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

LmGSTF3 Overexpression Enhances Cadmium Tolerance in Lemna minor

Glutathione S-transferase (GST) has been established to play an important role in regulating the responses of plants to stress, although its function and mechanisms of action in the cadmium (Cd)-tolerant Lemna minor remain unclear. In this study, we sought to identify a Cd-responsive GST gene from Lemna minor for functional analysis and mechanistic characterization. We accordingly identified a member of the GST gene family, LmGSTF3, which plays a positive role in adaptation of Lemna minor to Cd. Having successfully obtained overexpressing (OE) strains via genetic transformation, we established that these strains were characterized by elevated Cd tolerance compared with the wild-type strain, as evidenced by significant increases in growth rate, chlorophyll content, antioxidant enzyme activities, and Cd removal rate. At the transcriptome level, the OE strains were found to have a stronger regulatory ability in response to Cd, particularly with respect to photoprotection, antioxidant defense, and glycolytic metabolism, which may be key factors contributing to the Cd tolerance of Lemna minor. Our findings provide a basis for further elucidating the biochemical and molecular mechanisms underlying the Cd tolerance conferred by GST genes in Lemna minor and will potentially contribute to the utilization of Lemna minor in remediating aquatic pollution.

The Developmental Cycle of Spirodela polyrhiza Turions: A Model for Turion-Based Duckweed Overwintering?

Duckweeds are widely distributed small, simply constructed aquatic higher plants (the Lemnaceae) found on quiet freshwater surfaces. Species inhabiting temperate climates may have to cope with long periods of severe cold during the winter season. Several duckweeds form compact resting structures from the assimilatory fronds of the growing season that can bridge inhospitable conditions in a quiescent state. Of these, turions separate from the mother fronds and overwinter on the water body bottom in a dormant state. They can surface, germinate, and sprout to resume active growth upon warming in the spring. The turions of the largest duckweed, Spirodela polyrhiza, have been intensively examined as to ultrastructure, the factors governing their formation and release from dormancy, and the signals driving their germination and sprouting and the accompanying starch degradation. Comparative transcriptomics of assimilatory fronds and dormant turions are revealing the molecular features of this developmental cycle. The results illustrate an elegant sequence of reactions that ensures aquatic survival of even severe winters by frost avoidance in a vegetative mode. Since little is known about other duckweed resting fronds, the S. polyrhiza turion developmental cycle cannot be considered to be representative of duckweed resting fronds in general but can serve as a reference for corresponding investigations.

Duckweed: Beyond an Efficient Plant Model System

Duckweed (Lemnaceae) rises as a crucial model system due to its unique characteristics and wide-ranging utility. The significance of physiological research and phytoremediation highlights the intricate potential of duckweed in the current era of plant biology. Special attention to duckweed has been brought due to its distinctive features of nutrient uptake, ion transport dynamics, detoxification, intricate signaling, and stress tolerance. In addition, duckweed can alleviate environmental pollutants and enhance sustainability by participating in bioremediation processes and wastewater treatment. Furthermore, insights into the genomic complexity of Lemnaceae species and the flourishing field of transgenic development highlight the opportunities for genetic manipulation and biotechnological innovations. Novel methods for the germplasm conservation of duckweed can be adopted to preserve genetic diversity for future research endeavors and breeding programs. This review centers around prospects in duckweed research promoting interdisciplinary collaborations and technological advancements to drive its full potential as a model organism.

Characterisation of a Spontaneous Mutant of Lemna gibba G3 (Lemnaceae)

A spontaneous mutant of the duckweed Lemna gibba clone no. 7796 (known as strain G3, WT) was discovered. In this mutant clone, L. gibba clone no. 9602 (mt), the morphological parameters (frond length, frond width, root length, root diameter) indicated an enlarged size. A change in the frond shape was indicated by the decreased frond length/width ratio, which could have taxonomic consequences. Several different cell types in both the frond and the root were also enlarged. Flow cytometric measurements disclosed the genome size of the WT as 557 Mbp/1C and that of the mt strain as 1153 Mbp/1C. This represents the results of polyploidisation of a diploid clone to a tetraploid one. The mutant clone flowered under the influence of long day-treatment in half-strength Hutner's medium in striking contrast to the diploid WT. Low concentration of salicylic acid (<1 µM) induced flowering in the tetraploid mutant but not in the diploid plants. The transcript levels of nuclear-encoded genes of the photosynthetic apparatus (CAB, RBCS) showed higher abundance in light and less dramatic decline in darkness in the mt than in WT, while this was not the case with plastid-encoded genes (RBCL, PSAA, PSBA, PSBC).

Sustainable Stress Management: Aquatic Plants vs. Terrestrial Plants

The Indo-German Science and Technology Centre (IGSTC) funded an Indo-German Workshop on Sustainable Stress Management: Aquatic plants vs. Terrestrial plants (IGW-SSMAT) which was jointly organized at the Friedrich Schiller University of Jena, Germany from 25 to 27 July 2022 by Prof. Dr. Ralf Oelmüller, Friedrich Schiller University of Jena, Germany as the German coordinator and Dr. K. Sowjanya Sree, Central University of Kerala, India as the Indian Coordinator. The workshop constituted researchers working in this field from both India and Germany and brought together these experts in the field of sustainable stress management for scientific discussions, brainstorming and networking.

Evaluation of the Potential of Duckweed as a Human Food, Bioethanol Production Feedstock, and Antileukaemia Drug

This study evaluated the potential of duckweed as a human food, ethanol feedstock, and anticancer drug. First, the nutritional value of wild duckweed was reported for the first time. Its main composition was similar to that of artificially cultivated duckweed, and thus, wild duckweed can serve as a great human food source. In addition, high-starch duckweed induced by nutrient starvation was fermented into bioethanol. A yield of 0.262 g/g, the highest duckweed-ethanol yield reported thus far, was achieved, indicating that duckweed is an excellent feedstock for ethanol production. Finally, the anticancer effects of duckweed flavonoids (DFs) were assessed for the first time using acute myeloid leukaemia (AML) cells as models in vitro and in vivo. The results revealed that DFs possessed antileukaemia activity and were safe and effective for AML therapy. In conclusion, duckweed was demonstrated to be helpful for humans for food security, energy crisis remediation, and tumour treatment.

Duckweeds for Phytoremediation of Polluted Water

Tiny aquatic plants from the Lemnaceae family, commonly known as duckweeds, are often regarded as detrimental to the environment because of their ability to quickly populate and cover the surfaces of bodies of water. Due to their rapid vegetative propagation, duckweeds have one of the fastest growth rates among flowering plants and can accumulate large amounts of biomass in relatively short time periods. Due to the high yield of valuable biomass and ease of harvest, duckweeds can be used as feedstock for biofuels, animal feed, and other applications. Thanks to their efficient absorption of nitrogen- and phosphate-containing pollutants, duckweeds play an important role in the restorative ecology of water reservoirs. Moreover, compared to other species, duckweed species and ecotypes demonstrate exceptionally high adaptivity to a variety of environmental factors; indeed, duckweeds remove and convert many contaminants, such as nitrogen, into plant biomass. The global distribution of duckweeds and their tolerance of ammonia, heavy metals, other pollutants, and stresses are the major factors highlighting their potential for use in purifying agricultural, municipal, and some industrial wastewater. In summary, duckweeds are a powerful tool for bioremediation that can reduce anthropogenic pollution in aquatic ecosystems and prevent water eutrophication in a simple, inexpensive ecologically friendly way. Here we review the potential for using duckweeds in phytoremediation of several major water pollutants: mineral nitrogen and phosphorus, various organic chemicals, and heavy metals.

Species- and Metal-Specific Responses of the Ionome of Three Duckweed Species under Chromate and Nickel Treatments

In this study, growth and ionomic responses of three duckweed species were analyzed, namely Lemna minor, Landoltia punctata, and Spirodela polyrhiza, were exposed for short-term periods to hexavalent chromium or nickel under laboratory conditions. It was found that different duckweed species had distinct ionomic patterns that can change considerably due to metal treatments. The results also show that, because of the stress-induced increase in leaf mass-to-area ratio, the studied species showed different order of metal uptake efficiency if plant area was used as unit of reference instead of the traditional dry weight-based approach. Furthermore, this study revealed that μXRF is applicable in mapping elemental distributions in duckweed fronds. By using this method, we found that within-frond and within-colony compartmentation of metallic ions were strongly metal- and in part species-specific. Analysis of duckweed ionomics is a valuable approach in exploring factors that affect bioaccumulation of trace pollutants by these plants. Apart from remediating industrial effluents, this aspect will gain relevance in food and feed safety when duckweed biomass is produced for nutritional purposes.

Strategies for Intraspecific Genotyping of Duckweed: Comparison of Five Orthogonal Methods Applied to the Giant Duckweed Spirodela polyrhiza

The predominantly vegetative propagating duckweeds are of growing commercial interest. Since clonal accessions within a respective species can vary considerably with respect to their physiological as well as biochemical traits, it is critical to be able to track the clones of species of interest after their characterization. Here, we compared the efficacy of five different genotyping methods for Spirodela polyrhiza, a species with very low intraspecific sequence variations, including polymorphic NB-ARC-related loci, tubulin-gene-based polymorphism (TBP), simple sequence repeat variations (SSR), multiplexed ISSR genotyping by sequencing (MIG-seq), and low-coverage, reduced-representation genome sequencing (GBS). Four of the five approaches could distinguish 20 to 22 genotypes out of the 23 investigated clones, while TBP resolved just seven genotypes. The choice for a particular method for intraspecific genotyping can depend on the research question and the project budget, while the combination of orthogonal methods may increase the confidence and resolution for the results obtained.

Intraspecific Diversity in Aquatic Ecosystems: Comparison between Spirodela polyrhiza and Lemna minor in Natural Populations of Duckweed

Samples of two duckweed species, Spirodela polyrhiza and Lemna minor, were collected around small ponds and investigated concerning the question of whether natural populations of duckweeds constitute a single clone, or whether clonal diversity exists. Amplified fragment length polymorphism was used as a molecular method to distinguish clones of the same species. Possible intraspecific diversity was evaluated by average-linkage clustering. The main criterion to distinguish one clone from another was the 95% significance level of the Jaccard dissimilarity index for replicated samples. Within natural populations of L. minor, significant intraspecific genetic differences were detected. In each of the three small ponds harbouring populations of L. minor, based on twelve samples, between four and nine distinct clones were detected. Natural populations of L. minor consist of a mixture of several clones representing intraspecific biodiversity in an aquatic ecosystem. Moreover, identical distinct clones were discovered in more than one pond, located at a distance of 1 km and 2.4 km from each other. Evidently, fronds of L. minor were transported between these different ponds. The genetic differences for S. polyrhiza, however, were below the error-threshold of the method within a pond to detect distinct clones, but were pronounced between samples of two different ponds.