Duckweed rising at Chengdu: summary of the 1st International Conference on Duckweed Application and Research

Duckweeds, plants of the Lemnaceae family, have the distinction of being the smallest angiosperms in the world with the fastest doubling time. Together with its naturally ability to thrive on abundant anthropogenic wastewater, these plants hold tremendous potential to helping solve critical water, climate and fuel issues facing our planet this century. With the conviction that rapid deployment and optimization of the duckweed platform for biomass production will depend on close integration between basic and applied research of these aquatic plants, the first International Conference on Duckweed Research and Applications (ICDRA) was organized and took place in Chengdu, China, from October 7th to 10th of 2011. Co-organized with Rutgers University of New Jersey (USA), this Conference attracted participants from Germany, Denmark, Japan, Australia, in addition to those from the US and China. The following are concise summaries of the various oral presentations and final discussions over the 2.5 day conference that serve to highlight current research interests and applied research that are paving the way for the imminent deployment of this novel aquatic crop. We believe the sharing of this information with the broad Plant Biology community is an important step toward the renaissance of this excellent plant model that will have important impact on our quest for sustainable development of the world.     

Multifaceted roles of duckweed in aquatic phytoremediation and bioproducts synthesis

Duckweed (Lemnaceae) is a fast‐growing aquatic vascular plant. It has drawn an increasing attention worldwide due to its application in value‐added nutritional products and in sewage disposal. In particular, duckweed is a promising feedstock for bioenergy production. In this review, we summarized applications of duckweed from the following four aspects. Firstly, duckweed could utilize nitrogen, phosphorus, and inorganic nutrition in wastewater and reduces water eutrophication efficiently. During these processes, microorganisms play an important role in promoting duckweed growth and improving its tolerance to stresses. We also introduced our pilot‐scale test using duckweed for wastewater treatment and biomass production simultaneously. Secondly, its capability of fast accumulation of large amounts of starch makes duckweed a promising bioenergy feedstock, catering the currently increasing demand for bioethanol production. Pretreatment conditions prior to fermentation can be optimized to improve the conversion efficiency from starch to bioethanol. Furthermore, duckweed serves as an ideal source for food supply or animal feed because the composition of amino acids in duckweed is similar to that of whey protein, which is easily digested and assimilated by human and other animals. Finally, severing as a natural plant factory, duckweed has shown great potential in the production of pharmaceuticals and dietary supplements. With the surge of omics data and the development of Clustered Regularly Interspaced Short Palindromic Repeats technology, remodeling of the metabolic pathway in duckweed for synthetic biology study will be attainable in the future.     

Ammonium and nitrate uptake by the floating plant Landoltia punctata

BACKGROUND AND AIMS: Plants from the family Lemnaceae are widely used in ecological engineering projects to purify wastewater and eutrophic water bodies. However, the biology of nutrient uptake mechanisms in plants of this family is still poorly understood. There is controversy over whether Lemnaceae roots are involved in nutrient uptake. No information is available on nitrogen (N) preferences and capacity of Landoltia punctata (dotted duckweed), one of the best prospective species in Lemnaceae for phytomelioration and biomass production. The aim of this study was to assess L. punctata plants for their ability to take up NH4+ and NO3- by both roots and fronds. METHODS: NO3- and NH4+ fluxes were estimated by a non-invasive ion-selective microelectrode technique. This technique allows direct measurements of ion fluxes across the root or frond surface of an intact plant. KEY RESULTS: Landoltia punctata plants took up NH4+ and NO3- by both fronds and roots. Spatial distribution of NH4+ and NO3- fluxes demonstrated that, although ion fluxes at the most distal parts of the root were uneven, the mature part of the root was involved in N uptake. Despite the absolute flux values for NH4+ and NO3- being lower in roots than at the frond surface, the overall capacity of roots to take up ions was similar to that of fronds because the surface area of roots was larger. L. punctata plants preferred to take up NH4+ over NO3- when both N sources were available. CONCLUSIONS: Landoltia punctata plants take up nitrogen by both roots and fronds. When both sources of N are available, plants prefer to take up NH4+, but will take up NO3- when it is the only N source.     

Wetland degradation leads to homogenization of the biota at local and landscape scales

1. We examined whether the anthropogenic degradation of wetlands leads to homogenization of the biota at local and/or landscape scales and, if so, what specific factors account for such an effect. We compared 16 isolated wetlands (Michigan, U.S.A.) that varied in surrounding land use: half had developed, and half undeveloped, riparian zones. Samples of macrophytes, epiphytic diatoms, zooplankton, macroinvertebrates and water chemistry were collected along three transects in each wetland. 2. Developed wetlands were more nutrient‐rich with higher Cl concentrations. The plant community at developed sites was dominated by Lemnaceae (duckweed), while undeveloped wetlands were dominated by rooted, floating‐leaved vegetation and sensitive plant species. Undeveloped wetlands contained heterogeneous and species‐rich plant communities, greater species richness of zooplankton and diatoms, and heterogeneous zooplankton distributions as compared to developed sites. 3. A comparison among wetlands showed that diatom and zooplankton assemblages in developed wetlands were nested subsets of richer biota found in less developed wetlands. Conversely, plant communities were more heterogeneously distributed among developed wetlands at the landscape level. This may be attributable to patchy invasions by exotic species, which were a feature of the degraded wetlands within developed landscapes. 4. Our results indicate that several taxonomic groups showed similar, probably inter‐dependent, responses to wetland degradation and habitat homogenization at both the local and landscape scales. This change in community structure from a species‐rich and heterogeneous community dominated by floating‐leaved plants in undeveloped wetlands, to nutrient‐rich wetlands dominated by duckweed may represent a shift to an alternate stable state.     

Effect of 8-Hydroxyquinoline on Flowering and Endogenous Levels of Iron and Copper in Lemna paucicostata, Strain LP6

Lemna paucicostata LP₆, a strain of duckweed isolated locally,does not flower under any photoperiodic schedule when grownin Bonner and Devirian or other media routinely employed invarious laboratories for studies on flowering in Lemnaceae.Flowering in this strain could be induced, however, by 8-hydroxyquinoline(8-HQ)—a well-known copper chelating agent—irrespectiveof the length of the photoperiod. To our knowledge, this isthe first report where a direct induction of flowering in aduckweed by 8-HQ has been observed. Atomic absorption analysisof the plant material revealed that the endogenous level ofcopper is significantly higher in the plants treated with 8-HQ.This is contrary to the general assumption that chelating agentsinfluence flowering of duckweeds by causing a reduction in theuptake of copper ions and making them less available to theplants. (Received May 23, 1983; Accepted July 22, 1983)     

Wasserlinsen als Nutzpflanzen

Duckweeds as crop plants Members of the plant family Lemnaceae (duckweeds) are not only interesting because they represent the smallest flowering plants; they possess also the fastest rates of producing biomass. As aquatic plants, duckweed production is not in competition with other agricultural crops that require fertile land while the cultivation of duckweeds does not contribute to further eutrophication of surface water. Instead, they can be cultivated on municipal or agricultural waste water and remove the nutrients during their propagation and growth. Duckweeds can thus be used for cleaning of waste water and the resulting biomass can be valuable starting material for animal feeds and the production of biofuels. Research focusing on these goals has begun to transfer from research laboratories to pilot plants in different parts of the world, e.g. in New Jersey and North Carolina, USA; Chengdu, P. R. China; and Armidale, Australia.     

Key to the determination of taxa of Lemnaceae: an update

Species of Lemnaceae have a high potential for fast biomass production, and this is increasingly gaining attention among researchers in basic plant sciences as well as among entrepreneurs for feed, food and energy production. Hence, the correct identification of the species being used for different duckweed research and applications is becoming indispensable. Here, we present an updated identification key based on morphological markers to the currently accepted 36 species of duckweeds, considering all taxonomic revisions since the publication of the previous key by E. Landolt in 1986. We also provide supplementary morphological characterization and the geographical occurrence of each species of Lemnaceae.     

Characterization of photosynthetically active duckweed (<Emphasis Type="Italic">Wolffia australiana</Emphasis>) in vitro culture by Respiration Activity Monitoring System (RAMOS)

The feasibility of oxygen transfer rate (OTR) measurement to non-destructively monitor plant propagation and vitality of photosynthetically active plant in vitro culture of duckweed (Wolffia australiana, Lemnaceae) was tested using Respiration Activity Monitoring System (RAMOS). As a result, OTR proofed to be a sensitive indicator for plant vitality. The culture characterization under day/night light conditions, however, revealed a complex interaction between oxygen production and consumption, rendering OTR measurement an unsuitable tool to track plant propagation. However, RAMOS was found to be a useful tool in preliminary studies for process development of photosynthetically active plant in vitro cultures.     

Temporal changes of local and regional processes in the assembly of herbivorous insect communities

Herbivorous insect communities are structured by multiple processes operating locally (e.g. bottom–up effects of plants) and regionally (e.g. dispersal limitation). Although the relative strength of these processes has been well documented, how it varies in time is less understood, especially in relation with the temporal dynamics of plant communities. If temporal turnover of local plant species composition is too rapid for insect community assembly to keep up with, bottom–up effects are expected to be weak. Here, in plant and herbivorous insect communities in Japanese grasslands, we studied how the relative importance of local (bottom–up effects of plants, structures of plant communities and top–down effects of predators) and regional (dispersal limitation) processes varies over the growing season. In addition, we tested the hypothesis that larger temporal turnover of plant species composition is related to the weaker bottom–up effects, that is, the lower explanation power of plant communities for insect communities. We found that, throughout the growing season, the insect species composition was mainly explained by local variables (plant species composition, vegetation height and predator abundance), and their explanation power was higher during later phases of the season (late summer). Furthermore, the variation not explained by plant species composition was correlated with the degree of temporal turnover of plants, suggesting that insect communities failed to track the temporal turnover of plant species. These results were pronounced when we focused on leaf sucker insects, whose host plant range is presumably more limited. We conclude that herbivorous insect communities are mainly regulated by local processes, especially bottom–up effects from plants, while stochasticity may have played a role in early phases of the season. Furthermore, we underscore the importance of considering relative time scale of community assembly and environmental shifts, especially in systems characterized by dynamic changes.     

贵州省浮萍形态学鉴定与遗传多样性分析

为调查贵州省浮萍种间亲缘关系及变异情况, 研究在贵州省辖9个地级行政区的多处水环境中共采集到 41份浮萍种质, 通过形态鉴定这些种质中33个属于绿萍属(Lemna), 6个属于紫萍属(Spirodela), 2个属于斑萍属/兰氏萍属(Landoltia)。利用叶绿体atpF-atpH间隔序列和rpS16内含子序列进行分子生物学分析和鉴定, 41份浮萍种质聚类到Lemna aequinoctialis、Lemna minor、Spirodela polyrhiza和Landoltia punctata 4个种。atpF-atpH间隔序列和rpS16内含子序列的单倍型多样性分别为0.98700和0.64700, 群体突变率分别为0.15380和0.14334, 平均每kb核苷酸差异数为47.20000和61.72200, 核苷酸多态性分别为0.08725和0.09158。研究讨论了中国亚热带温润季风地区浮萍的遗传多样性, 为浮萍亲缘关系分析提供了依据, 也为亚热带湿润季风气候地区植物的种属鉴定及资源化利用奠定了基础。     

Rethinking false spring risk

Temperate plants are at risk of being exposed to late spring freezes. These freeze events—often called false springs—are one of the strongest factors determining temperate plants species range limits and can impose high ecological and economic damage. As climate change may alter the prevalence and severity of false springs, our ability to forecast such events has become more critical, and it has led to a growing body of research. Many false spring studies largely simplify the myriad complexities involved in assessing false spring risks and damage. While these studies have helped advance the field and may provide useful estimates at large scales, studies at the individual to community levels must integrate more complexity for accurate predictions of plant damage from late spring freezes. Here, we review current metrics of false spring, and how, when, and where plants are most at risk of freeze damage. We highlight how life stage, functional group, species differences in morphology and phenology, and regional climatic differences contribute to the damage potential of false springs. More studies aimed at understanding relationships among species tolerance and avoidance strategies, climatic regimes, and the environmental cues that underlie spring phenology would improve predictions at all biological levels. An integrated approach to assessing past and future spring freeze damage would provide novel insights into fundamental plant biology and offer more robust predictions as climate change progresses, which are essential for mitigating the adverse ecological and economic effects of false springs.     

Genomics in a changing arctic: critical questions await the molecular ecologist

Molecular ecology is poised to tackle a host of interesting questions in the coming years. The Arctic provides a unique and rapidly changing environment with a suite of emerging research needs that can be addressed through genetics and genomics. Here we highlight recent research on boreal and tundra ecosystems and put forth a series of questions related to plant and microbial responses to climate change that can benefit from technologies and analytical approaches contained within the molecular ecologist's toolbox. These questions include understanding (i) the mechanisms of plant acquisition and uptake of N in cold soils, (ii) how these processes are mediated by root traits, (iii) the role played by the plant microbiome in cycling C and nutrients within high‐latitude ecosystems and (iv) plant adaptation to extreme Arctic climates. We highlight how contributions can be made in these areas through studies that target model and nonmodel organisms and emphasize that the sequencing of the Populus and Salix genomes provides a valuable resource for scientific discoveries related to the plant microbiome and plant adaptation in the Arctic. Moreover, there exists an exciting role to play in model development, including incorporating genetic and evolutionary knowledge into ecosystem and Earth System Models. In this regard, the molecular ecologist provides a valuable perspective on plant genetics as a driver for community biodiversity, and how ecological and evolutionary forces govern community dynamics in a rapidly changing climate.     

Survey of duckweed diversity in Lake Chao and total fatty acid,triacylglycerol, profiles of representative strains

Lemnaceae (duckweeds) are widely distributed aquatic flowering plants. Their high growth rate, starch content and suitability for bioremediation make them potential feedstock for biofuels. However, few natural duckweed resources have been investigated in China, and there is no information about total fatty acid (TFA) and triacylglycerol (TAG) composition of duckweeds from China. Here, the genetic diversity of a natural duckweed population collected from Lake Chao, China, was investigated using multilocus sequence typing (MLST). The 54 strains were categorised into four species in four genera, representing 12 distinct sequence types. Strains representing Lemna aequinoctialis and Spirodela polyrhiza were predominant. Interestingly, a surprisingly high degree of genetic diversification within L. aequinoctialis was observed. The four duckweed species revealed a uniform fatty acid composition, with three fatty acids, palmitic acid, linoleic acid and linolenic acid, accounting for more than 80% of the TFA. The TFA in biomass varied among species, ranging from 1.05% (of dry weight, DW) for L. punctata and S. polyrhiza to 1.62% for Wolffia globosa. The four duckweed species contained similar TAG contents, 0.02% mg·DW^?1. The fatty acid profiles of TAG were different from those of TFA, and also varied among the four species. The survey investigated the genetic diversity of duckweeds from Lake Chao, and provides an initial insight into TFA and TAG of four duckweed species, indicating that intraspecific and interspecific variations exist in the content and composition of both TFA and TAG in comparison with other studies.     

Evolutionary community ecology of plant-associated arthropods in terrestrial ecosystems

In the 21st century, researchers have attempted a synthesis between community ecology and evolutionary biology. This emerging research area, which aims to synthesize community ecology and evolutionary biology, is evolutionary community ecology. Evolutionary community ecology addresses how intraspecific trait variation in community members is essential for predicting community properties and, how community properties are a key component of the selective forces that determine genetic and phenotypic variation in a community member. In this paper, I review recent findings in evolutionary community ecology in plant-associated arthropods in terrestrial ecosystems. I discuss roles of both genetic variation and phenotypic plasticity as a source of trait variation in plants in shaping plant-associated arthropod communities. Also, I discuss effects of genetic variation in herbivores on plant-associated arthropod communities. Furthermore, I highlight community context evolution in which multiple species interactions and community composition affect trait evolution of a community member. Finally, I argue that future studies should investigate a feedback loop between community and evolutionary dynamics beyond unidirectional studies on effects of evolution on a community or vice versa. This approach will provide major insights into mechanistic principles for making predictions of community ecology.     

Plant systems biology: network matters

Systems biology is all about networks. A recent trend has been to associate systems biology exclusively with the study of gene regulatory or protein-interaction networks. However, systems biology approaches can be applied at many other scales, from the subatomic to the ecosystem scales. In this review, we describe studies at the sub-cellular, tissue, whole plant and crop scales and highlight how these studies can be related to systems biology. We discuss the properties of system approaches at each scale as well as their current limits, and pinpoint in each case advances unique to the considered scale but representing potential for the other scales. We conclude by examining plant models bridging different scales and considering the future prospects of plant systems biology.     

Plants Replacing Plants: The Future of Community Modeling and Research

Over the past few decades, several conceptual and mathematical models of plant community organization and dynamics have been put forward. While each of these models has attempted to explain important plant community patterns by attributing them to some aspect of plant niches, or to a higher-level process, their predictive success has been very limited. Here I explore why this has happened by reviewing and summarizing each model individually by highlighting the plant community pattern each is trying to explain and predict, by identifying the mechanisms, tolerances, and/or processes authors propose are producing those patterns and describing how they work within the model, and by examining the assumptions of each model. I then discuss common misconceptions and shortcomings among the models, and finally propose a unifying synthesis and comprehensive framework that can serve as a basis for future plant community modeling and research. This synthesis is composed of three key ideas (1) that plant-plant replacements are the “fundamental process” of plant communities which produce every community-level terrestrial plant pattern, (2) that plants respond to mechanisms and tolerances which work both in spaces inside plants and in those spaces outside plants that influence them and/or they may be able to influence, and (3) that those responses make up plant niches which may be able to predict how plants replace themselves over time and space. Consequently I suggest to future field researchers that the best way to understand plant community patterns is to study plant-plant replacements, first by sampling long-term vegetation plots in order to map them, and then by manipulating mechanisms and tolerances in field experiments in order to understand what causes them.     

Empirical and theoretical challenges in aboveground–belowground ecology

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground–belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground–belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.     

Evolutionary ecology of plant-microbe interactions: soil microbial structure alters selection on plant traits

? Below-ground microbial communities influence plant diversity, plant productivity, and plant community composition. Given these strong ecological effects, are interactions with below-ground microbes also important for understanding natural selection on plant traits? ? Here, we manipulated below-ground microbial communities and the soil moisture environment on replicated populations of Brassica rapa to examine how microbial community structure influences selection on plant traits and mediates plant responses to abiotic environmental stress. ? In soils with experimentally simplified microbial communities, plants were smaller, had reduced chlorophyll content, produced fewer flowers, and were less fecund when compared with plant populations grown in association with more complex soil microbial communities. Selection on plant growth and phenological traits also was stronger when plants were grown in simplified, less diverse soil microbial communities, and these effects typically were consistent across soil moisture treatments. ? Our results suggest that microbial community structure affects patterns of natural selection on plant traits. Thus, the below-ground microbial community can influence evolutionary processes, just as recent studies have demonstrated that microbial diversity can influence plant community and ecosystem processes.     

Sink trap: duckweed and dye attractant reduce mosquito populations

Duckweeds, such as Lemna minor Linnaeus (Alismatales: Lemnaceae), are common in aquatic habitats and have been suggested to reduce larval mosquito survivorship via mechanical and chemical effects. Furthermore, pond dyes are used increasingly in aquatic habitats to enhance their aesthetics, although they have been shown to attract mosquito oviposition. The present study examined the coupled effects of L. minor and black pond dye on the oviposition selectivity of Culex pipiens Linnaeus (Diptera: Culicidae) mosquitoes in a series of laboratory choice tests. Subsequently, using outdoor mesocosms, the combined influence of duckweed and pond dye on mosquito abundances in aquatic habitats was quantified. Mosquitoes were strongly attracted to duckweed, and oviposited significantly greater numbers of egg rafts in duckweed-treated water compared with untreated controls, even when the duckweed was ground. The presence of pond dye interacted with the duckweed and further enhanced positive selectivity towards duckweed-treated water. The presence of duckweed caused significant and sustained reductions in larval mosquito numbers, whereas the relative effects of dye were not evident. The use of floating aquatic plants such as duckweed, combined with dye, may help reduce mosquito populations via the establishment of population sinks, characterized by high rates of oviposition coupled with high levels of larval mortality.     

Prevalence and transmission of antimicrobial resistance among <Emphasis Type="Italic">Aeromonas</Emphasis> populations from a duckweed aquaculture based hospital sewage water recycling system in Bangladesh

In order to investigate the influence of a duckweed aquaculture based hospital sewage water recycling plant on the prevalence and dissemination of antibiotic resistance, we made use of an existing collection of 1,315 Aeromonas isolates that were previously typed by the biochemical fingerprinting PhP-AE system. In these treatment plant, hospital raw sewage water is first collected in a settlement pond (referred to as sewage water in this study) and is then transferred to a lagoon, where the duckweed (Lemnaceae) is grown (referred to as lagoon). The duckweed is harvested and used as feed for the fish in a separate pond (referred to as fish pond). From this collection, representatives of 288 PhP types were subjected to antibiotic susceptibility testing for eight antimicrobials by broth microdilution method. The overall resistance rates among Aeromonas isolates from the treatment plant were highest for ampicillin (87%) and erythromycin (79%) followed by cephalothin (58%), nalidixic acid (52%), streptomycin (51%), tetracycline (31%), chloramphenicol (13%) and gentamicin (8%). A significantly lower prevalence of antibiotic resistance was found in Aeromonas from environmental control water, patient stool samples, duckweed and fish compared to sewage water isolates. The prevalence of resistance in the sewage water was not significantly reduced compared to the lagoon water and fish pond. Throughout the treatment system, the frequencies of resistant strains were found to diminish during the sewage water purification process, i.e. in the lagoon where sewage water is used to grow the duckweed. However, the frequency of resistant strains again increased in the fish pond where sewage grown duckweed is used for aquaculture. Among the selected isolates, two multiresistant clonal groups of Aeromonas caviae HG4 were identified that exhibited indistinguishable PhP and amplified fragment length polymorphism fingerprints and shared a common plasmid of approximately 5 kb. Representatives of both groups were recovered from almost every part of the sewage treatment plant but not in the control ponds nor in human samples, which suggests that specific multiresistant Aeromonas clones are able to persist and spread throughout the entire purification process.