An Oxidoreductase Is Involved in Cercosporin Degradation by the Bacterium Xanthomonas campestris pv. zinniae

Abstract

The polyketide toxin cercosporin plays a key role in pathogenesis by fungal species of the genus Cercospora. The bacterium Xanthomonas campestris pv. zinniae is able to rapidly degrade this toxin. Growth of X. campestris pv. zinniae strains in cercosporin-containing medium leads to the breakdown of cercosporin and to the formation of xanosporic acid, a nontoxic breakdown product. Five non-cercosporin-degrading mutants of a strain that rapidly degrades cercosporin (XCZ-3) were generated by ethyl methanesulfonate mutagenesis and were then transformed with a genomic library from the wild-type strain. All five mutants were complemented with the same genomic clone, which encoded a putative transcriptional regulator and an oxidoreductase. Simultaneous expression of these two genes was necessary to complement the mutant phenotype. Sequence analysis of the mutants showed that all five mutants had point mutations in the oxidoreductase gene and no mutations in the regulator. Quantitative reverse transcription-PCR (RT-PCR) showed that the expression of both of these genes in the wild-type strain is upregulated after exposure to cercosporin. Both the oxidoreductase and transcriptional regulator genes were transformed into three non-cercosporin-degrading bacteria to determine if they are sufficient for cercosporin degradation. Quantitative RT-PCR analysis confirmed that the oxidoreductase was expressed in all transconjugants. However, none of the transconjugants were able to degrade cercosporin, suggesting that additional factors are required for cercosporin degradation. Further study of cercosporin degradation in X. campestris pv. zinniae may allow for the engineering of Cercosporaresistant plants by using a suite of genes.

Functional characterization of three genes encoding putative oxidoreductases required for cercosporin toxin biosynthesis in the fungus Cercospora nicotianae

“…zinniae oxidoreductase (accession no. AAY86766) which has recently been shown to be involved in cercosporin degradation (Taylor et al, 2006). We propose that the function of CTB7 in the cercosporin biosynthetic pathway is probably to catalyse a hydration or reduction step during ring closure to form the polyketomethylene skeleton of cercosporin prior to the methylation steps.…”

Resistance to Gray Leaf Spot of Maize: Genetic Architecture and Mechanisms Elucidated through Nested Association Mapping and Near-Isogenic Line Analysis

“…Oxidoreductases have also been implicated in the degradation of cercosporin into non-toxic xanosporic acid. Taylor et al [ 57 ] identified mutant strains of Xanthomonas campestris that could no longer degrade cercosporin, while its wild-type progenitor had this ability. All mutants in the Taylor et al [ 57 ] study could be complemented with a genomic clone with homologous sequence to a transcriptional regulator and an oxidoreductase.…”

Phytotoxic secondary metabolites and peptides produced by plant pathogenicDothideomycetefungi

“…Mutant screening and expression profiling in the presence of cercosporin suggested that an oxidoreductase is involved in cercosporin degradation by the bacterial species Xanthomonas campestris pv. zinniae (Taylor et al, 2006). However, expression of this oxidoreductase in noncercosporin-degrading bacteria did not result in cercosporin degradation, suggesting that other factors are required as well.…”

“…Accordingly, over-expression of the flavin adenine dinucleotide (FAD)dependent pyridine nucleotide reductase Cpd1p in Saccharomyces cerevisiae resulted in resistance to cercosporin (Ververidis et al, 2001). zinniae (Taylor et al, 2006). Transgenic tobacco plants expressing CDP1 are resistant to cercosporin, indicating that this gene could be used as a source of resistance against cercosporin-producing pathogens (Panagiotis et al, 2007).…”