Importance of Duckweeds in Basic Research and Their Industrial 1 Applications Paul Fourounjian, Tamra Fakhoorian and Xuan Hieu Cao Abstract fuel prices rose and the US Department of The Lemnaceae family, commonly called Energy funded the sequencing of the Spirodela duckweeds, is 37 species of the smallest and polyrhiza genome. This launched not only the simplest flowering plants found floating on genomic investigations detailed in this book, nutrient-rich waters worldwide. Their small but the regrowth of duckweed industrial size and rapid clonal growth in aseptic condi- applications. Thanks to their ability to quickly tions made them a stable and simple model for absorb nitrogen, phosphorous, and other nutri- plant research especially from 1950 to 1990, ents while removing pathogens and growing at when they were used to study plant physiology a rate of 13–38 dry tons/hectare year in water and biochemistry including auxin synthesis treatment lagoons, scientists are currently and sulfur metabolism. Duckweed research exploring ways that duckweed can convert then saw a resurgence in 2008 when global agricultural and municipal wastewater into clean water and a high-protein animal feed. The potential of these plants for phytoremedi- ation of heavy metals and organic compounds fi This chapter was revised and signi cantly expanded also allows the possibility to clean the wastew- upon, with the guidance of T. F., from the chapter “The Importance and Potential of Duckweeds as a Model and ater from heavy industry while providing Crop Plant for Biomass-Based Applications and biofuels and even plastics. Finally, thanks to ” Beyond, in the Handbook on Environmental Materials their superb nutritional profile Wolffia species Management, which X. H. C. and P. F. wrote for Springer Nature a year ago (Cao et al. 2018). We hope grown in clean conditions promise to become this chapter thoroughly explains non-genomic research one of the healthiest and most environmentally and application topics, especially for those who are unfamiliar with the family. friendly vegetables. Given the importance of these incredible plants, it is no wonder P. Fourounjian (&) researchers are investigating the genetic mech- Waksman Institute of Microbiology, anisms that make it all possible. Rutgers University, Piscataway 08854, USA e-mail: [email protected] T. Fakhoorian The International Lemna Association, Mayfield, KY 42066, USA X. H. Cao Institute of Biology/Plant Physiology, University of Halle-Wittenberg Martin-Luther-Weinbergweg, 10, 06120 Halle, Germany © Springer Nature Switzerland AG 2020 1 X. H. Cao et al. (eds.), The Duckweed Genomes, Compendium of Plant Genomes, https://doi.org/10.1007/978-3-030-11045-1_1 2 P. Fourounjian et al. 1.1 Introduction Duckweed (known as monocotyledon family Lemnaceae or recently classified as subfamily Lemnoideae in the arum or aroid family Araceae) is a small group of aquatic plants with only five genera (Spirodela, Landoltia, Lemna, Wolffia, and Wolffiella) and 37 species (see Landolt 1986; Nauheimer et al. 2012; Sree et al. 2016). Except for Wolffiella (commonly named as bogmat) that is restricted to the Americas and Africa, species of other duckweed genera occur around the whole world. Although highly adaptable across a broad Fig. 1.1 Morphology of five representative species for range of climates, most diverse species of duck- duckweed genera. Spirodela: Spirodela polyrhiza; Lan- doltia: Landoltia punctata; Lemna: Lemna minor; Wolf- weed appear in the subtropical or tropical zones. fiella: Wolffiella lingulata; Wolffia: Wolffia arrhiza. Bar: Duckweed species tend to be associated with 1cm nutrient-rich or eutrophic freshwater environ- ments with quiet or slow-moving flow. However, they are extremely rare in deserts and are absent Only occasionally or very rarely, several in the cold polar regions (Arctic and Antarctica). species of duckweeds produce microscopic fl Duckweed species are the smallest flowering owers in nature as well as under in vitro con- plants with minute sizes from 0.5 mm to less ditions (Fu et al. 2017; Schmitz and Kelm 2017; than two cm (Landolt 1986). Species of duck- Sree et al. 2015a). In Spirodela and Lemna (be- weed can be easily distinguished morphologi- longing to the subfamily Lemnoideae), the fl cally from species of any other flowering plants, owering organs (1 membranous scale, 2 sta- even closely related aquatic plants, due to their mens, and 1 pistil) originate in the same pouches highly reduced body structure. The leaflike body in which the daughter fronds are normally fi of the duckweed species, sometimes called a formed. In the subfamily Wolf oideae (consisting fi fi frond or thallus, is a modified stem with only few of Wolf ella and Wolf a), generative and vege- cellular differentiations (Fig. 1.1). The growth of tative reproductions are spatially separated fl duckweed vegetatively occurs by budding within occupying the oral cavity on the upper surface the pouches or cavities of the basal sections of of the frond and the budding pouch, respectively. fl the fronds. Each daughter frond emerging from Duckweed fronds are free oating on or near the pouch of mother bud already contains two the surface of the water, often forming dense new generations of daughter fronds. Therefore, mats in suitable climatic and nutrient conditions. under optimal conditions, the growth rate of In unfavorable weather, such as drought or fl duckweed is nearly exponential. The frond freezing winter seasons, in addition to owering, number of fast-growing species (e.g., Lemna several duckweed species are able to form special “ ” aequinoctialis, Wolffiella hyalina, and Wolffia resting fronds (in the dormant phase) to persist microscopica) almost doubles within 24 h until conditions return that can support growth. (Ziegler et al. 2015; Sree et al. 2015b), presenting In place of a frond, the greater duckweed (Spir- the fastest growing flowering plants. With a odela polyrhiza) produces a starch-rich tissue miniaturized body plan and such rapid growth called a turion, which sinks to the bottom of the leading to maximum fitness, duckweed has water. Turion production has been reported also fi arguably been interpreted as an example of the for Lemna turionifera, L. aequinoctialis, Wolf a fi fi hypothetical Darwin–Wallace Demon for the brasiliensis, Wolf a borealis, Wolf a angusta, fi fi fi lifetime reproductive success (Kutschera and Wolf a australiana, Wolf a arrhiza, Wolf a fi Niklas 2015). columbiana, and Wolf a globosa. These turions 1 Importance of Duckweeds in Basic Research and Their Industrial … 3 do not grow any further but can germinate and in a quite simple and straight forward manner, start a new life cycle from the bottom of the combination of different techniques or using water body or mud when the water temperature additional barcodes may help to unambiguously reaches about 15 °C. In addition, resting fronds and economically assign remaining duckweed of the ivy duckweed (Lemna trisulca) and species. Wolffiella gladiata with reduced air spaces can The Lemnaceae family was one of the earliest accumulate starch and still rather slowly grow on model plants due to their ease of aseptic culti- the bottom of the water, forming new but similar vation in the laboratory and simple morphology. fronds. However, the common duckweed (Lemna The second volume of Landolt and Kandeler’s minor), gibbous duckweed (Lemna gibba), 1987 monographic study contains 360 pages Lemna perpusilla, and some strains of Lemna dedicated to the physiological research of the japonica produce starch-rich fronds that do not family in particular and plants as a whole (Lan- sink to the bottom of the water but are just dolt and Kandeler 1987). The professors who pressed down under the ice cover during freezing organized the first duckweed conference summed temperatures. Interestingly, formation of turions up the duckweed research stating that duckweeds as a survival and adaptive capacity of S. poly- were the main model for plant biology from 1950 rhiza strains collected from a wide geographical to 1990, when Arabidopsis and rice were used range seems to be genetically determined and for their sexual reproduction and applicability to highly influenced by the mean annual tempera- terrestrial crops (Zhao et al. 2012). In that time, ture of habitats (Kuehdorf et al. 2013). Further- investigations of duckweeds revealed the more, the family displays significant inter- and tryptophan-independent synthesis of auxin (Baldi intraspecies differences of cell physiology (e.g., et al. 1991), translational regulation in eukaryotes starch, protein, and oil contents) together with (Slovin and Tobin 1982), and seven of the first duckweed potential for industrial applications stable plant mutants (Posner 1962). Today, (Alvarado et al. 2008; Appenroth et al. 2017; physiological studies continue largely in the Hou et al. 2007; Mkandawire and Dudel 2005; fields of circadian rhythm research, xenobiotic Tang et al. 2015; Yan et al. 2013; Zhang et al. plant–microbe interactions, and phytoremedia- 2009). tion and toxicology. Starting in 2011, a biannual Due to their small and abbreviated structures, series of international duckweed conferences in morphological and physiological classification of research and applications has connected and the 37 duckweed species (Spirodela: 2 species; helped expand this research community and Landoltia:1;Lemna: 13; Wolffiella: 10; Wolffia: increased public awareness and recognition of 11) can be challenging. In the past decade, for duckweed economic and environmental impor- species assignment as well as resolving intras- tance (Zhao et al. 2012; Lam et al. 2014; pecies differences, several attempts have been Appenroth et al. 2015). Together with the com- carried out to employ molecular genotyping pletion of the Spirodela genome in the year 2014 techniques, including random amplified poly- and rapid advances in sequencing technologies, morphic DNA (RAPD; Martirosyan et al. 2008), this resurgence of interest has resulted in a pro- inter-simple sequence repeats (ISSR; Fu et al. liferation of genome and transcriptome sequen- 2017; Xue et al. 2012), simple sequence repeats ces for duckweed species and ecotypes discussed (SSR; Feng et al.