Coregulation of dimorphism and symbiosis by cyclic AMP signaling in the lichenized fungus <i>Umbilicaria muhlenbergii</i>

Abstract

<i>Umbilicaria muhlenbergii</i> is 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 in <i>U. muhlenbergii</i> Contact with algal cells of its photobiont <i>Trebouxia jamesii</i> induced 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 subunits <i>UmGPA2</i> and <i>UmGPA3</i> Whereas deletion of <i>UmGPA2</i> had only a minor effect on pseudohyphal growth, the Δ<i>Umgpa3</i> mutant was defective in yeast-to-pseudohypha transition induced by hyperosmotic stress or <i>T. jamesii</i> cells. IBMX treatment suppressed the defect of Δ<i>Umgpa3</i> in pseudohyphal growth. Transformants expressing the <i>UmGPA3</i>^G45V or <i>UmGPA3</i>^Q208L dominant active allele were enhanced in the yeast-to-pseudohypha transition and developed pseudohyphae under conditions noninducible to the wild type. Interestingly, <i>T. jamesii</i> cells in close contact with pseudohyphae of <i>UmGPA3</i>^G45V and <i>UmGPA3</i>^Q208L transformants 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 in <i>U. muhlenbergii</i>.