Elsinochrome A

Perylenequinones are a class of aromatic polyketides characterised by a highly conjugated pentacyclic core, which confers them with potent light-induced bioactivities and unique photophysical properties. Despite the biosynthetic gene clusters for the perylenequinones elsinochrome A (1), cercosporin (4) and hypocrellin A (6) being recently identified, key biosynthetic aspects remain elusive. Here, we first expressed the intactelcgene cluster encoding1from the wheat pathogenParastagonospora nodorumheterologously inAspergillus nidulanson a yeast-fungal artificial chromosome (YFAC). This led to the identification of a novel flavin-dependent monooxygenase, ElcH, responsible for oxidative enolate coupling of a perylenequinone intermediate to the hexacyclic dihydrobenzo(ghi)perylenequinone in1. In the absence of ElcH, the perylenequione intermediate formed a hexacyclic cyclohepta(ghi)perylenequinone systemviaan intramolecular aldol reaction resulting in6and a novel hypocrellin12with opposite helicity to1. Theoretical calculations supported that6and12resulted from atropisomerisation upon formation of the 7-membered ring. Using a bottom-up pathway reconstruction approach on a tripartite YFAC system developed in this study, we uncovered that both a berberine bridge enzyme-like oxidase ElcE and a laccase-like multicopper oxidase ElcG are involved in the double coupling of two naphthol intermediates to form the perylenequinone core. Gene swapping with the homologs from the biosynthetic pathway of4showed that cognate pairing of the two classes of oxidases is required for the formation of the perylenequinone core, suggesting the involvement of protein-protein interactions.

Parastagonospora nodorum is an important pathogen of wheat. The contribution of secondary metabolites to this pathosystem is poorly understood. A biosynthetic gene cluster (SNOG_08608-08616) has been shown to be upregulated during the late stage of P. nodorum wheat leaf infection. The gene cluster shares several homologues with the Cercospora nicotianae CTB gene cluster encoding the biosynthesis of cercosporin. Activation of the gene cluster by overexpression (OE) of the transcription factor gene (SNOG_08609) in P. nodorum resulted in the production of elsinochrome C, a perelyenequinone phytotoxin structurally similar to cercosporin. Heterologous expression of the polyketide synthase gene elcA from the gene cluster in Aspergillus nidulans resulted in the production of the polyketide precursor nortoralactone common to the cercosporin pathway. Elsinochrome C could be detected on wheat leaves infected with P. nodorum, but not in the elcA disruption mutant. The compound was shown to exhibit necrotic activity on wheat leaves in a light-dependent manner. Wheat seedling infection assays showed that ΔelcA exhibited reduced virulence compared with wild type, while infection by an OE strain overproducing elsinochrome C resulted in larger lesions on leaves. These data provided evidence that elsinochrome C contributes to the virulence of P. nodorum against wheat.

Biological control as an alternative to chemical pesticides is of increasing public interest. However, to ensure safe use of biocontrol methods, strategies to assess the possible risks need to be developed. The production of toxic metabolites is an aspect which has so far largely been neglected in the risk assessment and the registration process for biocontrol products. We have evaluated the risks of elsinochrome A (ELA) and leptosphaerodione production by the fungus Stagonospora convolvuli LA39, an effective biocontrol agent used against bindweeds. The toxicity of the two metabolites to bacteria, protozoa, fungi and plants was evaluated in in vitro assays. The most sensitive bacteria and fungi were already affected at 0.01-0.07 microM ELA, whereas plants were far less sensitive. Leptosphaerodione was less toxic than ELA. Subsequently, it was investigated whether ELA is present in the applied biocontrol product or LA39-treated bindweed and crop plants. In plants ELA was never detected and in the biocontrol product the ELA concentration was far too low to have toxic effects even on the most sensitive organisms. We conclude that the production of ELA by biocontrol strain LA39 does not pose a risk to the environment or to the consumer.