Selenate Reduction and Selenium Enrichment of Tea by the Endophytic Herbaspirillum sp. Strain WT00C

Significance of Herbaspirillum sp. in biodegradation and biodetoxification of herbicides, pesticides, hydrocarbons and heavy metals - A review

In today's industrialized world, contamination of soil and water with various substances has emerged as a pressing concern. Bioremediation, with its advantages of degradation or detoxification, non-polluting nature, and cost-effectiveness, has become a promising method due to technological advancements. Among the bioremediation agents, bacteria have been highly explored and documented as a productive organism. Recently, few studies have reported on the significance of Herbaspirillum sp., a Gram-negative bacterium, in bioremediating herbicides, pesticides, polycyclic aromatic hydrocarbons, metalloids, and heavy metals, as well as its role in augmenting phytoremediation efforts. Herbaspirillum sp. GW103 leached 66% of Cu from ore materials and significantly enhanced the phytoaccumulation of Pb and Zn in plumule and radical tissues of Zea mays L. plants. Additionally, Herbaspirillum sp. WT00C reduced Se6+ into Se0, resulting in an increased Se0 content in tea plants. Also, Herbaspirillum sp. proved effective in degrading 0.6 mM of 4-chlorophenol, 92.8% of pyrene, 77.4% of fluoranthene, and 16.4% of trifluralin from aqueous solution and soil-water system. Considering these findings, this review underscores the need for further exploration into the pathways of pollutant degradation, the enzymes pivotal in the degradation or detoxification processes, the influence of abiotic factors and pollutants on crucial gene expression, and the potential toxicity of intermediate products generated during the degradation process. This perspective reframes the numerical data to underscore the underutilized potential of Herbaspirillum sp. within the broader context of addressing a significant research gap. This shift in emphasis aligns more closely with the problem-necessity for solution-existing unexplored solution framework.

A novel selenite-tolerant rhizosphere bacterium Wautersiella enshiensis sp. nov., isolated from Chinese selenium hyperaccumulator, Cardamine hupingshanensis

Selenium (Se) is a dietary essential trace element for humans with various physiological functions and it could also be accumulated by some plant species, like Astragalus bisulcatus, Stanleya pinnata, and Cardamine hupinshanensis. A novel Gram-stain-negative, facultatively anaerobic, selenite-tolerant bacterium, designated strain YLX-1T , was isolated from the rhizosphere of a Se hyperaccumulating plant, Cardamine hupingshanensis in Enshi, China. Phylogenetic analysis based on 16 S rRNA gene sequences indicated that strain YLX-1T is a potential new species in the genus Wautersiella. Strain YLX-1T could grow in the temperature range of 4-37°C (optimally at 28°C) and in the pH range of 5-9 (optimum pH 7), which also could tolerate Se up to 6000 mg Se/L via producing extracellular red nano-Se with 100-300 nm size. However, it could predominantly accumulate selenocystine (SeCys2 ) in the cell under lower Se stress (1.5 mg Se/L). These results would help broaden our knowledge about the Se accumulation and transformation mechanism involved in rhizosphere bacteria like strain YLX-1T in C. hupingshanensis. Based on polyphasic data, we propose the creation of the new species Wautersiella enshiensis sp. nov., strain YLX-1T ( = CCTCC M 2013671) which will be promising to produce nano-Se as fertilizer, food additives or medicine.

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.

Selenium species transforming along soil-plant continuum and their beneficial roles for horticultural crops

Selenium (Se) acquirement from daily diet can help reduce the risk of many diseases. The edible parts of crop plants are the main source of dietary Se, while the Se content in crops is determined by Se bioavailability in soil. We summarize recent research on the biogeochemical cycle of Se driven by specific microorganisms and emphasize the oxidizing process in the Se cycle. Moreover, we discuss how plant root exudates and rhizosphere microorganisms affect soil Se availability. Finally, we cover beneficial microorganisms, including endophytes, that promote crop quality and improve crop tolerance to environmental stresses. Se availability to plants depends on the balance between adsorption and desorption, reduction, methylation and oxidation, which are determined by interactions among soil properties, microbial communities and plants. Reduction and methylation processes governed by bacteria or fungi lead to declined Se availability, while Se oxidation regulated by Se-oxidizing microorganisms increases Se availability to plants. Despite a much lower rate of Se oxidization compared to reduction and methylation, the potential roles of microbial communities in increasing Se bioavailability are probably largely underestimated. Enhancing Se oxidation and Se desorption are crucial for the promotion of Se bioavailability and uptake, particularly in Se-deficient soils. Beneficial roles of Se are reported in terms of improved crop growth and quality, and enhanced protection against fungal diseases and abiotic stress through improved photosynthetic traits, increased sugar and amino acid contents, and promoted defense systems. Understanding Se transformation along the plant-soil continuum is crucial for agricultural production and even for human health.

The molecular mechanisms of quality difference for Alpine Qingming green tea and Guyu green tea by integrating multi-omics

Introduction

Harvest time represents one of the crucial factors concerning the quality of alpine green tea. At present, the mechanisms of the tea quality changing with harvest time have been unrevealed.

Methods

In the current study, fresh tea leaves (qmlc and gylc) and processed leaves (qmgc and gygc) picked during Qingming Festival and Guyu Festival were analyzed by means of sensory evaluation, metabolomics, transcriptomic analysis, and high-throughput sequencing, as well as their endophytic bacteria (qm16s and gy16s).

Results

The results indicated qmgc possessed higher sensory quality than gygc which reflected from higher relative contents of amino acids, and soluble sugars but lower relative contents of catechins, theaflavins, and flavonols. These differential metabolites created features of light green color, prominent freshness, sweet aftertaste, and mild bitterness for qmgc.

Discussion

Flavone and flavonol biosynthesis and phenylalanine metabolism were uncovered as the key pathways to differentiate the quality of qmgc and gygc. Endophytic bacteria in leaves further influence the quality by regulating the growth of tea trees and enhancing their disease resistance. Our findings threw some new clues on the tea leaves picking to pursue the balance when facing the conflicts of product quality and economic benefits.

Effect of Sodium Selenite on the Metabolite Profile of Epichloë sp. Mycelia from Festuca sinensis in Solid Culture

Selenium (Se) is an essential micronutrient with many beneficial effects for humans and other living organisms. Numerous microorganisms in culture systems enrich and convert inorganic selenium to organic selenium. In this study, Epichloë sp. from Festuca sinensis was exposed to increasing Na2SeO3 concentrations (0, 0.1, 0.2, 0.3, and 0.4 mmol/L) in Petri dishes with potato dextrose agar (PDA) for 8 weeks. Epichloë sp. mycelia were immediately collected after mycelial diameters were measured at 4, 5, 6, 7, and 8 weeks of cultivation, respectively. Gas chromatography-mass spectrometer (GC-MS) analysis was performed on different groups of Epichloë sp. mycelia. Different changes were observed as Epichloë sp. was exposed to different selenite conditions and cultivation time. The colony diameter of Epichloë sp. decreased in response to increased selenite concentrations, whereas the inhibitory effects diminished over time. Seventy-two of the 203 identified metabolites did not differ significantly across selenite treatments within the same time point, while 82 compounds did not differ significantly between multiple time points of the same Se concentration. However, the relative levels of 122 metabolites increased the most under selenite conditions. Specifically, between the 4th and 8th weeks, there were increases in 2-keto-isovaleric acid, uridine, and maltose in selenite treatments compared to controls. Selenium increased glutathione levels and exhibited antioxidant properties in weeks 4, 5, and 7. Additionally, we observed that different doses of selenite could promote the production of carbohydrates such as isomaltose, cellobiose, and sucrose; fatty acids such as palmitoleic acid, palmitic acid, and stearic acid; and amino acids such as lysine and tyrosine in Epichloë sp. mycelia. Therefore, Epichloë sp. exposed to selenite stress may benefit from increased levels of some metabolite compounds.

Polysaccharides in Selenium-Enriched Tea: Extraction Performance under Innovative Technologies and Antioxidant Activities

Pulsed electric fields (PEF) and ultrasonic-assisted extraction (UE) were applied to improve the extraction performance of selenium-enriched tea polysaccharides (Se−TPSs) in mild conditions. Two combined extraction processes were investigated: (1) PEF strength at 10 kV/cm followed by conventional extraction (CE) at 50 °C for 60 min and (2) PEF+UE (PEF strength at 10 kV/cm followed by UE at 400 W for 60 min). The optimal extraction yields, and energy consumption rates were obtained at 36.86% and 41.53% and 78.78 kJ/mg and 133.91 kJ/mg, respectively. The Se−TPSs were analyzed and characterized by GPC, UV, and FT-IR, which evidenced the structural stability of the Se−TPSs during the extraction processes. It was found that PEF and UE could reduce the particle size diameter of the Se−TPS extract, as well as the proportion of uronic acid. Moreover, PEF could increase the selenium content in the Se−TPS extract by 160.14% due to a lower extraction temperature compared to conventional extraction. The antioxidant activities of the Se−TPSs in vitro were investigated using OH, O2−, and ABTS+ scavenging experiments, as well as a total antioxidant ability evaluation. It was found that the antioxidant activity of the Se−TPSs obtained using PEF₂+CE₂ was relatively high due to the potential synergistic effect between the selenium and polysaccharides. Based on these results, we speculate that PEF₂+CE₂ was the best extraction process for the Se−TPSs. Furthermore, this research indicates the application of selenium-enriched tea for functional food production.

Plant Growth-Promoting Activity of Herbaspirillum aquaticum ZXN111 on the Zijuan Tea Plant (Camellia sinensis var. assamica)

Herbaspirillum aquaticum ZXN111 which was isolated from the tea plant Zijuan can produce indole-3-acetic acid (IAA) and contain abiotic-stress tolerance gene 1-aminocyclopropane-1-carboxylate deaminase (accd). In this study, ZXN111 PGP activity and the molecular mechanism were investigated. The result showed that ACCD activity of wild-type ZXN111 was 0.4505 mM α-KB/mg·Pro·h, but mutants Δacc and Δacc-tyrb did not showed ACCD activity. IAA production by ZXN111 within 48 hrs was 20.4 μg/mL, while mutants of Δtyrb and Δacc-tyrb were lower than 3.6 μg/mL, indicating that indole-3-pyruvic acid is the primary IAA synthesis pathway. Potting tests found that ZXN111 displayed significant PGP activity to the tea plant Zijuan, but Δtyrb and Δacc-tyrb did not show PGP activity, indicating that IAA is critical to PGP activity. In a salt-stress test, ZXN111 did not enhance the tea plant NaCl tolerance by gene accd. The results of this study indicated that strain ZXN111 has potential for biofertilizer development on tea plantation.

Microbial reduction and resistance to selenium: Mechanisms, applications and prospects

Selenium is an essential trace element for humans, animals and microorganisms. Microbial transformations, in particular, selenium dissimilatory reduction and bioremediation applications have received increasing attention in recent years. This review focuses on multiple Se-reducing pathways under anaerobic and aerobic conditions, and the phylogenetic clustering of selenium reducing enzymes that are involved in these processes. It is emphasized that a selenium reductase may have more than one metabolic function, meanwhile, there are several Se(VI) and/or Se(IV) reduction pathways in a bacterial strain. It is noted that Se(IV)-reducing efficiency is inconsistent with Se(IV) resistance in bacteria. Moreover, we discussed the links of selenium transformations to biogeochemical cycling of other elements, roles of Se-reducing bacteria in soil, plant and digestion system, and the possibility of using functional genes involved in Se transformation as biomarker in different environments. In addition, we point out the gaps and perspectives both on Se transformation mechanisms and applications in terms of bioremediation, Se fortification or dietary supplementation.

Multiple exposures to high concentrations of selenate significantly improve selenate tolerability, red elemental selenium (Se0) and selenoprotein biosynthesis in Herbaspirillum camelliae WT00C

Herbaspirillum camelliae WT00C is a gram-negative endophyte isolated from the tea plant. It has an intact selenate metabolism pathway but poor selenate tolerability. In this study, microbiological properties of the strain WT00C were examined and compared with other three strains CT00C, NCT00C and NT00C, which were obtained respectively from four, six and eight rounds of 24-h exposures to 200 mM selenate. The selenate tolerability and the ability to generate red elemental selenium (Se0) and selenoproteins in H. camelliae WT00C has significantly improved by the forced evolution via 4-6 rounds of multiple exposures a high concentration of selenate. The original strain WT00C grew in 200 mM selenate with the lag phase of 12 h and 400 mM selenate with the lag phase of 60 h, whereas the strains CT00C and NCT00C grew in 800 mM selenate and showed a relatively short lag phase when they grew in 50-400 mM selenate. Besides selenate tolerance, the strains CT00C and NCT00C significantly improved the biosynthesis of red elemental selenium (Se⁰) and selenoproteins. Two strains exhibited more than 30% selenium conversion efficiency and 40% selenoprotein biosynthesis, compared to the original strain WT00C. These characteristics of the strains CT00C and NCT00C make them applicable in pharmaceuticals and feed industries. The strain NT00C obtained from eight rounds of 24-h exposures to 200 mM selenate was unable to grow in ≥ 400 mM selenate. Its selenium conversion efficiency and selenoprotein biosynthesis were similar to the strain WT00C, indicating that too many exposures may cause gene inactivation of some critical enzymes involving selenate metabolism and antioxidative stress. In addition, bacterial cells underwent obviously physiological and morphological changes, including gene activity, cell enlargement and surface-roughness alterations during the process of multiple exposures to high concentrations of selenate.

Degradation of epigallocatechin and epicatechin gallates by a novel tannase TanHcw from Herbaspirillum camelliae

Background

Herbaspirillum camelliae is a gram-negative endophyte isolated from the tea plant. Both strains WT00C and WT00F were found to hydrolyze epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate (ECG) to release gallic acid (GA) and display tannase activity. However, no tannase gene was annotated in the genome of H. camelliae WT00C.

Results

The 39 kDa protein, annotated as the prolyl oligopeptidase in the NCBI database, was finally identified as a novel tannase. Its gene was cloned, and the enzyme was expressed in E. coli and purified to homogeneity. Moreover, enzymatic characterizations of this novel tannase named Tan_Hcw were studied. Tan_Hcw was a secretary enzyme with a Sec/SPI signal peptide of 48 amino acids at the N-terminus, and it catalyzed the degradation of tannin, methyl gallate (MG), epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate (ECG). The optimal temperature and pH of Tan_Hcw activities were 30 °C, pH 6.0 for MG and 40 °C, pH 7.0 for both EGCG and ECG. Na⁺, K⁺ Mn²⁺ and Triton-X100, Tween80 increased the enzyme activity of Tan_Hcw, whereas Zn²⁺, Mg²⁺, Hg²⁺, EMSO, EDTA and β-mercaptoethanol inhibited enzyme activity. K_m, k_cat and k_cat /K_m of Tan_Hcw were 0.30 mM, 37.84 s⁻¹, 130.67 mM⁻¹ s⁻¹ for EGCG, 0.33 mM, 34.59 s⁻¹, 105.01 mM⁻¹ s⁻¹ for ECG and 0.82 mM, 14.64 s⁻¹, 18.17 mM⁻¹ s⁻¹ for MG, respectively.

Conclusion

A novel tannase Tan_Hcw from H. camelliae has been identified and characterized. The biological properties of Tan_Hcw suggest that it plays a crucial role in the specific colonization of H. camelliae in tea plants. Discovery of the tannase Tan_Hcw in this study gives us a reasonable explanation for the host specificity of H. camelliae. In addition, studying the characteristics of this enzyme offers the possibility of further defining its potential in industrial application.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01685-1.

Microbes: a potential tool for selenium biofortification

Selenium (Se) is a component of many enzymes and indispensable for human health due to its characteristics of reducing oxidative stress and enhancing immunity. Human beings take Se mainly from Se-containing crops. Taking measures to biofortify crops with Se may lead to improved public health. Se accumulation in plants mainly depends on the content and bioavailability of Se in soil. Beneficial microbes may change the chemical form and bioavailability of Se. This review highlights the potential role of microbes in promoting Se uptake and accumulation in crops and the related mechanisms. The potential approaches of microbial enhancement of Se biofortification can be summarized in the following four aspects: (1) microbes alter soil properties and impact the redox chemistry of Se to improve the bioavailability of Se in soil; (2) beneficial microbes regulate root morphology and stimulate the development of plants through the release of certain secretions, facilitating Se uptake in plants; (3) microbes upregulate the expression of certain genes and proteins that are related to Se metabolism in plants; and (4) the inoculation of microbes give rise to the generation of certain metabolites in plants contributing to Se absorption. Considering the ecological safety and economic feasibility, microbial enhancement is a potential tool for Se biofortification. For further study, the recombination and establishment of synthesis microbes is of potential benefit in Se-enrichment agriculture.

Biotransformation ability of endophytic fungi: from species evolution to industrial applications

Abstract

Increased understanding of the interactions between endophytic fungi and plants has led to the discovery of a new generation of chemical compounds and processes between endophytic fungi and plants. Due to the long-term co-evolution between fungal endophytes and host plants, endophytes have evolved special biotransformation abilities, which can have critical consequences on plant metabolic processes and their composition. Biotransformation or bioconversion can impact the synthesis and decomposition of hormones, sugars, amino acids, vitamins, lipids, proteins, and various secondary metabolites, including flavonoids, polysaccharides, and terpenes. Endophytic fungi produce enzymes and various bioactive secondary metabolites with industrial value and can degrade or sequester inorganic and organic small molecules and macromolecules (e.g., toxins, pollutants, heavy metals). These fungi also have the ability to cause highly selective catalytic conversion of high-value compounds in an environmentally friendly manner, which can be important for the production/innovation of bioactive molecules, food and nutrition, agriculture, and environment. This work mainly summarized recent research progress in this field, providing a reference for further research and application of fungal endophytes.

Key points

•The industrial value of degradation of endophytes was summarized.

• The commercial value for the pharmaceutical industry is reviewed.

Graphical abstract

Two new selenite reducing bacterial isolates from paddy soil and the potential Se biofortification of paddy rice

Selenium (Se) is an essential element for human health. Se-enriched agricultural products can promote people's intake of Se. Microorganisms play an important role in Se cycling. In this study, two new bacterial strains were isolated from paddy soil and were identified as Chitinophaga sp. and Comamonas testosteroni, respectively. More than 44% and 39% of 1.0 mM selenite were reduced in 84 h by them using yeast extract as carbon source, respectively. Scanning electron microscope (SEM) and Energy dispersive X-ray spectrometry (EDS) results indicated that the reduction product of selenite was nanometer Se. These strains could promote the available Se in soil and the content of Se in rice plants in pot experiments. Organic combined Se in soils was increased up to 35%, accompanied by the 92% and 130% increase of Se in rice plants. To our best knowledge, this is the first report of Se reduction by Chitinophaga. This work might provide a prospective strategy for microbial fortification of Se in corps.

Red elemental selenium (Se0 ) improves the immunoactivities of EPC cells, crucian carp and zebrafish against spring viraemia of carp virus

Selenium, as an essential trace element, interferes through selenoproteins in many physiological processes of plants and mammals. Its antiviral activity has recently attracted much attention because selenium improves the antiviral capacity of animal cells against a few viruses relevant to human diseases. In this study, the red elemental selenium was purified from the fermentative culture of Herbaspirillum camelliae WT00C and then used to culture epithelioma papulosum cyprinid (EPC) cells or feed crucian carp and zebrafish. Finally, its antiviral effects were investigated at the cell level and living fishes after spring viraemia of carp virus infection. At the cell level, 5, 10 and 20 μg ml^-1 red elemental selenium significantly induced the expression of interferon (IFN) and ISG15 genes in EPC cells. The viral TCID₅₀ (50% tissue culture infective dose) values in the EPC cells incubated with 5, 10 and 20 μg ml^-1 red elemental selenium were significantly less than those of the control. More expression of IFN and ISG15 genes and less TCID₅₀ values indicate that red elemental selenium indeed improves the antiviral capability of EPC cells. In the crucian carp fed with the food containing 5 and 10 μg g^-1 red elemental selenium, IFN expressions showed 13- and 39-fold increases at the 16th day of post-injection, and its expression was dependent on selenium concentrations. Meanwhile, no fish death occurred in all the experimental groups. In the zebrafish fed with the red worm containing 5 μg g^-1 red elemental selenium, IFN and Mx expressions and survival rate were significantly higher than those of the control. The results of this study show that red elemental selenium indeed improves the antiviral activity of fish. The antiviral effects of selenium mainly come from its immune regulation through its incorporation into selenoproteins. The optimum level of selenium contributes to improving fish immunity, whereas excess selenium causes excessive immune and inflammatory responses.

Developments in the study and applications of bacterial transformations of selenium species

Microbial bio-transformations of the essential trace element selenium are now recognized to occur among a wide variety of microorganisms. These transformations are used to convert this element into its assimilated form of selenocysteine, which is at the active center of a number of key enzymes, and to produce selenium nanoparticles, quantum dots, metal selenides, and methylated selenium species that are indispensable for biotechnological and bioremediation applications. The focus of this review is to present the state-of-the-art of all aspects of the investigations into the bacterial transformations of selenium species, and to consider the characterization and biotechnological uses of these transformations and their products.