Genetic Dissection Defines the Roles of Elsinochrome Phytotoxin for Fungal Pathogenesis and Conidiation of the Citrus Pathogen Elsinoë fawcettii

Abstract

Elsinochrome pigments produced by many phytopathogenic Elsinoë spp. are nonhost-selective toxins which react with oxygen molecules after light activation to produce highly toxic reactive oxygen species. The structures and chemical properties of four derivatives are well known. However, the biological roles of elsinochromes in fungal pathogenesis are poorly understood. Many isolates of Elsinoë fawcettii causing citrus scab are able to produce elsinochromes under axenic conditions. In this article, we report the cloning, expression, and functional characterization of the polyketide synthase-encoding gene, EfPKS1, which we show is required for the production of elsinochromes and fungal pathogenesis. Targeted disruption of EfPKS1 in E.fawcettii completely abrogated elsinochrome production, drastically reduced conidiation, and significantly decreased lesion formation on rough lemon leaves. All mutant phenotypes were restored to the wild type in fungal strains expressing a functional copy of EfPKS1. Accumulation of the EfPKS1 transcript and elsinochromes by a wild-type strain appears to be coordinately regulated by light, nutrients, and pH. The results indicate that the product of EfPKS1 is involved in the biosynthesis of elsinochromes via a fungal polyketide pathway, and that elsinochromes play an important role in fungal pathogenesis.

Determination of a transcriptional regulator-like gene involved in biosynthesis of elsinochrome phytotoxin by the citrus scab fungus, Elsinoë fawcettii

“…4c). In addition to elsinochrome deficiency, the TSF1 disruptants were also defective in conidiation and fungal pathogenesis to rough lemon (Table 3), similar to the mutant phenotypes resulting from disruption of the EfPKS1 gene (Liao & Chung, 2008b).…”

“…fruit in Florida and has been previously characterized (Liao & Chung, 2008a). The genetically modified EfPKS1 disruptant (D4) and two complementation strains (C1 and C2) were reported in a previous study (Liao & Chung, 2008b). Fungal isolates were cultured on potato dextrose agar (PDA, Difco).…”

“…Fungal pathogenicity was assessed on detached rough lemon (Citrus jambhiri Lush.) leaves inoculated with agar plugs bearing fungal mycelium as previously described (Liao & Chung, 2008b).…”

“…Accumulation of the gene transcripts was also examined in an EfPKS1 null mutant (D4) that is defective in elsinochrome production (Liao & Chung, 2008b). Northern-hybridization analysis revealed a marked reduction of transcripts of the RDT1, TSF1, PRF1, ECT1 and EfHP1 genes in the D4 mutant (Fig.…”

“…Further studies employing molecular and genetic approaches unambiguously confirmed a critical role for elsinochromes in lesion development and fungal pathogenesis. E. fawcettii mutants disrupted in the EfPKS1 gene, which encodes a fungal polyketide synthase, do not produce any detectable elsinochromes and have a drastically reduced ability to form lesions on citrus (Liao & Chung, 2008b).…”