Time-series transcriptomics sheds light on the rapid and dynamic response of the moss Physcomitrium patens to salt stress.

The moss Physcomitrium patens is a well-established model organism for studying plant responses to abiotic stress. As a non-vascular plant, it provides a valuable system for elucidating both conserved and lineage-specific adaptive mechanisms underlying abiotic stress. Previous transcriptomic studies have focused on severe salt stress (350 mM), while the response to moderate salt stress remained largely unexplored. Here, we investigated the transcriptional responses of P. patens to a moderate 150 mM NaCl treatment, using a high-resolution time-series RNA-sequencing approach. Our analysis revealed 2235 previously uncharacterized salt-responsive genes as part of a rapid, dynamic, and multi-layered response. Time-series clustering unveiled salt-specific gene expression patterns overlapping with salt-responsive pathways from angiosperms. We identified novel transcription factors involved in salt-induced transcriptional reprogramming, including PpMYB123, which we experimentally confirmed as a salt-responsive transcription factor. Overexpression of PpMYB123 altered the expression of enzymes involved in salt stress responses, such as those in phenylpropanoid and starch biosynthesis, suggesting a direct or indirect regulatory role. In summary, our findings highlight evolutionarily conserved patterns in stress-specific signaling pathways and in the underlying transcriptional regulation between bryophytes and angiosperms.