Phosphate solubilizing bacteria stimulate wheat rhizosphere and endosphere biological nitrogen fixation by improving phosphorus content

Plant roots affect free-living diazotroph communities in temperate grassland soils despite decades of fertilization

Fixation of atmospheric N2 by free-living diazotrophs accounts for an important proportion of nitrogen naturally introduced to temperate grasslands. The effect of plants or fertilization on the general microbial community has been extensively studied, yet an understanding of the potential combinatorial effects on the community structure and activity of free-living diazotrophs is lacking. In this study we provide a multilevel assessment of the single and interactive effects of different long-term fertilization treatments, plant species and vicinity to roots on the free-living diazotroph community in relation to the general microbial community in grassland soils. We sequenced the dinitrogenase reductase (nifH) and the 16S rRNA genes of bulk soil and root-associated compartments (rhizosphere soil, rhizoplane and root) of two grass species (Arrhenatherum elatius and Anthoxanthum odoratum) and two herb species (Galium album and Plantago lanceolata) growing in Austrian grassland soils treated with different fertilizers (N, P, NPK) since 1960. Overall, fertilization has the strongest effect on the diazotroph and general microbial community structure, however with vicinity to the root, the plant effect increases. Despite the long-term fertilization, plants strongly influence the diazotroph communities emphasizing the complexity of soil microbial communities' responses to changing nutrient conditions in temperate grasslands.

Antagonistic effect of Bacillus and Pseudomonas combinations against Fusarium oxysporum and their effect on disease resistance and growth promotion in watermelon

AIMS

We aimed to develop an effective bacterial combination that can combat Fusarium oxysporum infection in watermelon using in vitro and pot experiments.

METHODS AND RESULTS

In total, 53 strains of Bacillus and 4 strains of Pseudomonas were screened. Pseudomonas strains P3 and P4 and Bacillus strains XY-2-3, XY-13, and GJ-1-15 exhibited good antagonistic effects against F. oxysporum. P3 and P4 were identified as Pseudomonas chlororaphis and Pseudomonas fluorescens, respectively. XY-2-3 and GJ-1-15 were identified as B. velezensis, and XY-13 was identified as Bacillus amyloliquefaciens. The three Bacillus strains were antifungal, promoted the growth of watermelon seedlings and had genes to synthesize antagonistic metabolites such as bacilysin, surfactin, yndj, fengycin, iturin, and bacillomycin D. Combinations of Bacillus and Pseudomonas strains, namely, XY-2-3 + P4, GJ-1-15 + P4, XY-13 + P3, and XY-13 + P4, exhibited a good compatibility. These four combinations exhibited antagonistic effects against 11 pathogenic fungi, including various strains of F. oxysporum, Fusarium solani, and Rhizoctonia. Inoculation of these bacterial combinations significantly reduced the incidence of Fusarium wilt in watermelon, promoted plant growth, and improved soil nutrient availability. XY-13 + P4 was the most effective combination against Fusarium wilt in watermelon with the inhibition rate of 78.17%. The number of leaves; aboveground fresh and dry weights; chlorophyll, soil total nitrogen, and soil available phosphorus content increased by 26.8%, 72.12%, 60.47%, 16.97%, 20.16%, and 16.50%, respectively, after XY-13 + P4 inoculation compared with the uninoculated control. Moreover, total root length, root surface area, and root volume of watermelon seedlings were the highest after XY-13 + P3 inoculation, exhibiting increases by 265.83%, 316.79%, and 390.99%, respectively, compared with the uninoculated control.

CONCLUSIONS

XY-13 + P4 was the best bacterial combination for controlling Fusarium wilt in watermelon, promoting the growth of watermelon seedlings, and improving soil nutrient availability.

Phosphate-solubilizing fungi enhances the growth of Brassica chinensis L. and reduces arsenic uptake by reshaping the rhizosphere microbial community

High concentrations of arsenic in soil and plant systems are a threat to human health and ecosystems. The levels of phosphate ions in the soil strongly influence the soil efficacy and arsenic absorption by plants. This study investigated the effects of phosphate-solubilizing fungi (PSF) on environmental factors and structural changes in microbial community in soils contaminated with arsenic. Four experimental groups were created: control (CK), Penicillium GYAHH-CCT186 (W186), Aspergillus AHBB-CT196 (W196), and Penicillium GYAHH-CCT186 + Aspergillus AHBB-CT196 (W186 + W196), with Pakchoi (Brassica chinensis L.) as the test plant. Analysis of altered nutrient levels, enzyme activities and microbial community structure in the soil as well as the growth and physiological characteristics of Pakchoi, revealed a significant increase in the available phosphorus (AP), organic matter (OM), cation exchange capacity (CEC) and available arsenic (AAs) content of the soil following W186 + W196, W196 and W186 treatments. All experimental treatments enhanced the activity of soil β-glucosidase (β-GC) and soil catalase (S-CAT). W186 + W196 and W196 treatments significantly enhanced soil acid phosphatase (S-ACP) activity. Besides, W186 + W196 treatment significantly induced dehydrogenase (S-DHA) activity. Further, of the treatment with PSF increased the fresh weight, root length, plant height and chlorophyll levels while decreasing the arsenic accumulation in Pakchoi. Exposure to PSF also increased the activity of Ascomycota, Basidiomycota, Chytridiomycota, unclassified_Fungi, Mortierellomycota, Cryptomycota and Rozellomycota in the soil. The relative abundance of Ascomycota, Basidiomycota, and Mortierellomycota was positively correlated with the available nutrients (except iron) in the soil as well as enzyme activities. Consequently, the PSF improved the quality of soil and the safety of Pakchoi, suggesting that PSF can be utilized for the remediation of arsenic-contaminated soil.

Changes in Phenolic Profile and Total Phenol and Total Flavonoid Contents of Guadua angustifolia Kunth Plants under Organic and Conventional Fertilization

Agronomic management of a crop, including the application of fertilizers and biological inoculants, affects the phenol and flavonoid contents of plants producing these metabolites. Guadua angustifolia Kunth, a woody bamboo widely distributed in the Americas, produces several biologically active phenolic compounds. The aim of this study was to evaluate the effect of chemical and organic fertilizers together with the application of biological inoculants on the composition of phenolic compounds in G. angustifolia plants at the nursery stage. In 8-month-old plants, differences were observed in plant biomass (20.27 ± 7.68 g) and in the content of total phenols and flavonoids (21.89 ± 9.64 mg gallic acid equivalents/plant and 2.13 ± 0.98 mg quercetin equivalents/plant, respectively) when using the chemical fertilizer diammonium phosphate (DAP). No significant differences were found owing to the effect of the inoculants, although the plants with the application of Stenotrophomonas sp. on plants fertilized with DAP presented higher values of the metabolites (24.12 ± 6.72 mg gallic acid equivalents/plant and 2.39 ± 0.77 mg quercetin equivalents/plant). The chromatographic profile of phenolic metabolites is dominated by one glycosylated flavonoid, the concentration of which was favored by the application of the inoculants Azospirillum brasilense, Pseudomonas fluorescens, and Stenotrophomonas sp. In the case study, the combined use of DAP and bacterial inoculants is recommended for the production of G. angustifolia plant material with a high content of promising biologically active flavonoids or phenolics.

Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives

Nitrogen is abundant in the atmosphere but is generally the most limiting nutrient for plants. The inability of many crop plants, such as cereals, to directly utilize freely available atmospheric nitrogen gas means that their growth and production often rely heavily on the application of chemical fertilizers, which leads to greenhouse gas emissions and the eutrophication of water. By contrast, legumes gain access to nitrogen through symbiotic association with rhizobia. These bacteria convert nitrogen gas into biologically available ammonia in nodules through a process termed symbiotic biological nitrogen fixation, which plays a decisive role in ecosystem functioning. Engineering cereal crops that can fix nitrogen like legumes or associate with nitrogen-fixing microbiomes could help to avoid the problems caused by the overuse of synthetic nitrogen fertilizer. With the development of synthetic biology, various efforts have been undertaken with the aim of creating so-called "N-self-fertilizing" crops capable of performing autonomous nitrogen fixation to avoid the need for chemical fertilizers. In this review, we briefly summarize the history and current status of engineering N-self-fertilizing crops. We also propose several potential biotechnological approaches for incorporating biological nitrogen fixation capacity into non-legume plants.

Distribution characteristics of soil microbial communities and their responses to environmental factors in the sea buckthorn forest in the water-wind erosion crisscross region

Soil microorganisms are an important part of forest ecosystems, and their community structure and ecological adaptations are important for explaining soil material cycles in the fragile ecological areas. We used high-throughput sequencing technology to examine the species composition and diversity of soil bacterial and fungal communities in sea buckthorn forests at five sites in the water-wind erosion crisscross in northern Shaanxi (about 400 km long). The results are described as follows: (1) The soil bacterial community of the sea buckthorn forest in the study region was mainly dominated by Actinobacteria, Proteobacteria, and Acidobacteria, and the fungi community was mainly dominated by Ascomycota. (2) The coefficient of variation of alpha diversity of microbial communities was higher in the 0-10 cm soil layer than in the 10-20 cm soil layer. (3) Soil electrical conductivity (36.1%), available phosphorous (AP) (21.0%), available potassium (16.2%), total nitrogen (12.7%), and the meteorological factors average annual maximum temperature (33.3%) and average annual temperature (27.1%) were identified as the main drivers of structural changes in the bacterial community. Available potassium (39.4%), soil organic carbon (21.4%), available nitrogen (AN) (13.8%), and the meteorological factors average annual maximum wind speed (38.0%) and average annual temperature (26.8%) were identified as the main drivers of structural changes in the fungal community. The explanation rate of soil factors on changes in bacterial and fungal communities was 26.6 and 12.0%, respectively, whereas that of meteorological factors on changes in bacterial and fungal communities was 1.22 and 1.17%, respectively. The combined explanation rate of environmental factors (soil and meteorological factors) on bacterial and fungal communities was 72.2 and 86.6%, respectively. The results of the study offer valuable insights into the diversity of soil microbial communities in the water-wind erosion crisscross region and the mechanisms underlying their interaction with environmental factors.

Prospects for Using Phosphate-Solubilizing Microorganisms as Natural Fertilizers in Agriculture

Phosphates are known to be essential for plant growth and development, with phosphorus compounds being involved in various physiological and biochemical reactions. Phosphates are known as one of the most important factors limiting crop yields. The problem of phosphorus deficiency in the soil has traditionally been solved by applying phosphate fertilizers. However, chemical phosphate fertilizers are considered ineffective compared to the organic fertilizers manure and compost. Therefore, increasing the bioavailability of phosphates for plants is one of the primary goals of sustainable agriculture. Phosphate-solubilizing soil microorganisms can make soil-insoluble phosphate bioavailable for plants through solubilization and mineralization. These microorganisms are currently in the focus of interest due to their advantages, such as environmental friendliness, low cost, and high biological efficiency. In this regard, the solubilization of phosphates by soil microorganisms holds strong potential in research, and inoculation of soils or crops with phosphate-solubilizing bacteria is a promising strategy to improve plant phosphate uptake. In this review, we analyze all the species of phosphate-solubilizing bacteria described in the literature to date. We discuss key mechanisms of solubilization of mineral phosphates and mineralization of organic phosphate-containing compounds: organic acids secreted by bacteria for the mobilization of insoluble inorganic phosphates, and the enzymes hydrolyzing phosphorus-containing organic compounds. We demonstrate that phosphate-solubilizing microorganisms have enormous potency as biofertilizers since they increase phosphorus bioavailability for the plant, promote sustainable agriculture, improve soil fertility, and raise crop yields. The use of phosphate-solubilizing microbes is regarded as a new frontier in increasing plant productivity.

Microbial Inoculation Improves Growth, Nutritional and Physiological Aspects of Glycine max (L.) Merr

Considering a scenario where there is a low availability and increasing costs of fertilizers in the global agricultural market, as well as a finitude of important natural resources, such as phosphorus (P), this study tested the effect of the inoculation of rhizospheric or endophytic microorganisms isolated from Hymenaea courbaril and Butia purpurascens on the growth promotion of Glycine max (L.) Merr. The tests were conducted in a controlled greenhouse system, and the effects of biofertilization were evaluated using the following parameters: dry biomass, nutritional content, and photochemical and photosynthetic performance of plants. Seed biopriming was performed with four bacterial and four fungal isolates, and the results were compared to those of seeds treated with the commercial product Biomaphos^®. Overall, microbial inoculation had a positive effect on biomass accumulation in G. max, especially in strains PA12 (Paenibacillus alvei), SC5 (Bacillus cereus), and SC15 (Penicillium sheari). The non-inoculated control plants accumulated less nutrients, both in the whole plant and aerial part, and had reduced chlorophyll index and low photosynthetic rate (A) and photochemical efficiency. Strains PA12 (P. alvei), SC5 (B. cereus), and 328EF (Codinaeopsis sp.) stood out in the optimization of nutrient concentration, transpiration rate, and stomatal conductance. Plants inoculated with the bacterial strains PA12 (P. alvei) and SC5 (B. cereus) and with the fungal strains 328EF (Codinaeopsis sp.) and SC15 (P. sheari) showed the closest pattern to that observed in plants treated with Biomaphos^®, with the same trend of direction of the means associated with chlorophyll index, (A), dry mass, and concentration of important nutrients such as N, P, and Mg. We recommend the use of these isolates in field tests to validate these strains for the production of biological inoculants as part of the portfolio of bioinputs available for G. max.

Insight into soil nitrogen and phosphorus availability and agricultural sustainability by plant growth-promoting rhizobacteria

Nitrogen and phosphorus are critical for the vegetation ecosystem and two of the most insufficient nutrients in the soil. In agriculture practice, many chemical fertilizers are being applied to soil to improve soil nutrients and yield. This farming procedure poses considerable environmental risks which affect agricultural sustainability. As robust soil microorganisms, plant growth-promoting rhizobacteria (PGPR) have emerged as an environmentally friendly way of maintaining and improving the soil's available nitrogen and phosphorus. As a special PGPR, rhizospheric diazotrophs can fix nitrogen in the rhizosphere and promote plant growth. However, the mechanisms and influences of rhizospheric nitrogen fixation (NF) are not well researched as symbiotic NF lacks summarizing. Phosphate-solubilizing bacteria (PSB) are important members of PGPR. They can dissolve both insoluble mineral and organic phosphate in soil and enhance the phosphorus uptake of plants. The application of PSB can significantly increase plant biomass and yield. Co-inoculating PSB with other PGPR shows better performance in plant growth promotion, and the mechanisms are more complicated. Here, we provide a comprehensive review of rhizospheric NF and phosphate solubilization by PGPR. Deeper genetic insights would provide a better understanding of the NF mechanisms of PGPR, and co-inoculation with rhizospheric diazotrophs and PSB strains would be a strategy in enhancing the sustainability of soil nutrients.

Isolation, Mutagenesis, and Organic Acid Secretion of a Highly Efficient Phosphate-Solubilizing Fungus

The highly effective phosphate-solubilizing microorganisms are significant for making full use of the potential phosphorus resources in the soil and alleviating the shortage of phosphorus resources. In this study, a phosphate-solubilizing fungus was isolated from wheat and cotton rhizosphere soils in the lower reaches of the Yellow River in China and was identified as Penicillium oxalicum by morphological and ITS sequencing analysis. In order to obtain a fungus with more efficient phosphorus solubilization ability, we tested three positive mutant strains (P1, P2, and P3) and three negative mutant strains (N1, N2, and N3) through low-energy nitrogen ion implantation mutagenesis. Compared with the parental strain, the phosphate-solubilizing capacity of P1, P2, and P3 was enhanced by 56.88%, 42.26%, and 32.15%, respectively, and that of N1, N2, and N3 was weakened by 47.53%, 35.27%, and 30.86%, respectively. Compared with the parental strain, the total amount of organic acids secreted significantly increased in the three positive mutant strains and decreased in the negative mutant strains; the pH of culture medium was significantly lower in the positive mutant strains and higher in the negative mutant strains. The capacity of phosphate-solubilizing fungus to secrete organic acids and reduce the growth-medium pH was closely related to its phosphate-solubilizing ability. The changes in the amount of organic acids secreted by mutants can alter their acidification and phosphate-solubilizing capacity. In conclusion, this study offers a theoretical basis and strain materials for the exploration and application of phosphate-solubilizing fungi.

Multiple Potential Plant Growth Promotion Activities of Endemic Streptomyces spp. from Moroccan Sugar Beet Fields with Their Inhibitory Activities against Fusarium spp

The characterized 10 Streptomyces isolates were previously selected by their abilities to solubilize phosphates. To investigate whether these isolates represent multifaceted plant growth-promoting rhizobacteria (PGPR), their potassium-solubilizing, auxin-producing and inhibitory activities were determined. The 10 Streptomyces spp. yielded a variable biomass in the presence of insoluble orthoclase as the sole potassium (K) source, indicating that they were able to extract different amounts of K from this source for their own growth. Three strains (AZ, AYD and DE2) released soluble K from insoluble orthoclase in large amounts into the culture broth. The production levels ranged from 125.4 mg/L to 216.6 mg/L after 5 days of culture. Only two strains, Streptomyces enissocaesilis (BYC) and S. tunisiensis (AI), released a larger amount of soluble K from orthoclase and yielded much more biomass. This indicated that the rate of K released from this insoluble orthoclase exceeded its consumption rate for bacterial growth and that some strains solubilized K more efficiently than others. The results also suggest that the K solubilization process of AZ, AYD and DE2 strains, the most efficient K-solubilizing strains, involves a slight acidification of the medium. Furthermore, these 10 Streptomyces spp. were able to secrete indole acetic acid (IAA) in broth medium and ranged from 7.9 ± 0.1 µg/mL to 122.3 ± 0.1 µg/mL. The results of the antibiosis test proved the potential of the 10 tested strains to limit the growth of fungi and bacteria. In dual culture, S. bellus (AYD) had highest inhibitory effect against the three identified fungal causal agents of root rot of sugar beet: Fusarium equiseti and two F. fujikuroi at 55, 43 and 36%, respectively. Streptomyces enissocaesilis (BYC), S. bellus (AYD) and S. saprophyticus (DE2) exhibited higher multifaceted PGPR with their potassium-solubilizing, auxin-producing and inhibitory activities, which could be expected to lead to effectiveness in field trials of sugar beet.