CHARACTERIZING THE ATPENIN A5-LIKE BIOSYNTHETIC GENE CLUSTER IN THE NON-MODEL PLANT PATHOGEN SPHAERULINA MUSIVA

Sphaerulina musiva (syn. Septoria musiva), a stem canker pathogen of Populus species, is an unmitigated threat to ecosystems and plantations across North America. Native to the northeastern United States, this pathogen has been anthropogenically introduced to distant regions, disrupting previously unexposed ecosystems. Further study into the biology of this fungus is urgently needed to understand the ramifications of this movement. Secondary metabolites are specialized compounds that provide advantages to the producing organism, often play a critical role in mediating interactions between organisms and their surrounding environment.. The genome of S. musiva contains a biosynthetic gene cluster predicted to produce atpenin A5, a succinate dehydrogenase inhibitor. Phylogenetic analysis of this cluster highlights an ancient origin within the Ascomycota phylum, and suggests horizontal gene transfer events between S. musiva and the cypress canker pathogen Seiridium cardinale and between Eurotiomycetes. This evidence refutes previous studies that suggest the cluster originated solely from a Eurotiomycete lineage. Our research demonstrates that S. musiva produces high levels of atpenin A5 and that disruption of a clustered transcription factor, apnH, abolishes production. While infection assays across seven Populus genotypes revealed no statistically significant reduction in pathogenicity between wild-type (WT) and atpenin A5 deficient strains, we found that atpenin A5 produced by S. musiva suppress the growth of several poplar-associated fungi. Multiple sequence alignment and computational modeling trace resistance to atpenin A5 to mutations in residues of succinate dehydrogenase C. This work sheds light on the ecological function of an ancient secondary metabolite gene cluster that confers a competitive advantage to its bearer over other tree-associated fungi. Furthermore, it underscores the broader implications of fungal secondary metabolism in shaping interactions within managed forest ecosystems.