Research and application in duckweeds: A review

The free-floating aquatic duckweeds consist of the five genera, Spirodela, Landoltia, Lemna, Wolffiella and Wolffia, comprising 36 species within the monocot order of Alismatales. They are strongly adaptable to various environments and are widely distributed in a variety of climates all over the world except that Wolffiella species are restricted to the Americas and Africa. Among them, Spirodela species have large a flattened frond with 5−11 rhizoids while the rootless Wolffia species form an oval-shaped frond and are the smallest flowering plants. Duckweeds seldom flower and their rapid asexual propagation allows them to accumulate biomass quickly. They usually bud new generations (daughter fronds) from their reproductive pouches at the base of the mother fronds and the frond number almost doubles within 24 hunder good growth conditions yielding the fastest-growing flowering plants. They tend to be associated with nutrient-rich or eutrophic freshwater environments with slow-moving flow such as ponds, lakes, ditches, and paddy fields. While duckweeds become more reduced from Spirodela to Wolffia, their genome size varies 13-fold, ranging from 150 Mb in Spirodela polyrhiza to 1881 Mb in Wolffia arrhiza. With the development of long-read sequencing technology, the genomes of common duckweed species including S. polyrhiza, Lemna minor, Lemna gibba and Wolffia australiana have been sequenced and assembled. The underlying molecular mechanisms and regulatory networks in duckweeds under different treatments such as starvation, salt stress, heavy metal and radiation have also been revealed by transcriptome, proteome and metabolome analyses for the genera Spirodela, Landoltia, and Lemna enhancing their potential applications in the fields of phytoremediation and bioenergy. Furthermore, genetic transformation systems for some common duckweeds such as L. minor, Lemna aequinoctialis, S. polyrhiza and Wolffia globosa have been established, and especially the frond transformation systems have significantly improved transformation efficiencies (more than 90%). These characteristics have made duckweeds model systems for several decades for the study of plant physiology, biochemistry, ecotoxicology, and have contributed to knowledge of auxin biosynthesis, plant flowering and the circadian system. Duckweeds have also been widely used for the treatment of agricultural, municipal, and even industrial wastewater because they can uptake nitrogen and phosphorus as well as quickly accumulate heavy metals and organic pollutants. Besides, their accumulated biomass is rich in starch and protein and therefore can be used for feed applications and biofuels. In this review, we introduce the origin, classification and evolution of duckweeds, as well as research in morphology and anatomy, physiology, genetic transformation and omics studies. We further discuss the application of duckweeds in food, feed, bioenergy, and bioremediation. Finally, we highlight the challenges and future directions of duckweed research, providing a reference for basic biological research and resource utilization.