Coregulation of dimorphism and symbiosis by cyclic AMP signaling in the lichenized fungusUmbilicaria muhlenbergii
Umbilicaria muhlenbergiiis the only known dimorphic lichenized fungus that grows in the hyphal form in lichen thalli but as yeast cells in axenic cultures. However, the regulation of yeast-to-hypha transition and its relationship to the establishment of symbiosis are not clear. In this study, we show that nutrient limitation and hyperosmotic stress trigger the dimorphic change inU. muhlenbergii. Contact with algal cells of its photobiontTrebouxia jamesiiinduced pseudohyphal growth. Treatments with the cAMP diphosphoesterase inhibitor IBMX (3-isobutyl-1-methylxanthine) induced pseudohyphal/hyphal growth and resulted in the differentiation of heavily melanized, lichen cortex-like structures in culture, indicating the role of cAMP signaling in regulating dimorphism. To confirm this observation, we identified and characterized two Gα subunitsUmGPA2andUmGPA3. Whereas deletion ofUmGPA2had only a minor effect on pseudohyphal growth, the ΔUmgpa3mutant was defective in yeast-to-pseudohypha transition induced by hyperosmotic stress orT. jamesiicells. IBMX treatment suppressed the defect of ΔUmgpa3in pseudohyphal growth. Transformants expressing theUmGPA3G45VorUmGPA3Q208Ldominant active allele were enhanced in the yeast-to-pseudohypha transition and developed pseudohyphae under conditions noninducible to the wild type. Interestingly,T. jamesiicells in close contact with pseudohyphae ofUmGPA3G45VandUmGPA3Q208Ltransformants often collapsed and died after coincubation for over 72 h, indicating that improperly regulated pseudohyphal growth due to dominant active mutations may disrupt the initial establishment of symbiotic interaction between the photobiont and mycobiont. Taken together, these results show that the cAMP-PKA pathway plays a critical role in regulating dimorphism and symbiosis inU. muhlenbergii.