Water status shapes responses to ultraviolet radiation in two stress-tolerant mosses

Mosses cycle between hydrated, metabolically active periods and desiccated quiescence, with water status constraining when they can actively respond to stressors such as ultraviolet radiation (UVR). We tested how UVR responses depended on water status in the closely related mosses Syntrichia ruralis and S. caninervis. Using a fully factorial chronic UVR x hydration experiment, we combined transcriptomics with untargeted metabolomics of soluble and cell-wall bound fractions. Across both species, drying elicited the broadest transcriptomic and soluble metabolomic responses. The breadth of the UVR-associated transcriptomic response diverged between species across hydration states: the number of transcripts differing between UVR treatments increased during drying in S. ruralis but decreased in S. caninervis. Wall-associated chemistry showed a strong species-specific UVR signature, with nearly all significant interaction features attributable to species x UVR, while UVR x hydration interactions were rare. Nevertheless, conditional UVR responses in the wall-associated metabolome were much broader in hydrated than drying tissue. Water status therefore altered the extent of UVR associated changes in transcript and metabolite abundance, and the two species differed in how those responses were distributed across hydration states and molecular compartments.