Test system stability and natural variability of a Lemna gibba L. bioassay

Phytoremediation of toxic chemicals in aquatic environment with special emphasis on duckweed mediated approaches

The discharge of toxic chemicals into water bodies and their linked detrimental effects on health is a global concern. Phytoremediation, an environment-friendly plant-based technology, has gained intensive interest over the last decades. For the aquatic phytoremediation process, the commonly available duckweeds have recently attracted significant attention due to their capacity to grow in diverse ecological niches, fast growth characteristics, suitable morphology for easy handling of biomass, and capacity to remove and detoxify various potential toxic elements and compounds. This review presents the progress of duckweed-assisted aquatic phytoremediation of toxic chemicals. A brief background of general phytoremediation processes, including the different phytoremediation methods and advances in understanding their underlying mechanisms, has been described. A summary of different approaches commonly practiced to assess the growth of the plants and their metal removal capacity in the phytoremediation process has also been included. A vast majority of studies have established that duckweed is an efficient plant catalyst to accumulate toxic heavy metals and organic contaminants, such as pesticides, fluorides, toxins, and aromatic compounds, reducing their toxicity from water bodies. The potential of this plant-based phytoremediation process for its downstream applications in generating value-added products for the rural economy and industrial interest has been identified.

Ecotoxicity Study of Additives Composed of Zinc and Boron

The high use of additives containing zinc borate and their limited solubility in water both lead to its persistence and accumulation in biological systems. On the other hand, soluble forms of boron are easily available to plant roots and are taken up by plants. There are no ecotoxicological data available for zinc borate, the industrial utilization of which is widespread. Therefore, the potential toxicity of zinc borate and its dissociated compounds was evaluated. Based on two different ecotoxicology tests, their effect on plant growth was studied. Firstly, the impact on Lemna minor growth was investigated, including the effect on pigment content. Secondly, the inhibition of the root growth of higher plant species Sinapis alba (mustard), Lactuca sativa (lettuce) and Trifolium pretense (clover) was measured. The growth inhibition test on L. minor was more complex and sensitive compared to the plant seed germination test. Already low concentrations (10 mg/L) of ZnO, B₂O₃ and Zn₃BO₆ led to a decrease in frond growth and to an inhibition of the conversion of chlorophyll a to chlorophyll b. These results suggested that the stress caused by these additives caused damage to the photosynthetic apparatus. The highest inhibition of frond growth was detected in fronds treated with B₂O₃ (92-100%). In ZnO and Zn₃BO₆, the inhibition of frond growth was between 38 and 77%, with Zn₃BO₆ being slightly more toxic. In the seed germination test, the most sensitive species was lettuce, the growth of which was inhibited by 57, 83 and 53% in ZnO, B₂O₃ and Zn₃BO₆ treatments, respectively. However, the inhibitory effect on each plant was different. In lettuce and clover, the seed germination and root elongation decreased with increasing element concentrations. In contrast, in mustard, low concentrations of ZnO and Zn₃BO₆ supported the growth of roots. For that reason, more complex tests are essential to evaluate the additive toxicity in the environment.

Assessment of Various Toxicity Endpoints in Duckweed (Lemna minor) at the Physiological, Biochemical, and Molecular Levels as a Measure of Diuron Stress

The common, broad-spectrum herbicide diuron poses some risks to the environment due to its long persistence and high toxicity. Therefore, the effective monitoring of diuron residues will inform efforts to assess its impacts on ecosystems. In this study, we evaluated the toxicity targets of diuron in the model aquatic macrophyte Lemna minor at the physiological (growth and photosynthetic efficiency), biochemical (pigment biosynthesis and reactive oxygen species (ROS) levels), and molecular (rbcL transcript) levels. The toxicity of diuron was detectable after 48 h of exposure and the order of sensitivity of toxicity endpoints was gene transcription > maximum electron transport rate (ETR_max) > non-photochemical quenching (NPQ) > maximum quantum yield (F_v/F_m) > ROS > fresh weight > chlorophyll b > chlorophyll a > total frond area > carotenoids. Under diuron stress, pigment, ROS, and gene transcript levels increased while frond area, fresh weight, and photosynthesis (F_v/F_m and ETR_max) gradually decreased with the increasing duration of exposure. Notably, ROS levels, F_v/F_m, frond area, and fresh weight were highly correlated with diuron concentration. The growth endpoints (frond area and fresh weight) showed a strong negative correlation with ROS levels and a positive correlation with F_v/F_m and ETR_max. These findings shed light on the relative sensitivity of different endpoints for the assessment of diuron toxicity.

Evaluation of pharmaceutical toxic effects of non-standard endpoints on the macrophyte species Lemna minor and Lemna gibba

In the last years the environmental presence of pharmaceuticals has gained increasing attention. Research data show that these compounds can cause toxicological effects in different species of fish, mollusks and macroinvertebrates. However, the literature is scarce in terms of ecotoxicity data especially focusing on plants as test organisms. Ecotoxicological plant-based tests following the standard OEDC guideline 221 (OECD, 2006) are strongly restricted due to the recommended end-points: growth and yield of plants. It is necessary to develop and validate alternative macrophyte-based tests (non-standard endpoints), more sensible and providing additional information about the chemical contamination effects in plants. To attain this purpose, species from the Lemna genus were selected. Thus, the aim of this study was to analyze the toxic effects of pharmaceuticals in non-standard endpoints on two macrophyte species, Lemna minor and Lemna gibba. To this purpose an acute assay (96 h) was performed with L. minor and L. gibba exposed to chlorpromazine (CPZ), paracetamol (APAP), and diclofenac (DCF), in the following concentration ranges: 0 to 20 μg/L, 0 to 125 μg/L, and 0 to 100 μg/L, respectively. The analyzed endpoints were: levels of chlorophyll a and b, total chlorophyll, carotenoids, anthocyanins; chlorophyll fluorescence; and catalase activity. In general, higher concentrations of the tested pharmaceuticals caused significant effects on both Lemna species in terms of the different endpoints analyzed. In conclusion, acute exposures to CPZ, APAP, and DCF differently affected the defensive system of the tested species; among chlorophylls, chlorophyll b content was more affected, but pharmaceutical exposure was not able to cause alterations on chlorophyll fluorescence.

Development of a whole plant bioassay to test effects of potentized calcium carbonate in pillule formulation

OBJECTIVES

From a pharmaceutical point of view, we see a need to develop stable preclinical test systems to identify and investigate effects of potentized remedies as used in Anthroposophic Medicine and Homeopathy. We evaluated a plant bioassay regarding its capacity to distinguish homeopathic remedies from placebo, applied as sucrose pillules.

METHODS

Pea seed (Pisum sativum L) was soaked for 24 hours in water with dissolved homeopathic or placebo pillules, or in water only. Shoot length was measured 14 days after planting and treatment groups were compared by analysis of variance (ANOVA). The stability of the system was validated by systematic negative control experiments.

RESULTS

The system is suitable to test a common application form - sucrose pillules - of a potentized preparation without influence of the pharmaceutical carrier substance. A screening of 13 potentized preparations revealed Calcium carbonicum to affect pea shoot growth (p < 0.05). Three independent series of main experiments were performed with potentized Calcium carbonicum to assess reproducibility. Meta-analysis of all data revealed significant effects of Calcium carbonicum 12c and 30c on pea shoot growth (p < 0.05), which were however dependent on the date of experiment and/or the experimental series.

CONCLUSIONS

Potentized Calcium carbonicum, applied as sucrose pillules, influenced pea shoot growth in the assay investigated. However, due to the small effect size and due to the modulation of the effects by still unknown external factors, further optimization of this bioassay is necessary to be used in pharmaceutical quality control or in investigating the biological or pharmaceutical mode of action of potentized preparations.

Retrospective analyses of archive phytotoxicity test data can help in assessing internal dynamics and stability of growth in laboratory duckweed cultures

High growth potential of duckweed species (Lemnaceae family) has been utilized in wide range of research and practical applications. Based on literature data, however, it can be assumed that duckweed populations maintain constant growth rates only when short periods are considered but can vary over longer time scales. This intrinsic instability in growth can affect the interpretation of growth data. Duckweed phytotoxicity tests are usually performed according to highly standardized protocols. Therefore the archive data provide an opportunity for retrospective comparisons. In the present study we collected growth (frond number- and frond area-based relative growth rates) and morphology (average frond and colony sizes) data from control treatments of phytotoxicity tests. All the analyzed tests were carried out with the same Spirodela polyrhiza (L.) Schleid. (giant duckweed) clone (RDSC ID No. 5501) under the same experimental conditions over more than four years. We aimed to assess the overall variability of the above parameters and to test if intrinsic growth patterns affect growth data in short-term. In general, the results reflected high stability of the measured parameters in long term but also indicated that some temporal variability is inevitable which can bias the comparability of growth tests. The frond area-based relative growth rate resulted in smaller coefficient of variation than the usually preferred frond number-based one. The results also revealed a negative correlation between mean growth rates and their coefficients of variation. Therefore, it would be advisable to introduce higher minimal growth rates and/or maximized tolerable coefficients of variation for control cultures into the standard duckweed growth inhibition tests. Analyses of growth data aggregated on seasonal basis indicated faster growth and larger mean frond size in laboratory duckweed cultures from mid-autumn till mid-spring than during summer and early autumn. But, in shorter term (∼50 days) we did not observe distinct trends in growth suggesting that the successive frond generations have no effect on growth traits within this time-scale. Our results point to the importance of assessing intrinsic growth dynamics in duckweed cultures and also to the re-usability of the already collected phytotoxicity data in addressing new research questions.

Effects of treated industrial wastewaters and temperatures on growth and enzymatic activities of duckweed (Lemna minor L.)

The efficacy of the removal of contaminants from wastewater depends on physico-chemical properties of pollutants and the efficiency of treatment plant. Sometimes, low amounts of toxic compounds can be still present in the treated sewage. In this work we considered the effects of contaminant residues in treated wastewaters and of temperatures on Lemna minor L. Treated effluent waters were collected, analyzed and used as duckweed growth medium. In order to better understand the effects of micropollutants and seasonal variation, the plants were grown under ambient conditions for seven days in summer and winter. Relative growth rate, pigments and phenolic compounds concentrations were determined, as well as the activities of catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (G-POD) and polyphenol oxidase (PPO). The pollutant concentrations varied in the two seasons, depending on the industrial and municipal activities and efficiency of treatments. Treated waters contained heavy metals, nitrogenous and phosphorus compounds, surfactants and hydrocarbons. Compared to the control, duckweed growth of treated plants decreased by 25% in summer, while in the winter due to the lower temperatures and the presence of pollutants was completely impeded. The amounts of photosynthetic pigments of treated plants were not significantly affected in the summer, while they were higher than the control in the winter when the effluent had a high nitrogen amount. High CAT activity was registered in both seasons. Treated plants had significantly lower APX activity in the summer (53%) and winter (59%) respect to the controls. The observed inhibition of the peroxidase activities in the exposed plants, confirms the controversy existing in the literature about the variability of enzymatic response in stress condition.

Reproducibility of the effects of homeopathically potentised Argentum nitricum on the growth of Lemna gibba L. in a randomised and blinded bioassay

BACKGROUND

A previous study reported a significant statistical interaction between experiment date and treatment effect of Argentum nitricum 14x-30x on the growth rate of duckweed (Lemna gibba L.). The aim of the present study was to investigate the stability of the test system and intra-laboratory reproducibility of the effects found.

METHODS

Duckweed was treated with A. nitricum potencies (14x-30x) as well as succussed and unsuccussed water controls. The outcome parameter area-related growth rate for day 0-7 was determined by a computerised image analysis system in two series of independent randomised and blinded experiments. Systematic negative control (SNC) experiments were carried out to investigate test system stability. Statistical analysis was performed with full two-way analysis of variance (ANOVA) and protected Fisher's Least Significant Difference (LSD) test.

RESULTS

In the first repetition series we found a significant treatment effect (p = 0.016), while in the second series no effect was observed. The negative control experiments showed that the experimental system was stable. An a posteriori subgroup analysis concerning gibbosity revealed the importance of this growth state of L. gibba for successful reproduction of the statistically significant interaction in the original study; flat: no interaction (p = 0.762); slight gibbosity: no interaction (p = 0.356); medium gibbosity: significant interaction (p = 0.031), high gibbosity: highly significant interaction (p = 0.005).

CONCLUSIONS

With the original study design (disregarding gibbosity status of L. gibba) results of the original study could not be reproduced sensu stricto. We conclude that the growth state gibbosity is crucial for successful reproduction of the original study. Different physiological states of the test organisms used for bioassays for homeopathic basic research must carefully be considered.

Comparing the acute sensitivity of growth and photosynthetic endpoints in three Lemna species exposed to four herbicides

An ecological impact assessment of four herbicides (atrazine, diuron, paraquat and simazine) was assessed using the aquatic floating vascular plants, Lemna gibba, Lemna minor and Lemna paucicostata as test organisms. The sensitivity of several ecologically relevant parameters (increase in frond area, root length after regrowth, maximum and effective quantum yield of PSII and maximum electron transport rate (ETR_max), were compared after a 72 h exposure to herbicides. The present test methods require relatively small sample volume (3 mL), shorter exposure times (72 h), simple and quick analytical procedures as compared with standard Lemna assays. Sensitivity ranking of endpoints, based on EC₅₀ values, differed depending on the herbicide. The most toxic herbicides were diuron and paraquat and the most sensitive endpoints were root length (6.0-12.3 μg L^-1) and ETR_max (4.7-10.3 μg L^-1) for paraquat and effective quantum yield (6.8-10.4 μg L^-1) for diuron. Growth and chlorophyll a fluorescence parameters in all three Lemna species were sensitive enough to detect toxic levels of diuron and paraquat in water samples in excess of allowable concentrations set by international standards. CV values of all EC₅₀s obtained from the Lemna tests were in the range of 2.8-24.33%, indicating a high level of repeatability comparable to the desirable level of <30% for adoption of toxicity test methods as international standards. Our new Lemna methods may provide useful information for the assessment of toxicity risk of residual herbicides in aquatic ecosystems.

Reproducibility of effects of homeopathically potentised gibberellic acid on the growth of Lemna gibba L. in a randomised and blinded bioassay

BACKGROUND

Reproducibility of basic research investigations in homeopathy is challenging. This study investigated if formerly observed effects of homeopathically potentised gibberellic acid (GA3) on growth of duckweed (Lemna gibba L.) were reproducible.

METHODS

Duckweed was grown in potencies (14x-30x) of GA3 and one time succussed and unsuccussed water controls. Outcome parameter area-related growth rate was determined by a computerised image analysis system. Three series including five independent blinded and randomised potency experiments (PE) each were carried out. System stability was controlled by three series of five systematic negative control (SNC) experiments. Gibbosity (a specific growth state of L. gibba) was investigated as possibly essential factor for reactivity of L. gibba towards potentised GA3 in one series of potency and SNC experiments, respectively.

RESULTS

Only in the third series with gibbous L. gibba L. we observed a significant effect (p = 0.009, F-test) of the homeopathic treatment. However, growth rate increased in contrast to the former study, and most biologically active potency levels differed. Variability in PE was lower than in SNC experiments. The stability of the experimental system was verified by the SNC experiments.

CONCLUSIONS

Gibbosity seems to be a necessary condition for reactivity of L. gibba to potentised GA3. Further still unknown conditions seem to govern effect direction and the pattern of active and inactive potency levels. When designing new reproducibility studies, the physiological state of the test organism must be considered. Variability might be an interesting parameter to investigate effects of homeopathic remedies in basic research.

Comparing the sensitivity of geographically distinct Lemna minor populations to atrazine

The objectives of this study were to compare the sensitivities of field populations and a laboratory culture of a duckweed species (Lemna minor) to the herbicide atrazine using three different endpoints and to determine whether sensitivity to atrazine was affected by past exposure to the herbicide. L. minor cultures were purchased commercially or collected from field sites within an agricultural watershed and exposed to atrazine for 7 days under greenhouse conditions. Populations differed significantly in their sensitivity to atrazine. Biomass was more sensitive than frond number, while chlorophyll fluorescence was not a sensitive endpoint. Overall, the sensitivity of the various populations to atrazine was not strongly related to measures of past exposure to agriculture stressors. Positive correlations between biomass twenty-five percent inhibition concentrations (IC25s), biomass estimated marginal means and in-stream atrazine concentrations were observed, providing evidence that atrazine exposure is linked to a decrease in sensitivity to atrazine. However, IC25s generated for each population were similar, ranging from 19 to 40 and 57 to 92 μg/L atrazine for biomass and frond data respectively, and likely do not represent biologically significant differences in atrazine sensitivity. Given the small range in sensitivity observed between populations, commercial laboratory cultures appear to provide a good estimate of the sensitivity of field populations of L. minor to atrazine and should continue to be used in regulatory phytotoxicity testing.

Characterization of SSU5C promoter of a rbcS gene from duckweed (Lemna gibba)

Photosynthesis-associated nuclear genes are able to respond to multiple environmental and developmental signals. Studies have shown that light signals coordinate with hormone signaling pathways to control photomorphogenesis. A small subunit of ribulose-1,5 bisphosphate carboxylase/oxygenase (rbcS) gene promoter was cloned from duckweed (Lemna gibba). Sequence analysis revealed this promoter is different from the previously reported rbcs promoters and is named SSU5C. Analysis of T1 transgenic tobacco plants with a reporter gene under the control of the SSU5C promoter revealed that this promoter is tissue-specific and is positively regulated by red light. Promoter deletion analysis confirmed a region from position -152 to -49 relative to the start of transcription containing boxes X, Y and Z, and is identified to be critical for phytochrome responses. Further functional analysis of constructs of box-X, Y, Z, which was respectively fused to the basal SSU5C promoter, defined boxes X, Y and Z alone are able to direct phytochrome-regulated expression, indicating that boxes Y and Z are different from those of the SSU5B promoters in L. gibba. This promoter may be used for plant gene expression in a tissue-specific manner.

Duckweed (Lemna minor) as a model plant system for the study of human microbial pathogenesis

Background

Plant infection models provide certain advantages over animal models in the study of pathogenesis. However, current plant models face some limitations, e.g., plant and pathogen cannot co-culture in a contained environment. Development of such a plant model is needed to better illustrate host-pathogen interactions.

Methodology/Principal Findings

We describe a novel model plant system for the study of human pathogenic bacterial infection on a large scale. This system was initiated by co-cultivation of axenic duckweed (Lemna minor) plants with pathogenic bacteria in 24-well polystyrene cell culture plate. Pathogenesis of bacteria to duckweed was demonstrated with Pseudomonas aeruginosa and Staphylococcus aureus as two model pathogens. P. aeruginosa PAO1 caused severe detriment to duckweed as judged from inhibition to frond multiplication and chlorophyll formation. Using a GFP-marked PAO1 strain, we demonstrated that bacteria colonized on both fronds and roots and formed biofilms. Virulence of PAO1 to duckweed was attenuated in its quorum sensing (QS) mutants and in recombinant strains overexpressing the QS quenching enzymes. RN4220, a virulent strain of S. aureus, caused severe toxicity to duckweed while an avirulent strain showed little effect. Using this system for antimicrobial chemical selection, green tea polyphenols exhibited inhibitory activity against S. aureus virulence. This system was further confirmed to be effective as a pathogenesis model using a number of pathogenic bacterial species.

Conclusions/Significance

Our results demonstrate that duckweed can be used as a fast, inexpensive and reproducible model plant system for the study of host-pathogen interactions, could serve as an alternative choice for the study of some virulence factors, and could also potentially be used in large-scale screening for the discovery of antimicrobial chemicals.

Effects of potentised substances on growth rate of the water plant Lemna gibba L

OBJECTIVES

This study investigated, whether the growth rate of Lemna gibba L. (duckweed) can be influenced by the application of homeopathic potencies of gibberellic acid, kinetin, argentum nitricum, and lemna minor.

METHODS

Duckweed was grown in either potencies (14x-30x, decimal steps) or water controls (unsuccussed and succussed) over seven days. Frond (leaf-like structure) growth was measured using a non-destructive image analysis system. Growth rates were calculated for three time intervals (0-7, 0-3, 3-7 days). Five to six independent, randomized and blinded experiments were analysed for each of the four tested substances. Water control experiments were performed repeatedly to test the reliability of the experimental set-up (systematic negative controls).

RESULTS

The systematic negative control experiments did not yield any significant effects. Hence, false positive results could be excluded. The test system had a low coefficient of variation (1.5%). Out of the four tested substances gibberellic acid had the most pronounced effect (p=0.0002, F-test) on the main outcome parameter frond growth rate (r(area) day 0-7). Potency levels 15x, 17x, 18x, 23x and 24x reduced growth rate of Lemna gibba (p<0.05 against the pooled water control, LSD test).

CONCLUSIONS

Lemna gibba may be considered as a suitable test organism for further studies on the efficacy of homeopathic potencies. Evidence accumulates, that adjacent potency levels may strongly differ in their biological activity. Potential consequences for therapeutical application might be worth investigating.