A protocol for conducting 7-day daily renewal tests with Lemna gibba

Genetic transformation of Indian isolate of Lemna minor mediated by Agrobacterium tumefaciens and recovery of transgenic plants

Transgenic plants of an Indian isolate of Lemna minor have been developed for the first time using Agrobacterium tumefaciens and hard nodular cell masses 'nodular calli' developed on the BAP - pretreated daughter frond explants in B5 medium containing sucrose (1.0 %) with 2,4-D (5.0 μM) and 2-iP (50.0 μM) or 2,4-D (50.0 μM) and TDZ (5.0 μM) under light conditions. These calli were co-cultured with A. tumefaciens strain EHA105 harboring a binary vector that contained genes for β-glucuronidase with intron and neomycin phosphortransferase. Transformed cells selected on kanamycin selection medium were regenerated into fronds whose transgenic nature was confirmed by histochemical assay for GUS activity, PCR analysis and Southern hybridization. The frequency of transformation obtained was 3.8 % and a period of 11-13 weeks was required from initiation of cultures from explants to fully grown transgenic fronds. The pretreatment of daughter fronds with BAP, use of non-ionic surfactant, presence of acetosyringone in co-cultivation medium, co-culture duration of 3 d and 16 h photoperiod during culture were found crucial for callus induction, frond regeneration and transformation of L. minor. This transformation system can be used for the production of pharmaceutically important protein and in bioremediation.

Fate of sucralose through environmental and water treatment processes and impact on plant indicator species

The degradation and partitioning of sucralose during exposure to a variety of environmental and advanced treatment processes (ATP) and the effect of sucralose on indicator plant species were systematically assessed. Bench scale experiments were used to reproduce conditions from environmental processes (microbial degradation, hydrolysis, soil sorption) and ATPs (chlorination, ozonation, sorption to activated carbon, and UV radiation). Degradation only occurred to a limited extent during hydrolysis, ozonation, and microbial processes indicating that breakdown of sucralose will likely be slow and incomplete leading to accumulation in surface waters. Further, the persistence of sucralose was compared to suggested human tracer compounds, caffeine and acesulfame-K. In comparison sucralose exhibits similar or enhanced characteristics pertaining to persistence, prevalence, and facile detection and can therefore be considered an ideal tracer for anthropogenic activity. Ecological effects of sucralose were assessed by measuring sucrose uptake inhibition in plant cotelydons and aquatic plant growth impairment. Sucralose did not inhibit plant cotelydon sucrose uptake, nor did it effect frond number, wet weight, or growth rate in aquatic plant, Lemna gibba. Though sucralose does not appear toxic to plant growth, the peristent qualities of sucralose may lead to chronic low-dose exposure with largely unknown consequences for human and environmental health.

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.

Exploring Lemna gibba thresholds to nutrient and chemical stressors: differential effects of triclosan on internal stoichiometry and nitrate uptake across a nitrogen:phosphorus gradient

Nutrient enrichment often co-occurs with chemical stressors in aquatic ecosystems, but the impacts of these multiple stressors across nutrient gradients is poorly understood and not typically addressed in ecotoxicity studies of lower trophic level models. Moreover, laboratory assays performed to determine threshold responses of aquatic macrophytes to contaminants typically use growth and morphometric endpoints to establish threshold effects and seldom report other important functional responses of lower trophic levels. Using the aquatic macrophyte Lemna gibba, we examined influences of varying nitrogen (N) and phosphorus (P) levels in combination with triclosan, a widely used antimicrobial agent in consumer care products, on internal carbon (C):N:P and NO(3) (-) uptake kinetics. Triclosan modulated L. gibba tissue N and P content, and these stoichiometric responses for P-limited plants to triclosan exposure were more sensitive than growth endpoints employed in standardized phytotoxicity assays. Nitrate uptake capacities were also differentially inhibited by triclosan exposure according to external nutrient levels. Uptake rates for plants cultured and exposed under saturating N-levels were inhibited by more than threefold compared with N-limited plants. The results suggest that stoichiometric and nutrient uptake responses to chemical stressors provide useful information regarding adverse ecological thresholds not defined in standardized phytotoxicity assays with aquatic macrophytes. Our findings further indicate that site-specific impacts of chemicals associated with the wide ambient ranges of N and P typical of surface waters may be anticipated in lower trophic levels. Future studies should examine adverse effects of other stressors to these ecologically relevant endpoints, which may be useful in environmental assessment and management.

DNA barcoding of the Lemnaceae, a family of aquatic monocots

BACKGROUND

Members of the aquatic monocot family Lemnaceae (commonly called duckweeds) represent the smallest and fastest growing flowering plants. Their highly reduced morphology and infrequent flowering result in a dearth of characters for distinguishing between the nearly 38 species that exhibit these tiny, closely-related and often morphologically similar features within the same family of plants.

RESULTS

We developed a simple and rapid DNA-based molecular identification system for the Lemnaceae based on sequence polymorphisms. We compared the barcoding potential of the seven plastid-markers proposed by the CBOL (Consortium for the Barcode of Life) plant-working group to discriminate species within the land plants in 97 accessions representing 31 species from the family of Lemnaceae. A Lemnaceae-specific set of PCR and sequencing primers were designed for four plastid coding genes (rpoB, rpoC1, rbcL and matK) and three noncoding spacers (atpF-atpH, psbK-psbI and trnH-psbA) based on the Lemna minor chloroplast genome sequence. We assessed the ease of amplification and sequencing for these markers, examined the extent of the barcoding gap between intra- and inter-specific variation by pairwise distances, evaluated successful identifications based on direct sequence comparison of the "best close match" and the construction of a phylogenetic tree.

CONCLUSIONS

Based on its reliable amplification, straightforward sequence alignment, and rates of DNA variation between species and within species, we propose that the atpF-atpH noncoding spacer could serve as a universal DNA barcoding marker for species-level identification of duckweeds.

Genetic structure of the genus Lemna L. (Lemnaceae) as revealed by amplified fragment length polymorphism

Duckweeds (Lemnaceae) are extremely reduced in morphology, which made their taxonomy a challenge for a long time. The amplified fragment length polymorphism (AFLP) marker technique was applied to solve this problem. 84 clones of the genus Lemna were investigated representing all 13 accepted Lemna species. By neighbour-joining (NJ) analysis, 10 out of these 13 species were clearly recognized: L. minor, L. obscura, L. turionifera, L. japonica, L. disperma, L. aequinoctialis, L. perpusilla, L. trisulca, L. tenera, and L. minuta. However, L. valdiviana and L. yungensis could be distinguished neither by NJ cluster analysis nor by structure analysis. Moreover, the 16 analysed clones of L. gibba were assembled into four genetically differentiated groups. Only one of these groups, which includes the standard clones 7107 (G1) and 7741 (G3), represents obviously the "true" L. gibba. At least four of the clones investigated, so far considered as L. gibba (clones 8655a, 9481, 9436b, and Tra05-L), represent evidently close relatives to L. turionifera but do not form turions under any of the conditions tested. Another group of clones (6745, 6751, and 7922) corresponds to putative hybrids and may be identical with L. parodiana, a species not accepted until now because of the difficulties of delineation on morphology alone. In conclusion, AFLP analysis offers a solid base for the identification of Lemna clones, which is particularly important in view of Lemnaceae application in biomonitoring.

Herbicidal effects of sulfamethoxazole in Lemna gibba: using p-aminobenzoic acid as a biomarker of effect

Sulfamethoxazole (SMX) is among the most frequently detected antibiotics in the environment, heavily used in both human therapy and agriculture. Like other sulfonamides, SMX disrupts the folate biosynthetic pathway in bacteria, which was recently established as identical to that of plants, raising concerns over nontarget toxicity. Consequently, Lemna gibba was exposed to SMX to evaluate phytotoxic potency and mode of action (MOA) by HPLC-MS/MS measurement of p-aminobenzoic acid (pABA) metabolite levels, a precursor to folate biosynthesis and substrate of the target enzyme dihydropteroate synthase (DHPS). pABA levels were found to increase upon exposure to SMX following an exponential rise to a maxima regression model in a concentration-dependent manner. The EC50 for pABA content was 3.36 microg/L, 20 times lower than that of fresh weight (61.6 microg/L) and 40 times lower than frond number (132 microg/L) responses. These results suggest that, as in bacteria, sulfonamide antibiotics specifically disrupt folate biosynthesis via inhibition of DHPS. Analysis of pABA concentrations appears to provide a sulfonamide-specific biomarker of effect based on MOA with exceptional diagnostic capacity and sensitivity compared to traditional morphological end points. Using the EC50 for pABA content, a potential hazard was identified for L. gibba exposed to SMX, which would not have been detected based upon traditional standardized morphological approaches.