Format: ORAL

Anton Stepanenko1,2,3, Guimin Chen1, Phuong T. N. Hoang2,4, Todd Michael5, Eric Lam6, Ingo Schubert2, Nikolai Borisjuk1*

1 School of Life Sciences, Huaiyin Normal University, Huai’an, China 2 Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben, Germany 3 Institute of Cell Biology and Genetic Engineering, Kyiv, Ukraine 4 Faculty of Biology, Dalat University, Ðà Lat, Vietnam 5 Plant Molecular and Cellular Biology Laboratory, Salk Institute for Biological Studies, La Jolla, USA 6 Department of Plant Biology and Pathology, Rutgers the State University of New Jersey, New Brunswick, USA

Duckweeds, represented by 36 species in Lemnaceae family, are small, mostly vegetatively propagated aquatic plants with worldwide distribution and fast growth rate. Sequencing of 35S rDNA repeats in representative species of genus Spirodela, Landoltia, Wolffia and Lemna showed positive correlation of the repeats length with genome size, and significant sequence variability of the intergenic spacers. Our genome surveys of two most ancient duckweed species, S. polyrhiza (Michael et al., 2017) and S. intermedia (Hoang et al., 2020), revealed a very low representation of 35S and 5S rRNA genes, at around 100 copies per genome, and an unorthodox molecular structure of the usually conserved 35S rRNA transcription initiation site (TIS). The low rDNA copy number in Spirodela made possible to reveal detailed molecular organization of the species rDNA loci through a combination of molecular cytology, conventional sequencing and long DNA reads. The relatively GC-rich 35S rDNA and 5S rDNA arrays are imbedded in highly AT-enriched chromosome regions. Further sequence analysis demonstrated that the 5S rDNA repeat clusters, localized on two different chromosomal loci, and composed correspondingly of 40 and 60 repeat units have the non-transcribed spacers (NTS) of different size and different rates of sequence variability, suggesting contrasting evolutional dynamics of the two types of 5S rDNA units. In summary, our findings put duckweeds in the spotlight of rDNA research, promising new insights into basic principles of organization and regulation of rRNA genes. Michael et al. (2017) Plant Journal, 89(3):617-635. Hoang et al. (2020) Scientific Reports , 10(1):19230.