Phytohormones enhance heavy metal responses in Euglena gracilis: Evidence from uptake of Ni, Pb and Cd and linkages to hormonomic and metabolomic dynamics

Synergistic enhancement of plant growth and cadmium stress defense by Azospirillum brasilense and plant heme: Modulating the growth-defense relationship

Plant growth-promoting rhizobacteria (PGPR) play important roles in plant growth and defense under heavy metal (HM) stress. The direct integration of microbial and plant signals is key to the regulation of plant growth and HM stress defense, but the underlying mechanisms are still limited. Herein, we reveal a novel mechanism by which PGPR regulates plant growth-regulating substances in plant tissues and coordinates plant growth and defense in pak choi under cadmium (Cd) stress. This might be an efficient strategy and an extension of the mechanism by which plant-microbe interactions improve plant stress resistance. Azospirillum brasilense and heme synergistically reduced the shoot Cd content and promoted the growth of pak choi. The interaction between abscisic acid of microbial origin and heme improved Cd stress tolerance through enhancing Cd accumulation in the root cell wall. The interaction between A. brasilense and heme induced the growth-defense shift in plants under Cd stress. Plants sacrifice growth to enhance Cd stress defense, which then transforms into a dual promotion of both growth and defense. This study deepens our understanding of plant-microbe interactions and provides a novel strategy to improve plant growth and defense under HM stress, ensuring future food production and security.

Microalgae growth-promoting bacteria for cultivation strategies: Recent updates and progress

Microalgae growth-promoting bacteria (MGPB), both actinobacteria and non-actinobacteria, have received considerable attention recently because of their potential to develop microalgae-bacteria co-culture strategies for improved efficiency and sustainability of the water-energy-environment nexus. Owing to their diverse metabolic pathways and ability to adapt to diverse conditions, microalgal-MGPB co-cultures could be promising biological systems under uncertain environmental and nutrient conditions. This review proposes the recent updates and progress on MGPB for microalgae cultivation through co-culture strategies. Firstly, potential MGPB strains for microalgae cultivation are introduced. Following, microalgal-MGPB interaction mechanisms and applications of their co-cultures for biomass production and wastewater treatment are reviewed. Moreover, state-of-the-art studies on synthetic biology and metabolic network analysis, along with the challenges and prospects of opting these approaches for microalgal-MGPB co-cultures are presented. It is anticipated that these strategies may significantly improve the sustainability of microalgal-MGPB co-cultures for wastewater treatment, biomass valorization, and bioproducts synthesis in a circular bioeconomy paradigm.

The biomolecules of Euglena gracilis: Harnessing biology for natural solutions to future problems

Over the past decade, the autotrophic and heterotrophic protist Euglena gracilis (E. gracilis) has gained popularity across the studies of environmental science, biosynthesis experiments, and nutritional substitutes. The unique physiology and versatile metabolism of E. gracilis have been a recent topic of interest to many researchers who continue to understand the complexity and possibilities of using E. gracilis biomolecule production. In this review, we present a comprehensive representation of recent literature outlining the various uses of biomolecules derived from E. gracilis across the fields of natural product biosynthesis, as a nutritional substitute, and as bioremediation tools. In addition, we highlight effective strategies for altering metabolite production using abiotic stressors and growth conditions. To better understand metabolite biosynthesis and its role in E. gracilis, integrated studies involving genomics, metabolomics, and proteomics should be considered. Together, we show how the ongoing advancements in E. gracilis related research continue to broaden applications in the biosynthetic sector and highlight future works that would strengthen our understanding of overall Euglena metabolism.

Bio-pump cadmium phytoextraction efficiency promoted by phytohormones in Festuca arundinacea

The leaves of Festuca arundinacea can excrete cadmium (Cd) out onto the leaf surface, leading to a bio-pump phytoremediation strategy based on "root uptake-root-to-leaf translocation-leaf excretion". However, the bio-bump efficiency of soil Cd is a limiting factor for the implementation of this novel technology. Bio-bump remediation involves the bioprocess of plant root uptake from soil, root-to-leaf translocation, and leaf hydathode excretion. Here we show the significant effects of phytohormones in regulating the bio-pump phytoextraction efficiency. The results showed that salicylic acid and ethylene enhanced the whole process of Cd root uptake, root-to-leaf translocation, and leaf excretion, promoting the bio-pump phytoextraction efficiency by 63.6% and 73.8%, respectively. Gibberellin also greatly promoted Cd translocation, leaf excretion, and phytoextraction, but did not significantly impact Cd root uptake. Our results indicate that salicylic acid and ethylene could be recommended to promote bio-pump phytoextraction efficiency in F. arundinacea. Gibberellin might be used for a short-term promotion of the leaf Cd excretion.

The Effect of Two Siderophore-Producing Bacillus Strains on the Growth Promotion of Perennial Ryegrass under Cadmium Stress

Cadmium (Cd) is a highly toxic and cumulative environmental pollutant. Siderophores are heavy metal chelators with high affinity to heavy metals, such as Cd. Ryegrass (Lolium perenne L.) has a potential remediation capacity for soils contaminated by heavy metals. Consequently, using ryegrass alongside beneficial soil microorganisms that produce siderophores may be an effective means to remediate soils contaminated with Cd. In this study, the Bacillus strains WL1210 and CD303, which were previously isolated from the rhizospheres of Nitraria tangutorum in Wulan and Peganum harmala L. in Dachaidan, Qinghai, China, respectively, both arid and sandy environments, were evaluated for heavy metal pollution mitigation. Our quantitative analyses have discerned that the two bacterial strains possess commendable attributes of phosphorus (P) solubilization and potassium (K) dissolution, coupled with the capacity to produce phytohormones. To assess the heavy metal stress resilience of these strains, they were subjected to a cadmium concentration gradient, revealing their incremental growth despite cadmium presence, indicative of a pronounced tolerance threshold. The subsequent phylogenetic analysis, bolstered by robust genomic data from conserved housekeeping genes, including 16S rDNA, gyr B gene sequencing, as well as dnaK and recA, delineated a species-level phylogenetic tree, thereby confirming the strains as Bacillus atrophaeus. Additionally, we identified the types of iron-carrier-producing strains as catechol (WL1210) and carboxylic acid ferrophilin (CD303). A genomic analysis uncovered functional genes in strain CD303 associated with plant growth and iron carrier biosynthesis, such as fnr and iscA. Ryegrass seed germination assays, alongside morphological and physiological evaluations under diverse heavy metal stress, underscored the strains' potential to enhance ryegrass growth under high cadmium stress when treated with bacterial suspensions. This insight probes the strains' utility in leveraging alpine microbial resources and promoting ryegrass proliferation.

The impact of elevated sulfur and nitrogen levels on cadmium tolerance in Euglena species

Heavy metal (HM) pollution threatens human and ecosystem health. Current methods for remediating water contaminated with HMs are expensive and have limited effect. Therefore, bioremediation is being investigated as an environmentally and economically viable alternative. Freshwater protists Euglena gracilis and Euglena mutabilis were investigated for their tolerance to cadmium (Cd). A greater increase in cell numbers under Cd stress was noted for E. mutabilis but only E. gracilis showed an increase in Cd tolerance following pre-treatment with elevated concentrations of S or N. To gain insight regarding the nature of the increased tolerance RNA-sequencing was carried out on E. gracilis. This revealed transcript level changes among pretreated cells, and additional differences among cells exposed to CdCl2. Gene ontology (GO) enrichment analysis reflected changes in S and N metabolism, transmembrane transport, stress response, and physiological processes related to metal binding. Identifying these changes enhances our understanding of how these organisms adapt to HM polluted environments and allows us to target development of future pre-treatments to enhance the use of E. gracilis in bioremediation relating to heavy metals.

Removal of antibiotics by four microalgae-based systems for swine wastewater treatment under different phytohormone treatment

This study examined the removal of typical antibiotics from simulated swine wastewater. Microalgae-bacteria/fungi symbioses were constructed using Chlorella ellipsoidea, endophytic bacteria (S395-2), and Clonostachys rosea as biomaterials. The growth, photosynthetic performance, and removal of three types of antibiotics (tetracyclines, sulfonamides, and quinolones) induced by four phytohormones were analyzed in each system. The results showed that all four phytohormones effectively improved the tolerance of symbiotic strains against antibiotic stress; strigolactones (GR24) achieved the best performance. At 10-9 M, GR24 achieved the best removal of antibiotics by C. elliptica + S395-2 + C. rosea symbiosis. The average removals of tetracycline, sulfonamide, and quinolone by this system reached 96.2-99.4 %, 75.2-81.1 %, and 66.8-69.9 %, respectively. The results of this study help to develop appropriate bio enhancement strategies as well as design and operate algal-bacterial-fungal symbiotic processes for the treatment of antibiotics-containing wastewater.

Mitigating Effect of Trans-Zeatin on Cadmium Toxicity in Desmodesmus armatus

Phytohormones, particularly cytokinin trans-zeatin (tZ), were studied for their impact on the green alga Desmodesmus armatus under cadmium (Cd) stress, focusing on growth, metal accumulation, and stress response mechanisms. Using atomic absorption spectroscopy for the Cd level and high-performance liquid chromatography for photosynthetic pigments and phytochelatins, along with spectrophotometry for antioxidants and liquid chromatography-mass spectrometry for phytohormones, we found that tZ enhances Cd uptake in D. armatus, potentially improving phycoremediation of aquatic environments. Cytokinin mitigates Cd toxicity by regulating internal phytohormone levels and activating metal tolerance pathways, increasing phytochelatin synthase activity and phytochelatin accumulation essential for Cd sequestration. Treatment with tZ and Cd also resulted in increased cell proliferation, photosynthetic pigment and antioxidant levels, and antioxidant enzyme activities, reducing oxidative stress. This suggests that cytokinin-mediated mechanisms in D. armatus enhance its capacity for Cd uptake and tolerance, offering promising avenues for more effective aquatic phycoremediation techniques.

The mixed blessings of rare earth element supplements for tomatoes and ferns

Rare earth elements (REEs) constitute a key group of critical minerals that are strategic for the global low-carbon economy and several United Nations Sustainable Development Goals. Their expected escalating emissions into the environment from emerging anthropogenic sources can negatively affect natural ecosystems. However, their hormetic effects make these elements effective fertilizers to promote crop production. Here, we investigate the response of tomatoes and ferns to REE exposure (La, Gd, Yb). While ferns were unresponsive to REEs, these elements promote evident benefits in tomatoes, e.g., elevating nutrient uptake, higher photosynthetic capacity and phytohormone enhancement to allocate energy to green tissue and root development. Nevertheless, the non-selective cation uptake incurs risks of accumulating non-essential elements in edible tissues. These evident benefits of REEs on crops support applications in agricultural production systems, create added value to the global distribution and promote better material flow management of REEs as strategic and critical resources.

Plant response to heavy metal stress toxicity: the role of metabolomics and other omics tools

Metabolomic investigations offers a significant foundation for improved comprehension of the adaptability of plants to reconfigure the key metabolic pathways and their response to changing climatic conditions. Their application to ecophysiology and ecotoxicology help to assess potential risks caused by the contaminants, their modes of action and the elucidation of metabolic pathways associated with stress responses. Heavy metal stress is one of the most significant environmental hazards affecting the physiological and biochemical processes in plants. Metabolomic tools have been widely utilised in the massive characterisation of the molecular structure of plants at various stages for understanding the diverse aspects of the cellular functioning underlying heavy metal stress-responsive mechanisms. This review emphasises on the recent progressions in metabolomics in plants subjected to heavy metal stresses. Also, it discusses the possibility of facilitating effective management strategies concerning metabolites for mitigating the negative impacts of heavy metal contaminants on the growth and productivity of plants.

Molecular changes in phenolic compounds in Euglena gracilis cells grown under metal stress

Metal presence in the aquatic ecosystem has increased and diversified over the last decades due to anthropogenic sources. These contaminants cause abiotic stress on living organisms that lead to the production of oxidizing molecules. Phenolic compounds are part of the defense mechanisms countering metal toxicity. In this study, the production of phenolic compounds by Euglena gracilis under three different metal stressors (i.e. cadmium, copper, or cobalt) at sub-lethal concentration was assessed using an untargeted metabolomic approach by mass spectrometry combined with neuronal network analysis (i.e. Cytoscape). The metal stress had a greater impact on molecular diversity than on the number of phenolic compounds. The prevalence of sulfur- and nitrogen-rich phenolic compounds were found in Cd- and Cu-amended cultures. Together these results confirm the impact of metallic stress on phenolic compounds production, which could be utilized to assess the metal contamination in natural waters.