Assessing nickel tolerance of bacteria isolated from serpentine soils

The Plant Growth-Promoting Ability of Alfalfa Rhizobial Strains Under Nickel Stress

The growth and nutrient balance of legumes can be disrupted in soils with increased nickel (Ni) concentrations. The inoculation of legumes with rhizobia, symbiotic nitrogen-fixing bacteria, can be used for the alleviation of trace metal stress in plants. This study evaluated the Ni tolerance of alfalfa rhizobia isolates and some plant growth-promoting traits in the presence of Ni: indole-3-acetic acid (IAA) production, Ni biosorption potential, and the effect of rhizobia on alfalfa (Medicago sativa L.) growth. The strains were characterized as Shinorhizobium meliloti, Sinorhizobium medicae, and Rhizobium tibeticum. In total, 70% of the tested strains tolerate up to 0.8 mM Ni, while 15% of the strains tolerate 1.2 mM Ni. The production of IAA was maintained in the presence of Ni until bacterial growth was stopped by raising the Ni concentration. Alfalfa seed germination is significantly reduced in the presence of 0.5 mM Ni, while a significant reduction in 10-day-old seedling length already occurs at a Ni concentration of 0.03 mM. In the plant experiment, when alfalfa was inoculated with rhizobial strains, nodulation was maintained up to 0.05 mM Ni, but a significant reduction in nodule number was detected at 0.01 mM Ni. At the concentration of 0.005 mM Ni, inoculation with 12 particular rhizobial strains significantly improved the number of nodules per plant, plant height, and root length, as well as plant shoot dry weight, compared to non-inoculated plants with Ni addition. However, higher concentrations caused a reduction in all of these plant growth parameters compared to the plants without Ni. The selected rhizobia strains showed a Ni biosorption capacity of 20% in the in vitro assay. The inoculation of alfalfa with effective rhizobial strains improves growth parameters compared to non-inoculated plants in the presence of certain concentrations of Ni.

Unboxing PGPR-mediated management of abiotic stress and environmental cleanup: what lies inside?

Abiotic stresses including heavy metal toxicity, drought, salt and temperature extremes disrupt the plant growth and development and lowers crop output. Presence of environmental pollutants further causes plants suffering and restrict their ability to thrive. Overuse of chemical fertilizers to reduce the negative impact of these stresses is deteriorating the environment and induces various secondary stresses to plants. Therefore, an environmentally friendly strategy like utilizing plant growth-promoting rhizobacteria (PGPR) is a promising way to lessen the negative effects of stressors and to boost plant growth in stressful conditions. These are naturally occurring inhabitants of various environments, an essential component of the natural ecosystem and have remarkable abilities to promote plant growth. Furthermore, multifarious role of PGPR has recently been widely exploited to restore natural soil against a range of contaminants and to mitigate abiotic stress. For instance, PGPR may mitigate metal phytotoxicity by boosting metal translocation inside the plant and changing the metal bioavailability in the soil. PGPR have been also reported to mitigate other abiotic stress and to degrade environmental contaminants remarkably. Nevertheless, despite the substantial quantity of information that has been produced in the meantime, there has not been much advancement in either the knowledge of the processes behind the alleged positive benefits or in effective yield improvements by PGPR inoculation. This review focuses on addressing the progress accomplished in understanding various mechanisms behind the protective benefits of PGPR against a variety of abiotic stressors and in environmental cleanups and identifying the cause of the restricted applicability in real-world.

The profile of the soil microbiota in the Cerrado is influenced by land use

Extensive areas of the Cerrado biome have been deforested by the rapid advance of agricultural frontiers, especially by agricultural monocultures, and cultivated pastures. The objective of this study was to characterize the soil microbial community of an environment without anthropogenic interference and to compare it with soybean soil and pasture areas. For that, metagenomic sequencing techniques of the 16S rRNA gene were employed. Consistent changes in the profiles of diversity and abundance were described between communities in relation to the type of soil. The soil microbiome of the native environment was influenced by the pH level and content of Al³⁺, whereas the soil microbiomes cultivated with soybean and pasture were associated with the levels of nutrients N and P and the ions Ca²⁺ and Mg²⁺, respectively. The analysis of bacterial communities in the soil of the native environment showed a high abundance of members of the Proteobacteria phylum, with emphasis on the Bradyrhizobium and Burkholderia genera. In addition, significant levels of species of the Bacillus genus, and Dyella ginsengisoli, and Edaphobacter aggregans of the Acidobacteria phylum were detected. In the soil community with soybean cultivation, there was a predominance of Proteobacteria, mainly of the Sphingobium and Sphingomonas genera. In the pasture, the soil microbiota was dominated by the Firmicutes, which was almost entirely represented by the Bacillus genus. These results suggest an adaptation of the bacterial community to the soybean and pasture cultivations and will support understanding how environmental and anthropogenic factors shape the soil microbial community. KEY POINTS: • The Cerrado soil microbiota is sensitive to impacts on the biome. • Microbial communities have been altered at all taxonomic levels.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Enhanced bio-recovery of aluminum from low-grade bauxite using adapted fungal strains

Filamentous fungi have been proved to have a pronounced capability to recover metals from mineral ores. However, the metal recovery yield is reduced due to toxic effects triggered by various heavy metals present in the ore. The current study highlights the fungal adaptations to the toxic effects of metals at higher pulp densities for the enhanced bio-recovery of aluminum from low-grade bauxite. In the previous studies, a drastic decrease in the aluminum dissolution was observed when the bauxite pulp density was increased from 1 to 10% (w/v) due to the high metal toxicity and low tolerance of Aspergillus niger and Penicillium simplicissium to heavy metals. These fungi were adapted in order to increase heavy metal tolerance of these fungal strains and also to get maximum Al dissolution. A novel approach was employed for the adaptation of fungal strains using a liquid growth medium containing 5% bauxite pulp density supplemented with molasses as an energy source. The mycelia of adapted strains were harvested and subsequently cultured in a low-cost oat-agar medium. Batch experiments were performed to compare the aluminum leaching efficiencies in the direct one-step and the direct two-step bioleaching processes. FE-SEM analysis revealed the direct destructive and corrosive action by the bauxite-tolerant strains due to the extension and penetration of the vegetative mycelium filaments into the bauxite matrix. XRD analysis of the bioleached bauxite samples showed a considerable decline in oxide minerals such as corundum and gibbsite. Results showed a high amount of total Al (≥ 98%) was successfully bioleached and solubilized from low-grade bauxite by the adapted fungal strains grown in the presence of 5% pulp density and molasses as a low-cost substrate. Graphical abstract.