Abstract
The freshwater plant water lettuce (Pistia stratiotes L.) grows in warm climatic zones and is used for phytoremediation and biomass production. P. stratiotes belongs to the Araceae, an ecologically and structurally diverse early monocot family, but the phylogenetic relationships among Araceae members are poorly understood. Ribosomal DNAs (rDNAs), including the 35S and 5S rDNA, encode the RNA components of ribosomes and are widely used in phylogenetic and evolutionary studies of various plant taxa. Here, we comprehensively characterized the chromosomal locations and molecular organization of 35S and 5S rDNA genes in water lettuce using karyological and molecular methods. Fluorescence in situ hybridization revealed a single location for the 35S and 5S rDNA loci, each on a different pair of the species’ 28 chromosomes. Molecular cloning and nucleotide sequencing of 35S rDNA of P. stratiotes, the first representative Araceae sensu stricto in which such a study was performed, displayed typical structural characteristics. The full-length repeat showed high sequence conservation of the regions producing the 18S, 5.8S, and 25S rRNAs and divergence of the internal transcribed spacers ITS1 and ITS2 as well as the large intergenic spacer (IGS). Alignments of the deduced sequence of 18S rDNA with the sequences available for other Araceae and representatives of other clades were used for phylogenetic analysis. Examination of 11 IGS sequences revealed significant intra-genomic length variability due to variation in subrepeat number, with four types of units detected within the 35S rDNA locus of the P. stratiotes genome (estimated size 407 Mb/1C). Similarly, the 5S rDNA locus harbors gene units comprising a conserved 119-bp sequence encoding 5S rRNA and two types of non-transcribed spacer (NTS) sequences. Type I was classified into four subtypes, which apparently originated via progressive loss of subrepeats within the duplicated NTS region containing the 3’ part of the 5S rRNA gene. The minor Type II NTS is shorter than Type I and differs in nucleotide composition. Some DNA clones containing two or three consecutive 5S rDNA repeats harbored 5S rDNA genes with different types of NTSs, confirming the mosaic composition of the 5S rDNA locus.
Highlights
Water lettuce (Pistia stratiotes) belongs to a monospecific genus in the subfamily Aroideae of the ecologically and structurally diverse ancient monocot family Araceae sensu lato, a group of 118 genera comprising approximately 3,800 species (Henriquez et al, 2014)
Alignment of the obtained sequences showed that these sequences shared 100 and 99.6% similarity, respectively, with the corresponding ATP and PSB spacers of the P. stratiotes chloroplast genome deposited in GenBank
The P. stratiotes 35S Ribosomal DNA (rDNA) locus displayed at least four length variants of the gene, which differ in the number of repeats within the intergenic spacers (IGSs), but not in the promoter or transcribed sequences
Summary
Water lettuce (Pistia stratiotes) belongs to a monospecific genus in the subfamily Aroideae of the ecologically and structurally diverse ancient monocot family Araceae sensu lato ( known as aroids), a group of 118 genera comprising approximately 3,800 species (Henriquez et al, 2014). Water lettuce, which floats in fresh water, displays rapid, mostly vegetative propagation, and high biomass accumulation. In many locations, these features qualify P. stratiotes as an invasive species that is difficult to eliminate (Paolacci et al, 2018). P. stratiotes plants have tremendous potential for water bioremediation due to their capacity for fast and efficient assimilation of nitrogen and phosphate, heavy metals, and other water contaminants. Lu et al (2010) determined that water lettuce was superior to most other plants for efficient wastewater bioremediation due to its capability to annually remove 190–329 kg/ha of nitrogen and 25–34 kg/ha of phosphorus. Its high amounts of proteins and carbohydrates make P. stratiotes a valuable biomass resource for use as a green fertilizer or soil amendment (Kodituwakku and Yatawara, 2020) or as feedstock for the production of nitrogendoped biochar (Zhang et al, 2021)
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