Plant growth promoting bacteria (PGPR) induce antioxidant tolerance against salinity stress through biochemical and physiological mechanisms

Exiguobacterium aurantiacum SA100 induces antioxidant enzymes and salinity tolerance gene expression in wheat

This study evaluated the effects of Exiguobacterium aurantiacum SA100 on wheat (Triticum aestivum) growth under varying levels of salinity stress. Results indicated that SA100 significantly enhanced seed germination, root and shoot length, and fresh and dry biomass across salinity levels, particularly at 50 and 100 mM NaCl. Inoculation improved antioxidant enzyme activities (CAT, APX, POD, PPO), increased total phenolic content, and reduced oxidative damage by lowering MDA and H2O2 levels under 150 mM salinity. Ionic balance was maintained, with significant increases in K+, Mg++, and Ca++ and a reduction in Na+ accumulation. Gene expression analysis revealed upregulation of salt-tolerance genes (NAC7, NHX1, SOS1) and downregulation of stress-responsive genes (GS1, DREB2, DHN13, WRKY32). Principal component analysis confirmed that SA100 promotes salinity tolerance by modulating both biochemical and molecular responses. These findings suggest E. aurantiacum SA100 as a promising bioinoculant for enhancing wheat resilience under salinity stress.

Transformative strategies for saline soil restoration: Harnessing halotolerant microorganisms and advanced technologies

Soil salinity is a critical global challenge that severely impairs crop productivity and soil health by disrupting water uptake, nutrient acquisition, and ionic balance in plants, thereby posing a significant threat to food security. This review underscores innovative strategies to mitigate salinity stress, focusing on the pivotal role of halotolerant microorganisms and their synergistic interactions with plants. Halotolerant microorganisms enhance plant resilience through diverse mechanisms under salinity, including exopolysaccharide production, sodium sequestration, and phytohormone regulation. It improves ionic balance, nutrient uptake, and root development, facilitated by osmoregulatory and genetic adaptations. In this discussion, we explored emerging technologies, including genome editing (e.g., CRISPR-Cas9), synthetic biology, and advanced omics-based tools such as metagenomics and metatranscriptomics. These cutting-edge approaches offer profound insights into microbial diversity and their functional adaptations to saline environments. By leveraging these technologies, it is possible to design targeted bioremediation strategies through the customization of microbial functionalities to address specific environmental challenges effectively. Advanced methodologies, such as microbial volatile organic compounds (mVOCs), nanotechnology, and stress-tolerant microbial consortia, significantly enhance plant stress tolerance and facilitate soil restoration. Moreover, integrating digital technologies, including machine learning and artificial intelligence (AI), optimizes bioremediation processes by providing precise, scalable, and adaptable solutions tailored to diverse agricultural ecosystems. The synergistic application of halotolerant microbe-mediated approaches with advanced biotechnological and digital innovations presents a transformative strategy for saline soil restoration. Future research should focus on harmonizing these technologies and methodologies to maximize plant-microbe interactions and establish resilient, sustainable agricultural systems.

Role of plant growth-promoting rhizobacteria in sustainable agriculture: Addressing environmental and biological challenges

This review underscores the importance of plant growth-promoting rhizobacteria (PGPR), fostering sustainability to address various environmental and biological issues. PGPR helps crops withstand salinity, nutrient deficiencies, and drought stress while tackling agricultural threats. Sustainable agriculture has emerged as a response to the social and economic problems farming practices face. Plants encounter obstacles from biotic stressors such as bacteria, viruses, nematodes, arachnids, and weeds that impede their growth. Furthermore, PGPR enhances plant growth through improved nutrient absorption and defense against pests. Bacillus subtilis utilizes indirect methods to combat diseases and protect plants from various diseases and pests. Additionally, PGPR acts as a bio-fertilizer that mitigates drought stress effects on crops in various regions worldwide. This review proposes strategies to boost productivity and improve bio-inoculant efficiency under real-world conditions. PGPR demonstrates its role in combating threats by influencing plant defense mechanisms, initiating systemic resistance responses, and regulating gene expression related to pathogen detection and defense signaling pathways. It maintains a balanced root microbiome by suppressing harmful microbial proliferation while promoting beneficial microbial interactions. Despite the challenges posed by technology and ethical considerations surrounding their modification, integrating PGPR into farming methods holds promise for sustainable agriculture. Given the increasing impact of climate change, PGPR plays a crucial role in improving crop resilience, enhancing soil quality, and reducing dependence on synthetic agricultural inputs.

Effects on plant physiology in response to inoculation of growth-promoting bacteria: systematic review

Changes in physiological mechanisms resulting from the association of plant growth-promoting bacteria as well as the responses generated to stressful factors are of interest for sustainable agriculture. Based on this, the objective of this study was to gather insights from recent years (2012-2022) on the impacts on plant physiology of the use of inoculants from plant growth-promoting bacteria. To do this, the search for articles was done in three different databases, Science Direct, Springer Nature and Google Scholar, using the following descriptors: plant growth promoting bacteria, plant hormones, biological control, photosynthesis and abiotic stress. After selection, the included articles were systematized in the Excel program. Pearson Correlation and Principal Component Analysis were used for comparative analysis of physiological variables. 81 articles were included in the review, where a beneficial association was observed in 45 plant species distributed in 13 Orders and 13 Families, with emphasis on the Families Poaceae, Fabaceae, Solanaceae and Brassicaceae. 47 genera and 98 bacterial species were verified, where Bacillus and Pseudomonas represented 52% of the verified strains, with emphasis on Bacillus subtilis and Pseudomonas fluorescens. The main applications were growth promotion, productivity, control of biotic stress and abiotic stress. Positive regulation of photosynthesis was observed, modulating the gene expression of photosynthetic apparatus proteins, pigments, antioxidant production, increased hormonal and nutritional production, osmolyte content, antimicrobial production and decreased lipid peroxidation. Based on this review, it was possible to understand the multifaceted role of plant growth-promoting bacteria in contributing to the better direction of technology in agriculture.

Inoculation of methylotrophs mitigates heat and UV stress in mung bean (Vigna radiata L.) and enhances growth, antioxidant, and functional diversity

Climate change involves the induction of heat and solar ultraviolet (UV) radiation, which profoundly affects sustainable crop production. Increasing solar UV radiation negatively impacts the photosynthetic apparatus, plant-associated organisms, and plant health. The present study aimed to comprehensively assess methylotrophic bacteria to alleviate heat and UV radiation in Vigna radiata L. under pot studies and field conditions. Under normal and UVB stress conditions, inoculation of methylotrophs significantly enhanced seed germination (72.55%-96.70% (normal) and 51.67%-65.33% (stressed)) and improved plant growth parameters, total chlorophyll (25.80-48.16 mg/g (normal) and 9.13-27.88 mg/g (stressed)), and carotenoid (569.1-1067.1 μg/g (normal) and 287.8-903.4 μg/g (stressed)) contents. A similar enhancement in antioxidant properties such as superoxide dismutase (1-5 fold), peroxidase (1-9 fold), phenylalanine ammonia lyase (1-4 fold), and proline content (1-5 fold) was observed in response to UVB radiation and heat stress under pot studies. A community-level physiological profile (CLPP) study of leaf samples revealed enhanced AWCD in methylotrophs treated plants compared to the UVB-exposed controls. Furthermore, field studies in summer conditions confirmed that inoculation with methylotrophs had a positive effect on V. radiata growth and physiology. The methylotrophs inoculation increased pod formation (25.44-32.78 and 15.56-32.00) and yield (109.81-238.63 and 71.88-216.29 q/ha) under UV cut-off sheet covered and non-covered conditions, respectively. This study demonstrated the potential of methylotrophs to mitigate heat and solar (UV) radiation in plants and provide sustainable strategies for agriculture and the environment.

Effectiveness of salt priming and plant growth-promoting bacteria in mitigating salt-induced photosynthetic damage in melon

Seed priming and plant growth-promoting bacteria (PGPB) may alleviate salt stress effects. We exposed a salt-sensitive variety of melon to salinity following seed priming with NaCl and inoculation with Bacillus. Given the sensitivity of photosystem II (PSII) to salt stress, we utilized dark- and light-adapted chlorophyll fluorescence alongside analysis of leaf stomatal conductance of water vapour (Gsw). Priming increased total seed germination by 15.5% under salt-stress. NaCl priming with Bacillus inoculation (PB) increased total leaf area (LA) by 45% under control and 15% under stress. Under the control condition, priming (P) reduced membrane permeability (RMP) by 36% and PB by 55%, while under stress Bacillus (BS) reduced RMP by 10%. Although Bacillus inoculation (B) and priming (P) treatments did not show significant effects on some PSII efficiency parameters (FV/FM, ABS/RC, PIABS, FM), the BS treatment induced a significantly higher quantum efficiency of PSII (ΦPSII) and increased Gsw by 159% in the final week of the experiment. The BS treatment reduced electron transport rate per reaction center (ETO/RC) by 10% in comparison to the salt treatment, which showed less reaction centre damage. Bacillus inoculation and seed priming treatment under the stressed condition (PBS) induced an increase in electron transport rate of 40%. Salt stress started to show significant effects on PSII after 12 days, and adversely impacted all morphological and photosynthetic parameters after 22 days. Salt priming and PGPB mitigated the negative impacts of salt stress and may serve as effective tools in future-proofing saline agriculture.

Plant growth-promoting Bacillus amyloliquefaciens orchestrate homeostasis under nutrient deficiency exacerbated drought and salinity stress in Oryza sativa L. seedlings

MAIN CONCLUSION

Nutrient deficiency intensifies drought and salinity stress on rice growth. Bacillus amyloliquefaciens inoculation provides resilience through modulation in metabolic and gene regulation to enhance growth, nutrient uptake, and stress tolerance. Soil nutrient deficiencies amplify the detrimental effects of abiotic stresses, such as drought and salinity, creating substantial challenges for overall plant health and crop productivity. Traditional methods for developing stress-resistant varieties are often slow and labor-intensive. Previously, we demonstrated that plant growth-promoting rhizobacteria Bacillus amyloliquefaciens strain SN13 effectively alleviates stress induced by sub-optimum nutrient conditions in rice. In this study, we evaluated the effectiveness of SN13 in reducing the compounded impacts of drought and salinity under varying nutrient regimes in rice seedlings. The results demonstrated that PGPR inoculation not only improved the growth parameters, nutrient content, and physio-biochemical characteristics under nutrient-limited conditions, but also reduced the oxidative stress markers. The altered expression of stress-related and transcription factor genes (USP, DEF, CYP450, GST, MYB, and bZIP) revealed the regulatory effect of PGPR in enhancing stress tolerance through these genes. GC-MS-based untargeted metabolomic analysis revealed that PGPR significantly influenced various metabolic pathways, including galactose metabolism, fructose and mannose metabolism, and fatty acid biosynthesis pathways, suggesting that PGPR affects both energy production and stress-protective mechanisms, facilitating better growth and survival of rice seedlings.

Isolation and characterization of salt-stress-tolerant rhizosphere soil bacteria and their effects on plant growth-promoting properties

PGPR has a higher potential impact on agricultural crops. It enhances plant growth and development in a variety of adverse environmental conditions, including biotic and abiotic stresses. The PGPR is commercially vital since it is more efficient, safe for the environment, and beneficial to the economy. Nowadays, salt stress has an impact on the agricultural ecosystem. Salt-tolerant PGPR can directly stimulate plant growth and development by producing a variety of metabolites and phytohormones. The current study looked at the isolation of salt-tolerant bacterial species and their ability to stimulate plant development. Four bacterial species were chosen for their better salt stress tolerance (0-5%). They were identified by 16S rRNA sequencing: Solibacillus silvestris BR1, Peribacillus frigoritolerans BR2, Paenibacillus taichungensis CR1, and Solibacillus isronensis CR2. These strains were positive production of indole acetic acid with varying incubation periods (19.66 ± 1.528 to 646.111 ± 8.058 µg/mL), salt stress (ranging from 29.556 ± 1.171 to 147.8111 ± 2.086 µg/mL), phosphate solubilization (0.145 ± 0.011 to 0.921 ± 0.007 µg/mL), ammonium production (0.299 ± 0.047 to 1.202 ± 0.142 µg/mL), HCN production (0.308 ± 0.051 to 4.269 ± 0.069 µg/mL), and siderophore production (0.190 ± 0.064 to 1.543 ± 0.108 µg/mL) for control strains were used without salt stress. The production level was expressed using a standard curve containing various standards.

PGPR-Enabled bioremediation of pesticide and heavy metal-contaminated soil: A review of recent advances and emerging challenges

The excessive usage of agrochemicals, including pesticides, along with various reckless human actions, has ensued discriminating prevalence of pesticides and heavy metals (HMs) in crop plants and the environment. The enhanced exposure to these chemicals is a menace to living organisms. The pesticides may get bioaccumulated in the food chain, thereby leading to several deteriorative changes in the ecosystem health and a rise in the cases of some serious human ailments including cancer. Further, both HMs and pesticides cause some major metabolic disturbances in plants, which include oxidative burst, osmotic alterations and reduced levels of photosynthesis, leading to a decline in plant productivity. Moreover, the synergistic interaction between pesticides and HMs has a more serious impact on human and ecosystem health. Various attempts have been made to explore eco-friendly and environmentally sustainable methods of improving plant health under HMs and/or pesticide stress. Among these methods, the employment of PGPR can be a suitable and effective strategy for managing these contaminants and providing a long-term remedy. Although, the application of PGPR alone can alleviate HM-induced phytotoxicities; however, several recent reports advocate using PGPR with other micro- and macro-organisms, biochar, chelating agents, organic acids, plant growth regulators, etc., to further improve their stress ameliorative potential. Further, some PGPR are also capable of assisting in the degradation of pesticides or their sequestration, reducing their harmful effects on plants and the environment. This present review attempts to present the current status of our understanding of PGPR's potential in the remediation of pesticides and HMs-contaminated soil for the researchers working in the area.

Salt-tolerant plant growth-promoting bacteria as a versatile tool for combating salt stress in crop plants

Soil salinization poses a great threat to global agricultural ecosystems, and finding ways to improve the soils affected by salt and maintain soil health and sustainable productivity has become a major challenge. Various physical, chemical and biological approaches are being evaluated to address this escalating environmental issue. Among them, fully utilizing salt-tolerant plant growth-promoting bacteria (PGPB) has been labeled as a potential strategy to alleviate salt stress, since they can not only adapt well to saline soil environments but also enhance soil fertility and plant development under saline conditions. In the last few years, an increasing number of salt-tolerant PGPB have been excavated from specific ecological niches, and various mechanisms mediated by such bacterial strains, including but not limited to siderophore production, nitrogen fixation, enhanced nutrient availability, and phytohormone modulation, have been intensively studied to develop microbial inoculants in agriculture. This review outlines the positive impacts and growth-promoting mechanisms of a variety of salt-tolerant PGPB and opens up new avenues to commercialize cultivable microbes and reduce the detrimental impacts of salt stress on plant growth. Furthermore, considering the practical limitations of salt-tolerant PGPB in the implementation and potential integration of advanced biological techniques in salt-tolerant PGPB to enhance their effectiveness in promoting sustainable agriculture under salt stress are also accentuated.

Bacterial exopolysaccharide amendment improves the shelf life and functional efficacy of bioinoculant under salinity stress

AIM

Bacterial exopolysaccharides (EPS) possess numerous properties beneficial for the growth of microbes and plants under hostile conditions. The study aimed to develop a bioformulation with bacterial EPS to enhance the bioinoculant's shelf life and functional efficacy under salinity stress.

METHODS AND RESULTS

High EPS-producing and salt-tolerant bacterial strain (Bacillus haynessi SD2) exhibiting auxin-production, phosphate-solubilization, and biofilm-forming ability, was selected. EPS-based bioformulation of SD2 improved the growth of three legumes under salt stress, from which pigeonpea was selected for further experiments. SD2 improved the growth and lowered the accumulation of stress markers in plants under salt stress. Bioformulations with varying EPS concentrations (1% and 2%) were stored for 6 months at 4°C, 30°C, and 37°C to assess their shelf life and functional efficacy. The shelf life and efficacy of EPS-based bioformulation were sustained even after 6 months of storage at high temperature, enhancing pigeonpea growth under stress in both control and natural conditions. However, the efficacy of non EPS-based bioformulation declined following four months of storage. The bioformulation (with 1% EPS) modulated bacterial abundance in the plant's rhizosphere under stress conditions.

CONCLUSION

The study brings forth a new strategy for developing next-generation bioformulations with higher shelf life and efficacy for salinity stress management in pigeonpea.

Unboxing PGPR-mediated management of abiotic stress and environmental cleanup: what lies inside?

Abiotic stresses including heavy metal toxicity, drought, salt and temperature extremes disrupt the plant growth and development and lowers crop output. Presence of environmental pollutants further causes plants suffering and restrict their ability to thrive. Overuse of chemical fertilizers to reduce the negative impact of these stresses is deteriorating the environment and induces various secondary stresses to plants. Therefore, an environmentally friendly strategy like utilizing plant growth-promoting rhizobacteria (PGPR) is a promising way to lessen the negative effects of stressors and to boost plant growth in stressful conditions. These are naturally occurring inhabitants of various environments, an essential component of the natural ecosystem and have remarkable abilities to promote plant growth. Furthermore, multifarious role of PGPR has recently been widely exploited to restore natural soil against a range of contaminants and to mitigate abiotic stress. For instance, PGPR may mitigate metal phytotoxicity by boosting metal translocation inside the plant and changing the metal bioavailability in the soil. PGPR have been also reported to mitigate other abiotic stress and to degrade environmental contaminants remarkably. Nevertheless, despite the substantial quantity of information that has been produced in the meantime, there has not been much advancement in either the knowledge of the processes behind the alleged positive benefits or in effective yield improvements by PGPR inoculation. This review focuses on addressing the progress accomplished in understanding various mechanisms behind the protective benefits of PGPR against a variety of abiotic stressors and in environmental cleanups and identifying the cause of the restricted applicability in real-world.

The halotolerant exopolysaccharide-producing Rhizobium azibense increases the salt tolerance mechanism in Phaseolus vulgaris (L.) by improving growth, ion homeostasis, and antioxidant defensive enzymes

Globally, agricultural productivity is facing a serious problem due to soil salinity which often causes osmotic, ionic, and redox imbalances in plants. Applying halotolerant rhizobacterial inoculants having multifarious growth-regulating traits is thought to be an effective and advantageous approach to overcome salinity stress. Here, salt-tolerant (tolerating 300 mM NaCl), exopolysaccharide (EPS) producing Rhizobium azibense SR-26 (accession no. MG063740) was assessed for salt alleviation potential by inoculating Phaseolus vulgaris (L.) plants raised under varying NaCl regimes. The metabolically active cells of strain SR-26 produced a significant amount of phytohormones (indole-3-acetic acid, gibberellic acid, and cytokinin), ACC deaminase, ammonia, and siderophore under salt stress. Increasing NaCl concentration variably affected the EPS produced by SR-26. The P-solubilization activity of the SR-26 strain was positively impacted by NaCl, as demonstrated by OD shift in NaCl-treated/untreated NBRIP medium. The detrimental effect of NaCl on plants was lowered by inoculation of halotolerant strain SR-26. Following soil inoculation, R. azibense significantly (p ≤ 0.05) enhanced seed germination (10%), root (19%) shoot (23%) biomass, leaf area (18%), total chlorophyll (21%), and carotenoid content (32%) of P. vulgaris raised in soil added with 40 mM NaCl concentration. Furthermore, strain SR-26 modulated the relative leaf water content (RLWC), proline, total soluble protein (TSP), and sugar (TSS) of salt-exposed plants. Moreover, R. azibense inoculation lowered the concentrations of oxidative stress biomarkers; MDA (29%), H2O2 content (24%), electrolyte leakage (31%), membrane stability (36%) and Na+ ion uptake (28%) when applied to 40 mM NaCl-treated plants. Further, R. azibense increases the salt tolerance mechanism of P. vulgaris by upregulating the antioxidant defensive responses. Summarily, it is reasonable to propose that EPS-synthesizing halotolerant R. azibense SR-26 should be applied as the most cost-effective option for increasing the yields of legume crops specifically P. vulgaris in salinity-challenged soil systems.

Enhancement of the germination and growth of Panicum miliaceum and Brassica juncea in Cd- and Zn-contaminated soil inoculated with heavy-metal-tolerant Leifsonia sp. ZP3

The addition of plant-growth-promoting bacteria (PGPB) to heavy-metal-contaminated soils can significantly improve plant growth and productivity. This study isolated heavy-metal-tolerant bacteria with growth-promoting traits and investigated their inoculation effects on the germination rates and growth of millet (Panicum miliaceum) and mustard (Brassica juncea) in Cd- and Zn-contaminated soil. Leifsonia sp. ZP3, which is resistant to Cd (0.5 mM) and Zn (1 mM), was isolated from forest soil. The ZP3 strain exhibited plant-growth-promoting activity, including indole-3-acetic acid production, phosphate solubilization, catalase activity, and 2,2-diphenyl-1-picrylhydrazyl radical scavenging. In soil contaminated with low concentrations of Cd (0.232 ± 0.006 mM) and Zn (6.376 ± 0.256 mM), ZP3 inoculation significantly increased the germination rates of millet and mustard 8.35- and 31.60-fold, respectively, compared to the non-inoculated control group, while the shoot and root lengths of millet increased 1.77- and 4.44-fold (p < 0.05). The chlorophyll content and seedling vigor index were also 4.40 and 18.78 times higher in the ZP3-treated group than in the control group (p < 0.05). The shoot length of mustard increased 1.89-fold, and the seedling vigor index improved 53.11-fold with the addition of ZP3 to the contaminated soil (p < 0.05). In soil contaminated with high concentrations of Cd and Zn (0.327 ± 0.016 and 8.448 ± 0.250 mM, respectively), ZP3 inoculation led to a 1.98-fold increase in the shoot length and a 2.07-fold improvement in the seedling vigor index compared to the control (p < 0.05). The heavy-metal-tolerant bacterium ZP3 isolated in this study thus represents a promising microbial resource for improving the efficiency of phytoremediation in Cd- and Zn-contaminated soil.

Exploring the dynamics of ISR signaling in maize upon seed priming with plant growth promoting actinobacteria isolated from tea rhizosphere of Darjeeling

Plant growth-promoting rhizobacteria (PGPR) offer an eco-friendly alternative to agrochemicals for better plant growth and development. Here, we evaluated the plant growth promotion abilities of actinobacteria isolated from the tea (Camellia sinensis) rhizosphere of Darjeeling, India. 16 S rRNA gene ribotyping of 28 isolates demonstrated the presence of nine different culturable actinobacterial genera. Assessment of the in vitro PGP traits revealed that Micrococcus sp. AB420 exhibited the highest level of phosphate solubilization (i.e., 445 ± 2.1 µg/ml), whereas Kocuria sp. AB429 and Brachybacterium sp. AB440 showed the highest level of siderophore (25.8 ± 0.1%) and IAA production (101.4 ± 0.5 µg/ml), respectively. Biopriming of maize seeds with the individual actinobacterial isolate revealed statistically significant growth in the treated plants compared to controls. Among them, treatment with Paenarthrobacter sp. AB416 and Brachybacterium sp. AB439 exhibited the highest shoot and root length. Biopriming has also triggered significant enzymatic and non-enzymatic antioxidative defense reactions in maize seedlings both locally and systematically, providing a critical insight into their possible role in the reduction of reactive oxygen species (ROS) burden. To better understand the role of actinobacterial isolates in the modulation of plant defense, three selected actinobacterial isolates, AB426 (Brevibacterium sp.), AB427 (Streptomyces sp.), and AB440 (Brachybacterium sp.) were employed to evaluate the dynamics of induced systemic resistance (ISR) in maize. The expression profile of five key genes involved in SA and JA pathways revealed that bio-priming with actinobacteria (Brevibacterium sp. AB426 and Brachybacterium sp. AB440) preferably modulates the JA pathway rather than the SA pathway. The infection studies in bio-primed maize plants resulted in a delay in disease progression by the biotrophic pathogen Ustilago maydis in infected maize plants, suggesting the positive efficacy of bio-priming in aiding plants to cope with biotic stress. Conclusively, this study unravels the intrinsic mechanisms of PGPR-mediated ISR dynamics in bio-primed plants, offering a futuristic application of these microorganisms in the agricultural fields as an eco-friendly alternative.

Copper Oxide Nanoparticles Induced Growth and Physio-Biochemical Changes in Maize (Zea mays L.) in Saline Soil

Research on nanoparticles (NPs) is gaining great attention in modulating abiotic stress tolerance and improving crop productivity. Therefore, this investigation was carried out to evaluate the effects of copper oxide nanoparticles (CuO-NPs) on growth and biochemical characteristics in two maize hybrids (YH-5427 and FH-1046) grown under normal conditions or subjected to saline stress. A pot-culture experiment was carried out in the Botanical Research Area of "the University of Lahore", Lahore, Pakistan, in a completely randomized design. At two phenological stages, both maize hybrids were irrigated with the same amount of distilled water or NaCl solution (EC = 5 dS m-1) and subjected or not to foliar treatment with a suspension of CuO-NPs. The salt stress significantly reduced the photosynthetic parameters (photosynthetic rate, transpiration, stomatal conductance), while the sodium content in the shoot and root increased. The foliar spray with CuO-NPs improved the growth and photosynthetic attributes, along with the N, P, K, Ca, and Mg content in the roots and shoots. However, the maize hybrid YH-5427 responded better than the other hybrid to the saline stress when sprayed with CuO-NPs. Overall, the findings of the current investigation demonstrated that CuO-NPs can help to reduce the adverse effects of salinity stress on maize plants by improving growth and physio-biochemical attributes.

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.

Biopolymers as Seed-Coating Agent to Enhance Microbially Induced Tolerance of Barley to Phytopathogens

Infections of agricultural crops caused by pathogen ic fungi are among the most widespread and harmful, as they not only reduce the quantity of the harvest but also significantly deteriorate its quality. This study aims to develop unique seed-coating formulations incorporating biopolymers (polyhydroxyalkanoate and pullulan) and beneficial microorganisms for plant protection against phytopathogens. A microbial association of biocompatible endophytic bacteria has been created, including Pseudomonas flavescens D5, Bacillus aerophilus A2, Serratia proteamaculans B5, and Pseudomonas putida D7. These strains exhibited agronomically valuable properties: synthesis of the phytohormone IAA (from 45.2 to 69.2 µg mL-1), antagonistic activity against Fusarium oxysporum and Fusarium solani (growth inhibition zones from 1.8 to 3.0 cm), halotolerance (5-15% NaCl), and PHA production (2.77-4.54 g L-1). A pullulan synthesized by Aureobasidium pullulans C7 showed a low viscosity rate (from 395 Pa·s to 598 Pa·s) depending on the concentration of polysaccharide solutions. Therefore, at 8.0%, w/v concentration, viscosity virtually remained unchanged with increasing shear rate, indicating that it exhibits Newtonian flow behavior. The effectiveness of various antifungal seed coating formulations has been demonstrated to enhance the tolerance of barley plants to phytopathogens.

Effect of Plant Growth-Promoting Bacteria on Antioxidant Status, Acetolactate Synthase Activity, and Growth of Common Wheat and Canola Exposed to Metsulfuron-Methyl

Metsulfuron-methyl, a widely used herbicide, could cause damage to the sensitive plants in crop-rotation systems at extremely low levels in the soil. The potential of plant growth-promoting bacteria (PGPB) for enhancing the resistance of plants against herbicide stress has been discovered recently. Therefore, it is poorly understood how physiological processes occur in plants, while PGPB reduce the phytotoxicity of herbicides for agricultural crops. In greenhouse studies, the effect of strains Pseudomonas protegens DA1.2 and Pseudomonas chlororaphis 4CH on oxidative damage, acetolactate synthase (ALS), enzymatic and non-enzymatic antioxidants in canola (Brassica napus L.), and wheat (Triticum aestivum L.) were investigated under two levels (0.05 and 0.25 mg∙kg-1) of metsulfuron-methyl using spectrophotometric assays. The inoculation of herbicide-exposed wheat with bacteria significantly increased the shoots fresh weight (24-28%), amount of glutathione GSH (60-73%), and flavonoids (5-14%), as well as activity of ascorbate peroxidase (129-140%), superoxide dismutase SOD (35-49%), and ALS (50-57%). Bacterial treatment stimulated the activity of SOD (37-94%), ALS (65-73%), glutathione reductase (19-20%), and the accumulation of GSH (61-261%), flavonoids (17-22%), and shoots weight (27-33%) in herbicide-exposed canola. Simultaneous inoculation prevented lipid peroxidation induced by metsulfuron-methyl in sensitive plants. Based on the findings, it is possible that the protective role of bacterial strains against metsulfuron-metil is linked to antioxidant system activation.

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.

From salty to thriving: plant growth promoting bacteria as nature's allies in overcoming salinity stress in plants

Soil salinity is one of the main problems that affects global crop yield. Researchers have attempted to alleviate the effects of salt stress on plant growth using a variety of approaches, including genetic modification of salt-tolerant plants, screening the higher salt-tolerant genotypes, and the inoculation of beneficial plant microbiome, such as plant growth-promoting bacteria (PGPB). PGPB mainly exists in the rhizosphere soil, plant tissues and on the surfaces of leaves or stems, and can promote plant growth and increase plant tolerance to abiotic stress. Many halophytes recruit salt-resistant microorganisms, and therefore endophytic bacteria isolated from halophytes can help enhance plant stress responses. Beneficial plant-microbe interactions are widespread in nature, and microbial communities provide an opportunity to understand these beneficial interactions. In this study, we provide a brief overview of the current state of plant microbiomes and give particular emphasis on its influence factors and discuss various mechanisms used by PGPB in alleviating salt stress for plants. Then, we also describe the relationship between bacterial Type VI secretion system and plant growth promotion.

Effect of exogenous taurine on pea (Pisum sativum L.) plants under salinity and iron deficiency stress

Soil salinity and Fe deficiency affect plant growth and survival by changing nutrient availability and disrupting water balance. Natural and human activities, such as evaporation and deforestation, can intensify these soil conditions. Taurine, a novel growth regulator, holds promise in mediating plant defense responses. Its effects on defense responses are still unclear. Previously, taurine showed potential in improving clover tolerance to alkaline stress and manganese toxicity. Taurine impact on plant growth under Fe deficiency and salinity stress remains uninvestigated. A pot experiment was conducted to evaluate the effects of taurine on pea plant growth, ion uptake, and defense strategies in response to salt stress and Fe deficiency. Iron deficiency was established by substituting 0.1 mM FeSO4 for 0.1 mM Fe-EDTA in the nutrient solution. Salinity stress was induced by incorporating a mixture of NaCl, MgCl2, KCl, Na2SO4, Na2CO3, NaHCO3 and CaCl2 in a 1:1:1:1:1:1:1 ratio to produce a salinity concentration of 100 mM. The simultaneous imposition of salinity and Fe deficiency significantly exacerbated oxidative stress, as evidenced by elevated levels of relative membrane permeability, hydrogen peroxide (H2O2), superoxide radical (O2•-), methylglyoxal (MG), malondialdehyde (MDA), and increased activity of lipoxygenase (LOX). Salinity stress alone and the combination of salinity and Fe deficiency resulted in substantial accumulation of Na+ ions that impeded acquisition of essential nutrients. Taurine (100 and 200 mg L-1) notably improved osmotic adjustment and oxidative defense to diminish water imbalance and oxidative injury in plants under stress. These results suggest that exogenous taurine may serve as a promising means of mitigating the detrimental effects of salt stress and Fe deficiency in plants.

'A plant's major strength in rhizosphere': the plant growth promoting rhizobacteria

Human activities, industrialization and civilization have deteriorated the environment which eventually has led to alarming effects on plants and animals by heightened amounts of chemical pollutants and heavy metals in the environment, which create abiotic stress. Environmental conditions like drought, salinity, diminished macro-and micro-nutrients also contribute in abiotic stress, resulting in decrement of survival and growth of plants. Presence of pathogenic and competitive microorganisms, as well as pests lead to biotic stress and a plant alone can not defend itself. Thankfully, nature has rendered plant's rhizosphere with plant growth promoting rhizobacteria which maintain an allelopathic relationship with host plant to defend the plant and let it flourish in abiotic as well as biotic stress situations. This review discusses the mechanisms behind increase in plant growth via various direct and indirect traits expressed by associated microorganisms in the rhizosphere, along with their current scenario and promising future for sustainable agriculture. It also gives details of ten such bacterial species, viz. Acetobacter, Agrobacterium, Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia, Enterobacter and Frankia, whose association with the host plants is famed for enhancing plant's growth and survival.

Enhancing plant growth promoting rhizobacterial activities through consortium exposure: A review

Plant Growth Promoting Rhizobacteria (PGPR) has gained immense importance in the last decade due to its in-depth study and the role of the rhizosphere as an ecological unit in the biosphere. A putative PGPR is considered PGPR only when it may have a positive impact on the plant after inoculation. From the various pieces of literature, it has been found that these bacteria improve the growth of plants and their products through their plant growth-promoting activities. A microbial consortium has a positive effect on plant growth-promoting (PGP) activities evident by the literature. In the natural ecosystem, rhizobacteria interact synergistically and antagonistically with each other in the form of a consortium, but in a natural consortium, there are various oscillating environmental conditions that affect the potential mechanism of the consortium. For the sustainable development of our ecological environment, it is our utmost necessity to maintain the stability of the rhizobacterial consortium in fluctuating environmental conditions. In the last decade, various studies have been conducted to design synthetic rhizobacterial consortium that helps to integrate cross-feeding over microbial strains and reveal their social interactions. In this review, the authors have emphasized covering all the studies on designing synthetic rhizobacterial consortiums, their strategies, mechanism, and their application in the field of environmental ecology and biotechnology.

Nanoparticles assisted regulation of oxidative stress and antioxidant enzyme system in plants under salt stress: A review

The global biomass production from agricultural farmlands is facing severe constraints from abiotic stresses like soil salinization. Salinity-mediated stress triggered the overproduction of reactive oxygen species (ROS) that may result in oxidative burst in cell organelles and cause cell death in plants. ROS production is regulated by the redox homeostasis that helps in the readjustment of the cellular redox and energy state in plants. All these cellular redox related functions may play a decisive role in adaptation and acclimation to salinity stress in plants. The use of nanotechnology like nanoparticles (NPs) in plant physiology has become the new area of interest as they have potential to trigger the various enzymatic and non-enzymatic antioxidant capabilities of plants under varying salinity levels. Moreover, NPs application under salinity is also being favored due to their unique characteristics compared to traditional phytohormones, amino acids, nutrients, and organic osmolytes. Therefore, this article emphasized the core response of plants to acclimate the challenges of salt stress through auxiliary functions of ROS, antioxidant defense system and redox homeostasis. Furthermore, the role of different types of NPs mediated changes in biochemical, proteomic, and genetic expressions of plants under salt stress have been discussed. This article also discussed the potential limitations of NPs adoption in crop production especially under environmental stresses.

Bacillus Consortia Modulate Transcriptional and Metabolic Machinery of Arabidopsis Plants for Salt Tolerance

Rhizobacteria that are helpful to plants can lessen the impacts of salt stress, and they may hold promise for the development of sustainable agriculture in the future. The present study was intended to explicate consortia of salt-tolerant plant-beneficial rhizobacteria for the amelioration of salinity stress in Arabidopsis plants. Inoculation with both the consortia positively influenced the growth of plants as indicated by total chlorophyll content, MDA content, and antioxidant enzyme activities under stressful conditions. Both the multi-trait consortia altered the expression profiles of stress-related genes including CSD1, CAT1, Wrky, Ein, Etr, and ACO. Furthermore, the metabolomic analysis indicated that inoculated plants modulated the metabolic profiles to stimulate physiological and biochemical responses in Arabidopsis plants to mitigate salt stress. Our study affirms that the consortia of salt-tolerant bacterial strains modulate the transcriptional as well as metabolic machinery of plants to protect them from salinity stress. Nevertheless, the findings of this study revealed that consortia are composed of salt-tolerant bacterial strains viz. Bacillus safensis NBRI 12M, B. subtilis NBRI 28B, and B. subtilis NBRI 33N demonstrated significant improvement in Arabidopsis plants under saline stress conditions.