Responses of Landoltia punctata to cobalt and nickel: Removal, growth, photosynthesis, antioxidant system and starch metabolism

Screening potential antileukemia agents from duckweed: Integration of chemical profiling, network pharmacology, and experimental validation

INTRODUCTION

The identification of active dietary flavonoids in food is promising for novel drug discovery. The active ingredients of duckweed (a widely recognized food and herb with abundant flavonoids) that are associated with acute myeloid leukemia (AML) have yet to be identified, and their underlying mechanisms have not been elucidated.

OBJECTIVES

The objective of this study was to identify novel constituents exhibiting antileukemia activity in duckweed through the integration of chemical profiling, network pharmacology, and experimental validation.

METHODS

First, high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was used to characterize the primary constituents of duckweed. Subsequently, AML cell-xenograft tumor models were used to validate the anticancer effect of duckweed extract. Furthermore, network pharmacology analysis was conducted to predict the potential active compounds and drug targets against AML. Lastly, based on these findings, two monomers (apiin and luteoloside) were selected for experimental validation.

RESULTS

A total of 17 compounds, all of which are apigenin and luteolin derivatives, were identified in duckweed. The duckweed extract significantly inhibited AML cell growth in vivo. Furthermore, a total of 88 targets for duckweed against AML were predicted, with key targets including PTGS2, MYC, MDM2, VEGFA, CTNNB1, CASP3, EGFR, TP53, HSP90AA1, CCND1, MMP9, TNF, and MAPK1. GO and KEGG pathway enrichment analyses indicated that these targets were primarily involved in the apoptotic signaling pathway. Lastly, both apiin and luteoloside effectively induced apoptosis through CASP3 activation, and this effect could be partially reversed by a caspase inhibitor (Z-VAD).

CONCLUSION

Duckweed extract has an antileukemic effect, and apiin derived from duckweed shows potential as a treatment for AML.

Exploring the ecotoxicological impact of meropenem on Lemna minor: Growth, photosynthetic activity, and oxidative stress

Meropenem is a potent carbapenem antibiotic frequently used in medical settings. Several studies have confirmed the pervasive presence of these antibiotics in wastewater treatment plants and aquatic environments. However, the effects of these substances on non-target organisms, such as plants, have not been adequately monitored. Thus, this study aimed to assess the short-term impact of meropenem on the growth, photosynthesis, chlorophyll content, and enzyme activity of the macrophyte plant Lemna minor. The methods involved exposing the plant to meropenem under controlled conditions and assessing physiological and biochemical parameters to determine the impact on photosynthetic activity and oxidative stress. These analyses included growth rate, antioxidant enzyme activity, and photosynthetic capacity. The findings suggest that the growth rate of Lemna minor remained unaffected by meropenem at concentrations <200000 μgL-1. However, plants exposed to concentrations >20 μgL-1showed physiological alterations, such as decreased net photosynthesis rate (17%) and chlorophyll concentration (57%), compared to the control group. For acute toxicity assays, the calculated EC50 7-day and EC20 7-day were 1135 μgL-1and 33 μgL-1, respectively. In addition, in most treatments tested, meropenem caused an increase in the superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activity as a defense mechanism against oxidative stress. Our results suggest that meropenem affects photosynthetic processes and induces oxidative stress in the macrophyte plant Lemna minor. Further studies are needed to assess the physiological and metabolic interactions between antibiotics and primary producers at different long-term trophic levels.

Biomonitoring of heavy metals and their phytoremediation by duckweeds: Advances and prospects

Heavy metals (HMs) contamination of water bodies severely threatens human and ecosystem health. There is growing interest in the use of duckweeds for HMs biomonitoring and phytoremediation due to their fast growth, low cultivation costs, and excellent HM uptake efficiency. In this review, we summarize the current state of knowledge on duckweeds and their suitability for HM biomonitoring and phytoremediation. Duckweeds have been used for phytotoxicity assays since the 1930s. Some toxicity tests based on duckweeds have been listed in international guidelines. Duckweeds have also been recognized for their ability to facilitate HM phytoremediation in aquatic environments. Large-scale screening of duckweed germplasm optimized for HM biomonitoring and phytoremediation is still essential. We further discuss the morphological, physiological, and molecular effects of HMs on duckweeds. However, the existing data are clearly insufficient, especially in regard to dissection of the transcriptome, metabolome, proteome responses and molecular mechanisms of duckweeds under HM stresses. We also evaluate the influence of environmental factors, exogenous substances, duckweed community composition, and HM interactions on their HM sensitivity and HM accumulation, which need to be considered in practical application scenarios. Finally, we identify challenges and propose approaches for improving the effectiveness of duckweeds for bioremediation from the aspects of selection of duckweed strain, cultivation optimization, engineered duckweeds. We foresee great promise for duckweeds as phytoremediation agents, providing environmentally safe and economically efficient means for HM removal. However, the primary limiting issue is that so few researchers have recognized the outstanding advantages of duckweeds. We hope that this review can pique the interest and attention of more researchers.

Effect of ammonium sulfate combined with aqueous bio-chelator on Cd uptake by Cd-hyperaccumulator Solanum nigrum L

The efficacy of using plants to phytoremediate heavy metal (HM) contaminated soils can be improved using soil amendments. These amendments may both increase plant biomasses and HMs uptake. We aimed to determine the composite effect of ammonium sulfate ((NH4)2SO4) combined with the application of an aqueous stem-extracted bio-chelator (Bidens tripartita L) on the plant biomasses and cadmium (Cd) phytoextraction by Solanum nigrum L. The constant (NH4)2SO4 application mode plus bio-chelator additives collectively enhanced the shoot Cd extraction ability owing to the increased plant biomass and shoot Cd concentration by S. nigrum. The shoot Cd extraction and the soil Cd decreased concentration confirmed the optimal Cd phytoextraction pattern in K8 and K9 treatments (co-application of (NH4)2SO4 and twofold/threefold bio-chelators). Accordingly, Cd contamination risk in the soil (2 mg kg-1) could be completely eradicated (<0.2 mg kg-1) after three rounds of phytoremediation by S.nigrum based on K8 and K9 treatments through calculating soil Cd depletion. The microorganism counts and enzyme activities in rhizosphere soils at treatments with the combined soil additives apparently advanced. In general, co-application mode of (NH4)2SO4 and aqueous bio-chelator was likely to be a perfect substitute for conventional scavenger agents on account of its environmental friendliness and cost saving for field Cd contamination phytoremediation by S. nigrum.

Heavy metals/-metalloids (As) phytoremediation with Landoltia punctata and Lemna sp. (duckweeds): coupling with biorefinery prospects for sustainable phytotechnologies

Heavy metals/-metalloids can result in serious human health hazards. Phytoremediation is green bioresource technology for the remediation of heavy metals and arsenic (As). However, there exists a knowledge gap and systematic information on duckweed-based metal phytoremediation in an eco-sustainable way. Therefore, the present review offers a critical discussion on the effective use of duckweeds (genera Landoltia and Lemna)-based phytoremediation to decontaminate metallic contaminants from wastewater. Phytoextraction and rhizofiltration were the major mechanism in 'duckweed bioreactors' that can be dependent on physico-chemical factors and plant-microbe interactions. The biotechnological advances such as gene manipulations can accelerate the duckweed-based phytoremediation process. High starch and protein contents of the metal-loaded duckweed biomass facilitate their use as feedstock in biorefinery. Biorefinery prospects such as bioenergy production, value-added products, and biofertilizers can augment the circular economy approach. Coupling duckweed-based phytoremediation with biorefinery can help achieve Sustainable Development Goals (SDGs) and human well-being.

Modulation in the enzymatic antioxidants, MDA level and elicitation in conessine biomolecule in Holarrhena pubescens (medicinal tree) cultures exposed to different heavy metals: Ni, Co, Cr and As

Nodal explants of Holarrhena pubescens, an important medicinal tree, were cultured on Murashige and Skoog's medium (MS) containing 15 µM BA (control) alone and on medium supplemented with different concentrations (0, 1, 5, 25, 50, 100 and 200 mg/L) of heavy metals such as NiCl2, CoCl2, As2O3 and CrO3 to study their toxic effect. After 28 days of treatments, the nodal segments were harvested to assess the average number of shoots per explants, average shoot length, malondialdehyde content, proline content, conessine accumulation and antioxidant enzymatic activity. Among all the metals tried, best morphogenic response was achieved at 5 mg/L CrO3 where 80% culture differentiated an average of 3.21 ± 0.08 shoots per explant having 0.95 ± 0.018 cm average shoot length. Highest concentration (200 mg/L) of all the heavy metals proved lethal for morphogenesis. Maximum inhibition in average shoot number and average shoot length was observed in nodal explants treated with 25 mg/L As2O3 where an average of 0.49 ± 0.047 shoots having an average shoot length of 0.3 ± 0.02 cm. Contrarily, addition of heavy metals in culture medium proved strong elicitors, exhibiting significant enhancement in the biosynthesis of conessine, an important bioactive compound. HPLC analysis of the crude extract of in vitro grown untreated nodal cultures revealed an average of 117.06 ± 2.59 µg/g d. w. of conessine, whereas those treated with 100 mg/L of CoCl2 accounted for 297.1 ± 7.76 µg/g d. w. (an increase of 156% over control). Among the heavy metals tried, CoCl2 proved to be the best for conessine enhancement which was in the order of CoCl2 > Cr2O3 > NiCl2 > As2O3 in the nodal explants. Concomitantly, MDA content, the antioxidant enzymes activities of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GR) and ascorbate peroxidase (APX) were also observed to be differentially expressed with the increase in the heavy metals concentration from 1 to 200 mg/L. Free proline, too, increased up to 3.5-fold over control. The results obtained during the present investigation revealed that the overall response of the nodal explants in terms of morphogenesis, conessine content and antioxidant enzyme activities was metal specific as well as dose dependent.

Physiological and biochemical role of nickel in nodulation and biological nitrogen fixation in Vigna unguiculata L. Walp

Studies on the role of nickel (Ni) in photosynthetic and antioxidant metabolism, as well as in flavonoid synthesis and biological fixation nitrogen in cowpea crop are scarce. The aim of this study was to elucidate the role of Ni in metabolism, photosynthesis and nodulation of cowpea plants. A completely randomized experiment was performed in greenhouse, with cowpea plants cultivated under 0, 0.5, 1, 2, or 3 mg kg-1 Ni, as Ni sulfate. In the study the following parameters were evaluated: activity of urease, nitrate reductase, superoxide dismutase, catalase and ascorbate peroxidase; concentration of urea, n-compounds, photosynthetic pigments, flavonoids, H2O2 and MDA; estimative of gas exchange, and biomass as plants, yield and weight of 100 seeds. At whole-plant level, Ni affected root biomass, number of seeds per pot, and yield, increasing it at 0.5 mg kg-1 and leading to inhibition at 2-3 mg kg-1 (e.g. number of seeds per pot and nodulation). The whole-plant level enhancement by 0.5 mg Ni kg-1 occurred along with increased photosynthetic pigments, photosynthesis, ureides, and catalase, and decreased hydrogen peroxide concentration. This study presents fundamental new insights regarding Ni effect on N metabolism, and nodulation that can be helpful to increase cowpea yield. Considering the increasing population and its demand for staple food, these results contribute to the enhancement of agricultural techniques that increase crop productivity and help to maintain human food security.

Overexpression of the Phosphoserine Phosphatase-Encoding Gene (AtPSP1) Promotes Starch Accumulation in Lemna turionifera 5511 under Sulfur Deficiency

Duckweeds are well known for their high accumulation of starch under stress conditions, along with inhibited growth. The phosphorylation pathway of serine biosynthesis (PPSB) was reported as playing a vital role in linking the carbon, nitrogen, and sulfur metabolism in this plant. The overexpression of AtPSP1, the last key enzyme of the PPSB pathway in duckweed, was found to stimulate the accumulation of starch under sulfur-deficient conditions. The growth- and photosynthesis-related parameters were higher in the AtPSP1 transgenic plants than in the WT. The transcriptional analysis showed that the expression of several genes in starch synthesis, TCA, and sulfur absorption, transportation, and assimilation was significantly up- or downregulated. The study suggests that PSP engineering could improve starch accumulation in Lemna turionifera 5511 by coordinating the carbon metabolism and sulfur assimilation under sulfur-deficient conditions.

Duckweed Is a Promising Feedstock of Biofuels: Advantages and Approaches

With the growing scarcity of traditional sources of energy and the accompanying acute environmental challenges, biofuels based on biomass are favored as the most promising alternative. As one of the core raw materials for biomass energy, research on its production methods and synthesis mechanisms is emerging. In recent years, duckweed has been used as a high-quality new biomass feedstock for its advantages, including fast biomass accumulation, high starch content, high biomass conversion efficiency, and sewage remediation. This study provides a systematic review of the growth characteristics, starch metabolism pathways, and methods to improve starch accumulation in the new energy plant, duckweed. The study also presents a prospect that might be used as a reference for the development of duckweed as a new energy-providing plant.

Duckweed: a potential phytosensor for heavy metals

Globally, heavy metal (HM) contamination is one of the primary causes of environmental pollution leading to decreased quality of life for those affected. In particular, HM contamination in groundwater poses a serious risk to human health and the potential for destabilization of aquatic ecosystems. At present, strategies to remove HM contamination from wastewater are inefficient, costly, laborious, and often the removal poses as much risk to the environment as the initial contamination. Phytoremediation, plant-based removal of contaminants from soil or water, has long been viewed as an economical and sustainable solution to remove toxic metals from the environment. However, to date, phytoremediation has demonstrated limited successes despite a large volume of literature supporting its potential. A key aspect for achieving robust and meaningful phytoremediation is the selection of a plant species that is well suited to the task. For the removal of pollutants from wastewater, hydrophytes, like duckweed, exhibit significant potential due to their rapid growth on nutrient-rich water, ease of collection, and ability to survive in various ecosystems. As a model for ecotoxicity studies, duckweed is an ideal candidate, as it is easy to cultivate under controlled and even sterile conditions, and the rapid growth enables multi-generational studies. Similarly, recent advances in the genetic engineering and genome-editing of duckweed will enable the transition from fundamental ecotoxicity studies to engineered solutions for phytoremediation of HMs. This review will provide insight into the suitability of duckweeds for phytoremediation of HMs and strategies for engineering next-generation duckweed to provide real-world environmental solutions.

Combined effects of temperature and nutrients on the toxicity of cadmium in duckweed (Lemna aequinoctialis)

Global anthropogenic changes are altering the temperature and nutrients of the ecosystem, which might also affect the extent of cadmium (Cd) toxicity in organisms. This study aimed to investigate the combined effects of temperature and nutrient availability (here, nitrogen [N] and phosphorus [P]) on Cd toxicity in duckweed (Lemna aequinoctialis). The growth parameters, nutrient uptake, and Cd tolerance of plantlets reached their highest values for duckweed grown in medium with 28 mg/L N and 2.4 mg/L P (N:P = 11.67) at 25 °C under 1 mg/L CdCl₂ exposure. Raising the temperature (from 18 °C to 25 °C) and levels of N and P (from 0.01 N/P to 2 N/P) enhanced photosynthetic capacity and nutrient uptake, thus promoting plant growth and diluting the toxic effects of Cd. Although Cd uptake increased with increasing temperature, duckweed with relatively high biomass exhibited a lower accumulation of the toxic metal because their growth rate exceeded Cd uptake rate. Increasing N and P supply also enhanced the tolerance of duckweed to Cd by limiting Cd bioavailability. Our study therefore suggests the importance of combined effects from temperature and nutrients for Cd toxicity and provides novel insights for a comprehensive analysis of Cd toxicity associated with the environmental factors of a particular ecosystem.

Accumulation of starch in duckweeds (Lemnaceae), potential energy plants

Starch can accumulate in both actively growing vegetative fronds and over-wintering propagules, or turions of duckweeds, small floating aquatic plants belonging to the family of the Lemnaceae. The starch synthesizing potential of 36 duckweed species varies enormously, and the starch contents actually occurring in the duckweed tissues are determined by growth conditions, various types of stress and the action of growth regulators. The present review examines the effects of phytohormones and growth retardants, heavy metals, nutrient deficiency and salinity on the accumulation of starch in duckweeds with a view to obtaining high yields of starch as a feedstock for biofuel production. Biotechnological approaches to degrading duckweed starch to its component sugars and the fermentation of these sugars to bio-alcohols are also discussed.

Nickel stress-tolerance in plant-bacterial associations

Nickel (Ni) is an essential element for plant growth and is a constituent of several metalloenzymes, such as urease, Ni-Fe hydrogenase, Ni-superoxide dismutase. However, in high concentrations, Ni is toxic and hazardous to plants, humans and animals. High levels of Ni inhibit plant germination, reduce chlorophyll content, and cause osmotic imbalance and oxidative stress. Sustainable plant-bacterial native associations are formed under Ni-stress, such as Ni hyperaccumulator plants and rhizobacteria showed tolerance to high levels of Ni. Both partners (plants and bacteria) are capable to reduce the Ni toxicity and developed different mechanisms and strategies which they manifest in plant-bacterial associations. In addition to physical barriers, such as plants cell walls, thick cuticles and trichomes, which reduce the elevated levels of Ni entrance, plants are mitigating the Ni toxicity using their own antioxidant defense mechanisms including enzymes and other antioxidants. Bacteria in its turn effectively protect plants from Ni stress and can be used in phytoremediation. PGPR (plant growth promotion rhizobacteria) possess various mechanisms of biological protection of plants at both whole population and single cell levels. In this review, we highlighted the current understanding of the bacterial induced protective mechanisms in plant-bacterial associations under Ni stress.

Enhanced phytoremediation of TNT and cobalt co-contaminated soil by AfSSB transformed plant

2,4,6-trinitrotoluene (TNT) and cobalt (Co) contaminants have posed a severe environmental problem in many countries. Phytoremediation is an environmentally friendly technology for the remediation of these contaminants. However, the toxicity of TNT and cobalt limit the efficacy of phytoremediation application. The present research showed that expressing the Acidithiobacillus ferrooxidans single-strand DNA-binding protein gene (AfSSB) can improve the tolerance of Arabidopsis and tall fescue to TNT and cobalt. Compared to control plants, the AfSSB transformed Arabidopsis and tall fescue exhibited enhanced phytoremediation of TNT and cobalt separately contaminated soil and co-contaminated soil. The comet analysis revealed that the AfSSB transformed Arabidopsis suffer reduced DNA damage than control plants under TNT or cobalt exposure. In addition, the proteomic analysis revealed that AfSSB improves TNT and cobalt tolerance by strengthening the reactive superoxide (ROS) scavenging system and the detoxification system. Results presented here serve as strong theoretical support for the phytoremediation potential of organic and metal pollutants mediated by single-strand DNA-binding protein genes. SUMMARIZES: This is the first report that AfSSB enhances phytoremediation of 2,4,6-trinitrotoluene and cobalt separately contaminated and co-contaminated soil.

Physiological and morphoanatomical effects of glyphosate in Eugenia uniflora, a Brazilian plant species native to the Atlantic Forest biome

The herbicide glyphosate can cause severe ecotoxicological effects on non-target organisms. Eugenia uniflora L. (Myrtaceae) is very important for in situ environmental biomonitoring due to its wide distribution in the Atlantic Forest biome. Thus, this study aimed to evaluate the response of E. uniflora leaves to glyphosate. Eight-month-old plants were exposed to an aerial application of the herbicide at concentrations of 0, 144, 432, 864, and 1440 g a. e. ha-1 (grams of acid equivalent per hectare). Evaluations were performed on the 12th day after the glyphosate application (DAA). An accumulation of shikimic acid in the leaves of E. uniflora was observed. Glyphosate altered the photosynthetic parameters of the treated plants, with a drastic decrease in the photosynthetic rate, stomatal conductance, transpiration, and pigment content. There was an increase in Ci/Ca, lipid peroxidation, and electrolyte extravasation levels. Glyphosate also promoted ultrastructural, anatomical and visible damage to the E. uniflora leaves. Our findings indicate that glyphosate is phytotoxic to the native species E. uniflora at the tested doses. The presence of visible damage suggests that E. uniflora has remarkable potential as a bioindicator of glyphosate in the environment, making it a possible species for future biomonitoring projects.

Duckweeds: their utilization, metabolites and cultivation

Duckweeds are floating plants of the family Lemnaceae, comprising 5 genera and 36 species. They typically live in ponds or lakes and are found worldwide, except the polar regions. There are two duckweed subfamilies-namely Lemnoidea and Wolffioideae, with 15 and 21 species, respectively. Additionally, they have characteristic reproduction methods. Several metabolites have also been reported in various duckweeds. Duckweeds have a wide range of adaptive capabilities and are particularly suitable for experiments requiring high productivity because of their speedy growth and reproduction rates. Duckweeds have been studied for their use as food/feed resources and pharmaceuticals, as well as for phytoremediation and industrial applications. Because there are numerous duckweed species, culture conditions should be optimized for industrial applications. Here, we review and summarize studies on duckweed species and their utilization, metabolites, and cultivation methods to support the extended application of duckweeds in future.

Application of abscisic acid and 6-benzylaminopurine modulated morpho-physiological and antioxidative defense responses of tomato (Solanum lycopersicum L.) by minimizing cobalt uptake

A hydroponic study was conducted to determine the effects of single and/or combined application of different doses (0, 5 and 10 μM L^-1) of abscisic acid (ABA) and 6-benzylaminopurine (BAP) on cobalt (Co) accumulation, morpho-physiological and antioxidative defense attributes of tomato (Solanum lycopersicum L.) exposed to severe Co stress (400 μM L^-1). The single Co treatment (T1), prominently decreased tomato growth, relative water contents, photosynthetic pigments (chlorophyll a and chlorophyll b), whereas enhanced oxidative stress and Co accumulation in shoot and root tissues. Nonetheless, the supplementation of ABA and 6-BAP via nutrient media significantly (P < 0.05) enhanced plant biomass, root morphology and chlorophyll contents of tomato, compared to only Co treatment (T1). Moreover, the oxidative stress indicators such as malondialdehyde, proline and H₂O₂ contents were ameliorated through activation of enzymatic antioxidant activities i.e. ascorbate peroxidase, superoxide dismutase, catalase, and peroxidase, in growth modulator treatments in comparison to T1. The Co uptake, translocation (TF) and bioaccumulation factor (BAF) by shoot and root tissues of tomato were significantly reduced under all the treatments than that of T1. The supply of 6-BAP alone or in combination with ABA at 10 μM L^-1 application (T7) rate was found the most effective to reduce Co accumulation in the roots and shoots by 48.4% and 70.2% respectively than T1 treatment. It can be concluded that two plant growth modulators could improve the stress tolerance by inhibition of Co uptake in tomato plants.

Intraspecific variations in cadmium tolerance and phytoaccumulation in giant duckweed (Spirodela polyrhiza)

Duckweeds are widely recognized for the heavy metal phytoremediation. However, the intraspecific variations in biological responses of duckweeds to heavy metal remain largely unknown. Here, the toxicity and phytoaccumulation of cadmium (Cd) were synchronously evaluated in 30 accessions of giant duckweed (Spirodela polyrhiza) collected from different provenances in Southern China. Exposure to 1 μM Cd decreased relative growth rates of dry weight, fronds number and fronds area, as well as photosynthetic pigment contents, while it increased H₂O₂ accumulation, lipid peroxidation and activities of anti-oxidant enzymes in the majority of accessions. Cd treatment led to remarkable Cd accumulation but little changes in the starch content in giant duckweed. The biological responses to Cd varied among the accessions. Further correlation analysis indicated that growth traits and Cd concentration were positively correlated with Cd accumulation, while the contents of chlorophyll, H₂O₂ and MDA were negatively associated with Cd accumulation. Our results proved the great intraspecific variation in Cd tolerance of giant duckweed, suggesting a valuable natural resource for Cd phytoremediation. Moreover, different mechanisms may be exploited by S. polyrhiza for phytoaccumulation, but growth maintenance, Cd uptake and antioxidative enzyme-independent ROS-scavenging under Cd exposure are the common mechanisms contributing to Cd accumulation ability.

Tolerance of Landoltia punctata to arsenate: an evaluation of the potential use in phytoremediation programs

Plants used in phytoremediation should accumulate and tolerate a specific pollutant. Here, we aimed at evaluating a possible arsenic (As) accumulation and mechanisms of tolerance against As-induced damage in Landoltia punctata to explore this species for phytoremediation. Plants were subjected to increasing As levels. As absorption was higher with increasing As levels. The activity of superoxide dismutase and glutathione reductase as well as anthocyanin levels increased with As levels. Catalase and peroxidase activities increased in plants subjected to As levels up to 1.0 mg L^-1 and decreased at higher levels. Due to the antioxidant system, higher levels of reactive oxygen species were restrained in plants under low levels of As. However, the levels of superoxide anion, hydrogen peroxide, and lipid peroxidation increased in response to the impaired antioxidant system induced by the highest As levels. Biomass decreased in plants exposed to As and scanning electron microscopy revealed root structural damage in the root cap of plants under 3.0 mg L^-1As. This work highlights that L. punctata can be considered a hyperaccumulator species and has potential for As phytoremediation when levels are lower than 1.0 mg L^-1-a concentration 100-fold higher than that recommended for drinking water. Novelty Statement: Landoltia punctata can be considered a hyperaccumulator species and has the potential for arsenic phytoremediation when levels are lower than 1.0 mg L^-1.

Metabolic modifications to Ni excess in L. minor: Role of organic-, amino- and fatty acid profiles

In this study, the effects of excess nickel (Ni) (100 μM and 200 μM) on growth, antioxidant production, fatty acid, organic and amino acids profiles were examined in Lemna minor L. After 7 days of Ni treatment, chlorosis, growth inhibition and ROS overproduction were observed, accompanied by Ni accumulation. Interestingly, decreased malondialdehyde (MDA) levels were recorded in fronds upon Ni exposure. Fatty acid profiles in Ni-treated L. minor were characterized by increases in saturated- and decreases in unsaturated fatty acids. Ni excess increased the activities of antioxidant enzymes such as superoxide dismutase (SOD), guiacol peroxidase (GPX), and glutathione reductase (GR), and non-enzymatic antioxidants such as glutathione (GSH) and ascorbic acid (AsA); however, deactivation of ascorbate peroxidase (APX) and catalase (CAT) activities were also observed. Disruption of amino acid metabolism in Ni-exposed fronds was evidenced by the accumulation of cysteine, arginine, threonine, valine, isoleucine, leucine, lysine and phenylalanine, as well as reduced levels of tyrosine, alanine, aspartate and proline. Approximately 299%-396%, 139%-254% and 56%-97% concentration increments in citric, malic and oxalic acids, respectively, were concomitantly observed with significant decreases in tartaric, acetic, and fumaric acids in fronds subjected to Ni stress. Taken together, these results indicated that Ni stress induced negative effects on plant physiological, biochemical and morphological processes; however, it is likely that the coordination of metabolites and antioxidants may ameliorate the damaging effects of Ni accumulation.

Evaluation of cadmium hyperaccumulation and tolerance potential of Myriophyllum aquaticum

Enrichment of the hyperaccumulator bank is important for phytoremediation, and studying new hyperaccumulators has become a research hotspot. In this study, cadmium (Cd), the main representative factor of heavy-metal-polluted water, was the research object, and the Cd bioenrichment ability and tolerance of Myriophyllum aquaticum were studied for the first time. The experiment was conducted for 28 days by establishing experimental groups with different Cd concentrations (0, 10, 20, 40, 80, and 160 mg/L). The results show that M. aquaticum is a new Cd hyperaccumulator. There was no notable damage in the 40 mg/L Cd treatment group, and the Cd enrichment ability of M. aquaticum reached 17,970 ± 1020.01 mg/kg, while the bioconcentration factor (BCF) reached 449.25. At the same time, the antioxidant system (superoxide dismutase (SOD) and peroxidase (POD)) and proline (Pro) levels of M. aquaticum maintained normal plant physiology, but there were physiological anomalies in M. aquaticum at high concentrations and under long-term treatment. The results show that M. aquaticum has a high Cd bioenrichment ability and tolerance in water and can be used for phytoremediation of river water polluted by Cd.

Aquatic Plants, Landoltia punctata, and Azolla filiculoides as Bio-Converters of Wastewater to Biofuel

The aquatic plants, Azolla filiculoides, and Landoltia punctate, were used as complementing phytoremediators of wastewater containing high levels of phosphate, which simulates the effluents from textile, dyeing, and laundry detergent industries. Their complementarities are based on differences in capacities to uptake nitrogen and phosphate components from wastewater. Sequential treatment by L. punctata followed by A. filiculoides led to complete removal of NH4, NO3, and up to 93% reduction of PO4. In experiments where L. punctata treatment was followed by fresh L. punctata, PO4 concentration was reduced by 65%. The toxicity of wastewater assessed by shrimps, Paratya australiensis, showed a four-fold reduction of their mortality (LC50 value) after treatment. Collected dry biomass was used as an alternative carbon source for heterotrophic marine protists, thraustochytrids, which produced up to 35% dry weight of lipids rich in palmitic acid (50% of total fatty acids), the key fatty acid for biodiesel production. The fermentation of treated L. punctata biomass by Enterobacter cloacae yielded up to 2.14 mol H2/mole of reduced sugar, which is comparable with leading terrestrial feedstocks. A. filiculoides and L. punctata can be used as a new generation of feedstock, which can treat different types of wastewater and represent renewable and sustainable feedstock for bioenergy production.

Physiological and Transcriptomic Analysis Reveals Distorted Ion Homeostasis and Responses in the Freshwater Plant Spirodela polyrhiza L. under Salt Stress

Duckweeds are a family of freshwater angiosperms with morphology reduced to fronds and propagation by vegetative budding. Unlike other angiosperm plants such as Arabidopsis and rice that have physical barriers between their photosynthetic organs and soils, the photosynthetic organs of duckweeds face directly to their nutrient suppliers (waters), therefore, their responses to salinity may be distinct. In this research, we found that the duckweed Spirodela polyrhiza L. accumulated high content of sodium and reduced potassium and calcium contents in large amounts under salt stress. Fresh weight, Rubisco and AGPase activities, and starch content were significantly decreaseded in the first day but recovered gradually in the following days and accumulated more starch than control from Day 3 to Day 5 when treated with 100 mM and 150 mM NaCl. A total of 2156 differentially expressed genes were identified. Overall, the genes related to ethylene metabolism, major CHO degradation, lipid degradation, N-metabolism, secondary metabolism of flavonoids, and abiotic stress were significantly increased, while those involved in cell cycle and organization, cell wall, mitochondrial electron transport of ATP synthesis, light reaction of photosynthesis, auxin metabolism, and tetrapyrrole synthesis were greatly inhibited. Moreover, salt stress also significantly influenced the expression of transcription factors that are mainly involved in abiotic stress and cell differentiation. However, most of the osmosensing calcium antiporters (OSCA) and the potassium inward channels were downregulated, Na⁺/H⁺ antiporters (SOS1 and NHX) and a Na⁺/Ca²⁺ exchanger were slightly upregulated, but most of them did not respond significantly to salt stress. These results indicated that the ion homeostasis was strongly disturbed. Finally, the shared and distinct regulatory networks of salt stress responses between duckweeds and other plants were intensively discussed. Taken together, these findings provide novel insights into the underlying mechanisms of salt stress response in duckweeds, and can be served as a useful foundation for salt tolerance improvement of duckweeds for the application in salinity conditions.

Duckweed biomarkers for identifying toxic water contaminants?

Surface or ground waters can be contaminated with numerous toxic substances. The duckweeds Lemna minor and Lemna gibba are widely used for assaying waterborne toxicity to higher plants in terms of growth inhibition and photosynthetic pigment reduction. These tests cannot, however, in themselves determine the nature of the agents responsible for toxicity. Morphological, developmental, physiological, biochemical, and genetic responses of duckweeds to exposure to toxic water contaminants constitute biomarkers of toxic effect. In principle, the very detection of these biomarkers should enable the contaminants having elicited them (and being responsible for the toxicity) to be identified. However, in practice, this is severely compromised by insufficient specificity of biomarkers for their corresponding toxicants and by the lack of documentation of biomarker/toxin relationships. The present contribution illustrates the difficulties of using known water contaminant-related duckweed biomarkers to identify toxins, and discusses possibilities for achieving this goal.

Duckweed systems for eutrophic water purification through converting wastewater nutrients to high-starch biomass: comparative evaluation of three different genera (Spirodela polyrhiza, Lemna minor and Landoltia punctata) in monoculture or polyculture

This study provides a new insight into the application of duckweed in eutrophic water advanced treatment and starch production.

Clusia hilariana and Eugenia uniflora as bioindicators of atmospheric pollutants emitted by an iron pelletizing factory in Brazil

The objectives of this work were to evaluate if the pollution emitted by the pelletizing factory causes visual symptoms and/or anatomical changes in exposed Eugenia uniflora and Clusia hilariana, in active biomonitoring, at different distances from a pelletizing factory. We characterize the symptomatology, anatomical, and histochemistry alterations induced in the two species. There was no difference in the symptomatology in relation to the different distances of the emitting source. The foliar symptoms found in C. hilariana were chlorosis, necrosis, and foliar abscission and, in E. uniflora, were observed necrosis punctuais, purple spots in the leaves, and increase in the emission of new leaves completely purplish. The two species presented formation of a cicatrization tissue. E. uniflora presented reduction in the thickness of leaf. In C. hilariana, it was visualized hyperplasia of the cells and the adaxial epidermis did not appear collapsed due to thick cuticle and cuticular flanges. Leaves of C. hilariana showed positive staining for iron, protein, starch, and phenolic compounds. E. uniflora showed positive staining for total phenolic compounds and starch. Micromorphologically, there was accumulation of particulate matter on the leaf surface, obstruction of the stomata, and scaling of the epicuticular wax in both species. It was concluded that the visual and anatomical symptoms were efficient in the diagnosis of the stress factor. C. hilariana and E. uniflora showed to be good bioindicators of the atmospheric pollutants emitted by the pelletizing factory.