Abstract
Mycosphaerella fijiensis, causal agent of black Sigatoka disease of banana, is a Dothideomycete fungus closely related to fungi that produce polyketides important for plant pathogenicity. We utilized the M. fijiensis genome sequence to predict PKS genes and their gene clusters and make bioinformatics predictions about the types of compounds produced by these clusters. Eight PKS gene clusters were identified in the M. fijiensis genome, placing M. fijiensis into the 23rd percentile for the number of PKS genes compared to other Dothideomycetes. Analysis of the PKS domains identified three of the PKS enzymes as non-reducing and two as highly reducing. Gene clusters contained types of genes frequently found in PKS clusters including genes encoding transporters, oxidoreductases, methyltransferases, and non-ribosomal peptide synthases. Phylogenetic analysis identified a putative PKS cluster encoding melanin biosynthesis. None of the other clusters were closely aligned with genes encoding known polyketides, however three of the PKS genes fell into clades with clusters encoding alternapyrone, fumonisin, and solanapyrone produced by Alternaria and Fusarium species. A search for homologs among available genomic sequences from 103 Dothideomycetes identified close homologs (>80% similarity) for six of the PKS sequences. One of the PKS sequences was not similar (< 60% similarity) to sequences in any of the 103 genomes, suggesting that it encodes a unique compound. Comparison of the M. fijiensis PKS sequences with those of two other banana pathogens, M. musicola and M. eumusae, showed that these two species have close homologs to five of the M. fijiensis PKS sequences, but three others were not found in either species. RT-PCR and RNA-Seq analysis showed that the melanin PKS cluster was down-regulated in infected banana as compared to growth in culture. Three other clusters, however were strongly upregulated during disease development in banana, suggesting that they may encode polyketides important in pathogenicity.
Highlights
Mycosphaerella fijiensis is the causal agent of black Sigatoka, known as black leaf streak disease of banana and plantain
Since production of polyketides is a common strategy for pathogenesis among necrotrophic and hemibiotrophic fungi, the repertoire of PKS genes in M. fijiensis was compared to that of other Dothideomycete fungi
SMURF was used to predict PKS genes from 74 additional Dothideomycete genomes available from Joint Genome Institute (JGI) and NCBI. This analysis revealed that the M. fijiensis genome falls into the 23rd percentile for number of PKS genes compared to the other Dothideomycete genomes and into the 34th percentile for the number of PKS genes in the order Capnodiales, of which M. fijiensis is a member
Summary
Mycosphaerella fijiensis is the causal agent of black Sigatoka, known as black leaf streak disease of banana and plantain. Black Sigatoka was first described in Fiji in 1963, and since it has spread to most banana-growing regions around the world, including Latin America, Asia, Africa, and throughout the Pacific [1]. It has become one of the most important diseases of banana world-wide, causing up to 50% yield loss [2]. Development of fungicide-resistant strains is an on-going problem [3], and in developing countries, fungicide costs are prohibitive, and the disease results in severe yield losses [2]
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