Improving soybean growth under arsenic stress by inoculation with native arsenic-resistant bacteria

Characterization and identification of Pseudomonas sp. AW4, an arsenic-resistant and plant growth-promoting bacteria isolated from the soybean (Glycine max L.) rhizosphere

Pseudomonas sp. AW4 is a highly arsenic (As) resistant bacterium with plant growth promoting properties, originally isolated from the soybean (Glycine max L.) rhizosphere. In order to safely use this isolate in diverse bioformulations, its characterization needs to be completed and a reliable identification must be provided. In the present work, we analyzed the morpho-physiological, biochemical and genomic characteristics of Pseudomonas sp. AW4. Identification of the isolate varied according to the parameters analyzed, mainly biochemical and physiological tests or individual genes and phylogenetic analyses. In this regard, we performed massive sequencing of its genome, in order to consistently complete its characterization and identification. Pseudomonas sp. AW4 formed a monophyletic clade with P. urmiensis SWRI10, presenting 3.08 % of unique genes against this reference isolate. More than 70 % of AW4 genes were also shared with P. oryziphila strain 1257 NZ and with P. reidholzensis strain CCOS 865. The search for genes related to As resistance evidenced the presence of the operon arsHRBC. Taken together, results of the present work allow identification of this bacterium as Pseudomonas urmiensis AW4 and open up a number of opportunities to study this strain and understand the mechanisms of arsenic resistance and plant growth promotion.

Evaluation of the molecular mechanism underlying proline metabolic and catabolic pathways and some morpho-physiological traits of tobacco (Nicotiana tabacum L.) plants under arsenic stress

BACKGROUND

In recent decades, arsenic (As) toxicity has emerged as a significant challenge in many countries. It not only reduces the growth and performance of plants, but also poses a threat to human health. The synthesis of compatible solutes, particularly proline, is a mechanism plants utilize to cope with stress. Investigating the metabolic pathways of proline would deepen our understanding for future molecular breeding or genetic engineering efforts. Therefore, the aim of this study was to explore the metabolic and catabolic pathways of proline, as well as the morpho-physiological traits of tobacco, under As stress.

RESULTS

The results revealed a significant decrease in morphological traits and photosynthetic efficiency, chlorophyll content, and total soluble protein content with increasing As concentration. The results also showed that proline content, total soluble carbohydrates, hydrogen peroxide, and malondialdehyde, as well as the activity of two antioxidant enzymes, superoxide dismutase and ascorbate peroxidase, increased with increasing As concentration. At 10 mg As Kg-1 soil, the expression of Δ1-pyrroline-carboxylate synthetase (P5CS) and P5C reductase (P5CR) genes was not different from the control, but their expression increased significantly at 20 and 40 mg As Kg-1 soil. At 10 mg As Kg-1 soil, the expression of proline dehydrogenase (PDH) and P5C dehydrogenase (P5CDH) genes decreased sharply compared to the control but remained unchanged at 20 and 40 mg As Kg-1 soil. At 10 and 20 mg As Kg-1 soil, expression of the ornithine δ-aminotransferase (OAT) gene was unchanged compared to the control, but at 40 mg As Kg-1 soil, it increased sharply.

CONCLUSION

The results showed that the accumulation of proline at the lowest (10 mg As Kg-1 soil) tested As concentration was due to a decrease in the expression of proline catabolic genes (PDH and P5CDH), while the genes involved in proline synthesis did not play a role. At 20 mg As Kg-1 soil, proline accumulation was caused by the increased expression of genes (P5CS and P5CR) involved in the glutamate pathway of proline synthesis. Additionally, at the highest concentration of arsenic (40 mg As Kg-1 soil), the OAT gene, which is active in the ornithine pathway, was also involved in proline synthesis, along with the P5CS and P5CR genes.

Assessing the impact of arsenic on symbiotic and free-living PGPB: plant growth promoting traits, bacterial compatibility and adhesion on soybean seed

Arsenic (As) contamination in agricultural groundwater and soil is a significant economic and health problem worldwide. It inhibits soybean (Glycine max (L.) Merr.) nodulation and biological nitrogen fixation in symbiosis with Bradyrhizobium japonicum E109 (E109), a commonly used rhizobial strain for commercial biofertiliser formulation in Argentina. In the context of sustainable and climate-smart agriculture promoted by FAO, co-inoculating legumes with As-tolerant plant growth-promoting bacteria (PGPB) is suggested as a superior alternative to single inoculation. This study aimed to evaluate the impact of As on plant growth-promoting (PGP) traits -siderophore and indole acetic acid production, phosphate solubilisation, diazotrophic activity and hydrolytic enzymes activity- in E109 and three other PGPB strains: Pseudomonas sp. AW4 (AW4), Bacillus pumilus SF5 (SF5) and Bacillus toyonensis SFC 500-1E (Bt). In addition, bacterial compatibility and adhesion on soybean seed were evaluated. Arsenic significantly reduced PGP traits of E109 even at low concentrations, AW4's traits remained unchanged, while those of SF5 and Bt traits were affected (positively or negatively) only at the highest concentrations tested (500 µM arsenate, 250 µM arsenite). All PGPB strains were compatible with E109 under both control and As-stress conditions. Soybean seed adhesion was reduced for E109, only under As stress. Findings suggest that the effect of As on PGP traits is highly strain-dependent and influenced by As concentration and speciation. AW4, SF5, and Bt strains show promise for co-inoculation with E109 in soybean cultivation.

Microbial remediation and plant-microbe interaction under arsenic pollution

Arsenic contamination in aquatic and terrestrial ecosystem is a serious environmental issue. Both natural and anthropogenic processes can introduce it into the environment. The speciation of the As determine the level of its toxicity. Among the four oxidation states of As (-3, 0, +3, and + 5), As(III) and As(V) are the common species found in the environment, As(III) being the more toxic with adverse impact on the plants and animals including human health. Therefore, it is very necessary to remediate arsenic from the polluted water and soil. Different physicochemical as well as biological strategies can be used for the amelioration of arsenic polluted soil. Among the microbial approaches, oxidation of arsenite, methylation of arsenic, biosorption, bioprecipitation and bioaccumulation are the promising transformation activities in arsenic remediation. The purpose of this review is to discuss the significance of the microorganisms in As toxicity amelioration in soil, factors affecting the microbial remediation, interaction of the plants with As resistant bacteria, and the effect of microorganisms on plant arsenic tolerance mechanism. In addition, the exploration of genetic engineering of the bacteria has a huge importance in bioremediation strategies, as the engineered microbes are more potent in terms of remediation activity along with quick adaptively in As polluted sites.

Leveraging arsenic resistant plant growth-promoting rhizobacteria for arsenic abatement in crops

Arsenic is a toxic metalloid categorized under class 1 carcinogen and is detrimental to both plants and animals. Agricultural land in several countries is contaminated with arsenic, resulting in its accumulation in food grains. Increasing global food demand has made it essential to explore neglected lands like arsenic-contaminated lands for crop production. This has posed a severe threat to both food safety and security. Exploration of arsenic-resistant plant growth-promoting rhizobacteria (PGPR) is an environment-friendly approach that holds promise for both plant growth promotion and arsenic amelioration in food grains. However, their real-time performance is dependent upon several biotic and abiotic factors. Therefore, a detailed analysis of associated mechanisms and constraints becomes inevitable to explore the full potential of available arsenic-resistant PGPR germplasm. Authors in this review have highlighted the role and constraints of arsenic-resistant PGPR in reducing the arsenic toxicity in food crops, besides providing the details of arsenic transport in food grains.

Arsenic acquisition, toxicity and tolerance in plants - From physiology to remediation: A review

Globally, environmental contamination by potentially noxious metalloids like arsenic is becoming a critical concern to the living organisms. Arsenic is a non-essential metalloid for plants and can be acclimatised in plants to toxic levels. Arsenic acquisition by plants poses serious health risks in human due to its entry in the food chain. High arsenic regimes disturb plant water relations, promote the generation of reactive oxygen species (ROS) and induce oxidative outburst in plants. This review evidences a conceivable tie-up among arsenic levels, speciation, its availability, uptake, acquisition, transport, phytotoxicity and arsenic detoxification in plants. The role of different antioxidant enzymes to confer plant tolerance towards the enhanced arsenic distress has also been summed up. Additionally, the mechanisms involved in the modulation of different genes coupled with arsenic tolerance have been thoroughly discussed. This review is intended to present an overview to rationalise the contemporary progressions on the recent advances in phytoremediation approaches to overcome ecosystem contamination by arsenic.

Drivers and ecological consequences of arsenite detoxification in aged semi-aerobic landfill

Microbial populations responsible for arsenite [As(III)] detoxification were examined in aged refuse treated with 75 μM As(III) under semi-aerobic conditions. As(III) was rapidly oxidized to As(V) via microbial activity, and substantial As was fixed in the solid phase. The abundance of arsenite oxidase genes (aioA) was about four times higher in the moderate As(III) stressed treatment than in the untreated control. Network analysis of microbial community 16S rRNA genes based on MRT (random matrix theory) further illuminated details about microbe-microbe interactions, and showed six ecological clusters. A total of 166 "core" taxa were identified by within-module connectivity and among-module connectivity values. When compared with the control treatment without As(III), 12 putative keystone operational taxonomic units were positively correlated with As(III) oxidation, of which 10 of these were annotated to genera level. Eight genera were associated with As(III) detoxification: Pseudomonas, Paenalcaligenes, Proteiniphilum, Moheibacter, Mobilitalea, Anaerosporobacter, Syntrophomonas and Pusillimonas. Most of those putative keystone taxa were rare species in landfill, which suggests that low-abundance taxa might significantly contribute to As(III) oxidation.