Plant growth promoting bacteria as a tool to mitigate salt stress in crops: A review

The application of plant growth-promoting bacteria (PGPB) is vital for sustainable agriculture with continuous world population growth and an increase in soil salinity. Salinity is one of the severe abiotic stresses which lessens the productivity of agricultural lands. Plant growth-promoting bacteria are key players in solving this problem and can mitigate salinity stress. The highest of reported halotolerant Plant growth-promoting bacteria belonged to Firmicutes (approximately 50%), Proteobacteria (40%), and Actinobacteria (10%), respectively. The most dominant genera of halotolerant plant growth-promoting bacteria are Bacillus and Pseudomonas. Currently, the identification of new plant growth-promoting bacteria with special beneficial properties is increasingly needed. Moreover, for the effective use of plant growth-promoting bacteria in agriculture, the unknown molecular aspects of their function and interaction with plants must be defined. Omics and meta-omics studies can unreveal these unknown genes and pathways. However, more accurate omics studies need a detailed understanding of so far known molecular mechanisms of plant stress protection by plant growth-promoting bacteria. In this review, the molecular basis of salinity stress mitigation by plant growth-promoting bacteria is presented, the identified genes in the genomes of 20 halotolerant plant growth-promoting bacteria are assessed, and the prevalence of their involved genes is highlighted. The genes related to the synthesis of indole acetic acid (IAA) (70%), siderophores (60%), osmoprotectants (80%), chaperons (40%), 1-aminocyclopropane-1-carboxylate (ACC) deaminase (50%), and antioxidants (50%), phosphate solubilization (60%), and ion homeostasis (80%) were the most common detected genes in the genomes of evaluated halotolerant plant growth-promoting and salinity stress-alleviating bacteria. The most prevalent genes can be applied as candidates for designing molecular markers for screening of new halotolerant plant growth-promoting bacteria.

Global climate change results in frequent occurrences and/or long durations of abiotic stress. Field grown plants are affected by abiotic stress, and they modulate ethylene in response to abiotic stress exposure and use it as a signaling molecule in stress tolerance mechanisms. However, frequent occurrences and/or long durations of stress conditions can cause plants to induce ethylene levels higher than their thresholds, resulting in a reduction of plant growth and crop productivity. The use of plant growth-promoting bacteria (PGPB) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase has increased in various plant species to ameliorate the deleterious effects of stress-induced ethylene and promote plant growth despite abiotic stress conditions. Unfortunately, there are restrictions that limit the use of ACC deaminase-producing PGPB to protect plants from abiotic stresses. This review describes how abiotic stress induces ethylene and how stress-induced ethylene adversely affects plant growth. In addition, this review emphasizes the importance of the compatibility of PGPB strains and specific host plants and ACC deaminase activities in the reduction of stress ethylene and the promotion of plant growth, based on the research published in the last 10 years. Moreover, due to the restrictions in PGPB use, this review highlights the potential generation of transgenic plants expressing the AcdS gene that encodes the ACC deaminase enzyme as a substitute for PGPB in the future to support and uplift agricultural sustainability and food security globally.

Soil salinization refers to the buildup of salts in soil to a level toxic to plants. The major factors that contribute to soil salinity are the quality, the amount and the type of irrigation water used. The presented review discusses the different sources and causes of soil salinity. The effect of soil salinity on biological processes of plants is also discussed in detail. This is followed by a debate on the influence of salt on the nutrient uptake and growth of plants. Salinity decreases the soil osmotic potential and hinders water uptake by the plants. Soil salinity affects the plants K uptake, which plays a critical role in plant metabolism due to the high concentration of soluble sodium (<TEX>$Na^+$</TEX>) ions. Visual symptoms that appear in the plants as a result of salinity include stunted plant growth, marginal leaf necrosis and fruit distortions. Different strategies to ameliorate salt stress globally include breeding of salt tolerant cultivars, irrigation to leach excessive salt to improve soil physical and chemical properties. As part of an ecofriendly means to alleviate salt stress and an increasing considerable attention on this area, the review then focuses on the different plant growth promoting bacteria (PGPB) mediated mechanisms with a special emphasis on ACC deaminase producing bacteria. The various strategies adopted by PGPB to alleviate various stresses in plants include the production of different osmolytes, stress related phytohormones and production of molecules related to stress signaling such as bacterial 1-aminocyclopropane-1-carboxylate (ACC) derivatives. The use of PGPB with ACC deaminase producing trait could be effective in promoting plant growth in agricultural areas affected by different stresses including salt stress. Finally, the review ends with a discussion on the various PGPB activities and the potentiality of facultative halophilic/halotolerant PGPB in alleviating salt stress.

ABSTRACT: To explore the potential of Plant Growth-Promoting Bacteria (PGPB) such as Acidithiobacillus sp. in improving sugarcane tolerance to abiotic stresses like drought, salinity, and nutrient deficiencies, and optimizing carbon supply for sustainable cultivation.1 The use of Plant Growth-Promoting Bacteria (PGPB) has the potential for enhancing agricultural sustainability and productivity. PGPB supports plants in a variety of ways, including nutrient absorption, hormone production, and disease prevention. However, strain variability, formulation and distribution concerns, regulatory barriers, and socioeconomic constraints all offer obstacles to their widespread use. Addressing these challenges requires collaborative efforts from researchers, policymakers, extension workers, and farmers to identify effective PGPB strains, develop stable formulations, navigate regulatory processes, and provide technical support for successful integration into agricultural practices.2 Sugarcane plantlets in tissue culture survived heat stress, suggesting the bioformulation induces abiotic stress tolerance and carbon content was higher in tissue culture-treated, bio-inoculated plants compared to pot culture plants and controls. Despite these challenges, using PGPB has the potential to boost crop yields, minimise chemical inputs, and promote sustainable agriculture, therefore contributing to global food security and environmental stewardship.3 The study To explore the potential of Plant Growth-Promoting Bacteria (PGPB) such as Acidithiobacillus sp. in improving sugarcane tolerance to abiotic stresses like drought, salinity, and nutrient deficiencies, and optimizing carbon supply for sustainable cultivation.1 The use of Plant Growth-Promoting Bacteria (PGPB) has the potential for enhancing agricultural sustainability and productivity. PGPB supports plants in a variety of ways, including nutrient absorption, hormone production, and disease prevention. However, strain variability, formulation and distribution concerns, regulatory barriers, and socioeconomic constraints all offer obstacles to their widespread use. Addressing these challenges requires collaborative efforts from researchers, policymakers, extension workers, and farmers to identify effective PGPB strains, develop stable formulations, navigate regulatory processes, and provide technical support for successful integration into agricultural practices.2 Sugarcane plantlets in tissue culture survived heat stress, suggesting the bioformulation induces abiotic stress tolerance and carbon content was higher in tissue culture-treated, bio-inoculated plants compared to pot culture plants and controls. Despite these challenges, using PGPB has the potential to boost crop yields, minimise chemical inputs, and promote sustainable agriculture, therefore contributing to global food security and environmental stewardship.3 The study suggests that bioformulation induces abiotic stress tolerance and improves carbon supply in sugarcane.

Soil salinization, one of the most common causes of soil degradation, negatively affects plant growth, reproduction, and yield in plants. Saline conditions elicit some physiological changes to cope with the imposed osmotic and oxidative stresses. Inoculation of plants with some bacterial species that stimulate their growth, i.e., plant growth-promoting bacteria (PGPB), may help plants to counteract saline stress, thus improving the plant’s fitness. This manuscript reports the effects of the inoculation of a salt-sensitive cultivar of Brassica napus (canola) with five different PGPB species (separately), i.e., Azospirillum brasilense, Arthrobacter globiformis, Burkholderia ambifaria, Herbaspirillum seropedicae, and Pseudomonas sp. on plant salt stress physiological responses. The seeds were sown in saline soil (8 dS/m) and inoculated with bacterial suspensions. Seedlings were grown to the phenological stage of rosetta, when morphological and physiological features were determined. In the presence of the above-mentioned PGPB, salt exposed canola plants grew better than non-inoculated controls. The water loss was reduced in inoculated plants under saline conditions, due to a low level of membrane damage and the enhanced synthesis of the osmolyte proline, the latter depending on the bacterial strain inoculated. The reduction in membrane damage was also due to the increased antioxidant activity (i.e., higher amount of phenolic compounds, enhanced superoxide dismutase, and ascorbate peroxidase activities) in salt-stressed and inoculated Brassica napus. Furthermore, the salt-stressed and inoculated plants did not show detrimental effects to their photosynthetic apparatus, i.e., higher efficiency of PSII and low energy dissipation by heat for photosynthesis were detected. The improvement of the response to salt stress provided by PGPB paves the way to further use of PGPB as inoculants of plants grown in saline soils.

Paenibacillus yonginensis DCY84T induces changes in Arabidopsis thaliana gene expression against aluminum, drought, and salt stress

Humanity in the modern world is confronted with diverse problems at several levels. The environmental concern is probably the most important as it threatens different ecosystems, food, and farming as well as humans, animals, and plants. More specifically, salinization of agricultural soils is a global concern because of on one side, the permanent increase of the areas affected, and on the other side, the disastrous damage caused to various plants affecting hugely crop productivity and yields. Currently, great attention is directed towards the use of Plant Growth Promoting Bacteria (PGPB). This alternative method, which is healthy, safe, and ecological, seems to be very promising in terms of simultaneous salinity alleviation and improving crop productivity. This review attempts to deal with different aspects of the current advances concerning the use of PGPBs for saline stress alleviation. The objective is to explain, discuss, and present the current progress in this area of research. We firstly discuss the implication of PGPB on soil desalinization. We present the impacts of salinity on crops. We look for the different salinity origin and its impacts on plants. We discuss the impacts of salinity on soil. Then, we review various recent progress of hemophilic PGPB for sustainable agriculture. We categorize the mechanisms of PGPB toward salinity tolerance. We discuss the use of PGPB inoculants under salinity that can reduce chemical fertilization. Finally, we present some possible directions for future investigation. It seems that PGPBs use for saline stress alleviation gain more importance, investigations, and applications. Regarding the complexity of the mechanisms implicated in this domain, various aspects remain to be elucidated.

Salinity is an edaphic stress that dramatically restricts worldwide crop production. Nanomaterials and plant growth-promoting bacteria (PGPB) are currently used to alleviate the negative effects of various stresses on plant growth and development. This study investigates the protective effects of different levels of zinc oxide nanoparticles (ZnO-NPs) (0, 20, and 40 mg L−1) and PGPBs (no bacteria, Bacillus subtilis, Lactobacillus casei, Bacillus pumilus) on DNA damage and cytosine methylation changes in the tomato (Solanum lycopersicum L. ‘Linda’) seedlings under salinity stress (250 mM NaCl). Coupled Restriction Enzyme Digestion-Random Amplification (CRED-RA) and Randomly Amplified Polymorphic DNA (RAPD) approaches were used to analyze changes in cytosine methylation and to determine how genotoxic effects influence genomic stability. Salinity stress increased the polymorphism rate assessed by RAPD, while PGPB and ZnO-NPs reduced the adverse effects of salinity stress. Genomic template stability was increased by the PGPBs and ZnO-NPs application; this increase was significant when Lactobacillus casei and 40 mg L−1 of ZnO-NPs were used.A decreased level of DNA methylation was observed in all treatments. Taken together, the use of PGPB and ZnO-NPs had a general positive effect under salinity stress reducing genetic impairment in tomato seedlings.

BackgroundBiodynamic agriculture and the use of plant growth-promoting bacteria (PGPBs) have been demonstrated to offer various benefits for achieving agricultural sustainability. The aim of this study was to evaluate the effects of PGPBs Azotobacter and Azospirillum, compost, and compost with biodynamic preparations (BD) on the essential oil (EO) characteristics of lavender under salinity stress.Research methodsThe experiment was carried out in a greenhouse for 2 years and involved three factors: four PGPBs, three types of compost, and three levels of salinity stress.ResultsThe results indicated that the essential oil (EO) characteristics increased with 50 mM NaCl but decreased with 100 mM NaCl. Salt stress reduced the cell membrane stability (CMS) and auxin content, while increasing proline contents. However, the application of PGPBs, compost, and compost with biodynamic preparations had an opposite effect on CMS, auxin, and proline parameters compared to salt stress. Based on the results, the treatment that combined compost + BD with Azotobacter was found to be the most effective in enhancing the EO characteristics under both mild and severe salinity stress conditions.ConclusionsThe results of this study suggest that compost, biodynamic compost preparations, and PGPBs could be useful in enhancing the EO in medicinal plants and alleviating the adverse effects of salt stress on plants.Graphical

The use of microorganisms capable of promoting the growth and development of crops is generating interest at a global level as a sustainable technique in modern agriculture, especially in intensive farming systems, where the excessive use of synthetic fertilizers has led to environmental problems. The objective of this research was to evaluate the biofertilizing power of formulations enriched with plant growth-promoting bacteria (PGPB) (Azotobacter spp. to fix N and strains of Bacillus spp. to solubilize P and K not bioavailable for plants) to improve the fertility, quality, and productivity of a tomato crop and their potential use as an alternative to conventional fertilizers. Thus, NPK levels in soils, leaves, and fruits were evaluated; various parameters of fruit quality were measured; and an exhaustive analysis of the production and economic yields of the harvest was carried out. The results showed that the periodic supply of biofertilizers based on PGPB increased the harvest yield (20–32%) and favored the development of larger fruit sizes, which are economically more valuable, and the incomes increased even more than production (32–52%). The biofertilizers also demonstrated a positive effect on the solubilization of P and K in the soil, and the levels of P in leaves were also promoted. The capacity to mobilize the nutrients from soil to fruits was clearly favored when PGPB were inoculated periodically, and a reduction of up to 20% in synthetic fertilizers was accomplished (16, 34, and 23% increases for N, P, and K, respectively, against the treatment without PGPB and no fertigation reduction). Finally, the use of PGPB did not show appreciable differences regarding fruit quality parameters.

Biofortification of plants using Plant Growth-Promoting Bacteria (PGPB) represents a promising strategy in sustainable agriculture. This paper discusses the PGPB action in the context of their impact on phenolic compounds biosynthesis and the prospects for their application in agriculture. So far, no review article has summarized the significance of PGPB in increasing phenolic compounds in plants. PGPB, such as Pseudomonas, Bacillus, and Azospirillum, promote plant growth by producing phytohormones, enhancing nutrient availability, and stimulating the biosynthesis of secondary metabolites through the activation of Induced Systemic Resistance (ISR). The activation of ISR (Induced Systemic Resistance) by PGPB stimulates the phenylpropanoid pathway, which is the primary biosynthetic route for polyphenolic compounds, including phenolic acids and flavonoids, in plants. Studies indicate that PGPB may increase phenolic compounds content from 9% to over 200%, while simultaneously improving antioxidant activity. Through the secretion of phenolic compounds, PGPB also can mitigate abiotic stresses such as drought, salinity and heavy metal contamination. Among the phenolic compounds whose production in various plant parts can be stimulated by PGPB are flavonoids, such as quercetin, procyanidin B1, EGCG, and catechin, and phenolic acids, including caffeic acid, ferulic acid, and chlorogenic acid. Advancements in omics research will enable a more precise investigation of the impact of PGPB, including endophytic bacteria, on the biosynthetic pathways of phenolic compounds. In the future, this will translate into improved efficiency in stimulating the production of these compounds. Nevertheless, even now, the use of PGPB offers a sustainable alternative to genetic engineering, reducing reliance on chemical inputs in agriculture.

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.

Soil salinity is a growing agricultural challenge affecting millions of hectares of arable land globally.Excessive salt accumulation in the soil disrupts plant growth by limiting water uptake, causing ion toxicity, and reducing soil fertility.Traditional methods to address soil salinity, such as chemical amendments and physical leaching, are often costly, time-consuming, and environmentally taxing.An emerging and sustainable alternative is the use of plant growth-promoting bacteria (PGPB) to improve plant salt tolerance.Soil salinity has emerged as a formidable challenge to sustainable agriculture worldwide.The accumulation of soluble salts in the soil profile adversely affects soil structure, nutrient availability, and plant growth, ultimately reducing agricultural productivity.Soil salinity arises from both natural and anthropogenic factors [1-3].Naturally, saline soils develop in arid and semi-arid regions where evaporation exceeds precipitation, leaving salts to accumulate near the surface.Salinity negatively affects plant metabolism, leading to stunted growth, reduced yield, and, in severe cases, complete crop failure [4][5][6][7].Addressing this issue is critical given the increasing demand for food security and sustainable agriculture.Plant growth-promoting bacteria are naturally occurring microorganisms that enhance plant growth by various mechanisms [8][9][10][11].PGPB can thrive in saline environments and offer a sustainable approach to mitigate soil salinity as shown in Figure 1.Key benefits of PGPB include: Soil Structure Stabilization: Exopolysaccharides secreted by PGPB bind soil particles, improving soil structure and water retention capacity.PGPB contribute to soil improvement in several ways:1. Salt Tolerance Induction: PGPB produce metabolites like exopolysaccharides (EPS) that help plants maintain ionic balance and reduce salt stress.2. Nutrient Bioavailability: These bacteria solubilize essential nutrients like phosphorus and potassium, making them more accessible to plants.3. Phytohormone Production: PGPB synthesize hormones such as auxins, gibberellins, and cytokinins, which promote root growth and enhance plant resilience.4. ACC Deaminase Activity: By breaking down the stress hormone ethylene precursor (1-aminocyclopropane-1carboxylic acid), PGPB alleviate plant stress and promote growth under saline conditions.

Plants and their microbiomes, including plant growth-promoting bacteria (PGPB), can work as a team to reduce the adverse effects of different types of stress, including drought, heat, cold, and heavy metals stresses, as well as salinity in soils. These abiotic stresses are reviewed here, with an emphasis on salinity and its negative consequences on crops, due to their wide presence in cultivable soils around the world. Likewise, the factors that stimulate the salinity of soils and their impact on microbial diversity and plant physiology were also analyzed. In addition, the saline soils that exist in Mexico were analyzed as a case study. We also made some proposals for a more extensive use of bacterial bioinoculants in agriculture, particularly in developing countries. Finally, PGPB are highly relevant and extremely helpful in counteracting the toxic effects of soil salinity and improving crop growth and production; therefore, their use should be intensively promoted.

Combined ability of chromium (Cr) tolerant plant growth promoting bacteria (PGPB) and salicylic acid (SA) in attenuation of chromium stress in maize plants