How melanised lichens survive drying: transcriptomic insights into desiccation and UV-B cross-tolerance of Lobaria pulmonaria.

Lichens are known for their remarkable ability to cope with a wide range of environmental stresses, particularly desiccation. Under natural conditions, lichens often experience both desiccation and ultraviolet (UV) irradiation simultaneously. Such a combination of stresses may promote the development of cross-tolerance mechanisms, but there have been few studies on these mechanisms in lichens. Previously, we reported that in the lichen Lobaria pulmonaria, UV-B induces synthesis of the dark pigment melanin and changes in gene expression and energy metabolism. The aim of the current study was to elucidate the molecular basis of lichen cross-tolerance to desiccation and UV-B stresses by characterizing the transcriptomic and metabolomic responses of pale (UV-B untreated) and melanised (UV-B treated) thalli to desiccation and rehydration. Transcriptome profiling revealed a set of genes encoding antioxidative enzymes, heat shock proteins, and enzymes of secondary metabolism that are up-regulated in response to both desiccation and UV-B irradiation. Mycobiont genes encoding antioxidative enzymes and enzymes of secondary metabolism were more highly up-regulated in pale than in melanised thalli during desiccation. Furthermore, during desiccation UV-B exposed thalli accumulated higher levels of the sugar alcohol arabitol and displayed greater photosystem II (PSII) activity and relative electron transport rate (rETR) than pale thalli. In summary, we demonstrate that UV-B exposure causes a variety of responses in the lichen L. pulmonaria, including melanisation, and these responses confer enhanced tolerance to both UV-B and desiccation stresses, revealing the existence of cross-tolerance mechanisms.