Comparative effectiveness of Pseudomonas and Serratia sp. containing ACC-deaminase for improving growth and yield of wheat (Triticum aestivum L.) under salt-stressed conditions

Beneficial Effects of ACC Deaminase-Producing Rhizobacteria on the Drought Stress Resistance of Coffea arabica L

Given the challenges of climate change, effective adaptation strategies for crops like coffee are crucial. This study evaluated twelve 1-aminocyclopropane-1-carboxylate deaminase-producing bacterial strains selectively isolated from the rhizosphere of Coffea arabica L. cv. Costa Rica 95 in a plantation located in Veracruz, Mexico, focusing on their potential to enhance drought resistance. The strains, representing seven genera from the Gamma-proteobacteria and Bacteroidota groups, were characterized for growth-promoting traits, including ACC deaminase activity, indole-3-acetic acid (IAA) synthesis, phosphates solubilization, siderophore production, and nitrogen fixation. Strains of the genus Pantoea exhibited higher ACC deaminase activity, phosphate solubilization, and IAA synthesis, while others, such as Sphingobacterium and Chryseobacterium, showed limited plant growth-promoting traits. A pot experiment was conducted with coffee plants subjected to either full irrigation (soil with 85% volumetric water content) or drought (soil with 55% volumetric water content) conditions, along with inoculation with the isolated strains. Plants inoculated with Pantoea sp. RCa62 demonstrated improved growth metrics and physiological traits under drought, including higher leaf area, relative water content (RWC), biomass, and root development compared to uninoculated controls. Similar results were observed with Serratia sp. RCa28 and Pantoea sp. RCa31 under full irrigation conditions. Pantoea sp. RCa62 exhibited superior root development under stress, contributing to overall plant development. Proline accumulation was significantly higher in drought-stressed, non-inoculated plants compared to those inoculated with Pantoea sp. RCa62. This research highlights the potential of Pantoea sp. RCa62 to enhance coffee plant resilience to drought and underscores the need for field application and further validation of these bioinoculants in sustainable agricultural practices.

Inoculating maize (Zea mays L.) seeds with halotolerant rhizobacteria from wild halophytes improves physiological and biochemical responses of seedlings to salt stress

Salinity stress is a major environmental factor that poses a significant constraint to plant growth, threatening agricultural productivity and sustainability. This study investigated the potential of halotolerant bacteria, isolated from the rhizosphere of wild halotolerant plants in Turkey's Salt Lake basin, to enhance salt stress tolerance in maize (Zea mays L.). The rhizospheres of 17 different wild halotolerant plants were selected for bacterial isolation, resulting in the identification of 22 halotolerant bacteria using 16S rRNA sequence analysis. Among these, 19 isolates were found to possess positive activity for 1-aminocyclopropane-1-carboxylate (ACC) deaminase and nitrogen fixation. When the maize seeds inoculated with these 19 isolates were grown under normal conditions, four isolates‒TG-4 (Halomonas arcis), TG-8 (Marinococcus tarigensis), TG-12 (Halobacillus dabanensis), and TG-20 (Halomonas eurihalina)-significantly stimulated seedling growth and development. To evaluate the effect of these four isolates on salt tolerance, inoculated seeds were grown under various salt conditions (0.0, 75, 150, and 250 mM NaCl). The responses of plants to salt stress were analyzed by evaluating growth parameters, membrane damage, photosynthetic pigment and proline content, reactive oxygen species and lipid peroxidation levels, and antioxidant enzyme activities. According to the parameters, the results indicated that TG-4, TG-8, and TG-12, in particular, have the potential to function as plant growth-promoting rhizobacteria and effectively enhance salt stress tolerance in the maize seedlings. Overall, this research highlights the potential of halotolerant bacteria to improve salt stress tolerance in maize plants through multifaceted mechanisms, offering valuable insights for sustainable agriculture in saline environments.

Fostering climate-resilient agriculture with ACC-deaminase producing rhizobacterial biostimulants from the cold deserts of the Indian Himalayas

Climate change is one of the most significant threats to agricultural productivity, which necessitates a need for more resilient and sustainable farming practices. Rhizobacterial biostimulants that secrete 1-aminocyclopropane-1-carboxylate (ACC) deaminase and enhance crop resilience and yield can serve as a potential sustainable solution. The present study provides a comprehensive analysis of ACC-deaminase producing rhizobacteria (ACCD) isolated from cold deserts of the Indian trans-Himalayas and their efficacy to improve crop resilience and productivity under diverse climatic conditions. Thirty four efficient ACCD showed ACC deaminase activity ranging from 4.9 to 24484.3 nM α-ketobutyrate/h/mg/protein. These strains also exhibited broad-spectrum plant growth promotion (PGP) attributes, including tri-calcium phosphate (TCP) solubilization ranging from 2.4 to 687.5 μg/ml, siderophore production ranging from 62 to 224% and indole-3-acetic acid (IAA)-like auxin production ranging from 0.9 to 88.2 μg/ml. 16S rRNA gene sequencing of efficient strains showed their belonging to 10 genera, including Acinetobacter, Agrobacterium, Arthrobacter, Cellulomonas, Enterobacter, Microbacterium, Neomicrococcus, Priestia, Pseudomonas, and Rhizobium. Among these, Pseudomonas was the dominant genus with high ACC-deaminase activity and multiple PGP traits. These strains also showed growth under various stressed culture conditions, including acidity/alkalinity, different temperatures, desiccation, and salinity. Field applications of 4 efficient and stress-tolerant ACCD, including Pseudomonas geniculata, P. migulae, Priestia aryabhattai, and Rhizobium nepotum with reduced NPK dose under two different temperate climate conditions showed a significant improvement in growth and productivity of crops such as garlic, pea, potato, and wheat in slightly acidic soils and maize in saline-sodic alkaline soils. These findings indicated the broad-spectrum potential of these efficient and stress-tolerant ACCD strains to improve plant growth and productivity across diverse soil types and climatic conditions.

Drought stress tolerance and metabolomics of Medicago sativa induced by Bacillus amyloliquefaciens DGL1

Introduction

This study used Bacillus amyloliquefaciens DGL1 isolated from the arid sandy land of the Qinghai-Tibetan Plateau as the research strain and investigated the effects of DGL1 on the biomass, physiology, and metabolites of Medicago sativa under different intensities of drought stress to provide a high-quality bacterial source and a theoretical basis for the research and development of biological fertilizer suitable for arid areas.

Methods

The exopolysaccharides (EPS), 1-Aminocyclopropane-1-carboxylate deaminase (ACC), and phosphorus solubilizing capacity of DGL1 were determined. The effects of a DGL1 suspension on alfalfa biomass, physiological indexes, degree of peroxidation of cell membranes, and activity of antioxidant enzymes were determined after irrigating roots under drought stress. The effects on soil physicochemical properties were also evaluated, and metabolomics analysis was performed to explore the effect of DGL1 on the metabolites of alfalfa under drought stress.

Results

Strain DGL1 produced extracellular polysaccharide EPS and ACC deaminase and was capable of phosphorus solubilization. Treatment with DGL1 increased the biomass of alfalfa under different degrees of drought stress, significantly increased the activities of alfalfa antioxidant enzymes Super Oxide Dismutase (SOD), Peroxidase (POD), and catalase (CAT), reduced the content of MDA and H2O2, and increased the content of quick-acting phosphorus, quick-acting potassium, ammonium nitrogen, and nitrate nitrogen in the soil, thus improving soil fertility. Through metabolomics analysis, DGL1 was shown to affect amino acid metabolic pathways, such as arginine, leucine, glutamate, and tyrosine, as well as the levels of energy-providing polysaccharides and lipids, in alfalfa under 15% PEG-6000 drought stress, enhancing alfalfa's capacity to resist drought stress.

Discussion

Strain DGL1 enhances the drought suitability of alfalfa and has the potential for dryland development as a biological agent.

Long-term conservation tillage increase cotton rhizosphere sequestration of soil organic carbon by changing specific microbial CO2 fixation pathways in coastal saline soil

Coastal saline soil is an important reserve resource for arable land globally. Data from 10 years of continuous stubble return and subsoiling experiments have revealed that these two conservation tillage measures significantly improve cotton rhizosphere soil organic carbon sequestration in coastal saline soil. However, the contribution of microbial fixation of atmospheric carbon dioxide (CO2) has remained unclear. Here, metagenomics and metabolomics analyses were used to deeply explore the microbial CO2 fixation process in rhizosphere soil of coastal saline cotton fields under long-term stubble return and subsoiling. Metagenomics analysis showed that stubble return and subsoiling mainly optimized CO2 fixing microorganism (CFM) communities by increasing the abundance of Acidobacteria, Gemmatimonadetes, and Chloroflexi, and improving composition diversity. Conjoint metagenomics and metabolomics analyses investigated the effects of stubble return and subsoiling on the reverse tricarboxylic acid (rTCA) cycle. The conversion of citrate to oxaloacetate was inhibited in the citrate cleavage reaction of the rTCA cycle. More citrate was converted to acetyl-CoA, which enhanced the subsequent CO2 fixation process of acetyl-CoA conversion to pyruvate. In the rTCA cycle reductive carboxylation reaction from 2-oxoglutarate to isocitrate, synthesis of the oxalosuccinate intermediate product was inhibited, with strengthened CO2 fixation involving the direct conversion of 2-oxoglutarate to isocitrate. The collective results demonstrate that stubble return and subsoiling optimizes rhizosphere CFM communities by increasing microbial diversity, in turn increasing CO2 fixation by enhancing the utilization of rTCA and 3-hydroxypropionate/4-hydroxybutyrate cycles by CFMs. These events increase the microbial CO2 fixation in the cotton rhizosphere, thereby promoting the accumulation of microbial biomass, and ultimately improving rhizosphere soil organic carbon. This study clarifies the impact of conservation tillage measures on microbial CO2 fixation in cotton rhizosphere of coastal saline soil, and provides fundamental data for the improvement of carbon sequestration in saline soil in agricultural ecosystems.

Improvement of morphophysiological and anatomical attributes of plants under abiotic stress conditions using plant growth-promoting bacteria and safety treatments

Drought and salinity are the major abiotic stress factors negatively affecting the morphophysiological, biochemical, and anatomical characteristics of numerous plant species worldwide. The detrimental effects of these environmental factors can be seen in leaf and stem anatomical structures including the decrease in thickness of cell walls, palisade and spongy tissue, phloem and xylem tissue. Also, the disintegration of grana staking, and an increase in the size of mitochondria were observed under salinity and drought conditions. Drought and salt stresses can significantly decrease plant height, number of leaves and branches, leaf area, fresh and dry weight, or plant relative water content (RWC%) and concentration of photosynthetic pigments. On the other hand, stress-induced lipid peroxidation and malondialdehyde (MDA) production, electrolyte leakage (EL%), and production of reactive oxygen species (ROS) can increase under salinity and drought conditions. Antioxidant defense systems such as catalase, peroxidase, glutathione reductase, ascorbic acid, and gamma-aminobutyric acid are essential components under drought and salt stresses to protect the plant organelles from oxidative damage caused by ROS. The application of safe and eco-friendly treatments is a very important strategy to overcome the adverse effects of drought and salinity on the growth characteristics and yield of plants. It is shown that treatments with plant growth-promoting bacteria (PGPB) can improve morphoanatomical characteristics under salinity and drought stress. It is also shown that yeast extract, mannitol, proline, melatonin, silicon, chitosan, α-Tocopherols (vitamin E), and biochar alleviate the negative effects of drought and salinity stresses through the ROS scavenging resulting in the improvement of plant attributes and yield of the stressed plants. This review discusses the role of safety and eco-friendly treatments in alleviating the harmful effects of salinity and drought associated with the improvement of the anatomical, morphophysiological, and biochemical features in plants.

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.

Serratia spp. as plant growth-promoting bacteria alleviating salinity, drought, and nutrient imbalance stresses

In agricultural environments, plants are often exposed to abiotic stresses including temperature extremes, salt stress, drought, and heavy metal soil contamination, which leads to significant economic losses worldwide. Especially salt stress and drought pose serious challenges since they induce ionic toxicity, osmotic stress, and oxidative stress in plants. A potential solution can be the application of bacteria of the Serratia spp. known to promote plant growth under normal conditions Thus the mini-review aims to summarize the current knowledge on plant growth promotion by Serratia spp. (under the conditions of salinity stress, drought, and nutrient deficit) and highlight areas for development in the field. So far, it has been proven that Serratia spp. strains exhibit a variety of traits contributing to enhanced plant growth and stress tolerance, such as phytohormone production, ACC deaminase activity, nitrogen fixation, P and Zn solubilization, antioxidant properties improvement, and modulation of gene expression. Nevertheless, further research on Serratia spp. is needed, especially on two subjects: elucidating its mechanisms of action on plants at the molecular level and the effects of Serratia spp. on the indigenous soil and plant microbiota and, particularly, the rhizosphere. In both cases, it is advisable to use omics techniques to gain in-depth insights into the issues. Additionally, some strains of Serratia spp. may be phytopathogens, therefore studies to rule out this possibility are recommended prior to field trials. It is believed that by improving said knowledge the potential of Serratia spp. to stimulate plant growth will increase and strains from the genus will serve as an eco-friendly biofertilizer in sustainable agriculture more often.

Wheat genome architecture influences interactions with phytobeneficial microbial functional groups in the rhizosphere

Wheat has undergone a complex evolutionary history, which led to allopolyploidization and the hexaploid bread wheat Triticum aestivum. However, the significance of wheat genomic architecture for beneficial plant-microbe interactions is poorly understood, especially from a functional standpoint. In this study, we tested the hypothesis that wheat genomic architecture was an overriding factor determining root recruitment of microorganisms with particular plant-beneficial traits. We chose five wheat species representing genomic profiles AA (Triticum urartu), BB {SS} (Aegilops speltoides), DD (Aegilops tauschii), AABB (Triticum dicoccon) and AABBDD (Triticum aestivum) and assessed by quantitative polymerase chain reaction their ability to interact with free-nitrogen fixers, 1-aminocyclopropane-1-carboxylate deaminase producers, 2,4-diacetylphloroglucinol producers and auxin producers via the phenylpyruvate decarboxylase pathway, in combination with Illumina MiSeq metabarcoding analysis of N fixers (and of the total bacterial community). We found that the abundance of the microbial functional groups could fluctuate according to wheat genomic profile, as did the total bacterial abundance. N fixer diversity and total bacterial diversity were also influenced significantly by wheat genomic profile. Often, rather similar results were obtained for genomes DD (Ae. tauschii) and AABBDD (T. aestivum), pointing for the first time that the D genome could be particularly important for wheat-bacteria interactions.

Bacterial ACC deaminase: Insights into enzymology, biochemistry, genetics, and potential role in amelioration of environmental stress in crop plants

Growth and productivity of crop plants worldwide are often adversely affected by anthropogenic and natural stresses. Both biotic and abiotic stresses may impact future food security and sustainability; global climate change will only exacerbate the threat. Nearly all stresses induce ethylene production in plants, which is detrimental to their growth and survival when present at higher concentrations. Consequently, management of ethylene production in plants is becoming an attractive option for countering the stress hormone and its effect on crop yield and productivity. In plants, ACC (1-aminocyclopropane-1-carboxylate) serves as a precursor for ethylene production. Soil microorganisms and root-associated plant growth promoting rhizobacteria (PGPR) that possess ACC deaminase activity regulate growth and development of plants under harsh environmental conditions by limiting ethylene levels in plants; this enzyme is, therefore, often designated as a "stress modulator." TheACC deaminase enzyme, encoded by the AcdS gene, is tightly controlled and regulated depending upon environmental conditions. Gene regulatory components of AcdS are made up of the LRP protein-coding regulatory gene and other regulatory components that are activated via distinct mechanisms under aerobic and anaerobic conditions. ACC deaminase-positive PGPR strains can intensively promote growth and development of crops being cultivated under abiotic stresses including salt stress, water deficit, waterlogging, temperature extremes, and presence of heavy metals, pesticides and other organic contaminants. Strategies for combating environmental stresses in plants, and improving growth by introducing the acdS gene into crop plants via bacteria, have been investigated. In the recent past, some rapid methods and cutting-edge technologies based on molecular biotechnology and omics approaches involving proteomics, transcriptomics, metagenomics, and next generation sequencing (NGS) have been proposed to reveal the variety and potential of ACC deaminase-producing PGPR that thrive under external stresses. Multiple stress-tolerant ACC deaminase-producing PGPR strains have demonstrated great promise in providing plant resistance/tolerance to various stressors and, therefore, it could be advantageous over other soil/plant microbiome that can flourish under stressed environments.

Identification of endophytic fungi with ACC deaminase-producing isolated from halophyte Kosteletzkya Virginica

Seashore mallow (Kosteletzkya virginica), as a noninvasive perennial halophytic oilseed-producing dicot, is native from the Gulf to the Atlantic coasts of the U.S. The purpose of our research was to investigate 1-aminocyclopropane-1carboxylic acid deaminase (ACCD) producing endophytic fungi from K.virginica. A total of 59 endophytic fungal strains, isolated from roots in K.virginica of seedlings, were grouped into 12 genera including in Penicillium, Aspergillus, Fusarium, Trichoderma, Rhizopycnis sp., Ceriporia Donk, Trametes sp., Schizophyllum commune sp., Alternaria, Cladosporium, Cylindrocarpon, and Scytalidium according to sequences of ITS. The ACD activity of 10 endophytic fungi isolated was detected. T.asperellum had the highest ACC deaminase activity among all 10 isolated genera of fungal strains, followed by T. viride. Dry weight and fresh weight of plant, plant height, root length, SOD activity, and chlorophyll content of wheat and soybean inoculated with T.asperellum or T. viride was increased compared with non-inoculated control plants under non salt or salt stress. Further analysis showed that T.asperellum or T.viride strains induced downregulation of the expression of ethylene synthesis-related genes such as ACC oxidase (ACO) and ACC synthase (ACS), thereby reducing ethylene synthesis and damage to plants under salt stress. These endophytic fungi can be used as alternative bioinoculants to increase crop yield in saline soil.

Silicon and soil microorganisms improve rhizospheric soil health with bacterial community, plant growth, performance and yield

The interaction of silicon and soil microorganisms stimulates crop enhancement to ensure sustainable agriculture. Silicon may potentially increase nutrient availability in rhizosphere with improved plants' growth, development as it does not produce phytotoxicity. The rhizospheric microbiome accommodates a variety of microbial species that live in a small area of soil directly associated with the hidden half plants' system. Plant growth-promoting rhizobacteria (PGPR) play a major role in plant development in response to adverse climatic conditions. PGPRs may enhance the growth, quality, productivity in variety of crops, and mitigate abiotic stresses by reprogramming stress-induced physiological variations in plants via different mechanisms, such as synthesis of indole-3-acetic acid, 1-aminocyclopropane-1-carboxylate deaminase, exopolysaccharides, volatile organic compounds, atmospheric nitrogen fixation, and phosphate solubilization. Our article eye upon interactions of silicon and plant microbes which seems to be an opportunity for sustainable agriculture for series of crops and cropping systems in years to come, essential to safeguard the food security for masses.

The Importance of Microorganisms for Sustainable Agriculture-A Review

In the face of climate change, progressive degradation of the environment, including agricultural land negatively affecting plant growth and development, endangers plant productivity. Seeking efficient and sustainable agricultural techniques to replace agricultural chemicals is one of the most important challenges nowadays. The use of plant growth-promoting microorganisms is among the most promising approaches; however, molecular mechanisms underneath plant-microbe interactions are still poorly understood. In this review, we summarized the knowledge on plant-microbe interactions, highlighting the role of microbial and plant proteins and metabolites in the formation of symbiotic relationships. This review covers rhizosphere and phyllosphere microbiomes, the role of root exudates in plant-microorganism interactions, the functioning of the plant's immune system during the plant-microorganism interactions. We also emphasized the possible role of the stringent response and the evolutionarily conserved mechanism during the established interaction between plants and microorganisms. As a case study, we discussed fungi belonging to the genus Trichoderma. Our review aims to summarize the existing knowledge about plant-microorganism interactions and to highlight molecular pathways that need further investigation.

Potential of plant growth promoting bacterial consortium for improving the growth and yield of wheat under saline conditions

Owing to inconsistent results of a single bacterial strain, co-inoculation of more than one strain under salinity stress could be a more effective strategy to induce salt tolerance. Co-inoculation of more than one bacterial strain could be more effective due to the presence of several growths promoting traits. This study was conducted to evaluate the effectiveness of multi-strains bacterial consortium to promote wheat growth under salinity stress. Several plant growth promoting rhizobacteria (PGPR) had been isolated and tested for their ability to grow in increasing concentrations of sodium chloride (NaCl). Those rhizobacterial strains having tolerance against salinity were screened to evaluate their ability to promote wheat growth in the presence of salinity by conducting jar trials under axenic conditions. The rhizobacteria with promising results were tested for their compatibility with each other before developing multi-strain inoculum of PGPR. The compatible PGPR strains were characterized, and multi-strain inoculum was then evaluated for promoting wheat growth under axenic conditions at different salinity levels, i.e., 2.1 (normal soil), 6, 12, and 18 dS m^–1. The most promising combination was further evaluated by conducting a pot trial in the greenhouse. The results showed that compared to a single rhizobacterial strain, better growth-promoting effect was observed when rhizobacterial strains were co-inoculated. The multi-strain consortium of PGPR caused a significant positive impact on shoot length, root length, shoot fresh weight, and root fresh weight of wheat at the highest salinity level in the jar as well as in the pot trial. Results showed that the multi-strain consortium of PGPR caused significant positive effects on the biochemical traits of wheat by decreasing electrolyte leakage and increasing chlorophyll contents, relative water contents (RWC), and K/Na ratio. It can be concluded that a multi-strain consortium of PGPR (Ensifer adhaerens strain BK-30, Pseudomonas fluorescens strain SN5, and Bacillus megaterium strain SN15) could be more effective to combat the salinity stress owing to the presence of a variety of growth-promoting traits. However, further work is going on to evaluate the efficacy of multi-strain inoculum of PGPR under salt-affected field conditions.

Efforts towards overcoming drought stress in crops: Revisiting the mechanisms employed by plant growth-promoting bacteria

Globally, agriculture is under a lot of pressure due to rising population and corresponding increases in food demand. However, several variables, including improper mechanization, limited arable land, and the presence of several biotic and abiotic pressures, continually impact agricultural productivity. Drought is a notable destructive abiotic stress and may be the most serious challenge confronting sustainable agriculture, resulting in a significant crop output deficiency. Numerous morphological and physiological changes occur in plants as a result of drought stress. Hence, there is a need to create mitigation techniques since these changes might permanently harm the plant. Current methods used to reduce the effects of drought stress include the use of film farming, super-absorbent hydrogels, nanoparticles, biochar, and drought-resistant plant cultivars. However, most of these activities are money and labor-intensive, which offer limited plant improvement. The use of plant-growth-promoting bacteria (PGPB) has proven to be a preferred method that offers several indirect and direct advantages in drought mitigation. PGPB are critical biological elements which have favorable impacts on plants’ biochemical and physiological features, leading to improved sugar production, relative water content, leaf number, ascorbic acid levels, and photosynthetic pigment quantities. This present review revisited the impacts of PGPB in ameliorating the detrimental effects of drought stress on plants, explored the mechanism of action employed, as well as the major challenges encountered in their application for plant growth and development.

ACC deaminase producing rhizobacterium Enterobacter cloacae ZNP-4 enhance abiotic stress tolerance in wheat plant

Plant growth promoting rhizobacterium (PGPR) designated as ZNP-4, isolated from the rhizosphere of Ziziphus nummularia, was identified as Enterobacter cloacae following 16S rRNA sequence analysis. The isolated strain exhibited various plant growth promoting (PGP) traits. The 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) activity was evaluated under diverse physiological conditions that could be useful for minimizing the abiotic stress-induced inhibitory effects on wheat plants. The strain showed resistance to salt (NaCl) and metal (ZnSO₄) stress. The effect of E. cloacae ZNP-4 on the augmentation of plant growth was studied under salinity stress of 150 mM (T1 treatment) & 200 mM (T2 treatment) NaCl. The inoculation of strain ZNP-4 significantly improved the various growth parameters of wheat plant such as shoot length (41%), root length (31%), fresh weight (28%), dry weight (29%), photosynthetic pigments chlorophyll a (62%) and chlorophyll b (34%). Additionally, the strain was found to be efficient for minimizing the imposed Zn stress in terms of improving plant growth, biomass and photosynthetic pigments in pots containing different levels of metal stress of 150 mg kg^-1 (treatment T1) and 250 mg kg^-1 (treatment T2). Isolate ZNP-4 also improved the proline content and decreased malondialdehyde (MDA) level under both salinity and metal stress, therefore maintaining the membrane integrity. Furthermore, bacterial inoculation increased the activities of antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX). The positive effects of PGPR occurred concurrently with the decrease in abiotic stress-induced reactive oxygen species (ROS) molecules such as hydrogen peroxide (H₂O₂) and superoxide (O₂^-) contents. Overall, the observed results indicate that use of bacteria with such beneficial traits could be used as bio-fertilizers for many crops growing under stress conditions.

Co-inoculation of halotolerant potassium solubilizing Bacillus licheniformis and Aspergillus violaceofuscus improves tomato growth and potassium uptake in different soil types under salinity

Soil salinity is an important stress that negatively affects crop growth and productivity, causing extensive agricultural losses, worldwide. Potassium (K) solubilizing microorganisms (KSMs) can impart abiotic stress tolerance in plants in addition to nutrient solubilization. In this study, the salinity tolerance of KSMs Bacillus licheniformis and Aspergillus violaceofuscus originating from saxicolous habitats was examined using different concentrations of NaCl (0, 25, 50, 75, 100, and 125 mM) under in vitro conditions. The results indicated that both KSMs were capable of tolerating salinity. As B. licheniformis had a maximum growth in 100 mM NaCl at 37 °C, A. violaceofuscus had the maximum biomass and catalase (CAT) activity at 75 mM NaCl. However, maximum proline content was detected at 100 mM NaCl in both KSMs. Further, the ability of these KSMs to promote tomato growth individually and in combination with the presence or absence of mica was also examined in unsterilized or sterilized Alfisol and Vertisol soils under induced salinity in greenhouse conditions. The results of the greenhouse study revealed that inoculation of KSMs along with/without mica amendment significantly improved the morphological and physiological characteristics of tomato plants under salinity. Plant height, leaf area, biomass, relative water content, proline content, and CAT activity of dual inoculated plants were significantly higher than non-inoculated plants. Significant correlations existed between various soil, plant growth, soil pH and available K. From the results, it could be concluded that B. licheniformis and A. violaceofuscus are potential candidates for improving crop production in saline-stressed soils.

Pseudomonas putida and its close relatives: mixing and mastering the perfect tune for plants

Abstract

Plant growth–promoting rhizobacteria (PGPR) are a group of microorganisms of utmost interest in agricultural biotechnology for their stimulatory and protective effects on plants. Among the various PGPR species, some Pseudomonas putida strains combine outstanding traits such as phytohormone synthesis, nutrient solubilization, adaptation to different stress conditions, and excellent root colonization ability. In this review, we summarize the state of the art and the most relevant findings related to P. putida and its close relatives as PGPR, and we have compiled a detailed list of P. putida sensu stricto, sensu lato, and close relative strains that have been studied for their plant growth–promoting characteristics. However, the mere in vitro analysis of these characteristics does not guarantee correct plant performance under in vivo or field conditions. Therefore, the importance of studying adhesion and survival in the rhizosphere, as well as responses to environmental factors, is emphasized. Although numerous strains of this species have shown good performance in field trials, their use in commercial products is still very limited. Thus, we also analyze the opportunities and challenges related to the formulation and application of bioproducts based on these bacteria.

Key points

•The mini-review updates the knowledge on Pseudomonas putida as a PGPR.

• Some rhizosphere strains are able to improve plant growth under stress conditions.

• The metabolic versatility of this species encourages the development of a bioproduct.

Role of ACC deaminase producing bacteria for abiotic stress management and sustainable agriculture production

Plants are immobile and are exposed to various biotic and abiotic stresses, including heat, cold, drought, flooding, nutrient deficiency, heavy metal exposure, phytopathogens, and pest attacks. The stressors significantly affect agricultural productivity when exceed a certain threshold. It has been reported that most of the stressed plants are reported to have increased ethylene synthesis from its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene is a plant hormone that plays a vital role in the regulation of various physiological processes, such as respiration, nitrogen fixation, and photosynthesis. The increment in the plant hormone ethylene would reduce plant growth and development, and if the ethylene level increased beyond the limit, it could also result in plant death. Therefore, plant growth-promoting bacteria (PGPB) possessing ACC deaminase activity play an essential role in the management of biotic and abiotic stresses by hydrolysing 1-aminocyclopropane-1-carboxylic acid using ACC deaminase. In this review, the importance of ACC deaminase-producing bacteria in promoting plant growth under various abiotic stressors is discussed.

Significance of the Diversification of Wheat Species for the Assembly and Functioning of the Root-Associated Microbiome

Wheat, one of the major crops in the world, has had a complex history that includes genomic hybridizations between Triticum and Aegilops species and several domestication events, which resulted in various wild and domesticated species (especially Triticum aestivum and Triticum durum), many of them still existing today. The large body of information available on wheat-microbe interactions, however, was mostly obtained without considering the importance of wheat evolutionary history and its consequences for wheat microbial ecology. This review addresses our current understanding of the microbiome of wheat root and rhizosphere in light of the information available on pre- and post-domestication wheat history, including differences between wild and domesticated wheats, ancient and modern types of cultivars as well as individual cultivars within a given wheat species. This analysis highlighted two major trends. First, most data deal with the taxonomic diversity rather than the microbial functioning of root-associated wheat microbiota, with so far a bias toward bacteria and mycorrhizal fungi that will progressively attenuate thanks to the inclusion of markers encompassing other micro-eukaryotes and archaea. Second, the comparison of wheat genotypes has mostly focused on the comparison of T. aestivum cultivars, sometimes with little consideration for their particular genetic and physiological traits. It is expected that the development of current sequencing technologies will enable to revisit the diversity of the wheat microbiome. This will provide a renewed opportunity to better understand the significance of wheat evolutionary history, and also to obtain the baseline information needed to develop microbiome-based breeding strategies for sustainable wheat farming.

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

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

Multifarious effect of ACC deaminase and EPS producing Pseudomonas sp. and Serratia marcescens to augment drought stress tolerance and nutrient status of wheat

Drought is the prime abiotic stress that rigorously influences plant growth, yield and quality of crops. The current investigation illustrated the bio-protective characters of Serratia marcescens and Pseudomonas sp. to ameliorate drought stress tolerance, plant growth and nutrient status of wheat. The present study aimed for search of potential drought tolerant plant growth-promoting rhizobacteria (PGPR). All screened bacterial isolates exhibited potential plant growth promoting (PGP) attributes such as production of ACC deaminase, exo-polysaccharide, siderophore, ammonia, IAA, and efficiently solubilized zinc and phosphate under in vitro conditions. To assess the in situ plant growth promotion potential of PGPR, a greenhouse experiment was conducted by priming wheat seeds with screened plant PGPR. Improved water status, reactive oxygen species, osmolyte accumulation, chlorophyll and carotenoids content in plant leaves confirmed the excellent drought tolerance conferring ability of RRN II 2 and RRC I 5. Among all PGPR, RRN II 2 and RRC I 5 inoculated plants not only demonstrated greater harvest index but also exhibited more micronutrient (zinc and iron) content in wheat grains. Further, RRN II 2 and RRC I 5 were identified through 16S rDNA sequencing as S. marcescens and Pseudomonas sp., respectively. Furthermore, amplification of acdS gene (Amplified band size of acdS gene was ~ 1.8 Kb) also confirmed ACC deaminase enzyme producing ability of Pseudomonas sp. Moreover, correlation coefficient, principal component analysis and cluster analysis also demonstrated that nutrient status and values of agronomical parameters of wheat primed with S. marcescens and Pseudomonas sp. were at par with the positive control. Thus, the outcome of this comparative investigation indicates that Pseudomonas sp. and S. marcescens could be utilized as bioinoculant in wheat since they can improve the physiological status, productivity and nutrient status in wheat crop under drought.

Harnessing the Potential of Symbiotic Endophytic Fungi and Plant Growth-Promoting Rhizobacteria to Enhance Soil Quality in Saline Soils

Soil salinity is one of the most important abiotic stresses limiting crop growth and production worldwide. Some microorganisms can improve the plants’ tolerance to salinity. For this purpose, a greenhouse experiment was performed to understand the influence of various microorganisms on soil biological indices and wheat growth under different saline conditions. The factors varied in the experiment were the microbial treatment (rhizobacteria, mycorrhizal fungi, endophytic fungus, and control) and salinity stress (0.5, 8, and 14 dS m−1). Rhizobacteria were isolated from saline soils, but the fungi were prepared from a microbial bank. Overall, ten isolates were purified, and three with promising growth-promoting properties were identified using phenotypic and molecular methods. The selected isolates belonged to the genera Pseudomonas (P. aeruginosa Ur83 and P. fluorescens Ur67) and Stenotrophomonas (S. maltophilia Ur52). Soil quality indices were found to decrease with increasing salinity, but inoculation with microorganisms alleviated this decline. Inoculation with plant growth-promoting rhizobacteria (PGPRs) increased basal respiration, substrate-induced respiration, microbial biomass carbon, acid and alkaline phosphatase activities, and carbon availability by factors of 1.37, 1.27, 1.83, 3.07, 1.29, and 1.11, respectively. These results show that inoculation with symbiotic microorganisms can improve agricultural soil quality under saline conditions and may thus be valuable in agriculture.

Phomopsis liquidambaris reduces ethylene biosynthesis in rice under salt stress via inhibiting the activity of 1-aminocyclopropane-1-carboxylate deaminase

The endophytic fungus Phomopsis liquidambaris is characterized as a plant growth-promoting agent under salt stress, but its mechanism is unknown. Herein, 1-aminocyclopropane-1-carboxylate deaminase (ACCD) from the strain was confirmed that it had the ability of utilizing 1-aminocyclopropane-1-carboxylate as the sole nitrogen source. The full-length ACCD gene was 1152 bp, which encodes a mature protein of 384 amino acids with a molecular mass of 41.53 kDa. The ACCD activity was 3.9-fold in 3 mmol L^-1 ACC by qRT-PCR under salt stress comparing with no salt tress. Ethylene production was increased to 34.55-70.60% and reduced the growth of rice by 23-69.73% under salt stress. Inoculation of P. liquidambaris increased root-shoot length, fresh and dry weight, and overall growth of stressed rice seedlings. ACC accumulation, ACC synthase and ACC oxidase activities increased in salt-treated rice seedlings, while they were significantly reduced when P. liquidambaris was inoculated into rice by qRT-PCR. It therefore can be concluded that P. liquidambaris can be used as a plant growth promoting fungus against salt stress and other biotic or abiotic stresses.

Diversity and Taxonomic Distribution of Endophytic Bacterial Community in the Rice Plant and Its Prospective

Endophytic bacterial communities are beneficial communities for host plants that exist inside the surfaces of plant tissues, and their application improves plant growth. They benefit directly from the host plant by enhancing the nutrient amount of the plant’s intake and influencing the phytohormones, which are responsible for growth promotion and stress. Endophytic bacteria play an important role in plant-growth promotion (PGP) by regulating the indirect mechanism targeting pest and pathogens through hydrolytic enzymes, antibiotics, biocontrol potential, and nutrient restriction for pathogens. To attain these benefits, firstly bacterial communities must be colonized by plant tissues. The nature of colonization can be achieved by using a set of traits, including attachment behavior and motility speed, degradation of plant polymers, and plant defense evasion. The diversity of bacterial endophytes colonization depends on various factors, such as plants’ relationship with environmental factors. Generally, each endophytic bacteria has a wide host range, and they are used as bio-inoculants in the form of synthetic applications for sustainable agriculture systems and to protect the environment from chemical hazards. This review discusses and explores the taxonomic distribution of endophytic bacteria associated with different genotypes of rice plants and their origin, movement, and mechanism of PGP. In addition, this review accentuates compressive meta data of endophytic bacteria communities associated with different genotypes of rice plants, retrieves their plant-growth-promoting properties and their antagonism against plant pathogens, and discusses the indication of endophytic bacterial flora in rice plant tissues using various methods. The future direction deepens the study of novel endophytic bacterial communities and their identification from rice plants through innovative techniques and their application for sustainable agriculture systems.

Rhizobacteria Inoculation and Caffeic Acid Alleviated Drought Stress in Lentil Plants

Lentil (Lens culinaris Medik) is an important component of the human diet due to its high mineral and protein contents. Abiotic stresses, i.e., drought, decreases plant growth and yield. Drought causes the synthesis of reactive oxygen species, which decrease a plant’s starch contents and growth. However, ACC-deaminase (1-aminocyclopropane-1-carboxylate deaminase) producing rhizobacteria can alleviate drought stress by decreasing ethylene levels. On the other hand, caffeic acid (CA) can also positively affect cell expansion and turgor pressure maintenance under drought stress. Therefore, the current study was planned with an aim to assess the effect of CA (0, 20, 50 and 100 ppm) and ACC-deaminase rhizobacteria (Lysinibacillus fusiform, Bacillus amyloliquefaciens) on lentils under drought stress. The combined application of CA and ACC-deaminase containing rhizobacteria significantly improved plant height (55%), number of pods per plant (51%), 1000-grain weight (45%), nitrogen concentration (56%), phosphorus concentration (19%), potassium concentration (21%), chlorophyll (54%), relative water contents RWC (60%) and protein contents (55%). A significant decrease in electrolyte leakage (30%), proline contents (44%), and hydrogen peroxide contents (54%), along with an improvement in cell membrane stability (34% over control) validated the combined use of CA and rhizobacteria. In conclusion, co-application of CA (20 ppm) and ACC-deaminase producing rhizobacteria can significantly improve plant growth and yield for farmers under drought stress. More investigations are suggested at the field level to select the best rhizobacteria and CA level for lentils under drought.

Deep-Sea Actinobacteria Mitigate Salinity Stress in Tomato Seedlings and Their Biosafety Testing

Soil salinity is an enormous problem affecting global agricultural productivity. Deep-sea actinobacteria are interesting due to their salt tolerance mechanisms. In the present study, we aim to determine the ability of deep-sea Dermacoccus (D. barathri MT2.1^T and D. profundi MT2.2^T) to promote tomato seedlings under 150 mM NaCl compared with the terrestrial strain D. nishinomiyaensis DSM20448^T. All strains exhibit in vitro plant growth-promoting traits of indole-3-acetic acid production, phosphate solubilization, and siderophore production. Tomato seedlings inoculated with D. barathri MT2.1^T showed higher growth parameters (shoot and root length, dry weight, and chlorophyll content) than non-inoculated tomato and the terrestrial strain under 150 mM NaCl. In addition, hydrogen peroxide (H₂O₂) in leaves of tomatoes inoculated with deep-sea Dermacoccus was lower than the control seedlings. This observation suggested that deep-sea Dermacoccus mitigated salt stress by reducing oxidative stress caused by hydrogen peroxide. D. barathri MT2.1^T showed no harmful effects on Caenorhabditis elegans, Daphnia magna, Eisenia foetida, and Escherichia coli MC4100 in biosafety tests. This evidence suggests that D. barathri MT2.1^T would be safe for use in the environment. Our results highlight the potential of deep-sea Dermacoccus as a plant growth promoter for tomatoes under salinity stress.

Plant Growth Promoting Rhizobacteria, Arbuscular Mycorrhizal Fungi and Their Synergistic Interactions to Counteract the Negative Effects of Saline Soil on Agriculture: Key Macromolecules and Mechanisms

Soil saltiness is a noteworthy issue as it results in loss of profitability and development of agrarian harvests and decline in soil health. Microorganisms associated with plants contribute to their growth promotion and salinity tolerance by employing a multitude of macromolecules and pathways. Plant growth promoting rhizobacteria (PGPR) have an immediate impact on improving profitability based on higher crop yield. Some PGPR produce 1-aminocyclopropane-1-carboxylic (ACC) deaminase (EC 4.1.99.4), which controls ethylene production by diverting ACC into α-ketobutyrate and ammonia. ACC deaminase enhances germination rate and growth parameters of root and shoot in different harvests with and without salt stress. Arbuscular mycorrhizal fungi (AMF) show a symbiotic relationship with plants, which helps in efficient uptake of mineral nutrients and water by the plants and also provide protection to the plants against pathogens and various abiotic stresses. The dual inoculation of PGPR and AMF enhances nutrient uptake and productivity of several crops compared to a single inoculation in both normal and stressed environments. Positively interacting PGPR + AMF combination is an efficient and cost-effective recipe for improving plant tolerance against salinity stress, which can be an extremely useful approach for sustainable agriculture.

The Role of Plant Growth-Promoting Bacteria in Alleviating the Adverse Effects of Drought on Plants

Plant growth-promoting bacteria play an essential role in enhancing the physical, chemical and biological characters of soils by facilitating nutrient uptake and water flow, especially under abiotic stress conditions, which are major constrains to agricultural development and production. Drought is one of the most harmful abiotic stress and perhaps the most severe problem facing agricultural sustainability, leading to a severe shortage in crop productivity. Drought affects plant growth by causing hormonal and membrane stability perturbations, nutrient imbalance and physiological disorders. Furthermore, drought causes a remarkable decrease in leaf numbers, relative water content, sugar yield, root yield, chlorophyll a and b and ascorbic acid concentrations. However, the concentrations of total phenolic compounds, electrolyte leakage, lipid peroxidation, amounts of proline, and reactive oxygen species are considerably increased because of drought stress. This negative impact of drought can be eliminated by using plant growth-promoting bacteria (PGPB). Under drought conditions, application of PGPB can improve plant growth by adjusting hormonal balance, maintaining nutrient status and producing plant growth regulators. This role of PGPB positively affects physiological and biochemical characteristics, resulting in increased leaf numbers, sugar yield, relative water content, amounts of photosynthetic pigments and ascorbic acid. Conversely, lipid peroxidation, electrolyte leakage and amounts of proline, total phenolic compounds and reactive oxygen species are decreased under drought in the presence of PGPB. The current review gives an overview on the impact of drought on plants and the pivotal role of PGPB in mitigating the negative effects of drought by enhancing antioxidant defense systems and increasing plant growth and yield to improve sustainable agriculture.

Rhizosphere plant-microbe interactions under water stress

Climate change, with its extreme temperature, weather and precipitation patterns, is a major global concern of dryland farmers, who currently meet the challenges of climate change agronomically and with growth of drought-tolerant crops. Plants themselves compensate for water stress by modifying aerial surfaces to control transpiration and altering root hydraulic conductance to increase water uptake. These responses are complemented by metabolic changes involving phytohormone network-mediated activation of stress response pathways, resulting in decreased photosynthetic activity and the accumulation of metabolites to maintain osmotic and redox homeostasis. Phylogenetically diverse microbial communities sustained by plants contribute to host drought tolerance by modulating phytohormone levels in the rhizosphere and producing water-sequestering biofilms. Drylands of the Inland Pacific Northwest, USA, illustrate the interdependence of dryland crops and their associated microbiota. Indigenous Pseudomonas spp. selected there by long-term wheat monoculture suppress root diseases via the production of antibiotics, with soil moisture a critical determinant of the bacterial distribution, dynamics and activity. Those pseudomonads producing phenazine antibiotics on wheat had more abundant rhizosphere biofilms and provided improved tolerance to drought, suggesting a role of the antibiotic in alleviation of drought stress. The transcriptome and metabolome studies suggest the importance of wheat root exudate-derived osmoprotectants for the adaptation of these pseudomonads to the rhizosphere lifestyle and support the idea that the exchange of metabolites between plant roots and microorganisms profoundly affects and shapes the belowground plant microbiome under water stress.

Advances in Wheat Physiology in Response to Drought and the Role of Plant Growth Promoting Rhizobacteria to Trigger Drought Tolerance

In the coming century, climate change and the increasing human population are likely leading agriculture to face multiple challenges. Agricultural production has to increase while preserving natural resources and protecting the environment. Drought is one of the major abiotic problems, which limits the growth and productivity of crops and impacts 1–3% of all land.To cope with unfavorable water-deficit conditions, plants use through sophisticated and complex mechanisms that help to perceive the stress signal and enable optimal crop yield are required. Among crop production, wheat is estimated to feed about one-fifth of humanity, but faces more and more drought stress periods, partially due to climate change. Plant growth promoting rhizobacteria are a promising and interesting way to develop productive and sustainable agriculture despite environmental stress. The current review focuses on drought stress effects on wheat and how plant growth-promoting rhizobacteria trigger drought stress tolerance of wheat by highlighting several mechanisms. These bacteria can lead to better growth and higher yield through the production of phytohormones, osmolytes, antioxidants, volatile compounds, exopolysaccharides and 1-aminocyclopropane-1-carboxylate deaminase. Based on the available literature, we provide a comprehensive review of mechanisms involved in drought resilience and how bacteria may alleviate this constraint

Evaluation of interaction among indigenous rhizobacteria and Vigna unguiculata on remediation of metal-containing abandoned magnesite mine tailing

Abandoned magnesite mine heap causing pollution to nearby farmland and water reservoir. Thus the intention of this research was to screening metal mobilizing and absorbing bacteria from the rhizosphere section of V. unguiculata from farmland nearby to magnesite mine. Further, studied their stimulus effect on growth, biomass, and phytoextraction prospective of V unguiculata in mine tailing. The results of the physicochemical properties of mine tailing shows that four metals (Pb, Mn, Cd, and Zn) were crossing the permissible limit. Out of 27 isolates, 2 isolates (MMS15 and MMS17) were identified with maximum metal tolerance for up to 700 mg L^-1 (MIC) and metal mobilization (Pb 5.5 and 5.87, Mn 6.6 and 4.88, Cd 1.99 and 2.59, and Zn 6.55 and 6.94 mg kg^-1) and biosorption efficiency as Pb 3.74 and 3.74, Mn 4.9 and 4.7, Cd 2.41 and 3.96, and Zn 4.3 and 4.9 mg g^-1. These two strains were identified as members of B. cereus and Kosakonia sp. using 16S rRNA technique and labelled strains NDRMN001 and MGR1, respectively. The Kosakonia sp. MGR1 effectively fixes the nitrogen in the rate of 81.94% and B. cereus NDRMN001 solubilizes 69.98 ± 2.31 mg L^-1 of soluble phosphate. The experimental group's study results show that the group C (Kosakonia sp. MGR1 and B. cereus NDRMN001) has effectively stimulate the growth, biomass, and phytoextraction potential of V. unguiculata. The results conclude that the optimistic interaction between these two bacteria could be more significant to minimize the metal pollution in magnesite mine tailing.

Biofilm forming rhizobacteria enhance growth and salt tolerance in sunflower plants by stimulating antioxidant enzymes activity

Salinity stress is one of the major environmental stresses that impose global socio-economic impacts, as well as hindering crop productivity. Halotolerant plant growth-promoting rhizobacteria (PGPR) having potential to cope with salinity stress can be employed to counter this issue in eco-friendly way. In the present investigation, halotolerant PGPR strains, AP6 and PB5, were isolated from saline soil and characterized for their biochemical, molecular and physiological traits. Sequencing of 16 S rRNA gene and comparative analysis confirmed the taxonomic affiliation of AP6 with Bacillus licheniformis and PB5 with Pseudomonas plecoglossicida. The study was carried out in pots with different levels of induced soil salinity viz. 0, 5, 10 and 15 dSm^-1 to evaluate the potential of bacterial inoculants in counteracting salinity stress in sunflower at different plant growth stages (30, 45 and 60 days after sowing). Both the bacterial inoculants were capable of producing indole acetic acid and biofilm, solubilizing inorganic rock phosphate, and also expressed ACC deaminase activity. The PGPR inoculated plants showed significantly higher fresh and dry biomass, plant height, root length and yield plant^-1. Ameliorative significance of applied bacterial inoculants was also evidenced by mitigating oxidative stress through upregulation of catalase (CAT), superoxide dismutase (SOD) and guaiacol peroxidase (GPX) antioxidant enzymes. Increase in photosynthetic pigments, gas exchange activities and nutrient uptake are crucial salt stress adaptations, which were enhanced with the inoculation of salt tolerant biofilm producing PGPR in sunflower plants. Although increase in salinity stress levels has gradually decreased the plant's output compared to non-salinized plants, the plants inoculated with PGPR confronted salinity stress in much better way than uninoculated plants. Owing to the wide action spectrum of these bacterial inoculants, it was concluded that these biofilm PGPR could serve as effective bioinoculants and salinity stress alleviator for sunflower (oil seed crop) by increasing crop productivity in marginalized agricultural systems.

Mitigation of Heat Stress in Solanum lycopersicum L. by ACC-deaminase and Exopolysaccharide Producing Bacillus cereus: Effects on Biochemical Profiling

Soil microorganisms might be assessed for their capabilities of plant growth promotion in order to identify heat tolerant strategies for crop production. The planned study was conducted to determine the potential of heat tolerant plant growth promoting rhizobacteria (PGPR) in mitigating heat stress effects in tomato. Bacillus cereus was evaluated for plant growth promoting activities and assessed for 1-aminocyclopropane-1-carboxylate (ACC-deaminase) (0.76–C0.9 μM/mg protein/h), and exopolysaccharide (0.66–C0.91 mg/mL) under normal and heat stressed conditions. Plant growth regulators were evaluated through High Performance Liquid Chromatography. Bacterial inoculation effects on important physiological and biochemical parameters were evaluated under normal and heat stressed conditions in growth chamber. The morphological-physiological traits significantly revealed drastic effects on both of un-inoculated tomato varieties under heat stress conditions. Bacterial augmentation significantly promoted shoot, root length, leaf surface area, fresh and dry weight. Heat stress enhanced extracellular polymeric substances (EPS) production and cleavage of ACC into a-ketobutyrate and ammonia due to ACC-deaminase producing bacteria that significantly reduced the adverse effects of heat on tomato growth. In conclusion, the applied plant growth promoting rhizobacteria (PGPR) bacterial strain proved as potential candidate for improving tomato crop growing under heat stressed conditions. However, it is highly suggested to validate the current results by conducting field trials.

ACC deaminase in plant growth-promoting bacteria (PGPB): An efficient mechanism to counter salt stress in crops

Salinity in agricultural soil is a major problem around the world, with negative consequences for the growth and production of a wide range of crops. To counteract these harmful effects, plants sometimes have bacterial partners that contain the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which acts by degrading ACC (the precursor of ethylene in all higher plants). The enzymatic activity of ACC deaminase results in the production of α-ketobutyrate and ammonia, which, by lowering ACC levels, prevents excessive increases in the synthesis of ethylene under various stress conditions and is one of the most efficient mechanisms to induce plant tolerance to salt stress. In the present review, recent works on the role of ACC deaminase are discussed alongside its importance in promoting plant growth under conditions of salt stress in endophytic and rhizospheric bacteria, with some emphasis on Bacillus species. In addition, the toxic effects of soil salinity on plants and microbial biodiversity are analysed. Recent findings on the synergetic functioning of ACC deaminase and other bacterial mechanisms of salt stress tolerance, such as trehalose accumulation, are also summarized. Finally, we discuss the various advantages of ACC deaminase-producing bacilli as bioinoculants to address the problem of salinity in agricultural soils.

Regulation of antioxidant production, ion uptake and productivity in potato (Solanum tuberosum L.) plant inoculated with growth promoting salt tolerant Bacillus strains

The exchangeable sodium (Na+) in salt affected soils is a major constraint in potassium (K+) availability to plants that disturb ion transport and inhibit plant growth, adversely. Salt tolerant plant growth promoting rhizobacteria (PGPR) may regulate the Na+/K+ efflux and increase K+ uptake by the plant from the soil. Therefore, a pot study was performed to examine the effect of salt tolerant PGPR Bacillus sp. alone and in consortium, on antioxidant enzyme activity, ion uptake and potato (Solanum tuberosum L.) tuber yield in normal and salt affected soils. We observed that Bacillus sp. (strains SR-2-1 and SR-2-1/1) solubilized insoluble phosphorous and produced indole-3-acetic acid while only SR-2-1/1 produced ACC deaminase in culture medium supplemented with various concentrations of NaCl (0-6%). In the pot experiment, the consortium treatment of strains was found to increase relative leaf water contents whereas decreased the electrolyte leakage and antioxidant enzyme activity both in normal and salt affected soils. Similarly, consortium treatment decreased Na+ whereas increased K+, Ca+2, K+/Na+ and Ca+2/Na+ in plant dry matter in both soils. It has been investigated that inoculation of PGPR significantly (p < 0.05) increased plant biomass, number of tubers per plant and tuber weight as compared to un-inoculated plants in both soils. In addition, PGPR inoculation enhanced auxin production in root exudates of young potato plants and bacterial population dynamics in both soils. Na+ ion regulation (R2 = 0.95) and tuber weight (R2 = 0.90) in salt affected soil were significantly correlated with auxin production in the rhizosphere. Results of this study conferred that consortium of Bacillus strains (SR-2-1, SR-2-1/1) enhanced auxin production in the rhizosphere of potato plants and that ultimately regulated antioxidant enzyme production and uptake of Na+, K+ and Ca+2 in potato plants resulted into a higher tuber yield in both normal and salt affected soils.

Induction of tolerance to salinity in wheat genotypes by plant growth promoting endophytes: Involvement of ACC deaminase and antioxidant enzymes

Plant growth-promoting endophytes (PGPEs) can colonize the internal tissues of plants and are capable of promoting plant growth. These bacteria can improve plant tolerance against various biotic and abiotic stresses via the expression of antioxidant enzymes and the production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Two salt-tolerant PGPEs, Kocuria rhizophila: KF875448 (14ASP) and Cronobacter sakazakii: KM042090 (OF115), with ACC deaminase activity were investigated for their potential to ameliorate plant salinity stress. The wheat varieties Pasban 90 and Khirman were subjected to two levels of salt stress (80 and 160 mM NaCl) under greenhouse conditions by using a completely randomized design. Analyses of plant growth parameters, antioxidant enzyme activities, chlorophyll and plant mineral contents were conducted to investigate the stress tolerance induced by the PGPEs. The ACC utilization by the PGPEs directly relates to the promotion of plant growth due to the lowering of excess ethylene production under salt stress. High levels of NaCl exhibited negative effects in both varieties. However, inoculation with PGPEs increased the morphological traits and antioxidant activities of the plants while decreasing the Na⁺ contents in all treatments compared to uninoculated treatment. Wheat variety Pasban 90 was more tolerant than Khirman in to salt stress in all the measured morphological and biochemical parameters, while the bacterial strain OF115 performed significantly better in all morphological and biochemical parameters, such as fresh dry weight, root shoot length, proline and chlorophyll contents, compared to strain 14ASP. The K⁺/Na⁺ ratio in the tissues of bacterial treated plants was higher than the control, probably in order to maintain the nutrient balance. The results of our study revealed that the inoculation of plants by ACC deaminase-producing PGPEs is a potential tool for the enhancement of plant growth and stress tolerance. Moreover, endophytic bacteria allied with host plants are capable of enduring high saline conditions and can interact with plants in a very efficient way.

Enhancement of alfalfa yield and quality by plant growth-promoting rhizobacteria under saline-alkali conditions

BACKGROUND

Bacteria with 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity could decrease the ethylene level, confer resistance of plant, and stimulate plant growth under biotic and abiotic stress conditions.

RESULTS

Plant growth-promoting rhizobacteria (PGPR) strains Enterobacter aerogenes (LJL-5) and Pseudomonas aeruginosa (LJL-13) were obtained from the rhizosphere of alfalfa grown under saline-alkali conditions. The ACC deaminase activity of E. aerogenes (LJL-5) and Ps. aeruginosa (LJL-13) was approximately 2-5 µmol mg^-1  h^-1 . indole acetic acid synthesis was increased with the increasing concentration of l-tryptophan. Siderophore production and phosphate solubilization in Ps. aeruginosa (LJL-13) were higher than those in E. aerogenes (LJL-5). Compared to the non-inoculated seedlings (1.31 ng mL^-1  h^-1 ), inoculated alfalfa seedlings with E. aerogenes (LJL-5) (0.90 ng mL^-1  h^-1 ) and Ps. aeruginosa (LJL-13) (0.78 ng mL^-1  h^-1 ) emitted lower levels of ethylene. Under saline-alkali conditions in the greenhouse, inoculation with E. aerogenes (LJL-5) and Ps. aeruginosa (LJL-13) increased the biomass, soil and plant analyzer development (SPAD), and P content of alfalfa plants, and also induced the activity of antioxidant enzymes (superoxide dismutase, peroxidase and catalase), promoted the accumulation of antioxidant substances and removed various harmful substances. Under saline-alkali conditions in the field (2012, 2013, and 2014), inoculation of alfalfa with E. aerogenes (LJL-5) and Ps. aeruginosa (LJL-13) significantly increased the shoot height, fresh and dry weights, yield and crude protein content of alfalfa plants, but decreased the fiber content.

CONCLUSION

Two PGPR strains were isolated from the rhizosphere of alfalfa in saline-alkali conditions. Both strains could promote alfalfa growth in saline-alkali soil, and could be used as biofertilizer to promote plant growth under stress and reduce environmental pollution caused by fertilizers simultaneously. © 2018 Society of Chemical Industry.

Mechanisms of the IAA and ACC-deaminase producing strain of Trichoderma longibrachiatum T6 in enhancing wheat seedling tolerance to NaCl stress

BACKGROUND

Trichoderma species, a class of plant beneficial fungi, may provide opportunistic symbionts to induce plant tolerance to abiotic stresses. Here, we determined the possible mechanisms responsible for the indole acetic acid (IAA) and 1-aminocyclopropane-1-carboxylate-deaminase (ACC-deaminase) producing strain of Trichoderma longibrachiatum T6 (TL-6) in promoting wheat (Triticum aestivum L.) growth and enhancing plant tolerance to NaCl stress.

RESULTS

Wheat treated with or without TL-6 was grown under different levels of salt stress in controlled environmental conditions. TL-6 showed a high level of tolerance to 10 mg ml^- 1 of NaCl stress and the inhibitory effect was more pronounced at higher NaCl concentrations. Under NaCl stress, the activity of ACC-deaminase and IAA concentration in TL-6 were promoted, with the activity of ACC-deaminase increased by 26% at the salt concentration of 10 mg ml^- 1 and 31% at 20 mg ml^- 1, compared with non-saline stress; and the concentration of IAA was increased by 10 and 7%, respectively (P < 0.05). The increased ACC-deaminase and IAA concentration in the TL-6 strain may serve as an important signal to alleviate the negative effect of NaCl stress on wheat growth. As such, wheat seedlings with the ACC-deaminase and IAA producing strain of TL-6 treatment under NaCl stress increased the IAA concentration by an average of 11%, decreased the activity of ACC oxidase (ACO) by an average of 12% and ACC synthase (ACS) 13%, and decreased the level of ethylene synthesis and the content of ACC by 12 and 22%, respectively (P < 0.05). The TL-6 treatment decreased the transcriptional level of ethylene synthesis genes expression, and increased the IAA production genes expression significantly in wheat seedlings roots; down-regulated the expression of ACO genes by an average of 9% and ACS genes 12%, whereas up-regulated the expression of IAA genes by 10% (P < 0.05). TL-6 treatments under NaCl stress decreased the level of Na⁺ accumulation; and increased the uptake of K⁺ and the ratio of K⁺/Na⁺, and the transcriptional level of Na⁺/H⁺ antiporter gene expression in both shoots and roots.

CONCLUSIONS

Our results indicate that the strain of TL-6 effectively promoted wheat growth and enhanced plant tolerance to NaCl stress through the increased ACC-deaminase activity and IAA production in TL-6 stain that modulate the IAA and ethylene synthesis, and regulate the transcriptional levels of IAA and ethylene synthesis genes expression in wheat seedling roots under salt stress, and minimize ionic toxicity by disturbing the intracellular ionic homeostasis in the plant cells. These biochemical, physiological and molecular responses helped promote the wheat seedling growth and enhanced plant tolerance to salt stress.

Salt-tolerant and plant-growth-promoting bacteria isolated from high-yield paddy soil

Growth and productivity of rice is negatively affected by soil salinity. However, some salt-tolerant bacteria improve the health of plants under saline stress. In this study, 305 bacteria were isolated from paddy soil in Taoyuan, China. Among these, 162 strains were tested for salt-tolerance; 67.3%, 28.4%, and 9.3% of the strains could grow in media with NaCl concentrations of 50, 100, and 150 g/L, respectively. The phylogenic analysis of 74 of these 162 strains indicates that these bacteria belong to Bacillales (72%), Actinomycetales (22%), Rhizobiales (1%), and Oceanospirillales (4%). Among 162 strains, 30 salt-tolerant strains were screened for their plant-growth-promoting activities under axenic conditions at 3, 6, 9, and 12 g/L NaCl; 43%–97% of the strains could improve rice germination energy or germination capacity, while 63%–87% of the strains could increase shoot and root lengths. Among various plant-growth-promoting bacteria, TY0307 was the most effective strain for promoting the growth of rice, even at high salt stress. Its promotor effects were associated with its production of 1-aminocyclopropane-1-carboxycarboxylate deaminase, indole acetic acid, and siderophores; induction of proline accumulation; and reduction of the salt-induced malondialdehyde content. These results suggest that several strains isolated from paddy soil could improve rice salt tolerance and may be used in the development of biofertilizer.

Isolation and identification of Ammodendron bifolium endophytic bacteria and the action mechanism of selected isolates-induced seed germination and their effects on host osmotic-stress tolerance

This study aimed to identify Ammodendron bifolium endophytic bacteria, and to evaluate promoting mechanism of selected isolates on seed germination and their effects on host osmotic-stress tolerance. Forty-five strains were isolated from A. bifolium and were classified into 13 different genera by 16S rDNA gene sequence analysis. AY3, AY9 and AG18, which were identified as Staphylococcus, Kocuria, Bacillus sp., promoted host seed ethylene release during germination. Ethrel and 1-aminocyclopropane-1-carboxylic acid (ACC) imitated the effect of AY3, AY9 and AG18 on seed germination. The data suggest that ethylene mediates AY3-, AY9-, AG18-induced A. bifolium seed germination. In addition, osmotic stress prevented seed germination and radicle elongation. However, the inhibitory effect of osmotic stress on seed germination and radicle elongation were rescued by AY3, AY9 and AG18. The results show that AY3, AY9 and AG18 increased osmotic-stress tolerance in A. bifolium. AY3, AY9, AG18 induced A. bifolium seed germination through promoting ethylene production during endophytic bacteria-plant interaction, and increase osmotic-stress tolerance in A. bifolium. AY3, AY9 and AG18 are potential candidates for the protection of A. bifolium.

Use of plant growth promoting rhizobacteria (PGPRs) with multiple plant growth promoting traits in stress agriculture: Action mechanisms and future prospects

Increased incidence of abiotic stresses impacting adversely plant growth and productivity in major crops is being witnessed all over the world. Therefore, as a result of such stress factors, plant growth under the stress conditions will be less than the non-stress conditions. Growing concerns and global demand for correct, environmentally-friendly techniques exist to reduce the adverse effects of plant stress. Under such stressful conditions, the role of interactions of plant and beneficial microorganisms is of great significance. Application of plant growth promoting rhizobacteria (PGPRs) is a useful option to decrease these stresses and is now widely in practice. Plants inoculated with PGPRs induce morphological and biochemical modifications resulting in increased tolerance to abiotic stresses defined as IST (induced systemic tolerance). PGPRs increase plant growth and resistance to abiotic stresses through various mechanisms (more than one mechanism of action) such as production of ACC (1-aminocyclopropane-1-carboxylate) deaminase, reducing production of stress ethylene, modifications in phytohormonal content, induction of synthezing plant antioxidative enzymes, improvement in the uptake of essential mineral elements, extracellular polymeric substance (EPS) production, decrease in the absorbtion of excess nutrients/heavy metals, and induction of abiotic stress resistance genes. Experimental evidence also suggests that stimulated plant growth by these bacteria is the net result of various mechanisms of action that are activated simultaneously. In this review paper, we reviewed the action mechanisms through which PGPRs could alleviate abiotic stresses (salinity, drought, heavy metal toxicity, and nutritional imbalance) in plants. Use of PGPRs is predicted to become a suitable strategy and an emerging trend in sustainable enhancement of plant growth. Generally, ACC deaminase and IAA-producing bacteria can be a good option for optimal crop production and production of bio-fertilizers in the future due to having multiple potentials in alleviating stresses of salinity, drought, nutrient imbalance, and heavy metals toxicity in plants. This review paper also emphasizes future research needs about the combined utilization of stress tolerant-PGPRs with multiple plant growth promoting (PGP) characteristics under environmental stresses.

Effects of the inoculations using bacteria producing ACC deaminase on ethylene metabolism and growth of wheat grown under different soil water contents

Crop growth and productivity are often impacted by the increased ethylene content induced by adverse environmental conditions such drought. Inoculations with bacteria producing ACC deaminase is considered as a potential biological approach to improve the growth and tolerance of stressed plants by lowering endogenous ethylene level. In this study, germinated wheat seeds were inoculated using three species of the rhizobacteria, which were isolated from the rhizosphere of wheat growing in dryland, and sown in pots. After three weeks, wheat seedlings were exposed to non-limiting water condition, medium drought and severe drought, respectively, for six weeks. The results showed that, irrespective of rhizobacterial inoculations, decreased soil water contents stimulated wheat ethylene metabolism, which was reflected by the significantly increased activity of ACC synthetase and ACC oxidase, besides an increased content of ACC both in the roots and leaves, and an enhanced capacity of leaves to release ethylene, concomitant with a significant decline in shoot and roots biomass. The inoculations of all three rhizobacterial species under each water condition reduced ACC content in wheat leaves, but effects of the inoculations on ACC synthase and ACC oxidase activity in the leaves and roots, ACC content in the roots, the capacity of leaves to release ethylene, and wheat growth varied with water conditions and bacterial species. Hence, both soil water conditions and rhizobacterial inoculations acted on all the processes of ethylene metabolism, with the former being dominant. The inoculations under non-limiting water condition and medium drought promoted shoot and root growth of wheat plants.