Characterization and Expression Analysis of the Sterol C-5 Desaturase Gene <i>PcERG3</i> in the Mycobiont of the Lichen <i>Peltigera canina</i> Under Abiotic Stresses

Lichens, symbiotic organisms with a high tolerance to harsh environments, possess a greater diversity of sterols than other organisms. Sterols are involved in maintaining membrane integrity, hormone biosynthesis, and signal transduction. (1) Background: A characteristic feature of lichen sterols is a high degree of unsaturation, which influences membrane properties. Desaturases play an important role in the synthesis of unsaturated sterols, in particular, sterol C-5 desaturase (ERG3), which controls the conversion of episterol to ergosterol. Earlier, we demonstrated that the treatment of the lichen <i>Peltigera canina</i> with low and elevated temperatures results in changes in the levels of episterol and ergosterol. (2) Methods: Here, for the first time, we identified ERG3 in <i>P. canina</i> and, using an in silico analysis, we showed that PcERG3 belongs to the superfamily of fatty acid hydrolyases. A phylogenetic analysis was conducted to determine the evolutionary relationships of PcERG3. (3) Results: A phylogenetic analysis showed that PcERG3 clusters with ERG3 from other Peltigeralian and non-Peltigeralian lichens and also with ERG3 from free-living fungi. This suggests that PcERG3 has an ancient evolutionary origin and is related to fungi with lichenized ancestors, e.g., <i>Penicillium</i>. The differential expression of <i>PcERG3</i> in response to temperature stress, a dehydration/rehydration cycle, and heavy metal exposure suggests that it plays a crucial role in maintaining the balance between more and less saturated sterols and, more generally, in membrane functioning. The multifaceted response of <i>P. canina</i> to abiotic stresses was documented by simultaneously measuring changes in the expression of <i>PcERG3</i>, as well as the genes encoding the heat shock proteins, <i>PcHSP20</i> and <i>PcHSP98</i>, and <i>PcSOD1</i>, which encodes the antioxidant enzyme superoxide dismutase. (4) Conclusions: These findings suggest that PcERG3 is similar to the sterol C-5 desaturases from related and free-living fungi and plays important roles in the molecular mechanisms underlying the tolerance of lichens to environmental stress.