Rice-derived facultative endophytic Serratia liquefaciens F2 decreases rice grain arsenic accumulation in arsenic-polluted soil

The Roles of Plant-Growth-Promoting Rhizobacteria (PGPR)-Based Biostimulants for Agricultural Production Systems

The application of biostimulants has been proven to be an advantageous tool and an appropriate form of management towards the effective use of natural resources, food security, and the beneficial effects on plant growth and yield. Plant-growth-promoting rhizobacteria (PGPR) are microbes connected with plant roots that can increase plant growth by different methods such as producing plant hormones and molecules to improve plant growth or providing increased mineral nutrition. They can colonize all ecological niches of roots to all stages of crop development, and they can affect plant growth and development directly by modulating plant hormone levels and enhancing nutrient acquisition such as of potassium, phosphorus, nitrogen, and essential minerals, or indirectly via reducing the inhibitory impacts of different pathogens in the forms of biocontrol parameters. Many plant-associated species such as Pseudomonas, Acinetobacter, Streptomyces, Serratia, Arthrobacter, and Rhodococcus can increase plant growth by improving plant disease resistance, synthesizing growth-stimulating plant hormones, and suppressing pathogenic microorganisms. The application of biostimulants is both an environmentally friendly practice and a promising method that can enhance the sustainability of horticultural and agricultural production systems as well as promote the quantity and quality of foods. They can also reduce the global dependence on hazardous agricultural chemicals. Science Direct, Google Scholar, Springer Link, CAB Direct, Scopus, Springer Link, Taylor and Francis, Web of Science, and Wiley Online Library were checked, and the search was conducted on all manuscript sections in accordance with the terms Acinetobacter, Arthrobacter, Enterobacter, Ochrobactrum, Pseudomonas, Rhodococcus, Serratia, Streptomyces, Biostimulants, Plant growth promoting rhizobactera, and Stenotrophomonas. The aim of this manuscript is to survey the effects of plant-growth-promoting rhizobacteria by presenting case studies and successful paradigms in various agricultural and horticultural crops.

Identification and Characterization of Beneficial Soil Microbial Strains for the Formulation of Biofertilizers Based on Native Plant Growth-Promoting Microorganisms Isolated from Northern Mexico

Plant growth-promoting microorganisms (PGPM) benefit plant health by enhancing plant nutrient-use efficiency and protecting plants against biotic and abiotic stresses. This study aimed to isolate and characterize autochthonous PGPM from important agri-food crops and nonagricultural plants to formulate biofertilizers. Native microorganisms were isolated and evaluated for PGP traits (K, P, and Zn solubilization, N2-fixation, NH3-, IAA and siderophore production, and antifungal activity against Fusarium oxysporum). Isolates were tested on radish and broccoli seedlings, evaluating 19 individual isolates and 12 microbial consortia. Potential bacteria were identified through DNA sequencing. In total, 798 bacteria and 209 fungi were isolated. Isolates showed higher mineral solubilization activity than other mechanisms; 399 bacteria and 156 fungi presented mineral solubilization. Bacteria were relevant for nitrogen fixation, siderophore, IAA (29-176 mg/L), and ammonia production, while fungi for Fusarium growth inhibition (40-69%). Twenty-four bacteria and eighteen fungi were selected for their PGP traits. Bacteria had significantly (ANOVA, p < 0.05) better effects on plants than fungi; treatments improved plant height (23.06-51.32%), leaf diameter (25.43-82.91%), and fresh weight (54.18-85.45%) in both crops. Most potential species belonged to Pseudomonas, Pantoea, Serratia, and Rahnella genera. This work validated a high-throughput approach to screening hundreds of rhizospheric microorganisms with PGP potential isolated from rhizospheric samples.

Herbaspirillum sp. ZXN111 Colonization Characters to Different Tea Cultivars and the Effects on Tea Metabolites Profiling on Zijuan (Camellia sinensis var. assamica)

Herbaspirillum sp. ZXN111 and its mutants (Δacc, Δtyrb, and Δacc-tyrb), which show PGP activity on Zijuan, were tested for tea plants' colonization characteristics and the strain-dependent response of tea metabolites. The results showed that strain ZXN111 could widely colonize in different tea cultivars of Zijuan, Yunkang-10, Longjin 43, and Shuchazao, but with significant colonization preference to Zijuan, which might be ascribed to anthocyanins' chemotaxis. After 9 weeks of co-cultivation, l-theanine and theobromine in Zijuan leaves that were inoculated with wild-type ZXN111 were decreased, while theobromine, caffeine, and l-theanine that were inoculated with mutant Δacc were increased; especially l-theanine increased much significantly. Metabolomics analysis showed that tea metabolite profiling of inoculant groups was clearly separated from the control; therein, the flavanols were downregulated in ZXN111 and Δacc groups, but the l-theanine of the Δacc group was significantly upregulated compared to control and ZXN111 groups. These results indicated that strain ZXN111, especially of mutant Δacc, improved Zijuan tea flavor.

Bacterial Arsenic Metabolism and Its Role in Arsenic Bioremediation

Arsenic contaminations, often adversely influencing the living organisms, including plants, animals, and the microbial communities, are of grave apprehension. Many physical, chemical, and biological techniques are now being explored to minimize the adverse affects of arsenic toxicity. Bioremediation of arsenic species using arsenic loving bacteria has drawn much attention. Arsenate and arsenite are mostly uptaken by bacteria through aquaglycoporins and phosphate transporters. After entering arsenic inside bacterial cell arsenic get metabolized (e.g., reduction, oxidation, methylation, etc.) into different forms. Arsenite is sequentially methylated into monomethyl arsenic acid (MMA) and dimethyl arsenic acid (DMA), followed by a transformation of less toxic, volatile trimethyl arsenic acid (TMA). Passive remediation techniques, including adsorption, biomineralization, bioaccumulation, bioleaching, and so on are exploited by bacteria. Rhizospheric bacterial association with some specific plants enhances phytoextraction process. Arsenic-resistant rhizospheric bacteria have immense role in enhancement of crop plant growth and development, but their applications are not well studied till date. Emerging techniques like phytosuction separation (PS-S) have a promising future, but still light to be focused on these techniques. Plant-associated bioremediation processes like phytoextraction and phytosuction separation (PS-S) techniques might be modified by treating with potent bacteria for furtherance.

Effects of Cd-resistant fungi on uptake and translocation of Cd by soybean seedlings

In this study, three cadmium (Cd)-resistant fungal strains, temporarily named as F1, F2 and F3 were isolated from the roots of Cd-tolerant soybean cultivars, rhizosphere and bulk soils, respectively, in contaminated sites. Cd-resistant strains F1, F2 and F3 were characterized for their effect on biomass, Cd uptake and translocation of two soybean cultivars (Liaodou36 and Liaodou33) grown in Cd-contaminated soils. The results showed that Cd concentration decreased significantly in Cd-supplemented culture solutions inoculated with strains F1, F2 and F3 compared to non-inoculated controls, while cell counts significantly increased during the incubation. The increase in shoot biomass of two soybean cultivars inoculated with strains F1, F2 and F3 ranged from 13% to 29%, 16%-27% and 15%-32%, respectively, compared to controls. Strain F2 had a higher potential to reduce the water-soluble Cd content (23% and 40%) and EDTA-extractable Cd content in the rhizosphere soil of Liaodou36 and Liaodou33 seedlings compared to strains F1 and F3. A significant decrease of Cd contents was observed in the root and shoot of Liaodou33 inoculated with strain F2 compared to non-inoculated controls, and inoculation with strain F2 significantly reduced the TF and BCF of Liaodou33 in comparison with controls. Based on ITS rRNA gene sequence analyses, the strains F1, F2 and F3 were identified as Mucor circinelloides (similarity 99.81%), Curvularia lunata (similarity 99.31%) and Clonostachys rosea (similarity 99.17%). The results of our study demonstrated that the strain F2 had a higher Cd biosorption and immobilization potential than strains F1 and F3. The strain F2 promoted the growth and reduced Cd uptake and translocation of Liaodou33 in Cd-polluted soils. It is worth noting that our results might provide an effective technical support for Cd immobilization remediation and safe soybean production by inoculating moderate Cd-accumulating soybean cultivars with strain F2 in Cd-contaminated soils.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

Unveiling Endophytic Bacterial Community Structures of Different Rice Cultivars Grown in a Cadmium-Contaminated Paddy Field

Endophytic bacteria play potentially important roles in the processes of plant adaptation to the environment. Understanding the composition and dynamics of endophytic bacterial communities under heavy metal (HM) stress can reveal their impacts on host development and stress tolerance. In this study, we investigated root endophytic bacterial communities of different rice cultivars grown in a cadmium (Cd)-contaminated paddy field. These rice cultivars are classified into low (RBQ, 728B, and NX1B) and high (BB and S95B) levels of Cd-accumulating capacity. Our metagenomic analysis targeting 16S rRNA gene sequence data reveals that Proteobacteria, Firmicutes, Actinobacteria, Acidobacteria, Bacteroidetes, and Spirochaetes are predominant root endophytic bacterial phyla of the five rice cultivars that we studied. Principal coordinate analysis shows that the developmental stage of rice governs a larger source of variation in the bacterial communities compared to that of any specific rice cultivar or of the root Cd content. Endophytic bacterial communities during the reproductive stage of rice form a more highly interconnected network and exhibit higher operational taxonomic unit numbers, diversities, and abundance than those during the vegetative stage. Forty-five genera are significantly correlated with Cd content in rice root, notably including positive-correlating Geobacter and Haliangium; and negative-correlating Pseudomonas and Streptacidiphilus. Furthermore, Phylogenetic Investigation of Communities by Reconstruction of Unobserved States analysis shows that functional pathways, such as biosynthesis of siderophore and type II polyketide products, are significantly enhanced during the reproductive stage compared to those during the vegetative stage under Cd stress. The isolated endophytic bacteria from the Cd-contaminated rice roots display high Cd resistance and multiple traits that may promote plant growth, suggesting their potential application in alleviating HM stress on plants. This study describes in detail for the first time the assemblage of the bacterial endophytomes of rice roots under Cd stress and may provide insights into the interactions among endophytes, plants, and HM contamination.

Impact of rhizosphere microorganisms on arsenic (As) transformation and accumulation in a traditional Chinese medical plant

Panax notoginseng is an important traditional medicinal plant, but the commercial value is threatened by root-rot disease caused by rhizosphere microbes and a potential health risk caused by plant arsenic (As) accumulation. Whether rhizospheric microbes isolated from P. notoginseng rhizosphere soil could impact As uptake and transport into P. notoginseng is not yet known. Among the three root-rot disease-causing pathogens Fusarium flocciferum (PG 1), Fusarium oxysporum (PG 2), and Fusarium solani (PG 3) and one root-rot disease biocontrol fungus Trichoderma koningiopsis (FC 1) and five biocontrol-exerting bacterial species Bacillus siamensis (BC 1), Delftia acidovorans (BC 2), Brevibacillus formosus (BC 3), Mortierella alpine (BC 4), and Bacillus subtilis (BC 5), one As-resistant pathogen and four biocontrol microorganisms with As-resistant ability were identified. The As-transforming ability of the identified fungi and bacteria was ranked in the order of FC 1 > PG 1 and BC 2 > BC 3 > BC 1, respectively. Then, the As-resistant biocontrol and pathogenic microbes were initiated to colonize the rhizosphere of 1-year-old P. notoginseng seedlings growing in artificially As(V)-contaminated soil to evaluate the impact of microbe inoculation on P. notoginseng As uptake and transport capacity. Concentration of As in P. notoginseng tissues decreased in the order of the sequence stem > root > leaf. Compared to treatment without colonization by microorganism, inoculation with microorganisms increased As root uptake efficiency and root As concentration, especially under treatment of inoculation by BC 2 and PG 1 + BC 2. As transport efficiency from root to stem decreased by inoculation with microorganism, especially under treatment with inoculation of BC 2 and PG 1 + BC 2. However, the impact of microorganism colonization on As stem to leaf transport efficiency was not obvious. In summary, inoculation with rhizosphere microbes may increase As accumulation in P. notoginseng root, especially when using bacteria with high As transformation ability. Therefore, it is necessary to evaluate the As transformation capacity before applying biological control microorganism to the rhizosphere of P. notoginseng.

Phytoexclusion of heavy metals using low heavy metal accumulating cultivars: A green technology

Heavy metal (HM) pollution of farmland is a serious problem worldwide and consumption of HM-contaminated food products poses significant public health risks. Phytoexclusion using low HM accumulating cultivars (LACs) is a promising and practical technology to mitigate the risk of HM contamination of agricultural products grown in polluted soils, and does not alter cultivation practices, is easy to apply, and is economical. This review provides an overview of the major scientific advances accomplished in the field of LACs worldwide. The LACs concept and identification criteria are presented, and the known LACs among currently cultivated grain crops and vegetables are re-evaluated. The low HM accumulation by LACs is affected by crop ecophysiological features and soil physicochemical characteristics. Taking low Cd accumulating cultivars as an example, it is known that they can efficiently exclude Cd from entering their edible parts in three ways: 1) decrease in root Cd uptake by reducing organic acids secretion in the rhizosphere and transport protein production; 2) restriction of Cd translocation from roots to shoots via enhanced Cd retention in the cell wall and Cd sequestration in vacuoles; and 3) reduction in Cd translocation from shoots to grains by limiting Cd redirection and remobilization mediated through nodes. We propose an LAC application strategy focused on LACs and optimized to work with other agronomic measures according to the classification of HM risk level for LACs, providing a cost-effective and practical solution for safe utilization of large areas of farmland polluted with low to moderate levels of HMs.

Wheat-associated Pseudomonas taiwanensis WRS8 reduces cadmium uptake by increasing root surface cadmium adsorption and decreasing cadmium uptake and transport related gene expression in wheat

Metal-resistant bacteria can reduce Cd accumulation in plants, but mechanisms underlying this effect are poorly understood. In this study, a highly effective Cd-resistant WRS8 strain was obtained from the rhizoshere soil of Triticum aestivum L. Yangmai-13 and identified as Pseudomonas taiwanensis based on 16S rRNA gene sequence analysis. Strain WRS8 was investigated for its effects on Cd availability and wheat tissue Cd contents and the related mechanisms using a hydroponic culture experiment. In strain WRS8-inoculated solution, the Cd concentration reduced and the pH and cell-adsorbed Cd increased with time. Strain WRS8 increased the wheat root and above-ground tissue dry weights by 11-36% compared to the controls. In strain WRS8-inoculated wheat plants, the Cd contents of the roots and above-ground tissues decreased by 78-85% and 88-94% and the Cd bioconcentration and translocation factors decreased by 78-85% and 46-58% at days 3 and 10, respectively, compared with the controls. The root surface-adsorbed Cd contents increased by 99-121% in the WRS8 strain-inoculated wheat plants at days 3 and 10 compared to the controls. Furthermore, strain WRS8 colonized the wheat root surfaces and interiors and reduced the expression levels of the LCT1 and HMA2 genes involved in Cd accumulation and transport in wheat roots by 46% and 30%, respectively, compared to the controls. In the Cd-contaminated soils, strain WRS8 significantly reduced the available Cd content by 20-24% and increased the pH compared to the controls. These findings showed the important role of strain WRS8 in reducing solution and soil Cd availability and suggested that strain WRS8 reduced the wheat tissue Cd accumulation by increasing root surface Cd adsorption and decreasing wheat root Cd uptake and transport-related gene expression and may provide a new and effective wheat rhizobacteria-enhanced approach for reducing wheat Cd uptake in Cd-polluted environments.

A Gnotobiotic Model to Examine Plant and Microbiome Contributions to Survival under Arsenic Stress

So far, the relative importance of the plant and its microbiome in the development of early stages of plant seedling growth under arsenic stress has not been studied. To test the role of endophytic bacteria in increasing plant success under arsenic stress, gnotobiotic seeds of J. montana were inoculated with two endophytic bacteria: Pantoea conspicua MC-K1 (PGPB and As resistant bacteria) and Arthrobacter sp. MC-D3A (non-helper and non-As resistant bacteria) and an endobacteria mixture. In holobiotic seedlings (with seed-vectored microbes intact), neither the capacity of germination nor development of roots and lateral hairs was affected at 125 μM As(V). However, in gnotobiotic seedlings, the plants are negatively impacted by absence of a microbiome and presence of arsenic, resulting in reduced growth of roots and root hairs. The inoculation of a single PGPB (P. conspicua-MCK1) shows a tendency to the recovery of the plant, both in arsenic enriched and arsenic-free media, while the inoculation with Arthrobacter sp. does not help in the recovery of the plants. Inoculation with a bacterial mixture allows recovery of plants in arsenic free media; however, plants did not recover under arsenic stress, probably because of a bacterial interaction in the mixture.

Combined application of Bacillus sp. MN-54 and phosphorus improved growth and reduced lead uptake by maize in the lead-contaminated soil

Lead (Pb) is considered an important environmental contaminant due to its considerable toxicity to living organisms. It can enter and accumulate in plant tissues and become part of the food chain. In the present study, individual and combined effects of Bacillus sp. MN-54 and phosphorus (P) on maize growth and physiology were evaluated in Pb-contaminated soil. A pristine soil was artificially contaminated with two levels of Pb (i.e., 250 and 500 mg kg^-1 dry soil) and was transferred to plastic pots. Bacillus sp. MN-54 treated and untreated maize (DK-6714) seeds were planted in pots. Recommended doses of nutrients (N and K) were applied in each pot while P was applied in selective pots. Results showed that Pb stress hampered the maize growth and physiological attributes in a concentration-dependent manner, and significant reductions in seedling emergence, shoot and root lengths, fresh and dry biomasses, leaf area, chlorophyll content, rate of photosynthesis, and stomatal conductance were recorded compared with control. Application of Bacillus sp. MN-54 or P particularly in combination significantly reduced the toxic effects of Pb on maize. At higher Pb level (500 mg kg^-1), the combined application effectively reduced Pb uptake up to 42.4% and 50% by shoots, 30.8% and 33.9% by roots, and 18.4% and 26.2% in available Pb content in soil after 45 days and 90 days, respectively compared with that of control. Moreover, the use of Bacillus sp. MN-54 significantly improved the P uptake by maize plants by 44.4% as compared with that of control. Our findings suggest that the combined use of Bacillus sp. MN-54 and P could be effective and helpful in improving plant growth and Pb immobilization in Pb-contaminated soil.

Poly-γ-glutamic acid-producing bacteria reduced Cd uptake and effected the rhizosphere microbial communities of lettuce

Poly-γ-glutamic acid (γ-PGA) could efficiently stabilize heavy metals in the environment. This study characterized the effects of two plant growth-promoting and γ-PGA-producing bacteria Bacillus subtilis W7 and Bacillus amyloliquefaciens W25 on Cd immobilization and γ-PGA production in soil filtrate and on the biomass and Cd uptake by lettuce in Cd-contaminated soil, the impact of these strains on the rhizosphere soil bacterial community was also evaluated. The strains reduced Cd concentration (16-75 %) in soil filtrate and strain W25 had a higher ability of producing γ-PGA and immobilizing Cd than strain W7. Compared with the control, the strains significantly increased the biomass (41-85 %) and reduced Cd uptake (19-41 %) by lettuce, reduced available Cd content (25-37 %) and increased the relative abundance of γ-PGA-producing bacteria (24-30 %) in Cd-contaminated soil, among which the effects of strain W25 were better than that of strain W7. Besides, these isolates also increased soil pH value, urease activity and the relative abundance of plant growth-promoting and metal-immobilizing bacteria such as Sphingomonas and Bacillus. In summary, the two strains reduced soil available Cd and lettuce Cd uptake by increasing the pH value, urease activity and the abundance of γ-PGA-producing bacteria, and regulating bacterial community structure in rhizosphere soil.