Alternative oxidase is involved in oxidative stress resistance and melanin synthesis in Annulohypoxylon stygium, a companion fungus of Tremella fuciformis

Annulohypoxylon stygium, a companion fungus of Tremella fuciformis, provides nutrition for growth and development of T. fuciformis. Alternative oxidase (AOX), which is the terminal oxidase of the alternative respiration pathway, functions to transfer electrons from ubiquinol to O2 with the production of H2O. In this study, an AOX encoded gene, Asaox, was cloned from A. stygium. The coding sequence of Asaox gene contains 1056 nucleotides and encodes 351 amino acids. RNA interference was used to study the function of Asaox in A. stygium. The Asaox-silenced strains were confirmed by PCR, quantitative real-time PCR, and Southern blot. The Asaox-silenced strains exhibited increased relative growth inhibition rates when inoculated on PDA plates with 50 mM H2O2, compared with wild-type strain. The growth rate in sawdust medium, melanin content, and laccase activity of Asaox-silenced strains were decreased. The expression levels of tyr and pks genes were decreased in Asaox-silenced strains, indicating that there might be two melanin synthesis pathways, DHN and DOPA, in A. stygium. In summary, the results have demonstrated that Asaox gene was involved in oxidative stress resistance and melanin synthesis in A. stygium.