Azotobacter salinestris: A Novel Pesticide-Degrading and Prominent Biocontrol PGPR Bacteria

Unlocking the potential of ecofriendly guardians for biological control of plant diseases, crop protection and production in sustainable agriculture

Several beneficial microbial strains inhibit the growth of different phytopathogens and commercialized worldwide as biocontrol agents (BCAs) for plant disease management. These BCAs employ different strategies for growth inhibition of pathogens, which includes production of antibiotics, siderophores, lytic enzymes, bacteriocins, hydrogen cyanide, volatile organic compounds, biosurfactants and induction of systemic resistance. The efficacy of antagonistic strains could be further improved through genetic engineering for better disease suppression in sustainable farming practices. Some antagonistic microbial strains also possess plant-growth-promoting activities and their inoculation improved plant growth in addition to disease suppression. This review discusses the characterization of antagonistic microbes and their antimicrobial metabolites, and the application of these BCAs for disease control. The present review also provides a comprehensive summary of the genetic organization and regulation of the biosynthesis of different antimicrobial metabolites in antagonistic strains. Use of molecular engineering to improve production of metabolites in BCAs and their efficacy in disease control is also discussed. The application of these biopesticides will reduce use of conventional pesticides in disease control and help in achieving sustainable and eco-friendly agricultural systems.

The Influence of Plant Growth-Stimulating Bacteria on the Glutathione-S-Transferase Activity and the Toxic Effect of the Herbicide Metsulfuron-Methyl in Wheat and Canola Plants

The ability of some rhizosphere bacteria to mitigate herbicidal stress in cultivated plants may be useful in agriculture and bioremediation. There is poor understanding of how bacteria directly or through herbicide degradation affect the biochemical processes in plants exposed to sulfonylurea herbicides. In this study, treatment with a combination of herbicide metsulfuron-methyl (MSM) and bacteria (Pseudomonas protegens DA1.2 or P. chlororaphis 4CH) of wheat (Triticum aestivum L.) and canola (Brassica napus L.) plants was carried out. Activity of glutathione-S-transferase (GST), an important enzyme for the herbicide detoxification, and acetolactate synthase (ALS), a target for MSM in plants, was measured by spectrophotometric assays. MSM residues were analyzed using the HPLC-MS. Then, 24 h after bacterial treatment, GST activity increased by 75-91% in wheat and by 38-94% in canola. On the 30th day, a decrease in MSM in the soil associated with bacterial treatment was 54.6-79.7%. An increase in GST activity and acceleration of MSM degradation were accompanied by a decrease in inhibition of the ALS enzyme in plants, which indicated a mitigation of the toxic effect. The results obtained are evidence that rhizospheric bacteria can have beneficial effects on plants exposed to MSM due to the combination of abilities to directly affect detoxification enzymes in plants and degrade MSM in the soil.

Mitigation of the Negative Effect of Drought and Herbicide Treatment on Growth, Yield, and Stress Markers in Bread Wheat as a Result of the Use of the Plant Growth Regulator Azolen®

Most chemical pesticides, in addition to their main functions (protection against diseases, weeds, and pests), also have a noticeable inhibitory effect on target crops. In a laboratory experiment and two-year field experiments (Russia, Trans-Urals), a study was made of the effect of the biopreparation Azolen® (Azotobacter vinelandii IB-4) on plants of the Ekada 113 wheat variety under conditions of drought and stress caused by the exposure to the herbicide Chistalan (2.4-D and dicamba). The biopreparation and the herbicide were used separately and together on wheat during the tillering phase. Treatment with the biological preparation under stressful conditions had a significant effect on the hormonal balance of plants (a decrease in the amount of abscisic acid and a normalization of the balance of indolyl-3-acetic acid and cytokinins in shoots and roots of plants was noted), while the osmoprotective, antioxidant, and photosynthetic systems of plants were activated. In drought conditions, the treatment of plants with biological preparation prevented the inhibition of root growth caused by the use of the herbicide. This, in turn, improved the absorption of water by plants and ensured an increase in wheat yield (1.6 times). The results obtained give reason to believe that microbiological preparations can be used as antidotes that weaken the phytotoxic effect of herbicidal treatments, including in drought conditions.

Plant growth promoting rhizobacteria (PGPR) application for coping with salinity and drought: a bibliometric network multi-analysis

Rhizobacteria play a crucial role in plant growth and yield, stimulating primary production and improving stress resistance. Climate change has several consequences worldwide that affect arable land and agriculture. Studies on plant-soil-microorganism interactions to enhance plant productivity and/or resistance to abiotic stress may open new perspectives. This strategy aims to make agricultural-relevant plant species able to complete their biological cycle in extreme soils with the help of inoculated or primed plant growth-promoting rhizobacteria (PGPR). We provide an overview of the evolution of interest in PGPR research in the last 30 years through: (i) a quantitative search on the Scopus database; (ii) keyword frequencies and clustering analysis, and (iii) a keyword network and time-gradient analysis. The review of scientific literature on PGPR highlighted an increase in publications in the last 15 years, and a specific time gradient on subtopics, such as abiotic stresses. The rise in PGPR as a keyword co-occurring with salinity and drought stresses aligns with the growing number of papers from countries directly or partly affected by climate change. The study of PGPR, its features, and related applications will be a key challenge in the next decades, considering climate change effects on agriculture. The increased interest in PGPR leads to deeper knowledge focused specifically on researching agriculturally sustainable solutions for soils affected by salinity and drought.

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

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

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

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

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

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

Effect of Seed Priming with Endophytic Bacillus subtilis on Some Physio-Biochemical Parameters of Two Wheat Varieties Exposed to Drought after Selective Herbicide Application

Wheat plants are frequently exposed to combined herbicide and drought stress (HDS) which induces complex responses negatively, affects productivity, and is becoming more exacerbated with current climate change. In this work, we studied the influence of seed priming with endophytic bacteria Bacillus subtilis (strains 104 and 26D) on growth and tolerance of two wheat (Triticum aestivum L.) varieties (E70-drought tolerant; SY-drought susceptible) exposed to soil drought after application of selective herbicide Sekator® Turbo in pot experiments under controlled conditions; 17-day-old plants sprayed with herbicide and after 3 days were subjected to soil drought by stopping irrigating the plants for 7 days with subsequent resumption of normal irrigation (recovery). Additionally, the growth of tested strains (104, 26D) in the presence of different concentrations of herbicide Sekator® Turbo and drought (PEG-6000) were evaluated. It was established that both strains are herbicide and drought tolerant and capable to improve seed germination and early seedlings' growth under different herbicide and drought stress degrees. The results of pot experiments showed that HDS exposure declined growth (plant length, biomass), photosynthetic pigments (chlorophyll a and b), leaf area, and increased lipid peroxidation (LPO) and proline accumulation in plants, demonstrating higher damaging effects for SY variety. Strains 104 and 26D mitigated (in different levels) such negative impacts of HDS on growth of both varieties by increasing length of roots and shoots, biomass, photosynthetic pigments (chlorophyll a and b), and leaf area, reducing stress-caused LPO (i.e., malondialdehyde), and regulating proline biosynthesis, as well as contributing to a faster recovery of growth, photosynthetic pigments, and redox-status of plants in post-stress period in comparison with non-primed plants. These ultimately manifested in forming a better grain yield of both varieties primed with 104, 26D, and exposed to HDS. Thus, both strains 104 and 26D (which are herbicide and drought tolerant) may be used as seed priming agents to improve wheat HDS tolerance and grain yield; however, strain 104 more effectively protected plants of E70, while strain 26D-plants of SY. Further research should be focused on understanding the mechanisms that determine the strain and variety-specificity of endophytic symbiosis and the role of bacteria in the modulation of physiological states of primed plants under stress conditions, including HDS.

PGP-Bacterium Pseudomonas protegens Improves Bread Wheat Growth and Mitigates Herbicide and Drought Stress

The reaction of plants to simultaneous stress action and treatment with biological stimulants still remains poorly studied. Laboratory and field experiments have been conducted to study the growth and yield of bread wheat (Triticum aestivum L.) of the variety Ekada 113; stress markers and quantitative ratios of phytohormones in plants under insufficient soil moisture; the effects of spraying with herbicide containing 2,4-D and dicamba and growth-stimulating bacterium Pseudomonas protegens DA1.2; and combinations of these factors. Under water shortage conditions, spraying plants with Chistalan reduced their growth compared to non-sprayed plants, which was associated with inhibition of root growth and a decrease in the content of endogenous auxins in the plants. Under conditions of combined stress, the treatment of plants with the strain P. protegens DA1.2 increased the IAA/ABA ratio and prevented inhibition of root growth by auxin-like herbicide, ensuring water absorption by the roots as well as increased transpiration. As a result, the content of malondialdehyde oxidative stress marker was reduced. Bacterization improved the water balance of wheat plants under arid field conditions. The addition of bacterium P. protegens DA1.2 to the herbicide Chistalan increased relative water content in wheat leaves by 11% compared to plants treated with herbicide alone. Application of the bacterial strain P. protegens DA1.2 increased the amount of harvested grain from 2.0-2.2 t/ha to 3.2-3.6 t/ha. Thus, auxin-like herbicide Chistalan and auxin-producing bacterium P. protegens DA1.2 may affect the balance of phytohormones in different ways. This could be the potential reason for the improvement in wheat plants' growth during dry periods when the bacterium P. protegens DA1.2 is included in mixtures for weed control.

Unraveling the potential of pesticide-tolerant Pseudomonas sp. augmenting biological and physiological attributes of Vigna radiata (L.) under pesticide stress

In the agricultural domain, chemical pesticides are repetitively and harshly used to kill harmful pests, but they often pose a serious threat to microbial diversity, soil fertility and agricultural output. To deal with these problems, pesticide-tolerant plant growth promoting (PGP) rhizobacterial strains are often used to combat pesticidal toxicity. Here, Pseudomonas sp. PGR-11 (accession no. OM348534), recovered from a Vigna radiata (L.) rhizosphere, produced various growth regulating (GR) substances, including indole-3-acetic acid (IAA; 82.5 ± 9.2 μg mL^-1), enzyme 1-aminocyclopropane 1-carboxylate (ACC) deaminase (μM α-ketobutyrate mg^-1 protein h^-1), siderophores and ammonia. Strain PGR-11 grew well when cultured in growth medium with added metalaxyl (MTXL; 1200 μg mL^-1), carbendazim (CBZM; 800 μg mL^-1) and tebuconazole (TBZL; 1600 μg mL^-1). Pseudomonas sp. synthesized PGP substances even in the presence of increasing doses of pesticides. The phytotoxicity of the tested pesticides was assessed both in vitro and under pot-house conditions using a Vigna radiata (L.) crop. Increasing concentrations of chemical pesticides negatively impacted the growth, physiological and biochemical features. However, pesticide-tolerant Pseudomonas sp. relieved the toxicity and improved the biological attributes of the plant. Bio-inoculated plants showed significant enhancement in germination attributes, dry biomass, symbiotic features and yield features when compared to un-inoculated ones. Furthermore, with 100 μg metalaxyl kg^-1 soil, strain PGR-11 increased the chl-a, chl-b, total chlorophyll, carotenoids, SPAD index, photosystem efficiency (Fv/Fm), PSII quantum yield (FPSII), photochemical quenching (qP) and non-photochemical quenching (NpQ) content by 12, 19, 16, 27, 34, 41, 26, 29 and 33%, respectively, over un-inoculated but pesticide-treated plants. Additionally, inoculation of Pseudomonas sp. with 100 μg tebuconazole kg^-1 soil caused a significant (p ≤ 0.05) enhancement in transpiration rate (E), stomatal conductance (g _s), photosynthetic rate (P _N), vapor pressure deficit (kPa) and internal CO₂ concentration (C _i) of 19, 26, 23, 28 and 34%, respectively. Conclusively, the power to tolerate abnormally high pesticide concentration, the capacity to produce/secrete PGP substances even in a pesticide-stressed medium and the potential for improving/increasing the growth and physiology of plants by pesticide detoxification makes Pseudomonas sp. PGR-11 a fascinating choice for augmenting the productivity of V. radiata (L.) even in pesticide-stressed soils. The current findings will be helpful for exploring pesticide-tolerant ACC-deaminase-positive microbial strains as gifted entities for the environmental bioremediation of pesticides.

Characterization and diversity of native Azotobacter spp. isolated from semi-arid agroecosystems of Eastern Kenya

Declining food production in African agroecosystems is attributable to changes in weather patterns, soil infertility and limited farming inputs. The exploitation of plant growth-promoting soil microbes could remedy these problems. Such microbes include Azotobacter; free-living, nitrogen-fixing bacteria, which confer stress tolerance, avail phytohormones and aid in soil bioremediation. Here, we aimed to isolate, characterize and determine the biodiversity of native Azotobacter isolates from soils in semi-arid Eastern Kenya. Isolation was conducted on nitrogen-free Ashby's agar and the morphological, biochemical and molecular attributes evaluated. The isolates were sequenced using DNA amplicons of 27F and 1492R primers of the 16S rRNA gene loci. The Basic Local Alignment Search Tool (BLASTn) analysis of their sequences revealed the presence of three main Azotobacter species viz., Azotobacter vinelandii, Azotobacter salinestris and Azotobacter tropicalis. Kitui County recorded the highest number of recovered Azotobacter isolates (45.4%) and lowest diversity index (0.8761). Tharaka Nithi County showed the lowest occurrence (26.36%) with a diversity index of (1.057). The diversity was influenced by the soil pH, texture and total organic content. This study reports for the first time a wide diversity of Azotobacter species from a semi-arid agroecosystem in Kenya with potential for utilization as low-cost, free-living nitrogen-fixing bioinoculant.

Growth, respiratory activity and chlorpyrifos biodegradation in cultures of Azotobacter vinelandii ATCC 12837

This study aimed to evaluate the growth, respiratory activity, and biodegradation of chlorpyrifos in cultures of Azotobacter vinelandii ATCC 12837. A strategy based on the modification of culture media and aeration conditions was carried out to increase the cell concentration of A. vinelandii, in order to favor and determine its tolerance to chlorpyrifos and its degradation ability. The culture in shaken flasks, using sucrose as a carbon source, significantly improved the growth compared to media with mannitol. When the strain was cultivated under oxygen-limited (5.5, 11.25 mmol L⁻¹ h⁻¹) and no-oxygen-limited conditions (22 mmol L⁻¹ h⁻¹), the growth parameters were not affected. In cultures in a liquid medium with chlorpyrifos, the bacteria tolerated a high pesticide concentration (500 ppm) and the growth parameters were improved even under conditions with a reduced carbon source (sucrose 2 g L⁻¹). The strain degraded 99.6% of chlorpyrifos at 60 h of cultivation, in co-metabolism with sucrose; notably, A. vinelandii ATCC 12837 reduced by 50% the initial pesticide concentration in only 6 h (DT₅₀).

Isolation, identification and plant growth promotion ability of endophytic bacteria associated with lupine root nodule grown in Tunisian soil

The present study aims to characterize nodule endophytic bacteria of spontaneous lupine plants regarding their diversity and their plant growth promoting (PGP) traits. The potential of PGPR inoculation was investigated to improve white lupine growth across controlled, semi-natural and field conditions. Lupinus luteus and Lupinus angustifolius nodules were shown inhabited by a large diversity of endophytes. Several endophytes harbor numerous plant growth promotion traits such as phosphates solubilization, siderophores production and 1-aminocyclopropane-1-carboxylate deaminase activity. In vivo analysis confirmed the plant growth promotion ability of two strains (Paenibacillus glycanilyticus LJ121 and Pseudomonas brenneri LJ215) in both sterilized and semi-natural conditions. Under field conditions, the co-inoculation of lupine by these strains increased shoot N content and grain yield by 25% and 36%, respectively. These two strains Paenibacillus glycanilyticus LJ121 and Pseudomonas brenneri LJ215 are effective plant growth-promoting bacteria and they may be used to develop an eco-friendly biofertilizer to boost white lupine productivity.