Abstract
Pseudocercospora fijiensis, causal agent of black Sigatoka of banana, produces polyketide synthase (PKS) pathways shown to be important in disease development by related Dothideomycete fungi. Genome analysis of the P. fijiensis PKS8-1 gene identified it as part of a gene cluster including genes encoding two transcription factors, a regulatory protein, a glyoxylase/beta-lactamase-like protein, an MFS transporter, a cytochrome P450, two aldo/keto reductases, a dehydrogenase, and a decarboxylase. Genome analysis of the related pathogens Pseudocercospora musae, Pseudocercospora eumusae, and Pseudocercospora pini-densiflorae, identified orthologous clusters containing a nearly identical combination of genes. Phylogenetic analysis of PKS8-1 identified homology to PKS proteins in the monodictyphenone and cladofulvin pathways in Aspergillus nidulans and Cladosporium fulvum, respectively. Analysis of clustered genes showed that the PKS8-1 cluster shares genes for enzymes involved in the production of the emodin intermediate in the monodictyphenone and cladofulvin pathways, but differs in many genes, suggesting production of a different metabolic product. Time course analysis of gene expression in infected banana showed up-regulation of PKS8-1 and four of eight clustered genes as early as 2 weeks post-inoculation and remaining high through 9 weeks. Overexpression of the pathway through constitutive expression of an aflR-like transcription factor gene in the cluster resulted in increased expression in culture of PKS8-1 as well as the four clustered genes that are up-regulated in infected plants. No differences were seen in timing or severity of disease symptoms with the overexpression strains relative to controls, however gene expression analysis showed no difference in expression in planta by an overexpression strain relative to controls. Thus constitutive expression of the aflR-like gene is not sufficient to upregulate the pathway above normal expression in planta. Pathway expression during all phases of disease development and conservation of the pathway in related Pseudocercospora species support a role for this pathway in disease.
Highlights
Black Sigatoka disease, caused by the fungus Pseudocercospora fijiensis, is considered the most economically damaging disease of banana and plantain
PKS8-1 was found to be clustered in the genome with genes commonly found in secondary metabolite clusters including two transcription factor genes and genes encoding a glyoxylase/beta-lactamase-like protein, an MFS transporter, a cytochrome P450, two aldo/keto reductases, a dehydrogenase, and a regulatory protein similar to AflJ from the aflatoxin biosynthesis pathway in Aspergillus [18]
We show that the PKS8-1 gene cluster, one of seven polyketide synthase (PKS) gene clusters previously identified in the P. fijiensis genome [18], is conserved across many Dothideomycete species
Summary
Black Sigatoka disease, caused by the fungus Pseudocercospora fijiensis, is considered the most economically damaging disease of banana and plantain. The disease causes loss of photosynthetic capacity due to necrotic streaking on the banana leaves, as well as premature fruit ripening. The importance of this disease has led to extensive research efforts to understand the disease process, including sequencing of both the P. fijiensis and banana genomes [6,7,8,9]. Significant recent work has focused on defense responses, comparing disease development in susceptible vs resistant banana genotypes [6, 10,11,12], on characterization of effectors produced by P. fijiensis [13,14,15], and on the role of osmotic stress and signaling pathways in pathogenicity [16, 17]. Recent studies have focused on secondary metabolic pathways in the fungus that may serve as pathogenicity mechanisms [7, 18,19,20]
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