Azotobacter chroococcum inoculation can improve plant growth and resistance of maize to armyworm, Mythimna separata even under reduced nitrogen fertilizer application

Growth and yield of maize in response to reduced fertilizer application and its impacts on population dynamics and community biodiversity of insects and soil microbes

In the North China Plain, farmers are using excessive amounts of fertilizer for the production of high-yield crop yield, which indirectly causes pollution in agricultural production. To investigate an optimal rate of fertilizer application for summer maize, the fertilizer reduction experiments with 600 kg/ha NPK (N: P2O5: K2O = 28: 8: 10) as normal fertilizer application (NFA), (i.e., 100F), were conducted successively during 2020 and 2021 to study the effects of reduced fertilizer rates, including 90% (540 kg/ha; i.e., 90F), 80% (480 kg/ha; i.e., 80F), 62.5% (375 kg/ha; i.e., 62.5F) and 50% (300 kg/ha; i.e., 50F) of NFA, on the plant growth of maize, the dynamics of key population abundances and community diversity of insects, and the composition and diversity of microbial community and finally to find out the N-metabolic enzymes' activity in soil. Our findings revealed that the fertilizer reduction rates by 10% - 20% compared to the current 100% NFA, and it has not significantly affected the plant growth of maize, not only plant growth indexes but also foliar contents of nutrients, secondary metabolites, and N-metabolic enzymes' activity. Further, there was no significant alteration of the key population dynamics of the Asian corn borer (Ostrinia furnacalis) and the community diversity of insects on maize plants. It is interesting to note that the level of N-metabolic enzymes' activity and microbial community diversity in soil were also not affected. While the fertilizer reduction rate by 50% unequivocally reduced field corn yield compared to 100% NFA, significantly decreased the yield by 17.10%. The optimal fertilizer application was calculated as 547 kg/ha (i.e., 91.17% NFA) based on the simulation analysis of maize yields among the five fertilizer application treatments, and the fertilizer application reduced down to 486 kg/ha (i.e., 81.00% NFA) with a significant reduction of maize yield. These results indicated that reduced the fertilizer application by 8.83% - 19.00% is safe and feasible to mitigate pollution and promote sustainable production of maize crops in the region.

Reconstruction of the genome-scale metabolic network model of Sinorhizobium fredii CCBAU45436 for free-living and symbiotic states

Sinorhizobium fredii CCBAU45436 is an excellent rhizobium that plays an important role in agricultural production. However, there still needs more comprehensive understanding of the metabolic system of S. fredii CCBAU45436, which hinders its application in agriculture. Therefore, based on the first-generation metabolic model iCC541 we developed a new genome-scale metabolic model iAQY970, which contains 970 genes, 1,052 reactions, 942 metabolites and is scored 89% in the MEMOTE test. Cell growth phenotype predicted by iAQY970 is 81.7% consistent with the experimental data. The results of mapping the proteome data under free-living and symbiosis conditions to the model showed that the biomass production rate in the logarithmic phase was faster than that in the stable phase, and the nitrogen fixation efficiency of rhizobia parasitized in cultivated soybean was higher than that in wild-type soybean, which was consistent with the actual situation. In the symbiotic condition, there are 184 genes that would affect growth, of which 94 are essential; In the free-living condition, there are 143 genes that influence growth, of which 78 are essential. Among them, 86 of the 94 essential genes in the symbiotic condition were consistent with the prediction of iCC541, and 44 essential genes were confirmed by literature information; meanwhile, 30 genes were identified by DEG and 33 genes were identified by Geptop. In addition, we extracted four key nitrogen fixation modules from the model and predicted that sulfite reductase (EC 1.8.7.1) and nitrogenase (EC 1.18.6.1) as the target enzymes to enhance nitrogen fixation by MOMA, which provided a potential focus for strain optimization. Through the comprehensive metabolic model, we can better understand the metabolic capabilities of S. fredii CCBAU45436 and make full use of it in the future.

Endophytic fungi improved wheat resistance to Rhopalosiphum padi by decreasing its feeding efficiency and population fitness

The improvement of crop resistance to insect using endophytic fungi is an environmentally friendly and sustainable strategy for agricultural pest control. Clarifying the efficacy and mechanism of endophytic fungi in improving crop resistance to pest offers the opportunity for biological control. In this study, changes in the transcriptome and defense compounds of wheat inoculated with endophytic fungal strains (i.e., YC and BB) were evaluated, and the efficacy of endophytic fungi in improving wheat resistance to Rhopalosiphum padi was studied. The results showed that the numbers of upregulated differentially-expressed genes (DEGs) in wheat plants inoculated with endophytic fungal strains YC and BB were higher than those of the downregulated DEGs, irrespective of R. padi infestation. Defense-related metabolic pathways, such as plant hormone signal transduction and secondary metabolite biosynthesis pathways were significantly enriched. Endophytic fungal strains YC and BB significantly increased jasmonic acid, DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), total flavone, and tannin contents in wheat plants (P < 0.05) but decreased salicylic acid content. Variations in the contents of defense compounds were significantly correlated with decreased feeding, development, and reproduction of R. padi fed on wheat plants inoculated with strains YC and BB (|r| = 0.68-0.91, P < 0.05). The results suggested that endophytic fungi significantly decreased the feeding efficiency and population fitness [YC: (-11.13%) - (-22.07%); BB: (-10.98%) - (-22.20%)] of R. padi by altering the phytohormone pathway and secondary metabolite biosynthesis in wheat plants. This study helps in understanding of the efficacy of endophytic fungi in improving wheat resistance to insect and will be conducive to integrated pest management.

The efficacy of Azotobacter chroococcum in altering maize plant-defense responses to armyworm at elevated CO2 concentration

Elevated atmospheric carbon dioxide (eCO₂) concentrations can alter the carbon:nitrogen ratio and palatability of host plants for herbivorous insects, but rhizobacteria likely mitigate the alteration and influence physiological adaptation of insects. In this study, we conducted transcriptomic analysis of maize (Zea mays) response to Azotobacter chroococcum (AC) inoculation under eCO₂ conditions in contrast to ambient CO₂ (aCO₂), and studied the effects of plant-defense change of maize under eCO₂ on the oriental armyworm, Mythimna separata. Results showed that there were 16, 14, 16 and 135 differentially expressed genes that were associated with plant-defense response in maize leaves between aCO₂-CK and aCO₂-AC, eCO₂-CK and eCO₂-AC, aCO₂-CK and eCO₂-CK, aCO₂-AC and eCO₂-AC, respectively. Moreover, A. chroococcum inoculation and eCO₂ influenced plant hormone signal transduction of maize. Interestingly, A. chroococcum inoculation significantly decreased the contents of JA (jasmonic acid) and JA-Ile (isoleucine conjugate of JA) in leaves, but eCO₂ markedly increased contents of JA-Ile, JA and SA (salicylic acid). Compared to aCO₂, eCO₂ significantly decreased activity of protective enzyme (catalase), and increased activities of digestive (lipase and protease), protective (peroxidase) and detoxifying enzymes (carboxylesterase, Mixed-functional oxidase and glutathione s-transferase), prolonged developmental time, and decreased survival rate and body weight of larvae (P < 0.05). A. chroococcum inoculation significantly increased the activity of protective enzyme (catalase), and decreased the activities of detoxifying enzymes (carboxylesterase, glutathione s-transferase and mixed-functional oxidase), thus increased the growth rate and body weight of larvae in comparison with no-inoculation of A. chroococcum (P < 0.05). The indices of M. separata were significantly correlated with the foliar contents of JA, JA-Ile and SA (|r| = 0.44-0.85, P < 0.05), indicating that A. chroococcum inoculation altered the physiological adaptation of M. separata under eCO₂ by disturbing defense substances in maize. Our results in understanding effects of A. chroococcum inoculation on maize resistance to herbivorous insects will be valuable for agricultural pest control in the future at eCO₂ conditions.

Impacts of Ecological Shading by Roadside Trees on Tea Foliar Nutritional and Bioactive Components, Community Diversity of Insects and Soil Microbes in Tea Plantation

Roadside trees not only add aesthetic appeal to tea plantations, but also serve important ecological purposes for the shaded tea plants. In this study, we selected tea orchards with two access roads, from east to west (EW-road) and from south to north (SN-road), and the roadside trees formed three types of ecological shading of the adjoining tea plants; i.e., south shading (SS) by the roadside trees on the EW-road, and east shading and west shading (ES and WS) by the roadside trees on the SN-road. We studied the impacts of ecological shading by roadside trees on the tea plants, insects, and soil microbes in the tea plantation, by measuring the contents of soluble nutrients, bioactive compounds in the tea, and tea quality indices; and by investigating the population occurrence of key species of insects and calculating insect community indexes, while simultaneously assaying the soil microbiome. The results vividly demonstrated that the shading formed by roadside tree lines on the surrounding tea plantation (SS, ES, and WS) had adverse effects on the concentration of tea soluble sugars but enhanced the foliar contents of bioactive components and improved the overall tea quality, in contrast to the no-shading control tea plants. In addition, the roadside tree lines seemed to be beneficial for the tea plantation, as they reduced pest occurrence, and ES shading enhanced the microbial soil diversity in the rhizosphere of the tea plants.

Evaluation of metabolites in Iranian Licorice accessions under salinity stress and Azotobacter sp. inoculation

Licorice (Glycyrrhiza glabra L.) is an industrial medicinal plant that is potentially threatened by extinction. In this study, the effects of salinity (0 and 200 mM sodium chloride (NaCl)) and Azotobacter inoculation were evaluated on 16 licorice accessions. The results showed that salinity significantly reduced the fresh and dry biomass (FW and DW, respectively) of roots, compared to plants of the control group (a decrease of 15.92% and 17.26%, respectively). As a result of bacterial inoculation, the total sugar content of roots increased by 21.56% when salinity was applied, but increased by 14.01% without salinity. Salinity stress increased the content of glycyrrhizic acid (GA), phenols, and flavonoids in licorice roots by 104.6%, 117.2%, and 56.3%, respectively. Integrated bacterial inoculation and salt stress significantly increased the GA content in the accessions. Bajgah and Sepidan accessions had the highest GA contents (96.26 and 83.17 mg/g DW, respectively), while Eghlid accession had the lowest (41.98 mg/g DW). With the bacterial application, the maximum amounts of glabridin were obtained in Kashmar and Kermanshah accessions (2.04 and 1.98 mg/g DW, respectively). Bajgah and Kashmar accessions had higher amounts of rutin in their aerial parts (6.11 and 9.48 mg/g DW, respectively) when their roots were uninoculated. In conclusion, these results can assist in selecting promising licorice accessions for cultivation in harsh environments.

Impacts of Intercropped Maize Ecological Shading on Tea Foliar and Functional Components, Insect Pest Diversity and Soil Microbes

Ecological shading fueled by maize intercropping in tea plantations can improve tea quality and flavor, and efficiently control the population occurrence of main insect pests. In this study, tea plants were intercropped with maize in two planting directions from east to west (i.e., south shading (SS)) and from north to south (i.e., east shading (ES) and west shading (WS)) to form ecological shading, and the effects on tea quality, and the population occurrence and community diversity of insect pests and soil microbes were studied. When compared with the non-shading control, the tea foliar nutrition contents of free fatty acids have been significantly affected by the ecological shading. SS, ES, and WS all significantly increased the foliar content of theanine and caffeine and the catechin quality index in the leaves of tea plants, simultaneously significantly reducing the foliar content of total polyphenols and the phenol/ammonia ratio. Moreover, ES and WS both significantly reduced the population occurrences of Empoasca onukii and Trialeurodes vaporariorum. Ecological shading significantly affected the composition of soil microbial communities in tea plantations, in which WS significantly reduced the diversity of soil microorganisms.

Appraisal of MsepCSP14 for chemosensory functions in Mythimna separata

Chemosensory proteins (CSPs) have great contributions in performing diverse functions in insects. However, physiological appraisal of chemosensory protein genes still remains elusive in insects. We studied expression patterns and binding affinities of MsepCSP14, a chemosensory protein, in Mythimna separata. The distinct functions of MsepCSP14 were validated by employing different molecular techniques. The MsepCSP14 had high resemblance of sequence with chemosensory proteins of other insect family members. The MsepCSP14 expression was higher in antennal tissues of females than other tissues. Fluorescence binding assay validated that binding of nine out of 21 ligands to MsepCSP14 was higher at pH 7.4 than at pH 5.0. Three dimensional modeling (3D) and docking analysis predicted that amino acid residues of MsepCSP14 were involved in binding of compounds, and behavior assay displayed that adults of M. separata considerably responded to four volatiles from compounds demonstrating strong binding ability to MsepCSP14. Results of the present study suggest that MsepCSP14 is likely to mediate chemosensory functions in M. separata.

Decoyinine Induced Resistance in Rice against Small Brown Planthopper Laodelphax striatellus

Induced resistance against SBPH via microbial pesticides is considered as an eco-friendly and promising management approach. In this study, the induced resistance against SBPH in rice seedling by a new potential microbial pesticide, decoyinine (DCY), a secondary metabolite produced by Streptomyces hygroscopicus, was evaluated to investigate the effects of DCY on SBPH's biological and population parameters along with defense-related physiological and biochemical indices in rice against SBPH feeding. We found that DCY has potential to improve rice resistance and significantly reduced the fecundity of SBPH. Laboratory results revealed that DCY treated rice significantly changed SBPH's fecundity and population life table parameters. The concentrations of hydrogen peroxide (H2O2), soluble sugars and malondialdehyde (MDA) were significantly lower in DCY treated rice plants against SBPH infestation at 24, 48 and 96 hours post infestation (hpi), respectively. The concentrations of antioxidant enzymes, catalase (CAT) was significantly higher at 72 hpi, while super oxidase dismutase (SOD) and peroxidase (POD) concentrations were recorded higher at 96 hpi. The concentrations of synthases enzymes, phenyl alanine ammonia-lyase (PAL) was higher at 48 hpi, whereas polyphenol oxidase (PPO) concentration was maximum at 72 hpi against SBPH infestation. The results imply that DCY has unique properties to enhance rice resistance against SBPH by stimulating plant defensive responses. Microbial pesticides may be developed as an alternative to chemical pest control.

Plant Growth-Promoting Rhizobacteria Enhance Defense of Strawberry Plants Against Spider Mites

Plants mediate interactions between below- and above-ground microbial and animal communities. Microbial communities of the rhizosphere commonly include mutualistic symbionts such as mycorrhizal fungi, rhizobia and free-living plant growth-promoting rhizobacteria (PGPR) that may influence plant growth and/or its defense system against aboveground pathogens and herbivores. Here, we scrutinized the effects of three PGPR, Azotobacter chroococcum, Azospirillum brasilense, and Pseudomonas brassicacearum, on life history and population dynamics of two-spotted spider mites, Tetranychus urticae, feeding on aboveground tissue of strawberry plants, and examined associated plant growth and physiology parameters. Our experiments suggest that these three species of free-living rhizobacteria strengthen the constitutive, and/or induce the direct, anti-herbivore defense system of strawberry plants. All three bacterial species exerted adverse effects on life history and population dynamics of T. urticae and positive effects on flowering and physiology of whole strawberry plants. Spider mites, in each life stage and in total, needed longer time to develop on PGPR-treated plants and had lower immature survival rates than those fed on chemically fertilized and untreated plants. Reduced age-specific fecundity, longer developmental time and lower age-specific survival rates of mites feeding on rhizobacteria treated plants reduced their intrinsic rate of increase as compared to mites feeding on chemically fertilized and control plants. The mean abundance was lower in spider mite populations feeding on PGPR-treated strawberries than in those feeding on chemically fertilized and untreated plants. We argue that the three studied PGPR systemically strengthened and/or induced resistance in above-ground plant parts and enhanced the level of biochemical anti-herbivore defense. This was probably achieved by inducing or upregulating the production of secondary plant metabolites, such as phenolics, flavonoids and anthocyanins, which were previously shown to be involved in induced systemic resistance of strawberry plants. Overall, our study emphasizes that PGPR treatment can be a favorable strawberry plant cultivation measure because providing essential nutrients needed for proper plant growth and at the same time decreasing the life history performance and population growth of the notorious herbivorous pest T. urticae.

Rhamnolipid Enhances the Nitrogen Fixation Activity of Azotobacter chroococcum by Influencing Lysine Succinylation

The enhancement of nitrogen fixation activity of diazotrophs is essential for safe crop production. Lysine succinylation (KSuc) is widely present in eukaryotes and prokaryotes and regulates various biological process. However, knowledge of the extent of KSuc in nitrogen fixation of Azotobacter chroococcum is scarce. In this study, we found that 250 mg/l of rhamnolipid (RL) significantly increased the nitrogen fixation activity of A. chroococcum by 39%, as compared with the control. Real-time quantitative reverse transcription PCR (qRT-PCR) confirmed that RL could remarkably increase the transcript levels of nifA and nifHDK genes. In addition, a global KSuc of A. chroococcum was profiled using a 4D label-free quantitative proteomic approach. In total, 5,008 KSuc sites were identified on 1,376 succinylated proteins. Bioinformatics analysis showed that the addition of RL influence on the KSuc level, and the succinylated proteins were involved in various metabolic processes, particularly enriched in oxidative phosphorylation, tricarboxylic acid cycle (TCA) cycle, and nitrogen metabolism. Meanwhile, multiple succinylation sites on MoFe protein (NifDK) may influence nitrogenase activity. These results would provide an experimental basis for the regulation of biological nitrogen fixation with KSuc and shed new light on the mechanistic study of nitrogen fixation.

The combined use of arbuscular mycorrhizal fungi, biochar and nitrogen fertilizer is most beneficial to cultivate Cichorium intybus L. in Cd-contaminated soil

We assessed Cichorium intybus L. (chicory) growth, nutrients and Cd uptake in the presence of arbuscular mycorrhizal fungi (AMF), biochar (BC) and nitrogen (N) fertilizer applications in experimental pot studies. Mycorrhizal colonization was improved by BC addition but not N addition. However, in the presence of AMF, BC and N had positive effects on shoot biomass and plants grown in the presence of all 3 components yielded the highest levels of shoot biomass compared to pair-wise combinations or individual components singly added. AMF inoculation also increased root biomass and this effect was apparent in the combinations that included AMF without BC. Fungal inoculation also led to increased contents of phosphorus (P), magnesium (Mg) and copper (Cu) in both chicory shoots and roots and potassium (K) and manganese (Mn) in roots. Moreover, in the presence of BC, AMF inoculation evidently enhanced the shoot Ca content. In contrast, the presence of AMF inhibited plant Cd absorption and BC addition further inhibited root Cd concentration following the AMF inoculation. Applying N fertilizer alone increased Cd transfering from soil to plants. However, when combined with use of AMF and BC, Cd toxicity to plants was reduced. This study demonstrated that AMF inoculation combined with BC and N fertilizer could improve chicory growth, nutrient absorption and reduction of Cd uptake in Cd-contaminated soil.

AMF Inoculation Can Enhance Yield of Transgenic Bt Maize and Its Control Efficiency Against Mythimna separata Especially Under Elevated CO2

The promotion and application of transgenic Bt crops provides an approach for the prevention and control of target lepidopteran pests and effectively relieves the environmental pressure caused by the massive usage of chemical pesticides in fields. However, studies have shown that Bt crops will face a new risk due to a decrease in exogenous toxin content under elevated carbon dioxide (CO2) concentration, thus negatively affecting the ecological sustainability of Bt crops. Arbuscular mycorrhizal fungi (AMF) are important beneficial microorganisms that can effectively improve the nutrient status of host plants and are expected to relieve the ecological risk of Bt crops under increasing CO2 due to global climate change. In this study, the Bt maize and its parental line of non-transgenic Bt maize were selected and inoculated with a species of AMF (Funneliformis caledonium, synonyms: Glomus caledonium), in order to study the secondary defensive chemicals and yield of maize, and to explore the effects of F. caledonium inoculation on the growth, development, and reproduction of the pest Mythimna separata fed on Bt maize and non-Bt maize under ambient carbon dioxide concentration (aCO2) and elevated carbon dioxide concentration (eCO2). The results showed that eCO2 increased the AM fungal colonization, maize yield, and foliar contents of jasmonic acid (JA) and salicylic acid (SA), but decreased foliar Bt toxin content and Bt gene expression in Bt maize leaves. F. caledonium inoculation increased maize yield, foliar JA, SA contents, Bt toxin contents, and Bt gene expression in Bt maize leaves, and positively improved the growth, development, reproduction, and food utilization of the M. separata fed on non-Bt maize. However, F. caledonium inoculation was unfavorable for the fitness of M. separata fed on Bt maize, and the effect was intensified when combined with eCO2. It is indicated that F. caledonium inoculation had adverse effects on the production of non-Bt maize due to the high potential risk of population occurrence of M. separata, while it was just the opposite for Bt maize. Therefore, this study confirms that the AMF can increase the yield and promote the expression levels of its endogenous (JA, SA) and exogenous (Bt toxin) secondary defense substances of Bt maize under eCO2, and finally can enhance the insect resistance capacity of Bt crops, which will help ensure the sustainable utilization and safety of Bt crops under climate change.

Potential Use of Beneficial Microorganisms for Soil Amelioration, Phytopathogen Biocontrol, and Sustainable Crop Production in Smallholder Agroecosystems

Smallholder agroecosystems play a key role in the world's food security providing more than 50% of the food produced globally. These unique agroecosystems face a myriad of challenges and remain largely unsupported, yet they are thought to be a critical resource for feeding the projected increasing human population in the coming years. The new challenge to increase food production through agricultural intensification in shrinking per capita arable lands, dwindling world economies, and unpredictable climate change, has led to over-dependence on agrochemical inputs that are often costly and hazardous to both human and animal health and the environment. To ensure healthy crop production approaches, the search for alternative ecofriendly strategies that best fit to the smallholder systems have been proposed. The most common and widely accepted solution that has gained a lot of interest among researchers and smallholder farmers is the use of biological agents; mainly plant growth promoting microorganisms (PGPMs) that provide essential agroecosystem services within a holistic vision of enhancing farm productivity and environmental protection. PGPMs play critical roles in agroecological cycles fundamental for soil nutrient amelioration, crop nutrient improvement, plant tolerance to biotic and abiotic stresses, biocontrol of pests and diseases, and water uptake. This review explores different research strategies involving the use of beneficial microorganisms, within the unique context of smallholder agroecosystems, to promote sustainable maintenance of plant and soil health and enhance agroecosystem resilience against unpredictable climatic perturbations.