Cellular toxicity of elsinochrome phytotoxins produced by the pathogenic fungus, Elsinoë fawcettii causing citrus scab

Abstract

Summary• Elsinochromes are the red/orange pigments produced by many Elsinoë fungal species and are structurally similar to the phytotoxin, cercosporin. Here, pigments were extracted from cultures of a citrus pathogen, Elsinoë fawcettii and tested for cellular toxicity.• On irradiation with light, elsinochromes rapidly killed suspension cultured citrus and tobacco cells. The toxicity was decreased by adding the singlet oxygen ( 1 O 2 ) quenchers (bixin (carotenoid carboxylic acid), DABCO (1, 4-diazabicyco octane), ascorbate or reduced glutathione). Application of elsinochromes onto rough lemon leaves resulted in necrotic lesions, whereas lesion development was inhibited by the addition of bixin, DABCO or ascorbate, but not α -tocopherol. Incubation of rough lemon leaf discs with elsinochromes in the light induced a steady increase of electrolyte leakage.• Compared with two photosensitizing compounds, hematoporphyrin and cercosporin, the accumulation of 1 O 2 induced by elsinochromes after irradiation was indicated by successful detection of the cholesterol oxidation product, 5 α -hydroperoxide. Addition of a potent quencher, β -carotene prevented 5 α -hydroperoxide production. Elsinochromes generated superoxide ions ( ), whereas accumulation of was blocked by addition of the superoxide dismutase, a scavenger of , but not the 1 O 2 -quencher, DABCO. Our study indicated that elsinochromes are functioning as photosensitizing compounds that produce 1 O 2 and , and exert toxicity to plant cells.

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

“…The biological function of elsinochromes as phytotoxins toward host and nonhost plant cells both in vitro and in vivo was recently documented to be light-dependent (Liao & Chung, 2008a). Their cellular toxicities were further shown to be mediated through production of reactive oxygen species such as superoxide and singlet oxygen, which may damage cell membranes and induce electrolyte leakage from the cells (Liao & Chung, 2008a). Further studies employing molecular and genetic approaches unambiguously confirmed a critical role for elsinochromes in lesion development and fungal pathogenesis.…”

“…The wild-type CAL WH-1 isolate of Elsinoë fawcettii, used for cloning, transformation and targeted gene disruption, was cultured from scab-affected calamondin (Citrus madurensis Lour.) 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).…”

“…When preparation of protoplasts was desired, fungal isolates were grown in 50 ml potato dextrose broth (PDB, Difco) for 7 days, ground, mixed with fresh PDB (200 ml), and incubated for an additional 15 h. For DNA or RNA isolation, fungal isolates were grown on media with a layer of sterile cellophane (Choquer et al, 2005). Screening of elsinochrome-deficient mutants was performed on thin PDA as previously described (Choquer et al, 2005;Liao & Chung, 2008a).…”

“…The structures of elsinochromes were determined by NMR spectroscopic and X-ray crystallographic studies several decades ago (Weiss et al, 1965;Lousberg et al, 1969Lousberg et al, , 1970Meille et al, 1989;Mebius et al, 1990). The biological function of elsinochromes as phytotoxins toward host and nonhost plant cells both in vitro and in vivo was recently documented to be light-dependent (Liao & Chung, 2008a). Their cellular toxicities were further shown to be mediated through production of reactive oxygen species such as superoxide and singlet oxygen, which may damage cell membranes and induce electrolyte leakage from the cells (Liao & Chung, 2008a).…”

Isolation and Characterization of the Gibberellin Biosynthetic Gene Cluster in Sphaceloma manihoticola

“…The strain used in our study, Lu949, takes about 5 days to build colonies 0.5 cm in diameter and is not able to overgrow the agar plate if only a piece of mycelium is transferred as the inoculum. This has also been described for isolates of the related species Sphaceloma fawc- ettii, which forms Ͻ10-mm colonies in 30 days (28). The fungus synthesizes large amounts of exopolysaccharides, making it difficult to separate the mycelium from the culture medium.…”