Single-molecule Sequencing of the Desiccation-Tolerant Grass Oropetium Thomaeum

Oropetium thomaeum is a resurrection plant that can survive extreme water stress through desiccation to complete dryness, providing a model for drought tolerance; here, whole-genome sequencing and assembly of the Oropetium genome using single-molecule real-time sequencing is reported. Oropetium thomaeum is a resurrection plant that can endure extreme water stress through desiccation to complete dryness whilst retaining the ability to revive when water is available, providing a model for drought tolerance. These authors report whole-genome sequencing and assembly of the O. thomaeum genome, using only single-molecule real-time (SMRT) long-read sequencing. Understanding the genomic mechanisms of extreme desiccation tolerance in resurrection plants such as Oropetium may provide targets for engineering drought and stress tolerance in crop plants. Plant genomes, and eukaryotic genomes in general, are typically repetitive, polyploid and heterozygous, which complicates genome assembly1. The short read lengths of early Sanger and current next-generation sequencing platforms hinder assembly through complex repeat regions, and many draft and reference genomes are fragmented, lacking skewed GC and repetitive intergenic sequences, which are gaining importance due to projects like the Encyclopedia of DNA Elements (ENCODE)2. Here we report the whole-genome sequencing and assembly of the desiccation-tolerant grass Oropetium thomaeum. Using only single-molecule real-time sequencing, which generates long (>16 kilobases) reads with random errors, we assembled 99% (244 megabases) of the Oropetium genome into 625 contigs with an N50 length of 2.4 megabases. Oropetium is an example of a ‘near-complete’ draft genome which includes gapless coverage over gene space as well as intergenic sequences such as centromeres, telomeres, transposable elements and rRNA clusters that are typically unassembled in draft genomes. Oropetium has 28,466 protein-coding genes and 43% repeat sequences, yet with 30% more compact euchromatic regions it is the smallest known grass genome. The Oropetium genome demonstrates the utility of single-molecule real-time sequencing for assembling high-quality plant and other eukaryotic genomes, and serves as a valuable resource for the plant comparative genomics community.