Exogenously applied auxins and cytokinins ameliorate lead toxicity by inducing antioxidant defence system in green alga Acutodesmus obliquus

Phytohormones as a novel strategy for promoting phytoremediation in microalgae: Progress and prospects

Microalgal phytoremediation is a promising bioremediation approach that can achieve significant resource recovery while effectively removing pollutants. However, the toxicity of some pollutants in wastewater often induces stress responses in microalgae, reducing their pollutant removal efficiency. Recently, phytohormones have been identified as a novel solution to reduce these stress responses, enhancing microalgae growth and improving their ability to remove various pollutants from wastewater. This advancement significantly boosts the efficiency and viability of microalgal phytoremediation. In this paper, the pathways and challenges related to microalgal phytoremediation were systematically analyzed. On this basis, the promoting effects of phytohormones on the removal of nutrients, heavy metals, and emerging contaminants by microalgae and the related mechanisms were discussed. Additionally, the review also discusses the optimal use strategy of phytohormones, the ecological risks that may be faced in the use of phytohormones, and the feasible strategies to control the use cost of phytohormones. The goal is to provide insights and guidance for future research on the application of phytohormones in microalgal phytoremediation.

Purple non-sulfur bacteria for biotechnological applications

In this review, we focus on how purple non-sulfur bacteria can be leveraged for sustainable bioproduction to support the circular economy. We discuss the state of the field with respect to the use of purple bacteria for energy production, their role in wastewater treatment, as a fertilizer, and as a chassis for bioplastic production. We explore their ability to serve as single-cell protein and production platforms for fine chemicals from waste materials. We also introduce more Avant-Garde technologies that leverage the unique metabolisms of purple bacteria, including microbial electrosynthesis and co-culture. These technologies will be pivotal in our efforts to mitigate climate change and circularize the economy in the next two decades.

ONE-SENTENCE SUMMARY

Purple non-sulfur bacteria are utilized for a range of biotechnological applications, including the production of bio-energy, single cell protein, fertilizer, bioplastics, fine chemicals, in wastewater treatment and in novel applications like co-cultures and microbial electrosynthesis.

Resource recovery and contaminants of emerging concern mitigation by microalgae treating wastewater

This study aimed to investigate the recovery of agricultural biostimulants and biogas from microalgae treating wastewater, in the framework of a circular bioeconomy. To this end, municipal wastewater was treated in demonstrative raceway ponds, and microalgal biomass (Scenedesmus sp.) was then harvested and downstream processed to recover biostimulants and biogas in a biorefinery approach. The effect of microalgal biostimulants on plants was evaluated by means of bioassays, while the biogas produced was quantified in biochemical methane potential (BMP) tests. Furthermore, the fate of contaminants of emerging concern (CECs) over the process was also assessed. Bioassays confirmed the biostimulant effect of microalgae, which showed gibberellin-, auxin- and cytokinin-like activity in watercress seed germination, mung bean rooting, and wheat leaf chlorophyll retention. In addition, the downstream process applied to raw biomass acted as a pre-treatment to enhance anaerobic digestion performance. After biostimulant extraction, the residual biomass represented 91% of the methane yield from the raw biomass (276 mLCH4·g-1VS). The kinetic profile of the residual biomass was 43% higher than that of the unprocessed biomass. Co-digestion with primary sludge further increased biogas production by 24%. Finally, the concentration of CECs in wastewater was reduced by more than 80%, and only 6 out of 22 CECs analyzed were present in the biostimulant obtained. Most importantly, the concentration of those contaminants was lower than in biosolids that are commonly used in agriculture, ensuring environmental safety.

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.

Exploring the synergistic effects of indole acetic acid (IAA) and compost in the phytostabilization of nickel (Ni) in cauliflower rhizosphere

Heavy metals (HMs) contamination, owing to their potential links to various chronic diseases, poses a global threat to agriculture, environment, and human health. Nickel (Ni) is an essential element however, at higher concentration, it is highly phytotoxic, and affects major plant functions. Beneficial roles of plant growth regulators (PGRs) and organic amendments in mitigating the adverse impacts of HM on plant growth has gained the attention of scientific community worldwide. Here, we performed a greenhouse study to investigate the effect of indole-3-acetic acid (IAA @ 10- 5 M) and compost (1% w/w) individually and in combination in sustaining cauliflower growth and yield under Ni stress. In our results, combined application proved significantly better than individual applications in alleviating the adverse effects of Ni on cauliflower as it increased various plant attributes such as plant height (49%), root length (76%), curd height and diameter (68 and 134%), leaf area (75%), transpiration rate (36%), stomatal conductance (104%), water use efficiency (143%), flavonoid and phenolic contents (212 and 133%), soluble sugars and protein contents (202 and 199%), SPAD value (78%), chlorophyll 'a and b' (219 and 208%), carotenoid (335%), and NPK uptake (191, 79 and 92%) as compared to the control. Co-application of IAA and compost reduced Ni-induced electrolyte leakage (64%) and improved the antioxidant activities, including APX (55%), CAT (30%), SOD (43%), POD (55%), while reducing MDA and H2O2 contents (77 and 52%) compared to the control. The combined application also reduced Ni uptake in roots, shoots, and curd by 51, 78 and 72% respectively along with an increased relative production index (78%) as compared to the control. Hence, synergistic application of IAA and compost can mitigate Ni induced adverse impacts on cauliflower growth by immobilizing it in the soil.

Indole-3-acetic acid improves periphyton's resistance to ultraviolet-B: From physiological-biochemical properties and bacteria community to livestock-polluted water purification

Livestock-polluted water is a pressing water environmental issue in plateau pastoral regions, necessitating the adoption of eco-friendly solutions. Despite periphyton being a promising alternative, its efficacy is limited by the prevalence of intense ultraviolet radiation, particularly ultraviolet-B (UVB), in these regions. Therefore, this study employs molecular tools and small-scale trials to explore the crucial role of indole-3-acetic acid (IAA) in modulating periphyton characteristics and mediating nutrient removal from livestock-polluted water under UVB exposure. The results revealed that IAA augments periphyton's resilience to UVB stress through several pathways, including increasing periphyton's biomass, producing more extracellular polymeric substances (EPS), and enhancing antioxidant enzyme activities and photosynthetic activity of periphyton. Moreover, IAA addition increased periphyton's bacterial diversity, reshaped bacterial community structure, enhanced community stability, and elevated the R2 value of neutral processes in bacterial assembly from 0.257 to 0.651 under UVB. Practically, an IAA concentration of 50 mg/L was recommended. Small-scale trials confirmed the effectiveness of IAA in assisting UVB-stressed periphyton to remove nitrogen and phosphorus from livestock-polluted water, without the risk of nitrogen accumulation. These findings offer valuable insights into the protection of aquatic ecosystems in plateau pastoral regions based on periphyton property in an eco-friendly manner.

Role of Plant-Growth-Promoting Rhizobacteria in Plant Machinery for Soil Heavy Metal Detoxification

Heavy metals migrate easily and are difficult to degrade in the soil environment, which causes serious harm to the ecological environment and human health. Thus, soil heavy metal pollution has become one of the main environmental issues of global concern. Plant-growth-promoting rhizobacteria (PGPR) is a kind of microorganism that grows around the rhizosphere and can promote plant growth and increase crop yield. PGPR can change the bioavailability of heavy metals in the rhizosphere microenvironment, increase heavy metal uptake by phytoremediation plants, and enhance the phytoremediation efficiency of heavy-metal-contaminated soils. In recent years, the number of studies on the phytoremediation efficiency of heavy-metal-contaminated soil enhanced by PGPR has increased rapidly. This paper systematically reviews the mechanisms of PGPR that promote plant growth (including nitrogen fixation, phosphorus solubilization, potassium solubilization, iron solubilization, and plant hormone secretion) and the mechanisms of PGPR that enhance plant-heavy metal interactions (including chelation, the induction of systemic resistance, and the improvement of bioavailability). Future research on PGPR should address the challenges in heavy metal removal by PGPR-assisted phytoremediation.

Indole-3-acetic acid promotes growth in bloom-forming Microcystis via an antioxidant response

Interactions between bacteria and phytoplankton in the phycosphere facilitate and constrain biogeochemical cycling in aquatic ecosystems. Indole-3-acetic acid (IAA) is a bacterially produced chemical signal that promotes growth of phytoplankton and plants. Here, we explored the impact of IAA on bloom-forming cyanobacteria and their associated bacteria. Exposure to IAA and its precursor, tryptophan, resulted in a strong growth response in a bloom of the freshwater cyanobacterium, Microcystis. Metatranscriptome analysis revealed the induction of an antioxidant response in Microcystis upon exposure to IAA, potentially allowing populations to increase photosynthetic rate and overcome internally generated reactive oxygen. Our data reveal that co-occurring bacteria within the phycosphere microbiome exhibit a division of labor for supportive functions, such as nutrient mineralization and transport, vitamin synthesis, and reactive oxygen neutralization. These complex dynamics within the Microcystis phycosphere microbiome are an example of interactions within a microenvironment that can have ecosystem-scale consequences.

Effects of staged multiple phytohormones application on capillary-driven attached Chlorella sp. biofilm

Comparing with single phytohormone application, applying multiple phytohormones to microalgae-based wastewater treatment systems can offer more extensive growth-promoting and stress-protecting effects for microalgae, yet the advantage of stress-relieving salicylic acid (SA) under combined phytohormones application scenario has not been exploited. Employing the improved capillary-driven attached microalgae culturing device (CD-PBR) previously used for single phytohormone application, this study compared the effects of mixed and single phytohormone(s) addition under as low as 10-7 M dosage. In order to make the best of SA for its stress-relieving property, postponed SA addition combined with applying other phytohormone(s) at the beginning of microalgae cultivation was also investigated. Combination of 10-6 M 6-benzylaminopurine (6-BA) with 10-7 M SA was sufficient for enhancing growth-promoting effects and anti-oxidative responses for attached Chlorella sp., while indole-3-acetic acid (IAA) addition was unnecessary. Combination of 6-BA addition at the beginning while postponed SA addition on Day 4 could further sustain such beneficial effects, while removing up to 99.7% total nitrogen (TN) and 97.9% total phosphorus (TP) from the bulk liquid. These results provided innovative strategies on mixed phytohormones addition for microalgae.

Genetic evidence for algal auxin production in Chlamydomonas and its role in algal-bacterial mutualism

Interactions between algae and bacteria are ubiquitous and play fundamental roles in nutrient cycling and biomass production. Recent studies have shown that the plant auxin indole acetic acid (IAA) can mediate chemical crosstalk between algae and bacteria, resembling its role in plant-bacterial associations. Here, we report a mechanism for algal extracellular IAA production from L-tryptophan mediated by the enzyme L-amino acid oxidase (LAO1) in the model Chlamydomonas reinhardtii. High levels of IAA inhibit algal cell multiplication and chlorophyll degradation, and these inhibitory effects can be relieved by the presence of the plant-growth-promoting bacterium (PGPB) Methylobacterium aquaticum, whose growth is mutualistically enhanced by the presence of the alga. These findings reveal a complex interplay of microbial auxin production and degradation by algal-bacterial consortia and draws attention to potential ecophysiological roles of terrestrial microalgae and PGPB in association with land plants.

Foliar spraying of indoleacetic acid (IAA) enhances the phytostabilization of Pb in naturally tolerant ryegrass by limiting the root-to-shoot transfer of Pb and improving plant growth

Exogenous addition of IAA has the potential to improve the metal tolerance and phytostabilization of plants, but these effects have not been systematically investigated in naturally tolerant plants. Ryegrass (Lolium perenne L.) is a typical indigenous plant in the Lanping Pb/Zn mining area with high adaptability. This study investigated the phytostabilization ability and Pb tolerance mechanism of ryegrass in response to Pb, with or without foliar spraying of 0.1 mmol L-1 IAA. The results indicated that appropriate IAA treatment could be used to enhance the phytostabilization efficiency of naturally tolerant plants. Foliar spraying of IAA increased the aboveground and belowground biomass of ryegrass and improved root Pb phytostabilization. Compared to Pb-treated plants without exogenous IAA addition, Pb concentration in the shoots of ryegrass significantly decreased, then increased in the roots after the foliar spraying of IAA. In the 1,000 mg kg-1 Pb-treated plants, Pb concentration in the shoots decreased by 69.9% and increased by 79.1% in the roots after IAA treatment. IAA improved plant growth, especially in soils with higher Pb concentration. Foliar spraying of IAA increased shoot biomass by 35.9% and root biomass by 109.4% in 1,000 mg kg-1 Pb-treated plants, and increased shoot biomass by 196.5% and root biomass by 71.5% in 2,000 mg kg-1 Pb-treated plants. In addition, Pb stress significantly decreased the content of photosynthetic pigments and anti-oxidase activities in ryegrass, while foliar spraying of IAA remedied these negative impacts. In summary, foliar spraying of IAA could increase the biomass and improve the Pb tolerance of ryegrass.

Heavy metal stress in plants: Ways to alleviate with exogenous substances

Accumulation and enrichment of excessive heavy metals due to industrialization and modernization not only devastate our ecosystem, but also pose a threat to the global vegetation, especially crops. To improve plant resilience against heavy metal stress (HMS), numerous exogenous substances (ESs) have been tried as the alleviating agents. After a careful and thorough review of over 150 recently published literature, 93 reported ESs and their corresponding effects on alleviating HMS, we propose that 7 underlying mechanisms of ESs be categorized in plants for: 1) improving the capacity of the antioxidant system, 2) inducing the synthesis of osmoregulatory substances, 3) enhancing the photochemical system, 4) detouring the accumulation and migration of heavy metals, 5) regulating the secretion of endogenous hormones, 6) modulating gene expressions, and 7) participating in microbe-involved regulations. Recent research advances strongly indicate that ESs have proven to be effective in mitigating a potential negative impact of HMS on crops and other plants, but not enough to ultimately solve the devastating problem associated with excessive heavy metals. Therefore, much more research should be focused and carried out to eliminate HMS for the sustainable agriculture and clean environmental through minimizing towards prohibiting heavy metals from entering our ecosystem, phytodetoxicating polluted landscapes, retrieving heavy metals from detoxicating plants or crop, breeding for more tolerant cultivars for both high yield and tolerance against HMS, and seeking synergetic effect of multiply ESs on HMS alleviation in our feature researches.

Metal Toxicity across Different Thallus Sections of the Green Macroalga, Ulva australis

We aimed to identify functional differences between different sections of the thallus of Ulva australis and develop tissue-endpoint combinations to assess the toxicity of six metals (i.e., Ag, As, Cd, Cr, Cu, and Ni). EC₅₀ values for these metals in three sections of the thallus of Ulva were obtained for multiple endpoints: relative growth rate (RGR), chlorophyll a fluorescence, pigment contents, and the expression of the photosynthesis-related gene, rbcL. The responses of the endpoints varied across the respective thallus sections; overall, the most toxic metals were Ag and Cu. These endpoints were the best for evaluating metal toxicity: ETR_max of the middle thallus sections for Ag toxicity; RGR of the middle thallus section for As and Cd; ETR_max of the marginal thallus section for Cr; Chl b contents of the marginal thallus section for Cu; RGR of the basal thallus section for Ni. The EC₅₀ values for the inhibition of ETR_max in middle (0.06 mg∙L^-1) and Chl b in the marginal thallus sections (0.06 mg∙L^-1) were all lower than those of the quality standard for wastewater discharge values of Ag and Cu in Republic of Korea and the US, pointing to the suitability of U. australis-based endpoints for risk assessment.

Transcriptome studies on cadmium tolerance and biochar mitigating cadmium stress in muskmelon

Cadmium pollution in agricultural soil is a great threat to crop growth and human health. In this research, with 1%, 3% and 5% biochar applied to control soil cadmium pollution, melon was selected to be the experimental object for physiological detection and transcriptome analysis, through which we explored the mechanism of cadmium tolerance and biochar mitigating cadmium stress in muskmelon. Three set concentrations of biochar have a mitigative effect on muskmelon cadmium stress, and 5% biochar and 3% biochar respectively have the best and the worst alleviative effect. The alleviation of biochar to cadmium stress on muskmelon is primarily in the manner of inhibiting cadmium transfer, while the resistance of muskmelon to cadmium stress is through activating phenylpropanoid pathway and overexpressing stress related genes. Under cadmium treatment, 11 genes of the phenylpropane pathway and 19 stress-related genes including cytochrome P450 family protein genes and WRKY transcription factor genes were up-regulated, while 1%, 3%, 5% biochar addition significantly downregulated 3, 0, 7 phenylpropane pathway genes and 17, 5, 16 stress-related genes, respectively. Genes such as cytochrome P450 protein family genes, WRKY transcription factor genes, and annexin genes may play a key role in muskmelon's resistance to cadmium stress. The results show the key pathways and genes of cadmium stress resistance and the effect of different concentrations of biochar in alleviating cadmium stress, which provide a reference for the research of cadmium stress resistance in crops and the application of biochar in cadmium pollution in agricultural soil.

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

Over the last decade, significant effort has been made to understand phytohormonal functions (e.g., cytokinins (CKs) and abscisic acid (ABA)) in metal stress responses of higher plants and algae. Despite the potential for these phytohormones to improve industrial remediation by Euglena gracilis (Euglenophyceae), no such roles have been elucidated for this highly adaptive species and its response to heavy metals. This study demonstrates that toxic metals (nickel, lead, cadmium) modify hormonal activity profiles (i.e., CK forms and their concentrations) in E. gracilis. Furthermore, exogenous ABA or CK (tZ) enabled higher metal uptake efficiency (i.e., 9.35% in lead and 9.2% in cadmium uptake with CK) and alleviated metal toxicity through the regulation of endogenous CKs (i.e., total CK, isoprenoid CK) and gibberellin (GAs, GA₁ and GA₃) levels. These responses suggest that E. gracilis regulates multiple phytohormone signals during metal stress acclimation. A deeper approach, using untargeted metabolomic analyses, gave more detailed insight into phytohormone-controlled pathways and associated modified metabolites, which were frequently related to metal accumulation and the physiological acclimation to metal presence. Significant changes in the levels of cellular metabolites, especially those involved in acclimation to metal stress, were under the influence of phytohormones in algal cells. When grown under metal stress conditions, the presence of exogenous ABA or CKs, caused changes in cellular metabolites which included those from: lipid pathways, riboflavin metabolism, the biosynthesis of cofactors/vitamins, and carbohydrate metabolism. Also, bioactive secondary metabolites (e.g., terpenoids, alkaloids, flavonoids, carotenoids) were modified in algal cells treated with phytohormones. Thus, the study gives a detailed view on the regulatory functions of ABA and CKs in algal metal bioremediation strategies, which are attributed to enhanced metal uptake and in the fine-tuning of plant hormone levels during metal stress response. The results can guide efforts to develop efficient, low-cost and environmentally friendly methods for bioremediation.

Bio-removal of rare earth elements from hazardous industrial waste of CFL bulbs by the extremophile red alga Galdieria sulphuraria

In recent decades, a shift has been seen in the use of light-emitting diodes over incandescent lights and compact fluorescent lamps (CFL), which eventually led to an increase in wastes of electrical equipment (WEE), especially fluorescent lamps (FLs) and CFL light bulbs. These widely used CFL lights, and their wastes are good sources of rare earth elements (REEs), which are desirable in almost every modern technology. Increased demand for REEs and their irregular supply have exerted pressure on us to seek alternative sources that may fulfill this demand in an eco-friendly manner. Bio-removal of wastes containing REEs, and their recycling may be a solution to this problem and could balance environmental and economic benefits. To address this problem, the current study focuses on the use of the extremophilic red alga, Galdieria sulphuraria, for bioaccumulation/removal of REEs from hazardous industrial wastes of CFL bulbs and the physiological response of a synchronized culture of G. sulphuraria. A CFL acid extract significantly affected growth, photosynthetic pigments, quantum yield, and cell cycle progression of this alga. A synchronous culture was able to efficiently accumulate REEs from a CFL acid extract and efficiency was increased by including two phytohormones, i.e., 6-Benzylaminopurine (BAP - Cytokinin family) and 1-Naphthaleneacetic acid (NAA - Auxin family).

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

Plant hormones and neurotransmitter interactions mediate antioxidant defenses under induced oxidative stress in plants

Due to global climate change, abiotic stresses are affecting plant growth, productivity, and the quality of cultivated crops. Stressful conditions disrupt physiological activities and suppress defensive mechanisms, resulting in stress-sensitive plants. Consequently, plants implement various endogenous strategies, including plant hormone biosynthesis (e.g., abscisic acid, jasmonic acid, salicylic acid, brassinosteroids, indole-3-acetic acid, cytokinins, ethylene, gibberellic acid, and strigolactones) to withstand stress conditions. Combined or single abiotic stress disrupts the normal transportation of solutes, causes electron leakage, and triggers reactive oxygen species (ROS) production, creating oxidative stress in plants. Several enzymatic and non-enzymatic defense systems marshal a plant's antioxidant defenses. While stress responses and the protective role of the antioxidant defense system have been well-documented in recent investigations, the interrelationships among plant hormones, plant neurotransmitters (NTs, such as serotonin, melatonin, dopamine, acetylcholine, and γ-aminobutyric acid), and antioxidant defenses are not well explained. Thus, this review discusses recent advances in plant hormones, transgenic and metabolic developments, and the potential interaction of plant hormones with NTs in plant stress response and tolerance mechanisms. Furthermore, we discuss current challenges and future directions (transgenic breeding and genome editing) for metabolic improvement in plants using modern molecular tools. The interaction of plant hormones and NTs involved in regulating antioxidant defense systems, molecular hormone networks, and abiotic-induced oxidative stress tolerance in plants are also discussed.

Trace phenolic acids simultaneously enhance degradation of chlorophenol and biofuel production by Chlorella regularis

Coupling the cultivation of microalgae with wastewater treatment is a promising technology to recover bioresources from wastewater. However, toxic pollutants in wastewater seriously inhibit the growth of microalgae and the removal of pollutants. Phenolic acids are similar to phytohormones, could potentially relieve the toxicity to microalgae and simultaneously promote pollutant degradation and lipid accumulation. Chlorella and 4-chlorophenol (4-CP) were utilized to simulate the toxic wastewater treatment, and the roles of two typical phenolic acids, such as p-hydroxybenzoic acid (p-HBA) and caffeic acid (CA), were explored. The 0.2 μM concentration of p-HBA or CA improved the specific growth rate by 7.6% by enhancing photosynthesis and DNA replication. The oxidative damage caused by 4-CP was reduced by 30.3-49.7% via the synthesis of more antioxidant enzymes and the direct scavenging of free radicals by phenolic acids. Furthermore, the 4-CP removal rate increased by 27.0%, and toxic 4-CP was degraded into non-toxic compounds. The phenolic acids did not change the 4-CP degradation pathway but accelerated its removal and detoxification by enhancing the expression of 4-CP degradation enzymes. Simultaneously, lipid production increased by 20.5-23.1% due to the upregulation of enzymes related to fatty acid and triacylglycerol synthesis. Trace phenolic acids stimulated the mitogen-activated protein kinase signaling cascade and the calcium signaling pathway to regulate the physiology of the microalgae and protect cells from toxic stress. This study provides a promising new strategy for toxic wastewater treatment and bioresource recovery.

Comparative Transcriptome Analysis Provides Insights Into the Mechanism by Which 2,4-Dichlorophenoxyacetic Acid Improves Thermotolerance in Lentinula edodes

As the widest cultivated edible mushroom worldwide, Lentinula edodes suffers serious yield and quality losses from heat stress during growth and development, and in our previous study, exogenous 2,4-Dichlorophenoxyacetic acid (2,4-D) was found to improve the thermotolerance of L. edodes strain YS3357, but the molecular mechanism remains unclear. Here, we explored the potential protective mechanism of exogenous 2,4-D against heat stress by transcriptome analysis. 2,4-D possible improve the thermotolerance of L. edodes through regulating antioxidant genes, transcription factors, energy-provision system, membrane fluidity, and cell wall remodeling. Furthermore, 2,4-D was also found to regulate the saturation levels of fatty acids and ATP content in L. edodes mycelium under heat stress. This study proposed a regulatory network of 2,4-D in regulating L. edodes response to heat stress, providing a theoretical basis for improving L. edodes thermotolerance, and facilitating the understanding of the molecular mechanism of exogenous hormones in alleviating abiotic stress damage to macrofungi.

The Active Phytohormone in Microalgae: The Characteristics, Efficient Detection, and Their Adversity Resistance Applications

Phytohormones are a class of small organic molecules that are widely used in higher plants and microalgae as chemical messengers. Phytohormones play a regulatory role in the physiological metabolism of cells, including promoting cell division, increasing stress tolerance, and improving photosynthetic efficiency, and thereby increasing biomass, oil, chlorophyll, and protein content. However, traditional abiotic stress methods for inducing the accumulation of energy storage substances in microalgae, such as high light intensity, high salinity, and heavy metals, will affect the growth of microalgae and will ultimately limit the efficient accumulation of energy storage substances. Therefore, the addition of phytohormones not only helps to reduce production costs but also improves the efficiency of biofuel utilization. However, accurate and sensitive phytohormones determination and analytical methods are the basis for plant hormone research. In this study, the characteristics of phytohormones in microalgae and research progress for regulating the accumulation of energy storage substances in microalgae by exogenous phytohormones, combined with abiotic stress conditions at home and abroad, are summarized. The possible metabolic mechanism of phytohormones in microalgae is discussed, and possible future research directions are put forward, which provide a theoretical basis for the application of phytohormones in microalgae.

Co-application of indole-3-acetic acid/gibberellin and oxalic acid for phytoextraction of cadmium and lead with Sedum alfredii Hance from contaminated soil

Exogenous application of plant-growth promoting substances in combination with chelators is a common way to enhance the phytoextraction of heavy metals. A pot experiment was used to explore the influences of indole-3-acetic acid (IAA)/gibberellin (GA₃) alone or together with oxalic acid (OA) on the growth, physiological response, and nutrient contents of hyperaccumulator Sedum alfredii Hance, and cadmium (Cd) and lead (Pb) phytoextraction efficiency. The results showed that a foliar spray of IAA/GA₃ alone or together with OA increased plant growth. The largest shoot biomass with increase by 29.7% was produced by the 50 μmol L^-1 IAA combined with 2.5 mmol kg^-1 OA (50I+2.5OA) treatment as compared with the control treatment (CK). The presence of IAA and GA₃ enhanced the chlorophyll a, carotenoid, and potassium contents in leaves and decreased the malondialdehyde content. The Cd content in leaf and the translocation factor (TF_shoot) value from 50I+2.5OA treatment was increased by 4.29% and 21.4%, and the Pb content in stem and shoot, and the TF_shoot of Pb after applying 50 μmol L^-1 GA₃ combined with 2.5 mmol kg^-1 OA was enhanced by 32.5%, 13.4%, and 57.6%, compared with CK, respectively. The optimal Cd and Pb phytoextraction efficiency occurred from 50I+2.5OA treatment with increase by 82.4% and 79.3% as compared with CK, respectively. Therefore, the results showed that a combined application of 50 μmol L^-1 IAA and 2.5 mmol kg^-1 OA could effectively enhance S. alfredii Hance phytoremediation of Cd and Pb co-contaminated soil.

Plant programmed cell death meets auxin signalling

Both auxin signalling and programmed cell death (PCD) are essential components of a normally functioning plant. Auxin underpins plant growth and development, as well as regulating plant defences against environmental stresses. PCD, a genetically controlled pathway for selective elimination of redundant, damaged or infected cells, is also a key element of many developmental processes and stress response mechanisms in plants. An increasing body of evidence suggests that auxin signalling and PCD regulation are often connected. While generally auxin appears to suppress cell death, it has also been shown to promote PCD events, most likely via stimulation of ethylene biosynthesis. Intriguingly, certain cells undergoing PCD have also been suggested to control the distribution of auxin in plant tissues, by either releasing a burst of auxin or creating an anatomical barrier to auxin transport and distribution. These recent findings indicate novel roles of localized PCD events in the context of plant development such as control of root architecture, or tissue regeneration following injury, and suggest exciting possibilities for incorporation of this knowledge into crop improvement strategies.

Ascorbate and glutathione independently alleviate arsenate toxicity in brinjal but both require endogenous nitric oxide

In this study, we have explored the possible role of ascorbic acid (ASC) and glutathione (GSH) in alleviating arsenate (As^V ) toxicity in brinjal roots. Moreover, we have also focused our attention on the possible involvement of endogenous nitric oxide (NO) in accomplishing this task. As^V treatment negatively impacts the length and fresh weight of roots and shoots as well as the dry weight and fitness of roots, and this was accompanied by greater As accumulation in roots and shoots of brinjal. As^V treatment also declined the endogenous NO level by inhibiting Nitric Oxide Synthase-like (NOS-like) activity. Furthermore, As^V stimulated oxidative stress markers, caused protein damage by their carbonylation due to downregulation in antioxidants [particularly ascorbate (AsA)-GSH cycle], leading to disturbed cellular redox status. This, collectively, led to root cell death in brinjal. However, the addition of either ASC or GSH rescued brinjal roots from the toxic effects of As^V in. Interestingly, lycorine (an inhibitor of ASC biosynthesis) further increased As^V toxicity, while ASC rescued its effects. Moreover, buthionine sulphoximine (BSO, an inhibitor of GSH biosynthesis) interestingly increased further As^V toxicity, while GSH rescued the plant from the As toxic effects. An interesting notion of this study was that GSH rescued the toxic effect of lycorine, while ASC rescued the toxic effect of BSO, though the As^V toxicity mediated by either ASC or GSH was always accompanied by high endogenous NO level and NOS-like activity. All together, these results suggest that ASC and GSH independently mitigate As^V toxicity in brinjal roots, but both might be dependent on endogenous NO for accomplishing the As^V toxicity alleviatory tasks.

Hormones induce the metabolic growth and cytotoxin production of Microcystis aeruginosa under terpinolene stress

Several organic compounds released into the aquatic environment have a detrimental impact on humans and other organisms. There is a lack of knowledge about natural hormones and herbicides on non-target organisms, including cyanobacteria. In this study, the response of Microcystis aeruginosa to four phytohormones, indole-3-acetic acid (IAA; 10^-5), zeatin (ZT; 10^-5), abscisic acid (ABA; 10^-7), and brassinolide (BRL; 10^-9 mol/L), exposed to terpinolene (TPN; (0.44, 0.88, 1.17, or 1.62 mmol/L) at the cellular and genetic levels were investigated. The results showed that TPN could inhibit the growth and photosynthetic activities and stimulate microcystins (MCs) of M. aeruginosa at various levels through the co-occurrence of oxidative stress, antioxidant defense activities, and an imbalance of the antioxidative system. Hormones played critical roles in the growth promotion and photosynthetic activity by enhancing the antioxidant defense mechanisms and MCs production of M. aeruginosa under TPN stress in both hormone and TPN dose-dependent manner. The growth performance and photosynthetic activities of M. aeruginosa were significant with IAA (p < 0.01) and BSL (p < 0.05) compared to ZT and ABA, as TPN concentrations increased. Hormones stimulated the MCs production significantly BSL (p < 0.05) at various levels and protected the cells against TPN-induced oxidative stress and expression of mcyB and mcyD genes involve in MCs synthesis. Our results indicated that hormone contamination in eutrophic lakes might increase the risk of Microcystis aeruginosa bloom and microcystin production with the TPN association.

Ferulic acid exerts Nrf2-dependent protection against prenatal lead exposure-induced cognitive impairment in offspring mice

Prenatal and/or early postnatal exposure to lead (Pb) may be associated with deficits in cognitive function in the toddler offspring, and oxidative stress likely play a central role in mediating these adverse effects. Here, we tested the hypothesis that ameliorative effect of ferulic acid (FA) on lead-induced cognitive deficits attributed to its antioxidant properties in a nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-dependent manner in the context of prenatal Pb exposure. To test this hypothesis, Nrf2 knockout and C57BL/6 wild type mouse dams were exposed/unexposed to PbAc (250 ppm) during gestation day 5 to postnatal day 14 via drinking water, and FA (50 mg/kg)/vehicle was administered orally to dams for 31 d. Spatial learning and memory in pups was assessed by Morris water maze. Biochemical assays, real-time PCR, western blot techniques were employed to evaluate oxidative stress and signaling pathways in the brain of pups. We report that lead acetate (PbAc) leads to deficits in cognitive functions in offspring, which were partially attenuated by FA (P<.05). In parallel, pretreatment with FA also significantly inhibited the PbAc-induced oxidative stress, as indicated by a change in NAD+/NADH ratio, glutathione (GSH) and malondialdehyde contents (all P<.05). Interestingly, FA significantly elevated the glutamate cysteine ligase and heme oxygenase 1 at levels of transcription and translation in both mice exposed and unexposed to Pb, increasing de novo synthesis of GSH (all P<.05). Furthermore, maternal FA administration activates extracellular signal-regulated kinases 1 and 2 and promotes more Nrf2 nuclear accumulation by increasing the Nrf2 total protein in brain of offspring mice (all P<.05). Conversely, FA failed to influence Pb-induced both memory deficits and oxidative stress in offspring of Nrf2 knockout mice (all P≥.05), suggesting that Nrf2 is essential in mediating the cognition-enhancing and antioxidant effects of FA. Overall, our results demonstrate that FA protects against Pb-induced offspring's cognitive deficits, suggesting that it is a promising candidate for the treatment of Pb toxicity.

Bioregulators: unlocking their potential role in regulation of the plant oxidative defense system

Plant bioregulators play an important role in managing oxidative stress tolerance in plants. Utilizing their ability in stress sensitive crops through genetic engineering will be a meaningful approach to manage food production under the threat of climate change. Exploitation of the plant defense system against oxidative stress to engineer tolerant plants in the climate change scenario is a sustainable and meaningful strategy. Plant bioregulators (PBRs), which are important biotic factors, are known to play a vital role not only in the development of plants, but also in inducing tolerance in plants against various environmental extremes. These bioregulators include auxins, gibberellins, cytokinins, abscisic acid, brassinosteroids, polyamines, strigolactones, and ascorbic acid and provide protection against the oxidative stress-associated reactive oxygen species through modulation or activation of a plant's antioxidant system. Therefore, exploitation of their functioning and accumulation is of considerable significance for the development of plants more tolerant of harsh environmental conditions in order to tackle the issue of food security under the threat of climate change. Therefore, this review summarizes a new line of evidence that how PBRs act as inducers of oxidative stress resistance in plants and how they could be modulated in transgenic crops via introgression of genes. Reactive oxygen species production during oxidative stress events and their neutralization through an efficient antioxidants system is comprehensively detailed. Further, the use of exogenously applied PBRs in the induction of oxidative stress resistance is discussed. Recent advances in engineering transgenic plants with modified PBR gene expression to exploit the plant defense system against oxidative stress are discussed from an agricultural perspective.

The Role of Phytohormones in Enhancing Metal Remediation Capacity of Algae

Heavy metal (HM) contamination of the environment is a major issue worldwide, creating an ever-increasing demand for remediation techniques. Remediation with algae offers an ecologically safe, cost-effective, and efficient option for HM removal. Similar to plants, growth and development of algae are controlled by the hormonal system, where phytohormones are involved in HM tolerance and thus can regulate remediation ability; however, the underlying mechanisms of phytohormone function remain elusive. This review aims to draw a comprehensive model of phytohormone contributions to algal performance under HM stress. We focus on the mechanisms of HM biosorption, uptake and intracellular storage, and on how phytohormones interact with algal defence systems under HM exposure. We provide examples of successful utilization of algae in remediation, and of post-processing of algal materials. Finally, we discuss the advantages and risks of using algae for remediation. An in-depth understanding of these processes can inform effective HM remediation techniques.

Indole-3-butyric acid priming reduced cadmium toxicity in barley root tip via NO generation and enhanced glutathione peroxidase activity

Activation of GPX and enhanced NO level play a key role in IBA-mediated enhanced Cd tolerance in young barley roots. Application of exogenous indole-3-acetic acid (IAA) or an IAA precursor improves the tolerance of plants to heavy metals. However, the physiology of these tolerance mechanisms remains largely unknown. Therefore, we studied the priming effect of indole-3-butyric acid (IBA), an IAA precursor, on mild and severe cadmium (Cd) stress-induced responses in roots of young barley seedlings. IBA, similarly to mild Cd stress, significantly increased the glutathione peroxidase (GPX) activity in the apexes of barley roots, which remained elevated after the IBA pretreatment as well. IBA pretreatment-evoked high nitric oxide generation in roots effectively reduced the high superoxide level under the severe Cd stress, leading to less toxic peroxynitrite accumulation accompanied by markedly reduced Cd-induced cell death. On the other hand, the IBA-evoked changes in IAA homeostasis resulted in root growth reorientation from longitudinal elongation to radial swelling. However, the application of an IAA signaling inhibitor, following the activation of defense responses by IBA, was able to promote root growth even at high concentrations of Cd. Based on the results, it can be concluded that the application of IBA, as an effective activator of Cd tolerance mechanisms in young barley roots, and the subsequent use of an IAA signaling inhibitor for the inhibition of root morphogenic responses induced by altered auxin metabolism, results in a high degree of root Cd tolerance, helping it to withstand even the transient exposure to lethal Cd concentration without the absolute inhibition of root growth.

Reactive oxygen species and their applications toward enhanced lipid accumulation in oleaginous microorganisms

Oleaginous microorganisms are among the most promising alternative sources of lipids for oleochemicals and biofuels. However, in the course of lipid production, reactive oxygen species (ROS) are generated inevitably as byproducts of aerobic metabolisms. Although excessive accumulation of ROS leads to lipid peroxidation, DNA damage, and protein denaturation, ROS accumulation has been suggested to enhance lipid synthesis in these microorganisms. There are many unresolved questions concerning this dichotomous view of ROS influence on lipid accumulation. These include what level of ROS triggers lipid overproduction, what mechanisms and targets are vital and whether ROS act as toxic byproducts or cellular messengers in these microorganisms? Here we review the current state of knowledge on ROS generation, antioxidative defense system, the dual effects of ROS on microbial lipid production, and ROS-induced lipid peroxidation and accumulation mechanisms. Toward the end, the review summarizes strategies that enhance lipid production based on ROS manipulation.

Auxins and Cytokinins Regulate Phytohormone Homeostasis and Thiol-Mediated Detoxification in the Green Alga Acutodesmus obliquus Exposed to Lead Stress

Phytohormones, such as auxins and cytokinins, take part in the integration of growth control and stress response, but their role in algal adaptation to heavy metal remains to be elucidated. The current research indicated that lead (Pb), one of the most toxic metals in nature, causes severe depletion of endogenous cytokinins, auxins, and gibberellin and an increase in abscisic acid content in the green alga Acutodesmus obliquus. Exogenous auxins and cytokinins alleviate Pb toxicity through the regulation of the endogenous phytohormones’ levels. Exogenously applied auxins provoked the coordinated activation metal tolerance mechanisms leading to the increase in phytochelatin synthase activity and accumulation of phytochelatins and their precursors, which are essential for Pb sequestration. On the other hand, phytochelatin synthesis decreased in algal cells treated with cytokinins. Significant changes in the levels of low molecular weight metabolites, mainly involved in metal chelation and glutathione synthesis pathway under the influence of phytohormones in algal cells growing in the presence of Pb stress, were observed. This is the first report showing that auxins and cytokinins are important regulatory factors in algal adaptation strategies to heavy metal stress based on thiol-mediated compounds and the maintenance of phytohormone homeostasis.

The evolutionary origins of auxin transport: what we know and what we need to know

Auxin, represented by indole-3-acetic acid (IAA), has for a long time been studied mainly with respect to the development of land plants, and recent evidence confirms that canonical nuclear auxin signaling is a land plant apomorphy. Increasing sequential and physiological data show that the presence of auxin transport machinery pre-dates the emergence of canonical signaling. In this review, we summarize the present state of knowledge regarding the origins of auxin transport in the green lineage (Viridiplantae), integrating both data from wet lab experiments and sequence evidence on the presence of PIN-FORMED (PIN), PIN-LIKES (PILS), and AUXIN RESISTANT 1/LIKE-AUX1 (AUX1/LAX) homologs. We discuss a high divergence of auxin carrier homologs among algal lineages and emphasize the urgent need for the establishment of good molecular biology models from within the streptophyte green algae. We further postulate and discuss two hypotheses for the ancestral role of auxin in the green lineage. First, auxin was present as a by-product of cell metabolism and the evolution of its transport was stimulated by the need for IAA sequestration and cell detoxification. Second, auxin was primarily a signaling compound, possibly of bacterial origin, and its activity in the pre-plant green algae was a consequence of long-term co-existence with bacteria in shared ecological consortia.

Oxidative stress and toxicology of Cu2+ based on surface areas in mixed cultures of green alga and cyanobacteria: The pivotal role of H2O2

The toxicity of heavy metals in algal monocultures is well studied and is mediated by reactive oxygen and nitrogen species (ROS/RNS). However, little is known about the toxicity of heavy metals and the mechanisms involved in mixed cultures. Here we examine the oxidative stress and toxic effects of Cu²⁺ on the green alga Dunaliella salina (DS) and the cyanobacteria Synecochoccus elongatus (SE) in both mono- and mixed cultures. We find that both species benefit in mixed cultures and acquire higher resistance to Cu²⁺ toxicity, with a particularly marked effect on SE. DS has a larger surface area than SE, so increases in the number of DS cells compared to SE diminishes the proportion of SE surface area exposed to Cu²⁺, and contributes to increasing cyanobacterial resistance in mixed cultures. However, these mixed cultures also display as an unexpected property an increased resistance of DS in mixed cultures. SE and DS cells showed significant differences on the kinetics of H₂O₂ production and antioxidant capacities. The integrated (overall) redox response of mixed cultures, in terms of total amount of H₂O₂ produced, was proportional to the total surface area of algal species exposed to Cu²⁺, independent of algal composition in mixed systems. However, mixed cultures display emergent properties, as the time course of H₂O₂ accumulation is not a simple function of the composition of the mixed cultures. Emergent properties are also observed in the speed of membrane lipid oxidation by the two species, as measured using mixed cultures in which only one of the two species is labeled using the membrane oxidation indicator C₁₁-BODIPY^581/591. We suggest that, in addition to H₂O₂¸ other redox signals (e.g. NO) and allelochemicals (auxins, cytokinins, etc.) may be used to construct a complex inter-species communication network. This could allow mixed algal systems, whatever their composition, to integrate their cellular responses and perform as a coherent unit against toxic Cu²⁺ ions.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.