Microbial Derived Compounds, a Step Toward Enhancing Microbial Inoculants Technology for Sustainable Agriculture

Cell-Free Supernatant (CFS) from Bacillus subtilis EB2004S and Lactobacillus helveticus EL2006H Cultured at a Range of pH Levels Modulates Potato Plant Growth under Greenhouse Conditions

Agriculture involving industrial fertilizers is another major human made contributing factor to soil pH variation after natural factors such as soil parent rock, weathering time span, climate, and vegetation. The current study assessed the potential effect of cell-free supernatant (CFS) obtained from Bacillus subtilis EB2004S and Lactobacillus helveticus EL2006H cultured at three pH levels (5, 7, and 8) on potato (var Goldrush) growth enhancement in a greenhouse pot experiment. The results showed that CFSs obtained from B. subtilis EB2004S and L. helveticus EL2006H cultured at pH 5 significantly improved photosynthetic rates, stomatal conductance, root fresh weight, and whole plant fresh weight. interactive effects of pot pH and that of CFSs obtained from pH 5 influenced chlorophyll, plant height, and shoot and whole plant fresh weight. Moreover, treatment 52EB2004S~0.4% initiated early tuberization for potato grown at pH 7 and 8. Potato grown at pH 5, which received a 72EB2004S~0.4% CFS treatment, had greater whole plant fresh and dry weight than that treated with L. helveticus EL2006H CFS and a positive control. Taken together, the findings of this study are unique in that it probed the effect of CFS produced under differing pH conditions which revealed a new possibility to mitigate stresses in plants.

Cell-free supernatant of Devosia sp. (strain SL43) mitigates the adverse effects of salt stress on soybean (Glycine max L.) seed vigor index

Soil salinity is a major constraint for soybean production worldwide, and the exploitation of plant growth-promoting bacteria (PGPB) and their bioactive metabolite(s) can improve plant salinity tolerance. With this objective, two experiments were performed, aiming to test 4 culture media (YEM(A), TYE(A), TS(A), and LB(A)) for growing a novel Devosia sp. (strain SL43), and then evaluating cell-free supernatants (CFS) from the Devosia sp. on germination of soybean (Glycine max L.) seeds under salinity stress. Soybean seeds were subjected to three salinity levels (0, 100, and 125 mM NaCl) and 6 levels of Devosia sp. CFS dilution (0, 1:1, 1:100, 1:250, 1:500, 1:1000). The results indicated that 125 mM NaCl concentration caused the greatest reduction in the total number of germinated seeds (15%), germination rate (43.6%), root length (55.2%), root weight (39.3%), and seed vigor (68%), and it also increased mean germination time by 71.9%. However, Devosia-CFS improved soybean germination, and the greatest effect was obtained at 1:1 dilution. Under the highest salinity level, application of CFS at 1:1 dilution increased final germination (17.6%), germination rate (18.6%), root length (162.2%), root weight (239.4%), seed vigor index (318.7%), and also shortening mean germination time by 19.2%. The results indicated that seed vigor index was positively correlated with other traits except for mean germination time. Our study suggested that the highest productivity of Devoisa sp. was obtained from the YEM medium. Results also suggested that CFS produced by the novel Devosia sp. (SL43 strain) can successfully alleviate salt stress effects on soybean seed germination and manipulating the chemical composition of the growth medium can influence the effectiveness of these bioactive metabolites.

Lactobacillus helveticus EL2006H cell-free supernatant enhances growth variables in Zea mays (maize), Glycine max L. Merill (soybean) and Solanum tuberosum (potato) exposed to NaCl stress

Plant growth promoting microorganisms and their derived compounds, such as cell-free supernatant (CFS), enhance plant growth under stressed and non stressed conditions. Such technology is sustainable and environmentally friendly, which is desirable amidst the climate change threat. The current study evaluated the effect of CFS obtained from Lactobacillus helveticus EL2006H on its ability to enhance mean percentage germination and mean radicle length of corn and soybean, as well as growth parameters of potato, using treatment formulations that consisted of 0.2 and 1.0% [v/v] L. helveticus EL2006H CFS concentrations and 100 mM NaCl and 150 mM NaCl levels. Results show that treatment with 100 mM NaCl lowered percentage germination of corn by 52.63%, at 72 h, and soybean by 50%, at 48 h. Treatment with 100 NaCl +0.2% EL2006H enhanced percentage germination of soybean by 44.37%, at 48 h, in comparison to that of the 100 mM NaCl control. One hundred mM NaCl lowered radicle length of corn and soybean by 38.58 and 36.43%, respectively. Treatment with 100 Mm NaCl +1.0% EL2006H significantly increased radicle length of corn by 23.04%. Treatment with 100 mM NaCl +0.2% EL2006H significantly increased photosynthetic rate, leaf greenness and fresh weight of potato. Increasing NaCl concentration to 150 NaCl lowered the effectiveness of the 0.2% EL2006H CFS on the same growth variables of potato. In general, the lower CFS concentration of 0.2% was more efficient at enhancing germination in soybean while the higher concentration of 1.0% was more efficient at enhancing radicle length of corn. There was an observed variation in the effectiveness of L. helveticus EL2006H CFS across the different CFS concentrations, NaCl levels and crop species studied. In conclusion, based on findings of this study, CFS obtained from L. helveticus can be used as a bio stimulant to enhance growth of corn, soybean and potato. However, further studies need to be conducted, for validation, especially under field conditions, for commercial application.

Cell-free microbial culture filtrates as candidate biostimulants to enhance plant growth and yield and activate soil- and plant-associated beneficial microbiota

In this work we compiled information on current and emerging microbial-based fertilization practices, especially the use of cell-free microbial culture filtrates (CFs), to promote plant growth, yield and stress tolerance, and their effects on plant-associated beneficial microbiota. In addition, we identified limitations to bring microbial CFs to the market as biostimulants. In nature, plants act as metaorganisms, hosting microorganisms that communicate with the plants by exchanging semiochemicals through the phytosphere. Such symbiotic interactions are of high importance not only for plant yield and quality, but also for functioning of the soil microbiota. One environmentally sustainable practice to increasing crop productivity and/or protecting plants from (a)biotic stresses while reducing the excessive and inappropriate application of agrochemicals is based on the use of inoculants of beneficial microorganisms. However, this technology has a number of limitations, including inconsistencies in the field, specific growth requirements and host compatibility. Beneficial microorganisms release diffusible substances that promote plant growth and enhance yield and stress tolerance. Recently, evidence has been provided that this capacity also extends to phytopathogens. Consistently, soil application of microbial cell-free culture filtrates (CFs) has been found to promote growth and enhance the yield of horticultural crops. Recent studies have shown that the response of plants to soil application of microbial CFs is associated with strong proliferation of the resident beneficial soil microbiota. Therefore, the use of microbial CFs to enhance both crop yield and stress tolerance, and to activate beneficial soil microbiota could be a safe, efficient and environmentally friendly approach to minimize shortfalls related to the technology of microbial inoculation. In this review, we compile information on microbial CFs and the main constituents (especially volatile compounds) that promote plant growth, yield and stress tolerance, and their effects on plant-associated beneficial microbiota. In addition, we identify challenges and limitations for their use as biostimulants to bring them to the market and we propose remedial actions and give suggestions for future work.

Inhibitory Effects of Pepper Mild Mottle Virus Infection by Supernatants of Five Bacterial Cultures in Capsicum annuum L

Pepper mild mottle virus (PMMoV), one of the most prevalent viruses in chili pepper (Capsicum annuum L.) is a non-enveloped, rod-shaped, single-stranded positive-sense RNA virus classified in the genus Tobamovirus. The supernatants of five bacterial cultures (Pseudomonas putida [PP], Bacillus licheniformis [BLI], P. fluorescens [PF], Serratia marcescens [SER], and B. amyloliquifaciens [BA]) were analyzed to find novel antiviral agents to PMMoV in chili pepper. Foliar spraying with supernatants (1:1, v/v) obtained from Luria-Bertani broth cultures of PP, BLI, PF, SER, and BA inhibited PMMoV infection of chili pepper if applied before the PMMoV inoculation. Double-antibody sandwich enzyme-linked immunosorbent assay showed that treatments of five supernatants resulted in 51-66% reductions in PMMoV accumulation in the treated chili pepper. To identify key compounds in supernatants of PP, BLI, PF, SER, and BA, the supernatants were subjected to gas chromatography-mass spectrometry. The 24 different types of compounds were identified from the supernatants of PP, BLI, PF, SER, and BA. The compounds vary from supernatants of one bacterial culture to another which includes simple compounds-alkanes, ketones, alcohols, and an aromatic ring containing compounds. The compounds triggered the inhibitory effect on PMMoV propagation in chili pepper plants. In conclusion, the cultures could be used to further conduct tissue culture and field trial experiments as potential bio-control agents.

Microbe-Mediated Thermotolerance in Plants and Pertinent Mechanisms- A Meta-Analysis and Review

Microbial symbionts can mediate plant stress responses by enhancing thermal tolerance, but less attention has been paid to measuring these effects across plant-microbe studies. We performed a meta-analysis of published studies as well as discussed with relevant literature to determine how the symbionts influence plant responses under non-stressed versus thermal-stressed conditions. As compared to non-inoculated plants, inoculated plants had significantly higher biomass and photosynthesis under heat stress conditions. A significantly decreased accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2) indicated a lower oxidation level in the colonized plants, which was also correlated with the higher activity of catalase, peroxidase, glutathione reductase enzymes due to microbial colonization under heat stress. However, the activity of superoxide dismutase, ascorbate oxidase, ascorbate peroxidase, and proline were variable. Our meta-analysis revealed that microbial colonization influenced plant growth and physiology, but their effects were more noticeable when their host plants were exposed to high-temperature stress than when they grew under ambient temperature conditions. We discussed the mechanisms of microbial conferred plant thermotolerance, including at the molecular level based on the available literature. Further, we highlighted and proposed future directions toward exploring the effects of symbionts on the heat tolerances of plants for their implications in sustainable agricultural production.

Newly Isolated Paenibacillus monticola sp. nov., a Novel Plant Growth-Promoting Rhizobacteria Strain From High-Altitude Spruce Forests in the Qilian Mountains, China

Species in the genus Paenibacillus from special habitats have attracted great attention due to their plant growth-promoting traits. A novel plant growth-promoting rhizobacteria (PGPR) species in the genus Paenibacillus was isolated from spruce forest at the height of 3,150 m in the Qilian Mountains, Gansu province, China. The phylogenetic analysis based on 16S rRNA, rpoB, and nifH gene sequences demonstrated that strain LC-T2^T was affiliated in the genus Paenibacillus and exhibited the highest sequence similarity with Paenibacillus donghaensis KCTC 13049^T (97.4%). Average nucleotide identity (ANIb and ANIm) and digital DNA–DNA hybridization (dDDH) between strain LC-T2^T and P. donghaensis KCTC 13049^T were 72.6, 83.3, and 21.2%, respectively, indicating their genetic differences at the species level. These differences were further verified by polar lipids profiles, major fatty acid contents, and several distinct physiological characteristics. Meanwhile, the draft genome analysis provided insight into the genetic features to support its plant-associated lifestyle and habitat adaptation. Subsequently, the effects of volatile organic compound (VOC) emitted from strain LC-T2^T on the growth of Arabidopsis were evaluated. Application of strain LC-T2^T significantly improved root surface area, root projection area, and root fork numbers by 158.3, 158.3, and 241.2%, respectively, compared to control. Also, the effects of LC-T2^T on the growth of white clover (Trifolium repens L.) were further assessed by pot experiment. Application of LC-T2^T also significantly improved the growth of white clover with root fresh weight increased over three-folds compared to control. Furthermore, the viable bacterial genera of rhizosphere soil were detected in each treatment. The number of genera from LC-T2^T-inoculated rhizosphere soil was 1.7-fold higher than that of control, and some isolates were similar to strain LC-T2^T, indicating that LC-T2^T inoculation was effective in the rhizosphere soil of white clover. Overall, strain LC-T2^T should be attributed to a novel PGPR species within the genus Paenibacillus based on phylogenetic relatedness, genotypic features, and phenotypic and inoculation experiment, for which the name Paenibacillus monticola sp. nov. is proposed.

Cell-Free Supernatant Obtained From a Salt Tolerant Bacillus amyloliquefaciens Strain Enhances Germination and Radicle Length Under NaCl Stressed and Optimal Conditions

Seed germination and early plant growth are key stages in plant development that are, susceptible to salinity stress. Plant growth promoting microorganisms (PGPMs) produce substances, in their growth media, that could enhance plant growth under more optimal conditions, and or mitigate abiotic stresses, such as salinity. This study was carried out to elucidate the ability of a NaCl tolerant Bacillus amyloliquefaciens strain's cell-free supernatant to enhance germination and radicle length of corn and soybean, under optimal and NaCl stressed growth conditions. Three NaCl levels (0, 50, and 75 mM) and four cell-free supernatant concentrations (1.0, 0.2, 0.13, and 0.1% v/v) were used to formulate treatments that were used in the study. There were observed variations in the effect of treatments on mean radicle length and mean percentage germination of corn and soybean. Overall, the study showed that Bacillus amyloliquefaciens (BA) EB2003 cell-free supernatant could enhance mean percentage germination and or mean radicle length of corn and soybean. At optimal conditions (0 mM NaCl), 0.2% BA, 0.13% BA, and 0.1% BA concentrations resulted in 36.4, 39.70, and 39.91%, increase in mean radicle length of soybean, respectively. No significant observations were observed in mean radicle length of corn, and mean percentage germination of both corn and soybean. At 50 mM NaCl, 1.0% BA resulted in 48.65% increase in mean percentage germination of soybean, at 24 h. There was no observed effect of the cell-free supernatant on mean radicle length and mean percentage germination, at 72 and 48 h, in soybean. In corn however, at 50 mM NaCl, treatment with 0.2% BA and 0.13% BA enhanced mean radicle length by 23.73 and 37.5%, respectively. The resulting radicle lengths (43.675 and 49.7125 cm) were not significantly different from that of the 0 mM control. There was no observed significant effect of the cell-free supernatant on mean germination percentage of corn, at 50 mM NaCl. At 75 mM NaCl, none of the treatments enhanced mean radicle length or mean percentage germination to levels significantly higher than the 75 mM NaCl. Treatment with 1.0% BA, however, enhanced mean percentage germination to a level not significantly different from that of the 0 mM control, at 72 h. Likewise, in corn, none of the treatments enhanced radicle length to lengths significantly higher than the 75 mM control, although treatment with 1.0% BA, 0.13% BA, and 0.1% BA elongated radicles to lengths not significantly different from the 0 mM NaCl control. Treatment with 0.2% BA, 0.13% BA, and 0.1% BA resulted in mean percentage germination significantly higher than the 75 mM NaCl by 25.3% (in all 3), not significantly different from that of the 0 mM NaCl. In conclusion, concentration of cell-free supernatant and NaCl level influence the effect of Bacillus amyloliquefaciens strain EB2003A cell-free supernatant on mean percentage germination and mean radicle length of corn and soybean.

Microbial Derived Compounds Are a Promising Approach to Mitigating Salinity Stress in Agricultural Crops

The use of microbial derived compounds is a technological approach currently gaining popularity among researchers, with hopes of complementing, supplementing and addressing key issues associated with use of microbial cells for enhancing plant growth. The new technology is a promising approach to mitigating effects of salinity stress in agricultural crops, given that these compounds could be less prone to effects of salt stress, are required in small quantities and are easier to store and handle than microbial cells. Microorganism derived compounds such as thuricin17, lipochitooligosaccharides, phytohormones and volatile organic compounds have been reported to mitigate the effects of salt stress in agricultural crops such as soybean and wheat. This mini-review compiles current knowledge regarding the use of microbe derived compounds in mitigating salinity stress in crops, the mechanisms they employ as well as future prospects.