Halotolerant Plant Growth-promoting Rhizobacteria Research Articles

Dual PGPR-AMF Inoculation Offsets Salinity Stress Impact on the Fodder Halophyte Sulla carnosa by Concomitantly Modulating Plant ABA Content and Leaf Antioxidant Response

AbstractSalt-tolerant microbes are known to mitigate various biotic and abiotic stresses in plants. However, the intimate mechanisms involved, as well as their effects on the production of signaling molecules associated with the host plant–microbe interaction remain largely unknown. The present work aimed to investigate the role and potential uses of arbuscular mycorrhizal fungi (AMF) Rhizophagus intraradices and/or halotolerant plant growth-promoting rhizobacteria (PGPR) Bacillus subtilis in improving plant growth, functional biochemical synthesis and signaling of endogenous abscisic acid during plant response to short- and long-term salt stress in the forage halophyte Sulla carnosa. Plant growth attributes and biochemical traits were determined at 2 different time intervals (45 and 60 d after transplanting time) when salinity was raised from 100 to 200 mM NaCl. S. carnosa showed significant reduction in dry biomass in response to NaCl stress at the second harvest (200 mM NaCl); however inoculating plants with B. subtilis alone or associated with R. intraradices offset salt impact. Leaf electrolyte leakage was significantly increased by salinity but was significantly reduced following dual microbial inoculation. The applied bacterial inoculants also mitigated oxidative stress as reflected by the higher activities of catalase (APX) and superoxide dismutase (SOD) antioxidant enzymes and reduced H2O2 level. Inoculation with B. subtilis and R. intraradices upregulated 9-cisepoxycarotenoid dioxygenase 1 (NCED1) and SOD genes expression in S. carnosa plants upon salinity treatment. Furthermore, dual AMF-PGPR -inoculated plants accumulated significantly higher levels of abscisic acid (ABA) in both leaves and roots than non-inoculated and single inoculated plants under salinity stress at both harvest times, thereby accounting for their higher salt tolerance of salt-challenged S. carnosa plants. As a whole, the use of halophytic plants associated with beneficial soil microorganisms could improve the effectiveness of biological methods for saline soil rehabilitation. At the mechanistic level, ABA might represent a key player in the attenuation of salt impact in inoculated plants.

Halotolerant Bacillus sp. strain RA coordinates myo-inositol metabolism to confer salt tolerance to tomato.

Soil salinity is a worldwide problem threatening crop yields. Some plant growth-promoting rhizobacteria (PGPR) could survive in high salt environment and assist plant adaptation to stress. Nevertheless, the genomic and metabolic features, as well as the regulatory mechanisms promoting salt tolerance in plants by these bacteria remain largely unknown. In the current work, a novel halotolerant PGPR strain, namely, Bacillus sp. strain RA can enhance tomato tolerance to salt stress. Comparative genomic analysis of strain RA with its closely related species indicated a high level of evolutionary plasticity exhibited by strain-specific genes and evolutionary constraints driven by purifying selection, which facilitated its genomic adaptation to salt-affected soils. The transcriptome further showed that strain RA could tolerate salt stress by balancing energy metabolism via the reprogramming of biosynthetic pathways. Plants exude a plethora of metabolites that can strongly influence plant fitness. The accumulation of myo-inositol in leaves under salt stress was observed, leading to the promotion of plant growth triggered by Bacillus sp. strain RA. Importantly, myo-inositol serves as a selective force in the assembly of the phyllosphere microbiome and the recruitment of plant-beneficial species. It promotes destabilizing properties in phyllosphere bacterial co-occurrence networks, but not in fungal networks. Furthermore, interdomain interactions between bacteria and fungi were strengthened by myo-inositol in response to salt stress. This work highlights the genetic adaptation of RA to salt-affected soils and its ability to impact phyllosphere microorganisms through the adjustment of myo-inositol metabolites, thereby imparting enduring resistance against salt stress in tomato.

Remediation of Saline Soils Using Halo-Tolerant Plant Growth Promoting Rhizobacteria

Soil salinization poses a significant threat to global agriculture, affecting approximately 6.73m Ha land area in India. Salinity stress impacts plant growth and soil health negatively, leading to reduced crop yields and soil degradation. This review examines the sources and effects of soil salinity, highlighting the intricate interplay between salinity and soil nutrients and its remediation. Traditional methods for soil remediation often have detrimental long-term effects, prompting the exploration of alternative strategies such as the use of halo-tolerant plant growth-promoting rhizobacteria. HT-PGPR offer a promising solution for sustainable agriculture by enhancing soil fertility and plant resilience to salinity stress through various mechanisms. Furthermore, this review identifies research gaps in understanding the metabolic pathways and strain selection of HT-PGPR, as well as their interactions with soil microbiota. Future research directions include field-scale experiments to validate the effectiveness and economic viability of HT-PGPR inoculation for large-scale application in saline soils. Overall, leveraging the potential of HT-PGPR represents a critical step towards mitigating the global challenge of soil salinity and ensuring food security in the face of climate change.

Halotolerant Endophytic Bacteria Regulate Growth and Field Performance of Two Durum Wheat Genotypes with Contrasting Salinity Tolerance Potential.

Soil salinity hampers durum wheat plant growth and development at various stages. The detrimental effects of salinity on plant cellular and physiological processes necessitate strategies to alleviate its negative impact and improve overall crop yield. This study investigates the efficacy of plant growth-promoting rhizobacteria (PGPR) bacteria inoculation in mitigating salinity stress on two durum wheat genotypes with contrasting degrees of salinity tolerance (Tamaroi, salt-sensitive and Line 5004, salt-tolerant) under greenhouse and field conditions. For this purpose, two halotolerant-PGPR strains, Pseudomonas jordanii strain G34 and Oceanobacillus jordanicus strain GSFE11, were utilized for the inoculation. For the greenhouse experiment, the two selected genotypes were subjected to salinity at the flag leaf stage with continuous irrigation with a Hoagland solution supplemented with 50 mM NaCl. Field experiments were conducted across two locations with contrasting salinity levels over two growing seasons. At the end of both experiments, various parameters including total weight, spike weight, grain weight, spike number, spikelet number, grains per spike and thousand kernel weight were measured. The halotolerant PGPRs, P. jordanii strain G34 and O. jordanicus strain GSFE11, proved effective in alleviating salinity-induced adverse effects and enhancing growth under greenhouse and field conditions. However, bacterial inoculation significantly improved growth in the salt-sensitive genotype and such effects were not observed in the tolerant genotype, emphasizing genotype-specific responses. Notably, inoculation with O. jordanicus increased Na+ and Ca2+ uptake in the salt-tolerant "Line 5004" without hindering growth, suggesting one of its potential mechanisms for salt tolerance. This research demonstrates the potential of halotolerant-PGPR inoculation in enhancing durum wheat production in saline environments, but also underscores the importance of understanding genotype-specific responses for tailored interventions.

Integrated physiological, biochemical and transcriptomic analyses reveal the mechanism of salt tolerance induced by a halotolerant Serratia sp. NTN6 in maize

Salt stress is a serious environmental challenge that hinders crop growth and yield. Plant growth-promoting rhizobacteria (PGPR) can successfully and environmentally friendly boost plant development in saline soils. In this study, a halotolerant Serratia sp. NTN6 was isolated from the rhizosphere soil of halophyte Puccinellia tenuiflora and its plant growth-promoting properties were determined. NTN6 inoculation significantly promoted seed germination and seedling growth in maize under salt stress. Shoot height, stem thickness, root fresh weight, shoot fresh weight, root dry weight, and shoot dry weight of salt-stressed seedlings increased by 36.3%, 32.5%, 146.4%, 193.6%, 25.0%, and 109.6%, respectively, after inoculation with NTN6. NTN6-mediated maize salt tolerance was revealed via transcriptomic, physiological, and biochemical analyses. Under salt stress, NTN6 regulated the transcript levels of genes related to chlorophyll synthesis, photosynthesis apparatus, and carbon assimilation pathways, and thus increasing chlorophyll content, Fv/Fm, and net photosynthetic rate; NTN6 reduced the content of O2•–, H2O2, and MDA by regulating the transcripts and activities of antioxidant enzymes; NTN6 affected the transcriptional response of genes involved in ABA synthesis and signaling to salt stress and reduced ABA level. Finally, NTN6 boosted photosynthetic capacity in salt-stressed maize by modulating photochemical activity, C4 pathway and Calvin cycle as well as mitigating reactive oxygen species damage to the photosystem reaction center. This study first investigated the mechanism of Serratia sp.-induced salt tolerance in maize and these findings demonstrate the valuable contribution of halotolerant PGPR strain from halophytes in improving salinity tolerance in non-halophytic crops.

Mitigation of Salt Stress in Rice by the Halotolerant Plant Growth-Promoting Bacterium Enterobacter asburiae D2.

Salinity is a major abiotic stress that seriously affects crop growth worldwide. In this work, we aimed to isolate potential halotolerant plant growth-promoting rhizobacteria (PGPR) to mitigate the adverse impacts of salt stress in rice. An isolate, D2, with multiple plant growth-promoting (PGP) characteristics was identified as Enterobacter asburiae D2. Strain D2 could produce indole-3-acetic acid and siderophore. It also exhibited phosphate solubilization and 1-aminocyclopropane-1-carboxylic deaminase activity. Genome analysis further provided insights into the molecular mechanism of its PGP abilities. Strain D2 inoculation efficiently stimulated rice growth under both normal and saline conditions. Compared with the non-inoculated plants, a significant increase in plant height (18.1-34.7%), root length (25.9-57.1%), root dry weight (57.1-150%), and shoot dry weight (17.3-50.4%) was recorded in inoculated rice seedlings. Meanwhile, rice seedlings inoculated with strain D2 showed improvement in chlorophyll and proline content, while the oxidant damage was reduced in these plants in comparison with the control group. Moreover, the K+/Na+ ratio of the inoculated rice seedlings exposed to NaCl and Na2CO3 was higher than that of the uninoculated groups. These results imply that Enterobacter asburiae D2 is a potential PGPR that can be used for alleviation of salt stress in rice.

Thriving against the odds: Exploring halotolerant rhizobacteria in the saline rhizosphere of Tuzla lagoon's halophytes

Soil salinization poses a significant threat to global crop production, affecting around 20% of agricultural fields with the challenges of osmotic and ionic stress. The reclamation of salinity-affected agricultural lands is a pressing concern. To bridge existing gaps in our understanding of halotolerant plant growth-promoting rhizobacteria (PGPR), this research strategically focuses on isolating and characterizing these beneficial bacteria from the rhizosphere of halophytes in the Tuzla lagoon area. The objective is to contribute essential insights, fostering a deeper understanding of halotolerant PGPR for application in sustainable agricultural practices, particularly in alleviating salinity-induced stress in plants.All nine isolates demonstrated remarkable abilities, including the synthesis of siderophores, with isolates B15, C20, E2, and H9 showcasing phosphate solubilization. Additionally, isolates E2 and H9 displayed phytase production, and five isolates exhibited indole acetic acid synthesis. Although 1-aminocyclopropane-1-carboxylate deaminase production was absent, hydrogen cyanide synthesis was evident in all isolates except H9. Isolate E2 did not demonstrate cellulase synthesis, but biofilm formation was observed in all isolates except C23.To assess the salinity stress mitigation potential of the isolates, a randomized complete block design bioassay was conducted with ten treatments (nine bacterial isolates and a control) across four salt concentrations (0 mM, 100 mM, 200 mM, and 300 mM). While the isolates successfully demonstrated multiple plant growth-promoting traits, their capacity to significantly alleviate the effects of salinity stress on wheat plants suggests the need for further exploration. Comprehensive greenhouse/field trials under diverse physiological and environmental conditions are recommended to unlock the full potential of these isolates in effectively mitigating salinity stress in crops.

Halotolerant Plant Probiotic Bacterial Isolates of Mangrove Soils of Chidambaram and Thanjavur, India

Halotolerant plant growth-promoting rhizobacteria (HTPGPR) are beneficial microbes that can be exploited to mitigate the negative effects of soil salinity on crops. In the current investigation, eight saline soil samples collected from 2 mangrove ecosystems of Tamil Nadu (Chidambaram and Thanjavur) during December 2022 were used to isolate saline tolerant bacterial cultures at the Department of Agricultural Microbiology, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India. Using 5 different bacterial growth media, 48 rhizobacterial isolates (ST1 to ST48) were obtained. Salt tolerance ability (0, 5, 10, 15 and 20% NaCl) of these 48 isolates under in vitro conditions indicated their potential to tolerate up to 10% NaCl (1.71 M). None of the isolates could grow at 20% NaCl (3.42M). Bacterial isolates such as ST11, ST13, ST18, ST20 and ST27 showed minimum growth at 15%NaCl (2.57M). 33 isolates which could grow well at higher salt concentrations were selected. Among 33 isolates, 18 isolates with higher concentration of intracellular sodium content (ST1, ST2, ST3, ST7, ST8, ST9, ST11, ST12, ST13, ST17, ST18, ST20, ST27, ST30, ST32, ST34, ST39, and ST40) were selected and characterized qualitatively for their ability to mineralize phosphate, potassium, and zinc, and to produce HCN. Potential of these18 bacterial isolates to tolerate other abiotic stress factors such as pH and temperature was also studied. Among 18 isolates, the isolates ST7, ST17, and ST30 were found to be multimineral solubilizers. Bacterial isolate ST17 was found to prefer alkaline (pH 9.0) and mesophilic temperature (35ºC) for its growth at both saline (5% NaCl) and non saline condition.

Effectiveness of Phosphate and Zinc Solubilizing Paenarthrobacter nitroguajacolicus P1 as Halotolerant Rhizobacterium with Growth-Promoting Activity on Pistacia vera L.

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms to develop microbial fertilizers. Biofertilizers can accelerate plant growth and enhance crop yields. The current research aimed to isolate and identify rhizobacterium with plant growth-promoting activity in the rhizospheric region of pistachio trees in arid and salty region of Iran. In the present study, 26 bacterial isolates were isolated from the rhizospheric region of the pistachio trees. Plant growth-promoting characteristics of isolated bacteria, including the ability to solubilize phosphate and zinc, produce hydrolyzing enzymes, and hydrogen cyanide (HCN), as well as synthesize indole-3-acetic acid (IAA) were evaluated through in vitro assays. Based on these activities, five multifunctional bacterial strains designated P1, P10, P11, P17, and P19 were then applied and their effect was studied on the growth and physiological properties of Pistacia vera L. seedlings by pot experiments under normal conditions. Finally, the most efficient strain has been identified by analysis of the 16S rRNA gene sequence. According to the results, all the isolated bacteria exhibited considerable plant growth-promoting properties. They could produce amylase (n = 26, 2 ± 0.00-13 ± 0.42mm), lipase (n = 24, 2 ± 0.00-9 ± 0.23 mm), protease (n = 20, 1 ± 0.00-17 ± 0.0 mm), indole-3-acetic acid (n = 26, ranging from 5.05 ± 0.08 to 11.5 ± 0.11 μg/mL) and HCN (n = 24). Six isolates showed significant growth at 20% w/v NaCl. Inoculation of P1, P17, and P19 increased chlorophyll, carotenoid, and phenolic content in treated Pistacia vera L. seedlings. P1 and P11 inoculated plants showed an enhanced level of anthocyanin and proline. These most effective strains were catalase and Gram-positive bacterium and showed antibiotic sensitivity. They can consider as halotolerant PGPR, due to the growth in the presence of NaCl (20% w/v). Finally, P1 inoculated plants exhibited higher levels of sugar content. This strain showed the most similarity (99.92%-1322 bp) to Paenarthrobacter nitroguajacolicus based on 16S rRNA gene sequence. Based on the results, Paenarthrobacter nitroguajacolicus P1 with multiple PGPR can be applied as a promising candidate in the soil-Pistacia vera L. system to improve their productivity and health by increasing available nutrient content, improving photosynthetic parameters, and producing phytohormones and HCN.

Counteracting action of Bacillus stratosphericus and Staphylococcus succinus strains against deleterious salt effects on Zea mays L.

The salinization of soil is the process of progressive accumulation of salts such as sulfates, sodium, or chlorides into the soil. The increased level of salt has significant effects on glycophyte plants, such as rice, maize, and wheat, which are staple foods for the world's population. Consequently, it is important to develop biotechnologies that improve crops and clean up the soil. Among other remediation methods, there is an environmentally friendly approach to ameliorate the cultivation of glycophyte plants in saline soil, namely, the use of microorganisms tolerant to salt with growth-promoting features. Plant growth-promoting rhizobacteria (PGPR) can improve plant growth by colonizing their roots and playing a vital role in helping plants to establish and grow in nutrient-deficient conditions. Our research aimed to test in vivo halotolerant PGPR, isolated and characterized in vitro in a previous study conducted in our laboratory, inoculating them on maize seedlings to improve their growth in the presence of sodium chloride. The bacterial inoculation was performed using the seed-coating method, and the produced effects were evaluated by morphometric analysis, quantization of ion contents (sodium, potassium), produced biomass, both for epigeal (shoot) and hypogeal (root) organs, and by measuring salt-induced oxidative damage. The results showed an increase in biomass and sodium tolerance and even a reduction of oxidative stress in seedlings pretreated with a PGPR bacterial consortium (Staphylococcus succinus + Bacillus stratosphericus) over the control. Moreover, we observed that salt reduces growth and alters root system traits of maize seedlings, while bacterial treatment improves plant growth and partially restores the root architecture system in saline stress conditions. Therefore, the PGPR seed-coating or seedling treatment could be an effective strategy to enhance sustainable agriculture in saline soils due to the protection of the plants from their inhibitory effect.

Effects of halotolerant rhizobacteria on rice seedlings under salinity stress

Avirulent halotolerant plant growth-promoting rhizobacteria (HPGPR) located on the roots' periphery can reduce abiotic stressors (such as salinity and drought), enhance plant productivity. Salinity poses a significant challenge for growing agricultural products, like rice, in the coastal regions. It is crucial to enhance production because of limited arable land and the high growth rate of the population. This study targeted to identify HPGPR from legume root nodules and assessed their effects on rice plants experiencing salt stress in coastal regions of Bangladesh. Based on the culture morphology, biochemical, salt, pH, and temperature tolerance traits, sixteen bacteria were isolated from the root nodules of leguminous plants (Common bean, Yardlong bean, Dhaincha, and Shameplant). All the bacterial strains can tolerate 3 % salt concentration, and capable to survive at the highest 45 °C temperature and pH 11 (without isolate 1). Three preeminent bacteria, Agrobacterium tumefaciens (B1), Bacillus subtilis (B2), and Lysinibacillus fusiformis (B3) were specified through morpho-biochemical and molecular (16S rRNA gene sequence) exploration for inoculation. To assess the plant growth-promoting activities, germination tests are applied where bacterial inoculation increased germination in saline and non-saline conditions. Control group (C) showed 89.47 % and bacterial treated groups (C + B1, C + B2, and C + B3) 95 %, 90 %, and 75 % germination after 2 days of inoculation. In (1 % NaCl) saline condition control group revealed 40 % whereas three groups with bacteria showed 60 %, 40 %, and 70 % germination after 3 days, which increased 70 %, 90 %, 85 %, and 95 % respectively after 4 days of inoculation. The HPGPR significantly improved plant development metrics such as root length, shoot length, fresh and arid biomass yield, chlorophyll content, etc. Our results suggest that the salt-resistant bacteria (Halotolerant) have a great potential role in recuperating plant growth and would be cost-effective as a bio-inoculant in saline conditions to be used as a prospective bio-fertilizer for rice production. These findings indicate that the HPGPR has a substantially promising function in reviving plant development in an eco-friendly manner.

Novel halotolerant rhizobacterial strains mitigated the salt stress in-vitro and in-vivo and improved the growth of tomato plants

Increasing land salinization has become one of the most damaging threats to the global modern agriculture and has challenged food production and food security. Plant growth promoting rhizobacteria (PGPR) have emerged as efficient biological tools to improve plant growth in stressed environments. The present study was conducted to isolate, identify and characterize halotolerant PGPR from an unexplored saline area of Pakistan. The effect of the isolated strains was evaluated in-vitro and on the tomato varieties grown under 150 mM salt stress in the field. A total of 12 strains were isolated and further screened in-vitro to check their halotolerance. The isolates were screened using nutrient agar medium supplemented with different levels (5, 10, 15, 20 and 25%) of NaCl. After screening, two highly salt tolerant strains were selected and identified using 16S rRNA technique. Isolates B9 and B10 were successfully identified as Pseudarthrobacter oxydans (NR_026236.1) and Staphylococcus pasteuri (NR_024669.1). The isolates were tested qualitatively for their biochemical and plant growth promoting traits. These isolates promoted tomato growth under salinity stress when applied alone or in consortium. The isolates demonstrated catalase and protease activity, phosphorous solubilization (PSI: 2.45, 2.63) and produced indole acetic acid. The current findings established that the isolated strains were highly resistant and tolerant up to 25% NaCl stress as compared to the other reported strains. These halophiles possessed promising plant growth promoting traits hence, are assumed to have great potential to serve as bioinoculants for the alleviation of adverse effects of salinity on crops.

Evaluation of the growth-inducing efficacy of various Bacillus species on the salt-stressed tomato (Lycopersicon esculentum Mill.).

Plants are affected by salt stress in a variety of ways, including water deficiency, ion toxicity, nutrient imbalance, and oxidative stress, all of which can cause cellular damage or plant death. Halotolerant plant growth-promoting rhizobacteria (PGPR) could be a viable alternative for tomato plants growing in arid and semi-arid environments. The aim of this research was to isolate halotolerant plant growth promoting Bacillus sp. to promote tomato (Lycopersicon esculentum Mill.) growth and salt stress resistance. 107 PGPR strains were isolated from the rhizospheres of 'Kesudo' (Butea monosperma Lam.), 'Kawaria' (Cassia tora L.), and 'Arjun' (Terminalia arjuna Roxb.) plants to test their plant growth promoting abilities, including indole-3-acetic acid, phosphate solubilization, siderophore production, and ACC deaminase activity. Five bacterial strains (Bacillus pumilus (NCT4), Bacillus firmus (NCT1), Bacillus licheniformis (LCT4), Bacillus cereus (LAT3), and Bacillus safensis (LBM4)) were chosen for 16S rRNA on the basis of PGPR traits. Compared to PGPR untreated plants, tomato plants developed from PGPR-treated seeds had considerably increased germination percentage, seedling growth, plant height, dry weight, and leaf area. As comparison to PGPR non-inoculated plants, salt-stressed tomato plants treated with PGPR strains had higher levels of total soluble sugar, proline, and chlorophyll as well as higher levels of SOD, CAT, APX, and GR activity. PGPR-inoculated salt-stressed tomato plants had lower MDA, sodium, and chloride levels than non-inoculated plants. In addition, magnesium, calcium, potassium, phosphorus, and iron levels were higher in PGPR treated plants when subjected to salt stress. These results indicate that halotolerant PGPR strains can increase tomato productivity and tolerance to salt stress by removing salt stress's negative effects on plant growth.

Role of Halotolerant Plant Growth-Promoting Rhizobacteria in Mitigating Salinity Stress: Recent Advances and Possibilities

Soil salinity is one of the major abiotic constraints in agricultural ecosystems worldwide. High salinity levels have negative impacts on plant growth and yield, and affect soil physicochemical properties. Salinity also has adverse effects on the distribution and abundance of soil microorganisms. Salinity problems have previously been addressed in research, but most approaches, such as breeding for salt tolerant varieties and soil amelioration, are expensive and require years of efforts. Halotolerant plant growth-promoting rhizobacteria (HT-PGPR) secrete secondary metabolites, including osmoprotectants, exopolysaccharides, and volatile organic compounds. The importance of these compounds in promoting plant growth and reducing adverse effects under salinity stress has now been widely recognised. HT-PGPR are emerging as effective biological strategies for mitigating the harmful effects of high salinity; improving plant growth, development, and yield; and remediating degraded saline soils. This review describes the beneficial effects and growth-promoting mechanisms of various HT-PGPR, which are carried out by maintaining ion homeostasis, increasing nutrient availability, and the producing secondary metabolites, osmoprotectants, growth hormones, and volatile organic compounds. Exploring suitable HT-PGPR and applications in agriculture production systems can play a crucial role in reducing the adverse impacts of salinity stress and sustainable crop productivity.

Halotolerant Streptomyces spp. induce salt tolerance in maize through systemic induction of the antioxidant system and accumulation of proline

Plant Growth Promoting Rhizobacteria (PGPR) can mitigate the detrimental effects of salinity in plants. We characterized Streptomyces spp. as halotolerant PGPR and evaluated their influence on salt stress-tolerance mechanisms in maize plants. Bacterial halotolerance was evaluated at concentrations up to 900 mM NaCl, analyzing cell growth, viability, and production of secondary metabolites. The mechanisms of systemic tolerance in plants, such as the enzymatic activity of the antioxidant system and the production of osmolytes, were analyzed in bacterized plants under salt stress (100 and 300 mM NaCl). Isolates proved viable and produced indolic compounds and siderophores under NaCl conditions. Synthesis of indolic compounds and phosphate solubilization were induced in CLV179 by salt stress. The activity of the antioxidant enzymes and the synthesis of proline increased in bacterized plants submitted to salt stress, mainly those treated with isolate CLV97 and CLV179. Response of Streptomyces-treated plant to salinity was observed in roots and leaves, suggesting a mechanism of systemic tolerance. Therefore, bacterization with Streptomyces spp. alleviated the deleterious effects of salinity in maize plants by modulating the enzymatic and non-enzymatic antioxidant mechanisms.

A Bibliometric Review of Plant Growth-Promoting Rhizobacteria in Salt-Affected Soils

As a primary form of land degradation in arid and semi-arid areas, soil salinity can adversely affect plant nutrient balance, photosynthesis, protein synthesis, energy metabolism, and other functions. Plant growth-promoting rhizobacteria (PGPR) inoculation of plants is an environmentally friendly strategy to alleviate salt stress and improve salt tolerance. Based on the Web of Science Core Collection (WoSCC) database, in terms of the number of publications and citations, collaboration networks, and keywords, this bibliometric analysis employed VOSviewer 1.6.17 and HistCite Pro 2.1 software to map the scientific knowledge of related research, comprehensively review knowledge structure and provide an outlook on future research topics. The results showed that publications and citations increased exponentially between 1978 and 2021. Regarding knowledge structure, Asian nations conducted research in a more concentrated manner, developed close collaborative relationships, and produced rich research results. Halotolerant PGPR, sustainable agriculture, microbial community, soil salinization, microbiome, oxidative stress, and biofertilizer, are currently hot topics. This bibliometric study will provide a meaningful reference for investigating the field’s evolution and pinpointing the research frontiers.

Improved salinity and dust stress tolerance in the desert halophyte Haloxylon aphyllum by halotolerant plant growth-promoting rhizobacteria.

Because of global warming, desertification is increasing. One of the best strategies for combating desertification is reforestation of forests and biological operations of vegetation. However, events like soil salinity and dust storms, as the most important manifestations of desertification, prevent vegetation from settling in these areas. In this study, the effects of two halotolerant plant growth-promoting rhizobacterial strains, Bacillus pumilus HR and Zhihengliuella halotolerans SB, on physiological and nutritional status of the desert halophyte Haloxylon aphyllum under the stress of salinity (0, 300, and 600 mM NaCl) and dust (0 and 1.5 g m−2 month−1) were examined. Under dust application, the Z. halotolerans SB strain compared to the B. pumilus HR strain and the combination of these two bacterial strains improved the content of total chlorophyll (247 and 316%), carotenoid (94 and 107%), phosphorus (113 and 209%), magnesium (196 and 212%), and total dry biomass (13 and 28%) in H. aphyllum at salinity levels of 300 and 600 mM NaCl, respectively. Under conditions of combined application of dust and salinity, B. pumilus HR compared to Z. halotolerans SB and the combination of two strains at salinity levels of 300 and 600 mM NaCl, respectively, had better performance in increasing the content of iron (53 and 69%), calcium (38 and 161%), and seedling quality index (95 and 56%) in H. aphyllum. The results also showed that both bacterial strains and their combination were able to reduce the content of ascorbic acid, flavonoid, total phenol, proline, and malondialdehyde, and catalase activity, and ultimately improve the antioxidant capacity of H. aphyllum. This showed that the use of halotolerant rhizobacteria can stop the production of free radicals and thus prevent cell membrane damage and the formation of malondialdehyde under salinity and dust stress. The results of this study for the first time showed that halotolerant rhizobacteria can increase the seedling quality index of H. aphyllum under combined conditions of salinity and dust. The use of these bacteria can be useful in the optimal afforestation of H. aphyllum species in arid and semi-arid ecosystems.

Effects of multiple halotolerant rhizobacteria on the tolerance, growth, and yield of rice plants under salt stress.

Halotolerant bacteria get adapted to a saline environment through modified physiological/structural characteristics and may provide stress tolerance along with enhanced growth to the host plants by different direct and indirect mechanisms. This study reports on multiple halotolerant plant growth-promoting rhizobacteria isolated from the coastal soils in Bangladesh, in fields where the halophytic wild rice Oryza coarctata is endemic. The aim was to find halotolerant bacteria for potential use as biofertilizer under normal/salt-stressed conditions. In this study, eight different strains were selected from a total of 20 rhizobacterial isolates from the saline-prone regions of Debhata and Satkhira based on their higher salt tolerance. 16S rRNA gene sequencing results of the rhizobacterial strains revealed that they belonged to Halobacillus, Bacillus, Acinetobactor, and Enterobactor genera. A total of ten halotolerant rhizobacteria (the other 2 bacteria were previously isolated and already reported as beneficial for rice growth) were used as both single inoculants and in combinations and applied to rice growing in pots. To investigate their capability to improve rice growth, physiological parameters such as shoot and root length and weight, chlorophyll content at the seedling stage as well as survival and yield at the reproductive stage were measured in the absence or presence (in concentration 40 or 80 mmol/L) of NaCl and in the absence or presence of the rhizobacteria. At the reproductive stage, only 50% of the uninoculated plants survived without setting any grains in 80 mmol/L NaCl in contrast to 100% survival of the rice plants inoculated with a combination of the rhizobacteria. The combined halotolerant rhizobacterial inoculations showed significantly higher chlorophyll retention as well as yield under the maximum NaCl concentration applied compared to application of single species. Thus, the use of a combination of halotolerant rhizobacteria as bioinoculants for rice plants under moderate salinity can synergistically alleviate the effects of stress and promote rice growth and yield.

Characterization of Siccibacter sp. Strain C2 a Novel Rhizobacterium that Enhances Tolerance of Barley to Salt Stress.

Plant growth promoting rhizobacteria (PGPR) arouse an increasing interest as an eco-friendly solution for improving crop tolerance to environmental stresses. In this study, we report the characterization of a novel halotolerant PGPR strain (named C2) identified in a screen of rhizospheric bacterial isolates from southeast of Tunisia. Phylogenetic analysis showed that strain C2 is most likely affiliated to the genus Siccibacter with Siccibacter turicensis as the closest species (98.19%). This strain was able to perform phosphate solubilization and production of indole acetic acid (IAA), siderophores, hydrogen cyanide (HCN), as well as different hydrolytic enzymes (proteases, amylases, cellulases, and lipases). The potential of strain C2 in enhancing salt stress tolerance of Hordeum vulgare was also investigated. Our greenhouse inoculation assays showed that strain C2 promotes barley growth in the presence of 400mM NaCl by increasing biomass, root length, and chlorophyll contents. It has a positive effect on the photosynthetic efficiency, concomitantly with lower intercellular CO2 contents, compared to non-inoculated plants. Moreover, barley inoculation with strain C2 under salt stress, resulted in higher accumulation of proline and soluble sugars and alleviate the oxidative stress by decreasing hydrogen peroxide and malondialdehyde contents. Remarkably, this positive effect corroborates with a significant activation in the expression of a subset of barley stress responsive genes, including HVA1, HvDREB1, HvWRKY38 and HvP5CS. In summary, Siccibacter sp. strain C2 is able to enhance barley salt stress tolerance and should be leveraged in developing sustainable practices for cereal crop production.

Halotolerant native bacteria Enterobacter 64S1 and Pseudomonas 42P4 alleviate saline stress in tomato plants.

Salinity is one of the principal abiotic stresses that limit the growth and productivity of crops. The use of halotolerant plant growth-promoting rhizobacteria (PGPR) that increase the growth of salt-stressed crops is an environmentally friendly alternative to promote plant yield under salinity. The aim of this study was to test native PGPR, isolated according to their tolerance to NaCl, and to evaluate their influence on morphological, physiological, and biochemical traits promoted by salt stress in tomato plants. Enterobacter 64S1 and Pseudomonas 42P4 were selected as the most efficient strains in terms of salt tolerance. Both strains were classified as moderately resistant to salinity (NaCl) and maintained their plant growth-promoting activities, such as nitrogen fixation and phosphate solubilization, even in the presence of high levels of salt. The results of a greenhouse experiment demonstrated that PGPR inoculation increased root and shoot dry weight, stem diameter, plant height, and leaf area compared to control noninoculated plants under nonsaline stress conditions, reversing the effects of salinity. Inoculated plants showed increased tolerance to salt conditions by reducing electrolyte leakage (improved membrane stability) and lipid peroxidation and increasing chlorophyll quantum efficiency (Fv/Fm) and the performance index. Also, inoculation increased the accumulation of proline and antioxidant nonenzymatic compounds, such as carotenes and total phenolic compounds. The catalase and peroxidase activities increased with salinity, but the effect was reversed by Enterobacter 64S1. In conclusion, Enterobacter 64S1 and Pseudomonas 42P4 isolated from salt-affected regions have the potential to alleviate the deleterious effects of salt stress in tomato crops.