A Bioactive Fraction from Streptomyces sp. Enhances Maize Tolerance against Drought Stress

Sodium nitroprusside modulates oxidative and nitrosative processes in Lycopersicum esculentum L. under drought stress

KEY MESSAGE

Sodium nitroprusside mediates drought stress responses in tomatoes by modulating nitrosative and oxidative pathways, highlighting the interplay between nitric oxide, hydrogen sulfide, and antioxidant systems for enhanced drought tolerance. While nitric oxide (NO), a signalling molecule, enhances plant tolerance to abiotic stresses, its precise contribution to improving tomato tolerance to drought stress (DS) through modulating oxide-nitrosative processes is not yet fully understood. We aimed to examine the interaction of NO and nitrosative signaling, revealing how sodium nitroprusside (SNP) could mitigate the effects of DS on tomatoes. DS-seedlings endured 12% polyethylene glycol (PEG) in a 10% nutrient solution (NS) for 2 days, then transitioned to half-strength NS for 10 days alongside control plants. DS reduced total plant dry weight, chlorophyll a and b, Fv/Fm, leaf water potential (ΨI), and relative water content, but improved hydrogen peroxide (H2O2), proline, and NO content. The SNP reduced the DS-induced H2O2 generation by reducing thiol (-SH) and the carbonyl (-CO) groups. SNP increased not only NO but also the activity of L-cysteine desulfhydrase (L-DES), leading to the generation of H2S. Decreases in S-nitrosoglutathione reductase (GSNOR) and NADPH oxidase (NOX) suggest a potential regulatory mechanism in which S-nitrosylation [formation of S-nitrosothiol (SNO)] may influence protein function and signaling pathways during DS. Moreover, SNP improved ascorbate (AsA) and glutathione (GSH) and reduced oxidized glutathione (GSSG) levels in tomato plants under drought. Furthermore, the interaction of NO and H2S, mediated by L-DES activity, may serve as a vital cross-talk mechanism impacting plant responses to DS. Understanding these signaling interactions is crucial for developing innovative drought-tolerance strategies in crops.

Prospecting the Potential of Plant Growth-Promoting Microorganisms for Mitigating Drought Stress in Crop Plants

Drought is a global phenomenon affecting plant growth and productivity, the severity of which has impacts around the whole world. A number of approaches, such as agronomic, conventional breeding, and genetic engineering, are followed to increase drought resilience; however, they are often time consuming and non-sustainable. Plant growth-promoting microorganisms are used worldwide to mitigate drought stress in crop plants. These microorganisms exhibit multifarious traits, which not only help in improving plant and soil health, but also demonstrate capabilities in ameliorating drought stress. The present review highlights various adaptive strategies shown by these microbes in improving drought resilience, such as modulation of various growth hormones and osmoprotectant levels, modification of root morphology, exopolysaccharide production, and prevention of oxidative damage. Gene expression patterns providing an adaptive edge for further amelioration of drought stress have also been studied in detail. Furthermore, the practical applications of these microorganisms in soil are highlighted, emphasizing their potential to increase crop productivity without compromising long-term soil health. This review provides a comprehensive coverage of plant growth-promoting microorganisms-mediated drought mitigation strategies, insights into gene expression patterns, and practical applications, while also guiding future research directions.

Root chemistry and microbe interactions contribute to metal(loid) tolerance of an aromatic plant - Vetiver grass

Aromatic plants, such as vetiver grass (Chrysopogon zizanioides), possess strong abilities to resist environmental stresses. However, whether such abilities stem from the interaction between specific chemical characteristics and the associated microbes in roots and rhizosphere remains unclear. We conducted pot experiments to analyze stress-tolerant parameters, organic compounds, and bacterial communities in roots and rhizosphere of vetiver under typical metal(loid) stress [cadmium (Cd), arsenic (As), or Cd + As] over time. The results showed that the vetiver displayed limited toxic symptoms in terms of oxidative stress-antioxidant balance and chlorophyll content. The root low-molecular-weight organic acids (LMWOAs), fatty acids, and sterols were highly sensitive to growth stage (increased from the 4-month to the 8-month stage), and less sensitive to metal(loid) stress. The sugar contents in the rhizosphere soils also notably increased over time. Such endo and rhizosphere chemical changes strongly correlated with and enriched the functional bacteria including Streptomyces, which can resist stress and promote plant growth. The compound-bacteria interaction highly depended on growth stage. Vetiver demonstrated a progressive adaptation to stresses through metabolite modulation and cellular defense reinforcement. Our study evidenced that vetiver shapes the interaction between organic compounds and bacterial community in the root-soil interface and provides notable stress-resistant functions.

Streptomyces can be an excellent plant growth manager

Streptomyces, the most abundant and arguably the most important genus of actinomycetes, is an important source of biologically active compounds such as antibiotics, and extracellular hydrolytic enzymes. Since Streptomyces can have a beneficial symbiotic relationship with plants they can contribute to nutrition, health and fitness of the latter. This review article summarizes recent research contributions on the ability of Streptomyces to promote plant growth and improve plant tolerance to biotic and abiotic stress responses, as well as on the consequences, on plant health, of the enrichment of rhizospheric soils in Streptomyces species. This review summarizes the most recent reports of the contribution of Streptomyces to plant growth, health and fitness and suggests future research directions to promote the use of these bacteria for the development of a cleaner agriculture.

Simultaneous biodegradation of λ-cyhalothrin pesticide and Vicia faba growth promotion under greenhouse conditions

λ-cyhalothrin is a widely used synthetic pyrethroid insecticide and its persistence in plant, soil and water exerts a detrimental effect on humans as well as the environment. There are many studies regarding isolated bacteria capable of degrading λ-cyhalothrin in vitro. However, limited work has been done examining the microbial degradation of λ-cyhalothrin together with plant growth promotion under greenhouse conditions. In this study, 43 bacterial strains were isolated from heavily polluted soil with λ-cyhalothrin by the enrichment technique. The plant growth promotion characteristics of all isolates were evaluated. The results revealed that five isolates were potential in λ-cyhalothrin biodegradation at high concentration (1200 mg/L) within only 24 h together with their high plant growth promotion abilities. The morphological, biochemical and 16S rDNA sequence analyses identified the isolates as Bacillus subtilis strains. The GC/MS analysis revealed that the selected isolates reached high levels of degradation after only two days, the degradation percentage ranged from 95.72 to 99.52% after 48 h of incubation. Furthermore, the degradation pathway for complete detoxification and metabolism of λ-cyhalothrin was established. Moreover, greenhouse experiment was conducted, the results indicate that the application of seed coat significantly enhanced Vicia faba seedling growth and caused an increase from 38.4 to 40.2% percentage of fresh and dry weight, respectively compared to untreated control. All isolates were effective to remove the pesticide residues in Vicia faba seedlings and recorded the highest degradation percentage of 83.79 under greenhouse conditions. Therefore, it can be concluded that the Bacillus subtilis strains isolated in this study have a dual potential role in complete mineralization of λ-cyhalothrin residues in vivo as well as effective biofertilization for future use in sustainable agriculture.

Cold Acclimation in Brachypodium Is Accompanied by Changes in Above-Ground Bacterial and Fungal Communities

Shifts in microbiota undoubtedly support host plants faced with abiotic stress, including low temperatures. Cold-resistant perennials prepare for freeze stress during a period of cold acclimation that can be mimicked by transfer from growing conditions to a reduced photoperiod and a temperature of 4 °C for 2-6 days. After cold acclimation, the model cereal, Brachypodium distachyon, was characterized using metagenomics supplemented with amplicon sequencing (16S ribosomal RNA gene fragments and an internal transcribed spacer region). The bacterial and fungal rhizosphere remained largely unchanged from that of non-acclimated plants. However, leaf samples representing bacterial and fungal communities of the endo- and phyllospheres significantly changed. For example, a plant-beneficial bacterium, Streptomyces sp. M2, increased more than 200-fold in relative abundance in cold-acclimated leaves, and this increase correlated with a striking decrease in the abundance of Pseudomonas syringae (from 8% to zero). This change is of consequence to the host, since P. syringae is a ubiquitous ice-nucleating phytopathogen responsible for devastating frost events in crops. We posit that a responsive above-ground bacterial and fungal community interacts with Brachypodium's low temperature and anti-pathogen signalling networks to help ensure survival in subsequent freeze events, underscoring the importance of inter-kingdom partnerships in the response to cold stress.

Establishment of actinobacteria-Satureja hortensis interactions under future climate CO2-enhanced crop productivity in drought environments of Saudi Arabia

Drought is a significant global constraint on agricultural production and food security. As a promising approach to improve plant growth and yield under challenging conditions, plant growth-promoting actinobacteria has attracted much interest. Further, elevated levels of atmospheric CO2 (eCO2) may promote the plant-actinobacteria interactions which could be effective to improve the plant growth for food production. Herein, we have investigated the impact of actinobacteria and/or CO2 on biomass production, photosynthesis, macronutrients, levels of organic acids, amino acids, and essential oils as well as antioxidant activities of Satureja hortensis under water-deficit conditions. Among different actinobacterial isolates evaluated for development of secondary metabolites and biological activities, Ac9 was highly capable of producing flavonoids, and it also showed high antioxidant and microbial activities. It markedly induced the plant growth, photosynthesis, and global metabolic improvement, under water-deficit conditions. Interestingly, treatment with Ac9 in combination with eCO2 substantially minimized drought stress-induced biomass and photosynthesis reductions in Satureja hortensis. Improved photosynthesis by Ac9 and/or eCO2 induced the primary and secondary metabolisms in drought-stressed plants. The levels of the majority of the detected organic acids, essential oil, and amino acids were further improved as a result of the synergistic action of Ac9 and eCO2, as compared to the individual treatments. Furthermore, Ac9 or eCO2 significantly improved the antioxidant activities in stressed plants; however, much more positive impact was obtained by their synchronous application. Thus, the current study suggests that actinobacterial treatment induces global metabolic changes in water-stressed Satureja hortensis, the effects that have been much more strengthened under eCO2.

Soil enrichment with actinomycete mitigates the toxicity of arsenic oxide nanoparticles on wheat and maize growth and metabolism

The use of plant growth-promoting bacteria (PGPB) to enhance plant growth and protection against heavy metal toxicity has been extensively studied. However, its potentiality to reduce arsenate toxicity, a threat to plant growth and metabolism, has been hardly investigated. Moreover, the toxic effect of arsenic oxide nanoparticles (As-NPs) on plants and possible mechanisms for its alleviation has not yet been explored. In this study, the impact of the bioactive actinomycete Streptomyces spp. on the growth, physiology and stress-related metabolites, such as sugars and proline, on As-NPs-stressed wheat and maize plants was investigated. Soil amendment with arsenic oxide nanoparticles (As-NPs) induced the uptake and accumulation of As in the plants of both species, resulting in reduced growth and photosynthesis, but less marked in maize than in wheat plants. Under As-NPs-free conditions, Streptomyces spp. treatment markedly improved growth and photosynthesis in wheat only. The application of Streptomyces spp. reduced As accumulation, recovered the As-NPs-induced growth, photosynthesis inhibition, and oxidative damage in plants of both species. Wheat plants specifically accumulated soluble sugars, while both species accumulated proline. Under As-NPs stress, the ornithine pathway of proline biosynthesis was more important in maize than in wheat plants, while the glutamine pathway was dominant in wheat ones. The addition of Streptomyces spp. further induced the accumulation of proline and starch in both plant species. Overall, despite a different response to Streptomyces spp. under nontoxic conditions, the amendment of as-contaminated soil with Streptomyces spp. induced similar metabolic responses in the two tested species, which trigger stress recovery.

Diversity and function of culturable actinobacteria in the root-associated of Salvia miltiorrhiza Bunge

The root-associated actinobacteria play important roles in plant growth, nutrient use, and disease resistance due to their functional diversity. Salvia miltiorrhiza is a critical medicinal plant in China. The root actinobacterial community structure has been studied; however, the functions of root-associated actinobacteria of S. miltiorrhiza have not been elucidated. This study aimed to decipher the diversity and function of the culturable root-associated actinobacteria in plant growth using culture-dependent technology and culturable microbe metagenomes. We isolated 369 strains from the root-associated actinobacteria, belonging to four genera, among which Streptomyces was dominant. Besides, the functional prediction revealed some pathways related to plant growth, nitrogen and phosphorus metabolism, and antagonistic pathogens. We systematically described the diversity and functions of the culturable root-associated actinobacteria community. Our results demonstrated that the culturable root-associated actinobacteria of S. miltiorrhiza have rich functionalities, explaining the possible contribution of culturable root-associated actinobacteria to S. miltiorrhiza's growth and development. This study provides new insights into understanding the function of the culturable root-associated actinobacteria and can be used as a knowledge base for plant growth promoters and biological control agent development in agriculture.

Bioactive Potential of Several Actinobacteria Isolated from Microbiologically Barely Explored Desert Habitat, Saudi Arabia

Simple Summary

Bioactive natural products have been regarded as promising tools for treatment of various ailments. Among natural sources, actinomycetes have been widely explored for their potential bioactivity. In this regard, the present study has focused on the phytochemical content and biological activities of several actinobacteria isolates, which were investigated for their phenolic and flavonoid content, as well as their antioxidant, antibacterial and antiprotozoal activities. The most active isolates were further investigated for their antileukemic activity, where such isolates were shown to exert cytotoxic activity against the tested cell lines, following a mechanism that might be due to the ability of the active isolate extracts to reduce cyclooxygenase and lipoxygenase activities. Overall, isolation and characterization of the active molecule from the potential actinomycetes strains will pave the way for the development of drugs against human diseases such as blood cancer.

Abstract

Biomolecules from natural sources, including microbes, have been the basis of treatment of human diseases since the ancient times. Therefore, this study aimed to investigate the potential bioactivity of several actinobacteria isolates form Al-Jouf Desert, Saudi Arabia. Twenty-one actinobacterial isolates were tested for their antioxidant (flavonoids, phenolics, tocopherols and carotenoids) content, and biological activities, namely FRAP, DPPH, ABTS, SOS and XO inhibition, anti-hemolytic and anti-lipid peroxidation as well as their antibacterial and antiprotozoal activities. Accordingly, five isolates (i.e., Act 2, 12, 15, 19 and 21) were selected and their 90% ethanolic extracts were used. The phylogenetic analysis of the 16S rRNA sequences indicated that the most active isolates belong to genus Streptomyces. The genus Streptomyces has been documented as a prolific producer of biologically active secondary metabolites against different cancer types. Thus, the anti-blood cancer activity and the possible molecular mechanisms by which several Streptomyces species extracts inhibited the growth of different leukemia cells, i.e., HL-60, K562 and THP-1, were investigated. In general, the five active isolates showed cytotoxic activity against the tested cell lines in a dose dependent manner. Among the potent isolates, isolate Act 12 significantly decreased the cell viability and showed maximum cytotoxic activities against both HL-60 and K562 cells, while isolate Act 15 exhibited maximum cytotoxic activity against THP-1 cells. Moreover, Act 2 and Act 12 reduced cyclooxygenase (COX-2) and lipoxygenase (LOX) activity, which is involved in the proliferation and differentiation of cancer cells and may represent a possible molecular mechanism underlying leukemia growth inhibition. The bioactive antioxidant extracts of the selected Streptomyces species inhibited leukemia cell growth by reducing the COX-2 and LOX activity. Overall, our study not only introduced a promising natural alternative source for anticancer agents, but it also sheds light on the mechanism underlying the anticancer activity of isolated actinomycetes.

Actinomycetes Enrich Soil Rhizosphere and Improve Seed Quality as well as Productivity of Legumes by Boosting Nitrogen Availability and Metabolism

The use of actinomycetes for improving soil fertility and plant production is an attractive strategy for developing sustainable agricultural systems due to their effectiveness, eco-friendliness, and low production cost. Out of 17 species isolated from the soil rhizosphere of legume crops, 4 bioactive isolates were selected and their impact on 5 legumes: soybean, kidney bean, chickpea, lentil, and pea were evaluated. According to the morphological and molecular identification, these isolates belong to the genus Streptomyces. Here, we showed that these isolates increased soil nutrients and organic matter content and improved soil microbial populations. At the plant level, soil enrichment with actinomycetes increased photosynthetic reactions and eventually increased legume yield. Actinomycetes also increased nitrogen availability in soil and legume tissue and seeds, which induced the activity of key nitrogen metabolizing enzymes, e.g., glutamine synthetase, glutamate synthase, and nitrate reductase. In addition to increased nitrogen-containing amino acids levels, we also report high sugar, organic acids, and fatty acids as well as antioxidant phenolics, mineral, and vitamins levels in actinomycete treated legume seeds, which in turn improved their seed quality. Overall, this study shed the light on the impact of actinomycetes on enhancing the quality and productivity of legume crops by boosting the bioactive primary and secondary metabolites. Moreover, our findings emphasize the positive role of actinomycetes in improving the soil by enriching its microbial population. Therefore, our data reinforce the usage of actinomycetes as biofertilizers to provide sustainable food production and achieve biosafety.

The Prospective Beneficial Effects of Red Laser Exposure on Lactocaseibacillus casei Fermentation of Skim Milk

Probiotic lactic acid bacteria are crucial producers of fermented dairy products that are popular functional foods in many countries. The health benefits of probiotic bacteria are mainly attributed to their effective bioactive metabolites. The quality of fermented milk is mainly dependent on the bacterial strain used in the fermentation process. In this study, an innovative technique is used in order to enhance the activities of the probiotic bacteria, quality of fermented milk, and consequently the whole fermentation process. Red laser dosages, at the wavelength of 632.7 nm, were applied to the type strain Lacticaseibacillus casei NRRL-B-1922 before the fermentation of skim milk. The results revealed that the scavenging of 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) radical and total antioxidant capacity were significantly increased from 21% in untreated control to 56% after bacterial laser irradiation of 12 J/cm² dosage for 40 min. The antioxidant activity was found to be increased as the red laser dosage increased in a dose-response relationship. Additionally, the lactose fermentation in skim milk medium of 43.22 mg/mL initial concentration into organic acids was enhanced after L. casei irradiation and recorded 23.15 mg/mL compared to control group 28.35 mg/mL without bacterial pre-treatment. These results are correlated with increase of the β-Galactosidase activity, where the L. casei that has been exposed to 40 min of red laser exhibited the higher activity of a 0.37 unit/mL relative to the control 0.25 unit/mL. The assessment of this fermented milk after L. casei laser exposure for 10, 20, and 40 min indicates multiple biological effects, including assimilation of cholesterol as well as proteolytic and antibacterial activity. Our data on the exposure of L. casei to laser beam suggest promising application of red laser in the fermentation process of skim milk.