis (RDA) indicated that mOTUs in saline-alkali soil were associated with pH and ions, while mOTUs in normal soil were correlated with Ca2+ and K+. Comparative analyses identified significant differences in mOTUs between salt-tolerant and salt-sensitive varieties under both saline-alkali and normal soil conditions. Planctomycetes, Proteobacteria, and Actinobacteria were dominant in the bacterial community of saline-alkali soil, with significant enrichment compared to normal soil. The study explored the functioning of the soybean rhizosphere key microbiome by comparing metagenomic data to four databases related to the carbon, nitrogen, phosphorus, and sulfur cycles. A total of 141 KOs (KEGG orthologues) were identified, with 66 KOs related to the carbon cycle, 16 KOs related to the nitrogen cycle, 48 KOs associated with the phosphorus cycle, and 11 KOs linked to the sulfur cycle. Significant correlations were found between specific mOTUs, functional genes, and phenotypic traits, including per mu yield (PMY), grain weight, and effective pod number per plant.

Conclusion

Overall, this study provides comprehensive insights into the structure, function, and salt-related species of soil microorganisms in saline-alkali soil and their associations with salt tolerance and soybean phenotype. The identification of key microbial species and functional categories offers valuable information for understanding the mechanisms underlying plant-microbe interactions in challenging soil conditions.

Home-based microbial solution to boost crop growth in low-fertility soil

Soil microbial inoculants are expected to boost crop productivity under climate change and soil degradation. However, the efficiency of native vs commercialized microbial inoculants in soils with different fertility and impacts on resident microbial communities remain unclear. We investigated the differential plant growth responses to native synthetic microbial community (SynCom) and commercial plant growth-promoting rhizobacteria (PGPR). We quantified the microbial colonization and dynamic of niche structure to emphasize the home-field advantages for native microbial inoculants. A native SynCom of 21 bacterial strains, originating from three typical agricultural soils, conferred a special advantage in promoting maize growth under low-fertility conditions. The root : shoot ratio of fresh weight increased by 78-121% with SynCom but only 23-86% with PGPRs. This phenotype correlated with the potential robust colonization of SynCom and positive interactions with the resident community. Niche breadth analysis revealed that SynCom inoculation induced a neutral disturbance to the niche structure. However, even PGPRs failed to colonize the natural soil, they decreased niche breadth and increased niche overlap by 59.2-62.4%, exacerbating competition. These results suggest that the home-field advantage of native microbes may serve as a basis for engineering crop microbiomes to support food production in widely distributed poor soils.

Potential of halotolerant PGPRs in growth and yield augmentation of Triticum aestivum var. HD2687 and Zea mays var. PSCL4642 cultivars under saline conditions

This study aimed to culture and screen salt-tolerant Plant growth promoting rhizobacteria (PGPRs) from Bougainvillea glabra rhizosphere to improve wheat HD-2687 and maize PSCL-4642 cultivars under saline conditions up to the seedling stage. Twenty-four rhizobacterial isolates were screened for salt tolerance at different NaCl levels. Indole acetic acid (IAA) production, phosphate solubilization, and siderophore and hydrogen cyanide (HCN) production of salinity-tolerant isolates were tested. Positive salt-tolerant PGPRs were further subjected to seedling studies to examine the improvement in the development of experimental crops under 50, 100, 150, and 200 mM NaCl concentrations with/without bacterial inoculant. Of the 24 isolates, BoGl123 was the most promising PGPR, which showed the maximum phosphate solubilization, and IAA, siderophore and HCN production. It was further subjected to seedling studies. In comparison with controls, BoGl123 resulted in a higher radicle length in maize (34 mm, 87.4%) and wheat (26.8 mm, 85.8%) at the 50 mM salinity level. At the 100 mM NaCl level, the radicle length of wheat and maize seedlings was increased by 82.5% and 78.6%, respectively, compared with controls. At different NaCl concentrations, BoGl123 improved the plumule length of seedlings in both crops. The stress tolerance attributes and plant growth promotion (PGP) indicate the potential of Pseudomonas fluorescens BoGl123 to be used as a microbial inoculant in the cultivation of wheat and maize under stressful conditions.

Biocontrol and plant growth promotion by combined Bacillus spp. inoculation affecting pathogen and AMF communities in the wheat rhizosphere at low salt stress conditions

Applying plant growth-promoting rhizobacteria (PGPR) improves the efficiency of soil-borne disease control and is considered a sustainable practice. However, the effect of PGPR on the fungal community, especially pathogenic fungi and arbuscular mycorrhizal fungi (AMF), remains unclear. In this study, we examined the effects of a compound microbial agent (consisting of Bacillus subtilis HG-15 and Bacillus velezensis JC-K3) on the incidence and yield of wheat under low salt stress, as well as compared the diversity and community composition of the rhizosphere fungal and AMF communities of wheat in the CK (not inoculated bacterial agent) and BIO (inoculated with a bacterial agent) groups. Chlorophyll relative content (SPAD), net photosynthesis rate (Pn), transpiration rate (Tr), leaf water use efficiency (WUE _L), grains per spike and wheat yield in the BIO group increased more than in the CK group. The number of diseased plants and disease incidence was observed to be reduced. The relative efficacy reached 79.80%. We classified 1007 fungal operational taxonomic units (OTU) based on Miseq sequencing data: 11 phyla, 173 families, 319 genera, and 521 species. Fifty-four OTUs were classified from the AMF effective sequences, including 1 phylum, 3 families, 3 genera, and 17 species. The inoculation of bacterial agents reduced the relative abundance of pathogen genera such as Gibberella, Fusarium, Cladosporium, and Alternaria in wheat rhizosphere. It increased the relative abundance of AMF species such as Glomus-group-B-Glomus-lamellosu-VTX00193, Glomus-viscosum-VTX00063, and Glomus-Glo2-VTX00280. In addition, pH, EC, exchangeable K, available N, total N, organic matter, and olsen P were the main driving forces for shaping wheat rhizosphere fungi. The pH value was positively correlated with the relative abundance of fungal communities in soil, especially Gibberella, Cladosporium, Fusarium, and Alternaria. In summary, inoculation with Bacillus subtilis HG-15 and Bacillus velezensis JC-K3 affected wheat yield, incidence, rhizosphere soil chemical properties, rhizosphere fungi, and AMF fungal diversity and community. The findings may provide a theoretical foundation and strain support for constructing efficient PGPR-community and clarifying its mechanism of pathogenic bacteria inhibition.

Identification of Halophilic and Halotolerant Bacteria from the Root Soil of the Halophyte Sesuvium verrucosum Raf

Soil salinity is a condition that limits crop growth and productivity, and soil-dwelling bacteria from halophytic plant roots may be a viable strategy to cope with low productivity due to salt stress. Halophilic and halotolerant bacteria of the root soil of Sesuvium verrucosum were analyzed in this study as there is little evidence regarding its associated microbiology. Soil was sampled from the roots of Sesuvium verrucosum to obtain the cultivable bacteria. Their morphological characteristics were identified and they were molecularly identified by the 16S sequence. The growth capacity of the bacteria was determined at different levels of pH and salinity, and several growth promotion characteristics were identified, such as phosphorus solubilization, indole acetic acid production by the tryptophan-dependent (AIAt) and tryptophan-independent (IAA) pathways, ammonium production from organic sources, solubilization of carbonates, and zinc and sodium capture capacity. In addition, the bacteria that presented the best characteristics for germination variables of Solanum lycopersicum were evaluated. A total of 20 bacteria from root soil of Sesuvium verrucosum Raf. belonging to the phyla Proteobacteria (50%), Firmicutes (45%) and Actinobacteria (5%) were identified, with each one having different morphological characteristics. Among the bacterial isolates, 45% had the ability to resist different levels of salinity and pH, ranging from 0 to 20% of NaCl, and pH between 5 and 11. Moreover, these bacteria had the capacity to solubilize carbonates, phosphorus and zinc, capture sodium, produce ammonium from organic substrates and IAA (indole acetic acid), and promote enzymatic activity of amylases, proteases, lipases and cellulases. The bacteria evaluated on the germination of Solanum lycopersicum had an influence on germination at different salinity levels, with greater influence at 100 mM NaCl. This demonstrated that halophilic bacteria belonging to the rhizosphere of Sesuvium verrucosum have the ability to promote growth in extreme salinity conditions, making them candidates for the recovery of productivity in saline soils.

Diversity and functional traits of indigenous soil microbial flora associated with salinity and heavy metal concentrations in agricultural fields within the Indus Basin region, Pakistan

Soil salinization and heavy metal (HM) contamination are major challenges facing agricultural systems worldwide. Determining how soil microbial communities respond to these stress factors and identifying individual phylotypes with potential to tolerate these conditions while promoting plant growth could help prevent negative impacts on crop productivity. This study used amplicon sequencing and several bioinformatic programs to characterize differences in the composition and potential functional capabilities of soil bacterial, fungal, and archaeal communities in five agricultural fields that varied in salinity and HM concentrations within the Indus basin region of Pakistan. The composition of bacteria with the potential to fix atmospheric nitrogen (N) and produce the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase were also determined. Microbial communities were dominated by: Euryarchaeota (archaea), Actinobacteria, Proteobacteria, Planctomycetota, Firimicutes, Patescibacteria and Acidobacteria (bacteria), and Ascomycota (fungi), and all soils contained phylotypes capable of N-fixation and ACC-deaminase production. Salinity influenced bacterial, but not archaeal or fungal communities. Both salinity and HM altered the relative abundance of many phylotypes that could potentially promote or harm plant growth. These stress factors also appeared to influence the potential functional capabilities of the microbial communities, especially in their capacity to cycle phosphorous, produce siderophores, and act as symbiotrophs or pathotrophs. Results of this study confirm that farms in this region are at risk due to salinization and excessive levels of some toxic heavy metals, which could negatively impact crop and human health. Changes in soil microbial communities and their potential functional capabilities are also likely to affect several critical agroecosystem services related to nutrient cycling, pathogen suppression, and plant stress tolerance. Many potentially beneficial phylotypes were identified that appear to be salt and HM tolerant and could possibly be exploited to promote these services within this agroecosystem. Future efforts to isolate these phylotypes and determine whether they can indeed promote plant growth and/or carry out other important soil processes are recommended. At the same time, identifying ways to promote the abundance of these unique phylotypes either through modifying soil and crop management practices, or developing and applying them as inoculants, would be helpful for improving crop productivity in this region.

Endophytes from Halotolerant Plants Aimed to Overcome Salinity and Draught

The aim of our research was to study the endosphere of four halophytic plants: Salicornia europaea L., Salsola australis (R.Br.), Bassia sedoides (Pall.) and Kochia prostrata (L.) Schrad. from arid and saline areas of the Stavropol Territory, Russia. In total, 28 endophyte strains were isolated from the roots and stems of these halophytic plants. Most of the isolates (23 out of 28) were identified as Bacillus sp. while others belonged to the genera Oceanobacillus, Paenibacillus, Pantoea, Alcaligenes and Myroides. Three strains of Bacillus sp. (Se5R, Se1-1R, and Se1-3S), isolated from the S. europaea were capable of growth at 55 °C and in 10% of NaCl. Strains Se1-4S, Kp20-2S, and Bs11-2S Bacillus sp. (isolated from the S. australis, K. prostrata and B. sedoides, respectively) demonstrated strong plant growth promoting activity: 85-265% over control lettuce plants and a high degree of growth suppression (59.1-81.2%) of pathogenic fungi Fusarium oxysporum, Bipolaris sorokiniana and Rhizoctonia solani. Selected strains can be promising candidates for the development of bioinoculants to facilitate salt soil phytoremediation and be beneficial for mitigating the salt stress to the plants growing in salt-affected habitats.

Identification, characterization, and genome sequencing of Brevibacterium sediminis MG-1 isolate with growth-promoting properties

In recent years, plant growth-promoting rhizobacteria (PGPR) have received increased attention due to their prospective use as biofertilizers for the enhancement of crop growth and yields. However, there is a growing need to identify new PGPR isolates with additional beneficial properties. In this paper, we describe the identification of a new strain of a non-sporulating Gram-positive bacterium isolated from the rhizosphere of potato plants, classified as Brevibacterium sediminis MG-1 based on whole-genome sequencing. The bacteria are aerobic; they grow in a pH range of 6.0-10.0 (optimum 6.0), and a temperature range of 20-37 °C (optimum 30 °C). At 96 h of cultivation, strain MG-1 synthesizes 28.65 µg/ml of indole-3-acetic acid (IAA) when 500 µg/ml of l-tryptophan is added. It is a producer of catechol-type siderophores and ACC deaminase (213 ± 12.34 ng/ml) and shows halotolerance. Treatment of pea, rye, and wheat seeds with a suspension of MG-1 strain cells resulted in the stimulation of stem and root biomass accumulation by 12-26% and 6-25% (P < 0.05), respectively. Treatment of seeds with bacteria in the presence of high salt concentration reduced the negative effects of salt stress on plant growth by 18-50%. The hypothetical gene lin, encoding the bacteriocin Linocin-M18, RIPP-like proteins, and polyketide synthase type III (T3PKS) loci, gene clusters responsible for iron acquisition and metabolism of siderophores, as well as gene clusters responsible for auxin biosynthesis, were identified in the B. sediminis MG-1 genome. Thus, the rhizosphere-associated strain B. sediminis MG-1 has growth-stimulating properties and can be useful for the treatment of plants grown on soils with high salinity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03392-z.

Isolation and identification of plant growth-promoting rhizobacteria from tall fescue rhizosphere and their functions under salt stress

Soil salinity has become one of the major factors that threaten tall fescue growth and turf quality. Plants recruit diverse microorganisms in the rhizosphere to cope with salinity stress. In this study, 15 plant growth-promoting rhizobacteria (PGPR) were isolated from the salt-treated rhizosphere of tall fescue and were annotated to 10 genera, including Agrobacterium, Fictibacillus, Rhizobium, Bhargavaea, Microbacterium, Paenarthrobacter, Pseudarthrobacter, Bacillus, Halomonas, and Paracoccus. All strains could produce indole-3-acetic acid (IAA). Additionally, eight strains exhibited the ability to solubilize phosphate and potassium. Most strains could grow on the medium containing 600 mM NaCl, such as Bacillus zanthoxyli and Bacillus altitudinis. Furthermore, Bacillus zanthoxyli and Bacillus altitudinis were inoculated with tall fescue seeds and seedlings to determine their growth-promoting effect. The results showed that Bacillus altitudinis and mixed culture significantly increased the germination rate of tall fescue seeds. Bacillus zanthoxyli can significantly increase the tillers number and leaf width of seedlings under salt conditions. Through the synergistic effect of FaSOS1, FaHKT1, and FaHAK1 genes, Bacillus zanthoxyli helps to expel the excess Na⁺ from aboveground parts and absorb more K⁺ in roots to maintain ion homeostasis in tall fescue. Unexpectedly, we found that Bacillus altitudinis displayed an inapparent growth-promoting effect on seedlings under salt stress. Interestingly, the mixed culture of the two strains was also able to alleviate, to some extent, the effects of salt stress on tall fescue. This study provides a preliminary understanding of tall fescue rhizobacteria and highlights the role of Bacillus zanthoxyli in tall fescue growth and salt tolerance.

Bioremediation of Cadmium Toxicity in Wheat (Triticum aestivum L.) Plants Primed with L-Proline, Bacillus subtilis and Aspergillus niger

Cadmium toxicity is one of the deleterious abiotic factors that reduce wheat production. Two different cultivars (Akbar and Dilkash) were compared for their cadmium (0, 40 and 80 mg/kg) tolerance and responses towards Bacillus subtilis NA2, Aspergillus niger PMI-118 and L-proline. Both microbes were tested for heavy metal tolerance and production of various plant hormones and biological active enzyme characteristics under normal and cadmium stress. A completely randomized design (two cultivars × four treatments × three cadmium levels × three replicates) was adopted using distilled water as a control. The growth promotion potential of these strains under cadmium stress was determined by N-fixation, IAA synthesis, P-solubilization, amylase and proteases production. A pot experiment under controlled conditions was conducted to evaluate the effect of bacteria, fungi, and L-proline under cadmium stress. It was indicated from the result that plant biomass (46.43%), shoot length (22.40%), root length (25.06%), chlorophyll (17.17%), total sugars (27.07%), total proteins (86.01%) and ascorbic acid (83.27%) were improved with inoculation under control and cadmium stress. The accumulation of total flavonoids (48.64%), total phenolics (24.88%), hydrogen peroxide (53.96%) and activities of antioxidant enzymes CAT (26.37%) and APX (43.71%) were reduced in the plants treated with bacteria, fungi and L-proline than those under control. With parallel aids, Bacillus subtilis NA2 showed a higher cadmium tolerance and plant growth stability as compared to Aspergillus niger PMI-118 and L-proline and may be adopted in the future.

Antifungal Activity and Plant Growth-Promoting Properties of Bacillus mojovensis B1302 against Rhizoctonia Cerealis

Rhizoctonia cerealis is a worldwide soil-borne pathogenic fungus that significantly infects wheat and causes sharp eyespot in China. However, the biocontrol strains used for the control of Rhizoctonia cerealis are insufficient. In the present study, antagonistic strain B1302 from the rhizosphere of wheat were isolated and identified as Bacillus mojovensis based on their morphological, physiological, and biochemical characteristics, and their 16S rDNA sequence. Culture filtrate of strain B1302 had a broad antifungal spectrum. In order to improve the antifungal activity of B1302, response surface methodology (RSM) was used to optimize the culture conditions. The final medium composition and culture conditions were 13.2 g/L of wheat bran, 14.1 g/L of soybean meal, 224 r/min of rotation speed, 7.50 of initial pH, and 1.5 × 10⁸ CFU/mL of inoculation amount at 35 °C for a culture duration of 72 h. B. mojavensis B1302 inhibited the hyphae growth of R.cerealis and produced hydrolytic enzymes (protease, chitinase, and glucanase), IAA, and had N-fixing potentiality and P-solubilisation capacity. It can also promote wheat seedling growth in potted plants. The disease incidence and index of wheat seedlings were consistent with the effect of commercial pesticides under treatment with culture filtrate. The biocontrol efficacy of culture filtrate was significant—up to 65.25%. An animal toxicological safety analysis suggested that culture filtrate was safe for use and could be developed into an effective microbial fungicide to control wheat sharp eyespot.

Synthetic bacterial community derived from a desert rhizosphere confers salt stress resilience to tomato in the presence of a soil microbiome

The root bacterial microbiome is important for the general health of the plant. Additionally, it can enhance tolerance to abiotic stresses, exemplified by plant species found in extreme ecological niches like deserts. These complex microbe-plant interactions can be simplified by constructing synthetic bacterial communities or SynComs from the root microbiome. Furthermore, SynComs can be applied as biocontrol agents to protect crops against abiotic stresses such as high salinity. However, there is little knowledge on the design of a SynCom that offers a consistent protection against salt stress for plants growing in a natural and, therefore, non-sterile soil which is more realistic to an agricultural setting. Here we show that a SynCom of five bacterial strains, originating from the root of the desert plant Indigofera argentea, protected tomato plants growing in a non-sterile substrate against a high salt stress. This phenotype correlated with the differential expression of salt stress related genes and ion accumulation in tomato. Quantification of the SynCom strains indicated a low penetrance into the natural soil used as the non-sterile substrate. Our results demonstrate how a desert microbiome could be engineered into a simplified SynCom that protected tomato plants growing in a natural soil against an abiotic stress.

Bacillus thuringiensis PM25 ameliorates oxidative damage of salinity stress in maize via regulating growth, leaf pigments, antioxidant defense system, and stress responsive gene expression

Soil salinity is the major abiotic stress that disrupts nutrient uptake, hinders plant growth, and threatens agricultural production. Plant growth-promoting rhizobacteria (PGPR) are the most promising eco-friendly beneficial microorganisms that can be used to improve plant responses against biotic and abiotic stresses. In this study, a previously identified B. thuringiensis PM25 showed tolerance to salinity stress up to 3 M NaCl. The Halo-tolerant Bacillus thuringiensis PM25 demonstrated distinct salinity tolerance and enhance plant growth-promoting activities under salinity stress. Antibiotic-resistant Iturin C (ItuC) and bio-surfactant-producing (sfp and srfAA) genes that confer biotic and abiotic stresses were also amplified in B. thuringiensis PM25. Under salinity stress, the physiological and molecular processes were followed by the over-expression of stress-related genes (APX and SOD) in B. thuringiensis PM25. The results detected that B. thuringiensis PM25 inoculation substantially improved phenotypic traits, chlorophyll content, radical scavenging capability, and relative water content under salinity stress. Under salinity stress, the inoculation of B. thuringiensis PM25 significantly increased antioxidant enzyme levels in inoculated maize as compared to uninoculated plants. In addition, B. thuringiensis PM25-inoculation dramatically increased soluble sugars, proteins, total phenols, and flavonoids in maize as compared to uninoculated plants. The inoculation of B. thuringiensis PM25 significantly reduced oxidative burst in inoculated maize under salinity stress, compared to uninoculated plants. Furthermore, B. thuringiensis PM25-inoculated plants had higher levels of compatible solutes than uninoculated controls. The current results demonstrated that B. thuringiensis PM25 plays an important role in reducing salinity stress by influencing antioxidant defense systems and abiotic stress-related genes. These findings also suggest that multi-stress tolerant B. thuringiensis PM25 could enhance plant growth by mitigating salt stress, which might be used as an innovative tool for enhancing plant yield and productivity.

Rhizobiome engineering: Unveiling complex rhizosphere interactions to enhance plant growth and health

Crop plants are affected by a series of inhibitory environmental and biotic factors that decrease their growth and production. To counteract these adverse effects, plants work together with the microorganisms that inhabit their rhizosphere, which is part of the soil influenced by root exudates. The rhizosphere is a microecosystem where a series of complex interactions takes place between the resident microorganisms (rhizobiome) and plant roots. Therefore, this study analyzes the dynamics of plant-rhizobiome communication, the role of exudates (diffusible and volatile) as a factor in stimulating a diverse rhizobiome, and the differences between rhizobiomes of domesticated crops and wild plants. The study also analyzes different strategies to decipher the rhizobiome through both classical cultivation techniques and the so-called "omics" sciences. In addition, the rhizosphere engineering concept and the two general strategies to manipulate the rhizobiome, i.e., top down and bottom up engineering have been revisited. In addition, recent studies on the effects on the indigenous rhizobiome of inoculating plants with foreign strains, the impact on the endobiome, and the collateral effects on plant crops are discussed. Finally, understanding of the complex rhizosphere interactions and the biological repercussions of rhizobiome engineering as essential steps for improving plant growth and health is proposed, including under adverse conditions.

Mucilaginibacter sp. K Improves Growth and Induces Salt Tolerance in Nonhost Plants via Multilevel Mechanisms

Soil salinity negatively modulates plant growth and development, contributing to severe decreases in the growth and production of crops. Mucilaginibacter sp. K is a root endophytic bacterium that was previously reported by our laboratory to stimulate growth and confer salt tolerance in Arabidopsis (Arabidopsis thaliana). The main purpose of the present study is to elucidate the physiological and molecular machinery responsible for the prospective salt tolerance as imparted by Mucilaginibacter sp. K. We first report that auxin, gibberellin, and MPK6 signalings were required for strain K-induced growth promotion and salt tolerance in Arabidopsis. Then, this strain was assessed as a remediation strategy to improve maize performance under salinity stress. Under normal growth conditions, the seed vigor index, nitrogen content, and plant growth were significantly improved in maize. After NaCl exposure, strain K significantly promoted the growth of maize seedlings, ameliorated decline in chlorophyll content and reduced accretion of MDA and ROS compared with the control. The possible mechanisms involved in salt resistance in maize could be the improved activities of SOD and POD (antioxidative system) and SPS (sucrose biosynthesis), upregulated content of total soluble sugar and ABA, and reduced Na⁺ accumulation. These physiological changes were then confirmed by induced gene expression for ion transportation, photosynthesis, ABA biosynthesis, and carbon metabolism. In summary, these results suggest that strain K promotes plant growth through increases in photosynthesis and auxin- and MPK6-dependent pathways; it also bestows salt resistance on plants through protection against oxidative toxicity, Na⁺ imbalance, and osmotic stress, along with the activation of auxin-, gibberellin-, and MPK6-dependent signaling pathways. This is the first detailed report of maize growth promotion by a Mucilaginibacter sp. strain from wild plant. This strain could be used as a favorable biofertilizer and a salinity stress alleviator for maize, with further ascertainment as to its reliability of performance under field conditions and in the presence of salt stress.

Progress and Applications of Plant Growth-Promoting Bacteria in Salt Tolerance of Crops

Saline soils are a major challenge in agriculture, and salinization is increasing worldwide due to climate change and destructive agricultural practices. Excessive amounts of salt in soils cause imbalances in ion distribution, physiological dehydration, and oxidative stress in plants. Breeding and genetic engineering methods to improve plant salt tolerance and the better use of saline soils are being explored; however, these approaches can take decades to accomplish. A shorter-term approach to improve plant salt tolerance is to be inoculated with bacteria with high salt tolerance or adjusting the balance of bacteria in the rhizosphere, including endosymbiotic bacteria (living in roots or forming a symbiont) and exosymbiotic bacteria (living on roots). Rhizosphere bacteria promote plant growth and alleviate salt stress by providing minerals (such as nitrogen, phosphate, and potassium) and hormones (including auxin, cytokinin, and abscisic acid) or by reducing ethylene production. Plant growth-promoting rhizosphere bacteria are a promising tool to restore agricultural lands and improve plant growth in saline soils. In this review, we summarize the mechanisms of plant growth-promoting bacteria under salt stress and their applications for improving plant salt tolerance to provide a theoretical basis for further use in agricultural systems.

Plant growth-promoting characteristics of halotolerant endophytic bacteria isolated from Sporobolus specatus (Vahr) Kunth and Cyperus laevigatus L. of Ethiopian rift valley lakes

Understanding plant microbes' intimate relationship and search for beneficial microbes is a sustainable alternative to improve plant growth and yield under a wide range of biotic and abiotic stress conditions. More than 20% of the total global agricultural land is affected by salinity. High salinity challenges crop plants by affecting several metabolic pathways and decreasing plant growth and yield. Unlike chemical fertilizers and pesticides, endophytic microbes offer an eco-friendly approach to increasing crop yield via various metabolites during salinity stress. The objective of this study was to isolate and characterize endophytic halotolerant bacterial isolates from haloalkaliphytes, investigate their plant growth-promoting (PGP) properties and tolerance for various stress conditions. Sporobolus specatus (Vahr) Kunth and Cyperus laevigatus L. grass samples were collected from the shores of two Ethiopian soda lakes (Lakes Abijata, and Chitu, respectively). A total of 167 halotolerant endophytic bacterial isolates, that clustered into 21 ARDRA (Amplified ribosomal DNA restriction analysis) groups, affiliated to members of 11 bacterial genera, namely Halomonas, Agrobacterium, Exiguobacterium, Jonesia, Stenotrophomonas, Pseudomonas, Alishewanella, Kosakonia, Bacillus, Paracoccus and Pannonibacter, were identified based on 16S rRNA sequencing. Most of the strains were able to produce IAA (indole-3-acetic acid) and hydrogen cyanide, grow on a nitrogen-free medium and solubilize phosphate. In vitro tolerance tests reveal that isolates were tolerant to: 5.0-15% NaCl, up to 40% PEG 6000, temperatures up to 50 °C, and pH 5-11. These characteristics of the isolates indicate their potential PGP application under various plant stress conditions.

Salt stress mitigation in Lathyrus cicera by combining different microbial inocula

Arid and semi-arid areas are considered vulnerable to various environmental constraints which are further fortified by climate change. Salinity is one of the most serious abiotic factors affecting crop yield and soil fertility. Till now, no information is available on the effect of salinity on development and symbiotic nitrogen (N₂) fixation in the legume species Lathyrus cicera. Here, we evaluated the effect of different microbial inocula including nitrogen-fixing Rhizobium laguerreae, arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, a complex mixed inoculum of AMF isolated from rhizospheric soil in "Al Aitha", and various plant growth-promoting bacteria (PGPB) including Bacillus subtilus, Bacillus simplex and Bacillus megaterium combined with Rhizobium, the AMF consortium, or R. irregularis on alleviating salt stress in this legume. A pot trial was conducted to evaluate the ability of different microbial inocula to mitigate adverse effects of salinity on L. cicera plants. The results showed that salinity (100 mM NaCl) significantly reduced L. cicera plant growth. However, inoculation with different inocula enhanced plant growth and markedly promoted various biochemical traits. Moreover, the combined use of PGPB and AMF was found to be the most effective treatment in mitigating deleterious effects of salinity stress on L. cicera. In addition, this co-inoculation upregulated the expression of two marker genes (LcHKT1 and LcNHX7) related to salinity tolerance. Our findings suggest that the AMF/PGPB formulation has a great potential to be used as a biofertilizer to improve L. cicera plant growth and productivity under saline conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01205-4.

Soil Mineral Composition and Salinity Are the Main Factors Regulating the Bacterial Community Associated with the Roots of Coastal Sand Dune Halophytes

Soil salinity and mineral deficiency are major problems in agriculture. Many studies have reported that plant-associated microbiota, particularly rhizosphere and root microbiota, play a crucial role in tolerance against salinity and mineral deficiency. Nevertheless, there are still many unknown parts of plant–microbe interaction, especially regarding their role in halophyte adaptation to coastal ecosystems. Here, we report the bacterial community associated with the roots of coastal sand dune halophytes Spinifex littoreus and Calotropis gigantea, and the soil properties that affect their composition. Strong correlations were observed between root bacterial diversity and soil mineral composition, especially with soil Calcium (Ca), Titanium (Ti), Cuprum (Cu), and Zinc (Zn) content. Soil Ti and Zn content showed a positive correlation with bacterial diversity, while soil Ca and Cu had a negative effect on bacterial diversity. A strong correlation was also found between the abundance of several bacterial species with soil salinity and mineral content, suggesting that some bacteria are responsive to changes in soil salinity and mineral content. Some of the identified bacteria, such as Bacillus idriensis and Kibdelosporangium aridum, are known to have growth-promoting effects on plants. Together, the findings of this work provided valuable information regarding bacterial communities associated with the roots of sand dune halophytes and their interactions with soil properties. Furthermore, we also identified several bacterial species that might be involved in tolerance against stresses. Further work will be focused on isolation and transplantation of these potential microbes, to validate their role in plant tolerance against stresses, not only in their native hosts but also in crops.

A, A. WA, Bhubalan K, S. SNM, C. I. MS, Ng LC. Setting a Plausible Route for Saline Soil-Based Crop Cultivations by Application of Beneficial Halophyte-Associated Bacteria: A Review

The global scale of land salinization has always been a considerable concern for human livelihoods, mainly regarding the food-producing agricultural industries. The latest update suggested that the perpetual salinity problem claimed up to 900 million hectares of agricultural land worldwide, inducing salinity stress among salt-sensitive crops and ultimately reducing productivity and yield. Moreover, with the constant growth of the human population, sustainable solutions are vital to ensure food security and social welfare. Despite that, the current method of crop augmentations via selective breeding and genetic engineering only resulted in mild success. Therefore, using the biological approach of halotolerant plant growth-promoting bacteria (HT-PGPB) as bio-inoculants provides a promising crop enhancement strategy. HT-PGPB has been proven capable of forming a symbiotic relationship with the host plant by instilling induced salinity tolerance (IST) and multiple plant growth-promoting traits (PGP). Nevertheless, the mechanisms and prospects of HT-PGPB application of glycophytic rice crops remains incomprehensively reported. Thus, this review describes a plausible strategy of halophyte-associated HT-PGPB as the future catalyst for rice crop production in salt-dominated land and aims to meet the global Sustainable Development Goals (SDGs) of zero hunger.

Recent Advances in Bacterial Amelioration of Plant Drought and Salt Stress

Simple Summary

Salt and drought stress cause enormous crop losses worldwide. Several different approaches may be taken to address this problem, including increased use of irrigation, use of both traditional breeding and genetic engineering to develop salt-tolerant and drought-resistant crop plants, and the directed use of naturally occurring plant growth-promoting bacteria. Here, the mechanisms used by these plant growth-promoting bacteria are summarized and discussed. Moreover, recently reported studies of the effects that these organisms have on the growth of plants in the laboratory, the greenhouse, and the field under high salt and/or drought conditions is discussed in some detail. It is hoped that by understanding the mechanisms that these naturally occurring plant growth-promoting bacteria utilize to overcome damaging environmental stresses, it may be possible to employ these organisms to increase future agricultural productivity.

Abstract

The recent literature indicates that plant growth-promoting bacteria (PGPB) employ a range of mechanisms to augment a plant’s ability to ameliorate salt and drought stress. These mechanisms include synthesis of auxins, especially indoleacetic acid, which directly promotes plant growth; synthesis of antioxidant enzymes such as catalase, superoxide dismutase and peroxidase, which prevents the deleterious effects of reactive oxygen species; synthesis of small molecule osmolytes, e.g., trehalose and proline, which structures the water content within plant and bacterial cells and reduces plant turgor pressure; nitrogen fixation, which directly improves plant growth; synthesis of exopolysaccharides, which protects plant cells from water loss and stabilizes soil aggregates; synthesis of antibiotics, which protects stress-debilitated plants from soil pathogens; and synthesis of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which lowers the level of ACC and ethylene in plants, thereby decreasing stress-induced plant senescence. Many of the reports of overcoming these plant stresses indicate that the most successful PGPB possess several of these mechanisms; however, the involvement of any particular mechanism in plant protection is nearly always inferred and not proven.

Deciphering Cadmium (Cd) Tolerance in Newly Isolated Bacterial Strain, Ochrobactrum intermedium BB12, and Its Role in Alleviation of Cd Stress in Spinach Plant (Spinacia oleracea L.)

A cadmium (Cd)-tolerant bacterium Ochrobactrum intermedium BB12 was isolated from sewage waste collected from the municipal sewage dumping site of Bhopal, India. The bacterium showed multiple heavy metal tolerance ability and had the highest minimum inhibitory concentration of 150 mg L-1 of Cd. Growth kinetics, biosorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy studies on BB12 in the presence of Cd suggested biosorption as primary mode of interaction. SEM and TEM studies revealed surface deposition of Cd. FTIR spectra indicated nitrogen atom in exopolysaccharides secreted by BB12 to be the main site for Cd attachment. The potential of BB12 to alleviate the impact of Cd toxicity in spinach plants (Spinacia oleracea L.) var. F1-MULAYAM grown in the soil containing Cd at 25, 50, and 75 mg kg-1 was evaluated. Without bacterial inoculation, plants showed delayed germination, decrease in the chlorophyll content, and stunted growth at 50 and 75 mg kg-1 Cd content. Bacterial inoculation, however, resulted in the early germination, increased chlorophyll, and increase in shoot (28.33%) and root fresh weight (72.60%) at 50 mg kg-1 of Cd concentration after 75 days of sowing. Due to bacterial inoculation, elevated proline accumulation and lowered down superoxide dismutase (SOD) enzyme activity was observed in the Cd-stressed plants. The isolate BB12 was capable of alleviating Cd from the soil by biosorption as evident from significant reduction in the uptake/translocation and bioaccumulation of Cd in bacteria itself and in the plant parts of treated spinach. Potential PGP prospects and heavy metal bioremediation capability of BB12 can make the environmental application of the organism a promising approach to reduce Cd toxicity in the crops grown in metal-contaminated soils.

Tolerance of pesticides and antibiotics among beneficial soil microbes recovered from contaminated rhizosphere of edible crops

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Soil bacterial isolates were recovered from contaminated rhizosphere regions.

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Majority of bacterial isolatesshowed multifarious plant growth promoting (PGP) activities.

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Bacterial isolates exhibited a varied level of pesticide tolerance.

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Sensitivity/resistance pattern among isolates was variable

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Pesticides tolerance and antibiotic resistance among soil isolates were variably correlated

A total of 45 beneficial soil bacterial isolates (15 each of Pseudomonas, Azotobacter and phosphate solubilizing bacteria: PSB) recovered from polluted rhizosphere soils were morphologically and biochemically characterized. Bacterial isolates produced indole-3-acetic acid (IAA), phenolate siderophores; SA (salicylic acid) and 2, 3-dihydroxy benzoic acid (2, 3-DHBA), 1-amino cyclopropane 1-carboxylate (ACC) deaminase, solubilised insoluble phosphate (Pi), secreted exopolysaccharides (EPS) and produced ammonia and cyanogenic compound (HCN). Isolates were tested for their tolerance ability against 12 different agrochemicals (chemical pesticides) and 14 antibiotics. Among Pseudomonas, isolate PS1 showed maximum (2183 µg mL⁻¹) tolerance to all tested agrochemicals. Likewise, among all Azotobacter isolates (n = 15), AZ12 showed maximum (1766 µg mL⁻¹) while AZ7 had lowest (950 µg mL⁻¹) tolerance ability to all tested agrochemicals. Moreover, among phosphate solubilizing bacterial isolates, maximum (1970 µg mL⁻¹) and minimum (1308 µg mL⁻¹) tolerance to agrochemicals was represented by PSB8 and PSB13 isolates, respectively. The antibiotic sensitivity/resistance among isolates varied considerably. As an example, Pseudomonas spp. was susceptible to several antibiotics, and inhibition zone differed between 10 mm (polymyxin B) to 34 mm (nalidixic acid). Also, isolate PS2 showed resistance to erythromycin, ciprofloxacin, methicillin, novobiocin and penicillin. The resistance percentage to multiple antibiotics among Azotobacter isolates varied between 7 and 33%. Among PSB isolates, inhibition zone differed between 10 and 40 mm and maximum and minimum resistance percentage to multiple antibiotics was recorded as 47% and 20%, respectively. The persistence of pesticides in agricultural soil may contribute to an increase in multidrug resistance among soil microorganisms. In conclusion, plant growth promoting (PGP) substances releasing soil microorganisms comprising of inherent/intrinsic properties of pesticides tolerance and antibiotics resistance may provide an attractive, agronomically feasible, and long-term prospective alternative for the augmentation of edible crops. However, in future, more research is needed to uncover the molecular processes behind the development of pesticide tolerance and antibiotic resistance among soil microorganisms.

Halotolerant Rhizobacteria for Salinity-Stress Mitigation: Diversity, Mechanisms and Molecular Approaches

Agriculture is the best foundation for human livelihoods, and, in this respect, crop production has been forced to adopt sustainable farming practices. However, soil salinity severely affects crop growth, the degradation of soil quality, and fertility in many countries of the world. This results in the loss of profitability, the growth of agricultural yields, and the step-by-step decline of the soil nutrient content. Thus, researchers have focused on searching for halotolerant and plant growth-promoting bacteria (PGPB) to increase soil fertility and productivity. The beneficial bacteria are frequently connected with the plant rhizosphere and can alleviate plant growth under salinity stress through direct or indirect mechanisms. In this context, PGPB have attained a unique position. The responses include an increased rate of photosynthesis, high production of antioxidants, osmolyte accumulation, decreased Na+ ions, maintenance of the water balance, a high germination rate, and well-developed root and shoot elongation under salt-stress conditions. Therefore, the use of PGPB as bioformulations under salinity stress has been an emerging research avenue for the last few years, and applications of biopesticides and biofertilizers are being considered as alternative tools for sustainable agriculture, as they are ecofriendly and minimize all kinds of stresses. Halotolerant PGPB possess greater potential for use in salinity-affected soil as sustainable bioinoculants and for the bioremediation of salt-affected soil.

Diversity and Plant Growth-Promoting Ability of Endophytic, Halotolerant Bacteria Associated with Tetragonia tetragonioides (Pall.) Kuntze

The diversity of salt-tolerant cultivable endophytic bacteria associated with the halophyte New Zealand spinach (Tetragonia tetragonioides (Pall.) Kuntze) was studied, and their plant beneficial properties were evaluated. The bacteria isolated from leaves and roots belonged to Agrobacterium, Stenotrophomonas, Bacillus, Brevibacterium, Pseudomonas, Streptomyces, Pseudarthrobacter, Raoultella, Curtobacterium, and Pantoea. Isolates exhibited plant growth-promoting traits, including the production of a phytohormone (indole 3-acetic-acid), cell wall degrading enzymes, and hydrogen cyanide production. Furthermore, antifungal activity against the plant pathogenic fungi Fusarium solani, F. oxysporum, and Verticillium dahliae was detected. Ten out of twenty bacterial isolates were able to synthesize ACC deaminase, which plays a vital role in decreasing ethylene levels in plants. Regardless of the origin of isolated bacteria, root or leaf tissue, they stimulated plant root and shoot growth under 200 mM NaCl conditions. Our study suggests that halophytes such as New Zealand spinach are a promising source for isolating halotolerant plant-beneficial bacteria, which can be considered as potentially efficient biofertilizers in the bioremediation of salt-affected soils.

Interaction Between Halotolerant Phosphate-Solubilizing Bacteria (Providencia rettgeri Strain TPM23) and Rock Phosphate Improves Soil Biochemical Properties and Peanut Growth in Saline Soil

Soil salinity has adverse effects on soil microbial activity and nutrient cycles and therefore limits crop growth and yield. Amendments with halotolerant phosphate-solubilizing bacteria (PSB) and rock phosphate (RP) may improve properties of saline soil. In this study, we investigated the effects of RP either alone or in combination with PSB (Providencia rettgeri strain TPM23) on peanut growth and soil quality in a saline soil. With the combined application of RP and PSB, plant length and biomass (roots and shoots) and uptake of phosphorus (P), nitrogen (N), and potassium (K) increased significantly. Soil Na+ and Cl- contents decreased in the PR alone or PR combined with PSB treatment groups. There were strongly synergistic effects of RP and PSB on soil quality, including a decrease in pH. The soil available N, P, and K contents were significantly affected by the PSB treatments. In addition, the alkaline phosphomonoesterases, urease, and dehydrogenase activities increased significantly compared with the untreated group; highest alkaline phosphomonoesterases activity was observed in the RP and PSB treatment groups. The composition of rhizosphere soil bacterial communities was determined using 454-pyrosequencing of the 16S rRNA gene. In the PR alone or PR combined with PSB treatment groups, the structure of the soil bacterial community improved with increasing richness and diversity. With PSB inoculation, the relative abundance of Acidobacteria, Chloroflexi, and Planctomycetes increased. The three phyla were also positively correlated with soil available N and root dry weight. These results suggested microbiological mechanisms by which the combined use of RP and PSB improved saline soil and promoted plant growth. Overall, the study indicates the combined use of RP and PSB can be an economical and sustainable strategy to increase plant growth in P-deficient and salt-affected soils.

Performance of halotolerant bacteria associated with Sahara-inhabiting halophytes Atriplex halimus L. and Lygeum spartum L. ameliorate tomato plant growth and tolerance to saline stress: from selective isolation to genomic analysis of potential determinants

The use of halotolerant beneficial plant-growth-promoting (PGP) bacteria is considered as a promising eco-friendly approach to improve the salt tolerance of cash crops. One strategy to enhance the possibility of obtaining stress-alleviating bacteria is to screen salt impacted soils. In this study, amongst the 40 endophytic bacteria isolated from the roots of Sahara-inhabiting halophytes Atriplex halimus L. and Lygeum spartum L., 8 showed interesting NaCl tolerance in vitro. Their evaluation, through different tomato plant trials, permitted the isolate IS26 to be distinguished as the most effective seed inoculum for both plant growth promotion and mitigation of salt stress. On the basis of 16S rRNA gene sequence, the isolate was closely related to Stenotrophomonas rhizophila. It was then screened in vitro for multiple PGP traits and the strain-complete genome was sequenced and analysed to further decipher the genomic basis of the putative mechanisms underlying its osmoprotective and plant growth abilities. A remarkable number of genes putatively involved in mechanisms responsible for rhizosphere colonization, plant association, strong competition for nutrients, and the production of important plant growth regulator compounds, such as AIA and spermidine, were highlighted, as were substances protecting against stress, including different osmolytes like trehalose, glucosylglycerol, proline, and glycine betaine. By having genes related to complementary mechanisms of osmosensing, osmoregulation and osmoprotection, the strain confirmed its great capacity to adapt to highly saline environments. Moreover, the presence of various genes potentially related to multiple enzymatic antioxidant processes, able to reduce salt-induced overproduction of ROS, was also detected.

Understanding the potential of root microbiome influencing salt-tolerance in plants and mechanisms involved at the transcriptional and translational level

Soil salinity severely affects plant growth and development and imparts inevitable losses to crop productivity. Increasing the concentration of salts in the vicinity of plant roots has severe consequences at the morphological, biochemical, and molecular levels. These include loss of chlorophyll, decrease in photosynthetic rate, reduction in cell division, ROS generation, inactivation of antioxidative enzymes, alterations in phytohormone biosynthesis and signaling, and so forth. The association of microorganisms, viz. plant growth-promoting rhizobacteria, endophytes, and mycorrhiza, with plant roots constituting the root microbiome can confer a greater degree of salinity tolerance in addition to their inherent ability to promote growth and induce defense mechanisms. The mechanisms involved in induced stress tolerance bestowed by these microorganisms involve the modulation of phytohormone biosynthesis and signaling pathways (including indole acetic acid, gibberellic acid, brassinosteroids, abscisic acid, and jasmonic acid), accumulation of osmoprotectants (proline, glycine betaine, and sugar alcohols), and regulation of ion transporters (SOS1, NHX, HKT1). Apart from this, salt-tolerant microorganisms are known to induce the expression of salt-responsive genes via the action of several transcription factors, as well as by posttranscriptional and posttranslational modifications. Moreover, the potential of these salt-tolerant microflora can be employed for sustainably improving crop performance in saline environments. Therefore, this review will briefly focus on the key responses of plants under salinity stress and elucidate the mechanisms employed by the salt-tolerant microorganisms in improving plant tolerance under saline environments.

Supplying silicon alters microbial community and reduces soil cadmium bioavailability to promote health wheat growth and yield

Soil amendments of black bone (BB), biochar (BC), silicon fertilizer (SI), and leaf fertilizer (LF) play vital roles in decreasing cadmium (Cd) availability, thereby supporting healthy plant growth and food security in agroecosystems. However, the effect of their additions on soil microbial community and the resulting soil Cd bioavailability, plant Cd uptake and health growth are still unknown. Therefore, in this study, BB, BC, SI, and LF were selected to evaluate Cd amelioration in wheat grown in Cd-contaminated soils. The results showed that relative to the control, all amendments significantly decreased both soil Cd bioavailability and its uptake in plant tissues, promoting healthy wheat growth and yield. This induced-decrease effect in seeds was the most obvious, wherein the effect was the highest in SI (52.54%), followed by LF (43.31%), and lowest in BC (35.24%) and BB (31.98%). Moreover, the induced decrease in soil Cd bioavailability was the highest in SI (29.56%), followed by BC (28.85%), lowest in LF (17.55%), and BB (15.30%). The significant effect in SI likely resulted from a significant increase in both the soil bioavailable Si and microbial community (Acidobacteria and Thaumarchaeota), which significantly decreased soil Cd bioavailability towards plant roots. In particular, a co-occurrence network analysis indicated that soil microbes played a substantial role in wheat yield under Si amendment. Therefore, supplying Si alters the soil microbial community, positively and significantly interacting with soil bioavailable Si and decreasing Cd bioavailability in soils, thereby sustaining healthy crop development and food quality.

Plants Saline Environment in Perception with Rhizosphere Bacteria Containing 1-Aminocyclopropane-1-Carboxylate Deaminase

Soil salinity stress has become a serious roadblock for food production worldwide since it is one of the key factors affecting agricultural productivity. Salinity and drought are predicted to cause considerable loss of crops. To deal with this difficult situation, a variety of strategies have been developed, including plant breeding, plant genetic engineering, and a wide range of agricultural practices, including the use of plant growth-promoting rhizobacteria (PGPR) and seed biopriming techniques, to improve the plants' defenses against salinity stress, resulting in higher crop yields to meet future human food demand. In the present review, we updated and discussed the negative effects of salinity stress on plant morphological parameters and physio-biochemical attributes via various mechanisms and the beneficial roles of PGPR with 1-Aminocyclopropane-1-Carboxylate(ACC) deaminase activity as green bio-inoculants in reducing the impact of saline conditions. Furthermore, the applications of ACC deaminase-producing PGPR as a beneficial tool in seed biopriming techniques are updated and explored. This strategy shows promise in boosting quick seed germination, seedling vigor and plant growth uniformity. In addition, the contentious findings of the variation of antioxidants and osmolytes in ACC deaminase-producing PGPR treated plants are examined.

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

The Contrivance of Plant Growth Promoting Microbes to Mitigate Climate Change Impact in Agriculture

Combating the consequences of climate change is extremely important and critical in the context of feeding the world's population. Crop simulation models have been extensively studied recently to investigate the impact of climate change on agricultural productivity and food security. Drought and salinity are major environmental stresses that cause changes in the physiological, biochemical, and molecular processes in plants, resulting in significant crop productivity losses. Excessive use of chemicals has become a severe threat to human health and the environment. The use of beneficial microorganisms is an environmentally friendly method of increasing crop yield under environmental stress conditions. These microbes enhance plant growth through various mechanisms such as production of hormones, ACC deaminase, VOCs and EPS, and modulate hormone synthesis and other metabolites in plants. This review aims to decipher the effect of plant growth promoting bacteria (PGPB) on plant health under abiotic soil stresses associated with global climate change (viz., drought and salinity). The application of stress-resistant PGPB may not only help in the combating the effects of abiotic stressors, but also lead to mitigation of climate change. More thorough molecular level studies are needed in the future to assess their cumulative influence on plant development.

Characterization and Comparison of Bacterial Communities of an Invasive and Two Native Caribbean Seagrass Species Sheds Light on the Possible Influence of the Microbiome on Invasive Mechanisms

Invasive plants, including marine macrophytes, are one of the most important threats to biodiversity by displacing native species and organisms depending on them. Invasion success is dependent on interactions among living organisms, but their study has been mostly limited to negative interactions while positive interactions are mostly underlooked. Recent studies suggested that microorganisms associated with eukaryotic hosts may play a determinant role in the invasion process. Along with the knowledge of their structure, taxonomic composition, and potential functional profile, understanding how bacterial communities are associated with the invasive species and the threatened natives (species-specific/environmentally shaped/tissue-specific) can give us a holistic insight into the invasion mechanisms. Here, we aimed to compare the bacterial communities associated with leaves and roots of two native Caribbean seagrasses (Halodule wrightii and Thalassia testudinum) with those of the successful invader Halophila stipulacea, in the Caribbean island Curaçao, using 16S rRNA gene amplicon sequencing and functional prediction. Invasive seagrass microbiomes were more diverse and included three times more species-specific core OTUs than the natives. Associated bacterial communities were seagrass-specific, with higher similarities between natives than between invasive and native seagrasses for both communities associated with leaves and roots, despite their strong tissue differentiation. However, with a higher number of OTUs in common, the core community (i.e., OTUs occurring in at least 80% of the samples) of the native H. wrightii was more similar to that of the invader H. stipulacea than T. testudinum, which could reflect more similar essential needs (e.g., nutritional, adaptive, and physiological) between native and invasive, in contrast to the two natives that might share more environment-related OTUs. Relative to native seagrass species, the invasive H. stipulacea was enriched in halotolerant bacterial genera with plant growth-promoting properties (like Halomonas sp. and Lysinibacillus sp.) and other potential beneficial effects for hosts (e.g., heavy metal detoxifiers and quorum sensing inhibitors). Predicted functional profiles also revealed some advantageous traits on the invasive species such as detoxification pathways, protection against pathogens, and stress tolerance. Despite the predictive nature of our findings concerning the functional potential of the bacteria, this investigation provides novel and important insights into native vs. invasive seagrasses microbiome. We demonstrated that the bacterial community associated with the invasive seagrass H. stipulacea is different from native seagrasses, including some potentially beneficial bacteria, suggesting the importance of considering the microbiome dynamics as a possible and important influencing factor in the colonization of non-indigenous species. We suggest further comparison of H. stipulacea microbiome from its native range with that from both the Mediterranean and Caribbean habitats where this species has a contrasting invasion success. Also, our new findings open doors to a more in-depth investigation combining meta-omics with bacterial manipulation experiments in order to confirm any functional advantage in the microbiome of this invasive seagrass.

Multi-Trait Wheat Rhizobacteria from Calcareous Soil with Biocontrol Activity Promote Plant Growth and Mitigate Salinity Stress

Plant growth promoting rhizobacteria (PGPR) can be functional microbial fertilizers and/or biological control agents, contributing to an eco-spirit and safe solution for chemical replacement. Therefore, we have isolated rhizospheric arylsulfatase (ARS)-producing bacteria, belonging to Pseudomonas and Bacillus genus, from durum wheat crop grown on calcareous soil. These isolates harbouring plant growth promoting (PGP) traits were further evaluated in vitro for additional PGP traits, including indole compounds production and biocontrol activity against phytopathogens, limiting the group of multi-trait strains to eight. The selected bacterial strains were further evaluated for PGP attributes associated with biofilm formation, compatibility, salt tolerance ability and effect on plant growth. In vitro studies demonstrated that the multi-trait isolates, Bacillus (1.SG.7, 5.SG.3) and Pseudomonas (2.SG.20, 2.C.19) strains, enhanced the lateral roots abundance and shoots biomass, mitigated salinity stress, suggesting the utility of beneficial ARS-producing bacteria as potential microbial fertilizers. Furthermore, in vitro studies demonstrated that compatible combinations of multi-trait isolates, Bacillus sp. 1.SG.7 in a mixture coupled with 5.SG.3, and 2.C.19 with 5.SG.3 belonging to Bacillus and Pseudomonas, respectively, may enhance plant growth as compared to single inoculants.

Isolation of Endophytic Salt-Tolerant Plant Growth-Promoting Rhizobacteria From Oryza sativa and Evaluation of Their Plant Growth-Promoting Traits Under Salinity Stress Condition

The application of plant growth-promoting rhizobacteria (PGPR) as vital components for plant growth promotion against biotic and abiotic stresses could be a promising strategy to improve crop production in areas vulnerable to increasing salinity. Here, we isolated Seventy-five endophytic bacteria from roots of healthy Oryza sativa grown in a saline environment of the southern coastal region of Bangladesh. The endophytes in a culture of ~108 CFU/ml showed arrays of plant growth-promoting (PGP) activities: phytohormone (Indole acetic acid) production (1.20–60.13 μg/ ml), nutrient (phosphate) solubilization (0.02–1.81 μg/ml) and nitrogen fixation (70.24–198.70 μg/ml). Four genomically diverse groups were identified namely, Enterobacter, Achromobacter, Bacillus, and Stenotrophomonas using amplified ribosomal DNA restriction analysis followed by their respective 16S rDNA sequence analyses with that of the data available in NCBI GenBank. These four specific isolates showed tolerance to NaCl ranging from 1.37 to 2.57 mol/L in the nutrient agar medium. Under a 200 mmol/L salt stress in vitro, the bacteria in a culture of 108 CFU/ml exhibited competitive exopolysaccharide (EPS) production: Stenotrophomonas (65 μg/ml) and Bacillus (28 μg/ml), when compared to the positive control, Pseudomonas spp. (23.65 μg/ml), a phenomenon ably supported by their strong biofilm-producing abilities both in a microtiter plate assay, and in soil condition; and demonstrated by images of the scanning electron microscope (SEM). Overall, the isolated endophytic microorganisms revealed potential PGP activities that could be supported by their biofilm-forming ability under salinity stress, thereby building up a sustainable solution for ensuring food security in coastal agriculture under changing climate conditions.

Enterobacter hormaechei (MF957335) enhanced yield, disease and salinity tolerance in tomato

Soil salinity is one of the major limiting factors for poor crop yield in the world. Increasing salinity in the soil is a challenge for agriculture. In the recent past, plant growth-promoting rhizobacteria (PGPR) are being used to enhance plant growth in various conditions. However, the saline-tolerant PGPR are of great use for plant growth under saline condition. In the present study, saline-tolerant E. hormaechei (MF957335) was isolated from saline water. E. hormaechei (MF957335) was tested for its potassium and calcium solubilizing efficiency using Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM-EDX). E. hormaechei (MF957335) and K-Feldspar treatments significantly increased plant growth as compared to untreated plants (negative control). E. hormaechei (MF957335) significantly increased fresh biomass, shoot and root length of tomato plants. Among all the NaCl treatments, maximum fruits (9.66) were achieved in 250 mM NaCl + E. hormaechei treatment. Similar results with increased fruit numbers were obtained in K-Feldspar-treated plants. Apart from the plant growth, fresh biomass and fruit numbers, tomatoes from K-Feldspar-treated plants were large, fleshy and deep red colored. The study could demonstrate bioavailability of potassium from K-feldspar for tomato cultivation. Control plants tomato were small, non-fleshy, yellowish red, and infected with calcium deficiency disease blossom-end rot. The present study demonstrates the role of E. hormaechei (MF957335) in plant growth, yield promotion and disease tolerance by potassium and calcium solubilization, respectively. The study showed that E. hormaechei (MF957335) could be applied to saline and non-saline soils to enhance tomato yield.