Arsenite-oxidizing bacteria exhibiting plant growth promoting traits isolated from the rhizosphere of Oryza sativa L.: Implications for mitigation of arsenic contamination in paddies
Introduction
The indiscriminate use of arsenic (As)-contaminated groundwater for irrigation of agricultural land is a pressing issue of current public health globally. This is because the incorporation of this toxic metalloid into the food chain poses long term risks to human health [1]. In addition to human health impacts caused by ingestion of As-contaminated food, the potential for reduced crop yield due to the accumulation of this toxic metalloid in soil is alarming [2]. Mitigation of As in the food chain has thus received worldwide attention in recent years.
Arsenic accumulation in rice has been recognized as a catastrophe in southeast Asian countries, where rice is the daily staple food [3]. Compared to other cereals, rice is particularly susceptible to As accumulation because it is generally grown under submerged conditions where As mobility is high [4]. Growing rice is mainly dependent on groundwater used for irrigation. In southwest Taiwan, the incidence of elevated concentrations of As in groundwater is known for generating unique cases of endemic Blackfoot disease (BFD), a peripheral vascular disease (i.e., gangrene) [5], which is caused by drinking As-contaminated groundwater. Although the groundwater in the BFD area has not been utilized for drinking since 1990, it is still extensively used for irrigation of croplands, specifically paddies, resulting in the incorporation of this carcinogenic element into the food chain. Research that ensures a safe level of As in rice for consumption is currently very active and includes soil and water management, rhizosphere manipulation, breeding and genetic engineering [3]. Currently, the implementation of As-resistant and/or arsenite [As(III)]-oxidizing bacteria exhibiting PGP traits to lower As toxicity and uptake and support plant growth on As-stressed soils has gained significant attention because compared to other techniques, it is cost-effective and eco-friendly [6].
The fate and behavior of As in paddy soil depends largely on its speciation [4]. In well-aerated soils, arsenate [As(V)] is in a stable form since it is strongly adsorbed on metal (hydr) oxides [7]. Under reducing conditions e.g., in flooded paddy soil, As(V) is reduced to As(III) and becomes the dominant form in the flooded soil [4]. This promotes the incorporation of As(III) into rice since the dominant species taken up by rice roots is As(III) [7]. Oxidation of As(III) leads to the formation of the less toxic and less bioavailable As(V), which has much higher affinity for iron oxides as compared to As(III). This makes As(V) much less mobile than As(III) in soil [7]. It is assumed that the activity of As(III)-oxidizing bacteria may be elevated by root exudates and/or the oxygen released from the rice roots, resulting in more As(V) binding on iron minerals in soil and iron plaque on rice roots. These processes reduce As bioavailability and uptake in rice [1], [3]. In addition, As(III)-oxidation represents a possible means of As detoxification, and As(III)-oxidizing bacteria thus have a vital role in natural As-remediation processes in the environment [8]. Rice grown in As-contaminated soil harbors unique As-resistant micro-flora in the rhizosphere. Such micro-flora may have the advantage of secreting plant growth promoting substances like siderophores, indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase and solubilize phosphate to alleviate As-toxicity in rice [9], [10]. The significance of As(III)-oxidizing bacteria exhibiting potential PGP traits can be enormous in As-contaminated paddy soils since these bacteria are able to oxidize the more toxic and more bioavailable As(III) to less toxic and less bioavailable As(V) and are likely to reduce As uptake by rice, increase the tolerance of rice plants against As-stress, stimulate plant growth and contribute in this way to combating a reduction in crop yield in As-contaminated paddy soils.
Although there have been reports on As-resistant/transforming bacteria exhibiting potential PGP traits, the reports on As(III)-oxidizing bacteria exhibiting potential PGP traits and their implementation in soil conditions to evaluate their role in rice plant growth and reduction in As contamination in rice grain remain very elusive (supplementary Table S1). Noteworthy, the implementation of such novel As(III)-oxidizing plant growth promoting bacteria (PGPB) in soil conditions is necessary to get mechanistic interpretation (i.e., the role of this strain on plant growth and As tolerance) and the role of the strain in plant-soil relationships in As-stressed soil [9], [10]. In this study we report (i) isolation and identification of a novel As(III)-oxidizing bacterium from the rhizosphere of rice, (ii) its level of resistance to As(III) and As(V), (iii) detection of aoxB gene and its phylogenetic affirmation, (iv) exhibition of As(III)-oxidase enzyme activity, (v) kinetics of As(III)-oxidation, (vi) quantitative determination of potential PGP traits and (vii) effect of inoculation of As(III)-oxidizing bacteria exhibiting potential PGP traits on plant growth parameters, and As distribution and accumulation in Oryza sativa L.
Section snippets
Sampling and chemical analysis of rhizosphere soil and groundwater
The rhizosphere soil samples at the flowering stage (58 days after transplanting (DAT)) of rice (Oryza sativa L.) grown under the same treatment conditions (i.e., same cultivar and same doses of fertilizer application) and irrigated with As-rich groundwater were collected from three different locations in Hsuechia (23°12′4.02″N and 120°10′50.7″E), Tainan City, southwestern Taiwan. Groundwater used to irrigate rice fields in Hsuechia was also collected.
For the soil chemical analysis, the
Isolation, identification and characterization of the strain ASO-6
Seventy-two bacterial colonies grown on media containing As(III) were isolated from the rhizosphere of rice. Selection of the flowering stage for rhizosphere soil collection is based on the fact that it was considered a key point of time for crop-induced changes in the soil microbial community since root growth and root exudation of rice reaches its peak near the flowering stage [29]. Among the bacterial isolates, six isolates screened positive for an As(III)-oxidation reaction by the silver
Conclusions
The strain B. flexus ASO-6 isolated from rhizosphere of rice was resistant to high levels of As, exhibited elevated rates of As(III) oxidation and had intrinsic ability to produce IAA, ACC-diaminase, siderophores and to solubulize phosphate. The inoculation of rice plants with this strain significantly increased root biomass, straw yield, grain yield, chlorophyll and the carotenoid of the plants. Moreover, As uptake from root to shoot and As accumulation in straw and grain decreased
Acknowledgements
This work was supported by the National Science Council of Taiwan (NSC 103-2116-M-006 -010).
References (39)
- et al.
Arsenic in the environment: biology and chemistry
Sci. Total Environ.
(2007) - et al.
Exposure to inorganic arsenic from rice: a global health issue?
Environ. Pollut.
(2008) - et al.
Influence of inoculation of arsenic-resistant Staphylococcus arlettae on growth and arsenic uptake in Brassica juncea (L.) Czern. Var. R-46
J. Hazard. Mater.
(2013) - et al.
Depth-resolved abundance and diversity of arsenite-oxidizing bacteria in the groundwater of Beimen, a blackfoot disease endemic area of southwestern Taiwan
Water Res.
(2013) - et al.
Screening of plant growth-promoting traits in arsenic-resistant bacteria isolated from agricultural soil and their potential implication for arsenic bioremediation
J. Haz. Mater.
(2014) - et al.
Current statistical methods for estimating the Km and Vmax of Michaelis–Menten kinetics
Biochem. Edu.
(1996) - et al.
The purification and characterization of As(III) oxidase from Alcaligenes faecalis, a molybdenum containing hydroxylase
J. Biol. Chem.
(1992) - et al.
Significance of Bacillus subtilis strain SJ-101 as a bioinoculant for concurrent plant growth promotion and nickel accumulation in Brassica juncea
Chemosphere
(2006) - et al.
Universal chemical assay for detection and determination of siderophores
Anal. Biochem.
(1987) - et al.
Effect of multiple metal resistant bacteria from contaminated lake sediments on metal accumulation and plant growth
J. Hazard. Mater.
(2011)
Effects of inoculation of plant-growth promoting bacteria on Ni uptake by Indian mustard
Bioresour. Technol.
Arsenic-tolerant, arsenite-oxidising bacterial strains in the contaminated soils of West Bengal, India
Sci. Total Environ
Kinetics of arsenite oxidation by Variovorax sp. MM-1 isolated from a soil and identification of arsenite oxidase gene
J. Hazard. Mater.
The potential contribution of plant growth-promoting bacteria to reduce environmental degradation—a comprehensive evaluation
Appl. Soil Ecol.
Using soil bacteria to facilitate phytoremediation
Biotechnol. Adv.
Arsenic as a food chain contaminant: mechanisms of plant uptake and metabolism and mitigation strategies
Annu. Rev. Plant Biol.
Arsenic behavior in paddy fields during the cycle of flooded and non-flooded periods
Environ. Sci. Technol.
Effects and dose-response relationships of skin cancer and blackfoot disease with arsenic
Environ. Health Perspect.
Direct evidence showing the effect of root surface iron plaque on arsenite and arsenate uptake into rice (Oryza sativa) roots
New Phytol.
Cited by (71)
Beyond contamination: Enhancing plant tolerance to arsenic through phytobial remediation
2024, South African Journal of BotanyPseudomonas putida and salicylic acid key players: Impact on arsenic phytotoxicity of quinoa under soil salinity stress
2023, Biocatalysis and Agricultural BiotechnologyIonome profiling and arsenic speciation provide evidence of arsenite detoxification in rice by phosphate and arsenite-oxidizing bacteria
2023, Journal of Environmental Sciences (China)A combined study on Vallisneria spiralis and lanthanum modified bentonite to immobilize arsenic in sediments
2023, Environmental Research