Syntrichia ruralis: Emerging model moss genome reveals a conserved and previously unknown regulator of desiccation in flowering plants

Water scarcity poses a significant threat to ecosystems in the face of global climate change. Syntrichia ruralis, a dryland moss known for its desiccation tolerance, provides valuable insights into surviving water-limited conditions. In this study, the genome of S. ruralis was sequenced and assembled into 12 chromosomes encompassing 21,169 protein-coding genes. Additionally, 3,199 unplaced scaffolds were identified as non-nuclear and symbiont DNA. Transposable elements (TEs) constitute 51.24% of the genome. Notably, chromosome 12, the largest in size due to its high TE load, was identified as the putative sex chromosome. Comparative analysis with the closely related Syntrichia caninervis genome reveals significant large-scale synteny yet some rearrangements, as well as the occurrence of older duplication events that are shared by both. Desiccation and drought tolerance associated gene families, such as early light-inducible proteins (ELIPs) and late embryogenesis abundant (LEA) proteins, were characterized. In addition to a subset of LEA genes being species-specific, a comparative transcriptomic analysis revealed that some shared LEA genes respond differently to dehydration in these two species. Many ELIPs (9 out of 30) are the product of tandem duplication events. As expected, our analyses revealed the importance of the phytohormone abscisic acid (ABA) in the desiccation response of S. ruralis. A significant number of ABA responsive genes were found to be regulated by S. ruralis orthologs of ABA insensitive 3 (ABI3) and abscisic acid responsive element binding factor 2 (AREB2). Markedly, an uncharacterized, but deeply conserved MYB transcription factor, appears to act as a negative regulator of AREB2 in S. ruralis. Interestingly, we determined that the orthologous MYB TF is also involved in an ABA-dependent stress response in the model flowering plant A. thaliana. In sum, the new genomic resources from this emerging model moss offer new insights into the evolution of desiccation tolerance in land plants.