A polyketide synthase gene cluster associated with the sexual reproductive cycle of the banana pathogen, Pseudocercospora fijiensis

Disease spread of Pseudocercospora fijiensis, causal agent of the black Sigatoka disease of banana, depends on ascospores produced through the sexual reproductive cycle. We used phylogenetic analysis to identify P. fijiensis homologs (PKS8-4 and Hybrid8-3) to the PKS4 polyketide synthases (PKS) from Neurospora crassa and Sordaria macrospora involved in sexual reproduction. These sequences also formed a clade with lovastatin, compactin, and betaenone-producing PKS sequences. Transcriptome analysis showed that both the P. fijiensis Hybrid8-3 and PKS8-4 genes have higher expression in infected leaf tissue compared to in culture. Domain analysis showed that PKS8-4 is more similar than Hybrid8-3 to PKS4. pPKS8-4:GFP transcriptional fusion transformants showed expression of GFP in flask-shaped structures in mycelial cultures as well as in crosses between compatible and incompatible mating types. Confocal microscopy confirmed expression in spermagonia in leaf substomatal cavities, consistent with a role in sexual reproduction. A disruption mutant of pks8-4 retained normal pathogenicity on banana, and no differences were observed in growth, conidial production, and spermagonia production. GC-MS profiling of the mutant and wild type did not identify differences in polyketide metabolites, but did identify changes in saturated fatty acid methyl esters and alkene and alkane derivatives. To our knowledge, this is the first report of a polyketide synthase pathway associated with spermagonia.