Molecular Studies of <i>Umbilicaria muhlenbergii</i> and Fungal-Algal Interactions During Lichen Symbiosis

<p dir="ltr">Lichen formed by symbiotic interactions between fungal and algal cells can thrive on extremely environmental conditions and play an important role in ecology. <i>Umbilicaria muhlenbergii</i> is a dimorphic, lichen-forming fungus that is amenable to genetic manipulations and suitable for studying lichen symbiosis at the molecular level. To better understand the interactions of <i>U. muhlenbergii </i>with algal cells, we conducted transcriptome analysis with 10-day-old symbiotic complexes formed by co-incubation of the mycobiont and photobiont cells in comparison with isolated fungal and algal cells. In addition to genes that are likely associated with pseudohyphal growth and capsule formation, the upregulated fungal DEGs include putative effectors and GPCR genes, which may mediate fungal-algal interactions. Notably, several of them are related to MAPK and cAMP signaling, confirming the importance of these conserved signaling pathways for dimorphic transition and symbiotic interactions. Among the upregulated algal DEGs, we identified several genes related to plant immunity or defense signaling, suggesting immune responses in algal cells for lichenized interaction. In addition, we developed a CRISPR-mediated gene editing strategy to improve the efficiency of generating mutants in target genes. Targeted gene disruption mutants were obtained for the Ste12, Tec1, Efg1 and Gpr1 homologs in <i>U. muhlenbergii</i>. The <i>Umtec1 </i>mutants displayed a derepressed pseudohyphal growth defect, which was complemented by re-introducing the wild-type <i>UmTEC1</i> allele. Disruption of <i>UmTEC1</i> resulted in changes in cell morphology and growth patterns. As a downstream target of cAMP signaling, UmTEC1 has a conserved PKA phosphorylation site. Taken together, this study has identified several key regulators and genes important for dimorphic transition and symbiotic interactions with algal cells and developed an efficient approach for functional genomics studies in <i>U. muhlenbergii</i>, which will be beneficial to develop it into a model system for studying lichen symbiosis.</p>