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The oxidation of polycyclic aromatic compounds was studied in systems consisting of laccase from Trametes versicolor and so-called mediator compounds. The enzymatic oxidation of acenaphthene, acenaphthylene, anthracene, and fluorene was mediated by various laccase substrates (phenols and aromatic amines) or compounds produced and secreted by white rot fungi. The best natural mediators, such as phenol, aniline, 4-hydroxybenzoic acid, and 4-hydroxybenzyl alcohol were as efficient as the previously described synthetic compounds ABTS [2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid)] and 1-hydroxybenzotriazole. The oxidation efficiency increased proportionally with the redox potentials of the phenolic mediators up to a maximum value of 0.9 V and decreased thereafter with redox potentials exceeding this value. Natural compounds such as methionine, cysteine, and reduced glutathione, containing sulfhydryl groups, were also active as mediator compounds.The concerted action of fungal laccases and oxidizable lowmolecular-weight compounds (called mediators in some studies) was found to extend or permit oxidation of nonsubstrate compounds by this enzyme class; an overview of the chronological development of these processes has been recently presented (26). However, this system received widespread attention only when used for bleaching kraft pulp, thus showing promise for biotechnological application (11,12,13,14,15). Subsequently these laccase mediator systems (LMS), as they are commonly referred to, were also applied to the oxidation of various compounds, and they seems to be useful in preparative synthesis as well (20,31). Another area of interest with regard to the LMS originates from its application in the degradation of environmental chemicals such as polycyclic aromatic hydrocarbons (PAH) (9,16,22,25).The choice of the proper mediator substance plays a key role in the general applicability and effectiveness of the system. More than 100 possible mediator compounds have already been described (13), but the most commonly used are still 2,2Ј-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1-hydroxybenzotriazole (HBT). HBT and ABTS are oxidized by laccases to the radical (HBT˙), the cation radical (ABTS ϩ˙) , and the ABTS dication (ABTS 2ϩ ); the role of these oxidized species as the essential oxidants for aromatic alcohols has been recently demonstrated (10,26). In a previous study we reported that the oxidation of a high-molecularweight PAH model compound is performed by the LMS via ABTS 2ϩ and HBT˙by an indirect oxidation without direct contact of the substrate and enzyme (A. Majcherczyk and C. Johannes, submitted for publication).The most relevant disadvantages of all known effective mediator compounds are either high price or toxicity. Generally, the compounds applied are products of chemical synthesis, and it is still not clear whether the LMS plays a role in natural systems. The ability of white rot fungi secreting only laccases as oxidative enzymes to degrade lignin model compounds (37) or envir...

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Verticillium longisporum (VL) is one of the most devastating diseases in important oil crops from the family of Brassicaceae. The fungus resides for much time of its life cycle in the extracellular fluid of the vascular system, where it cannot be controlled by conventional fungicides. To obtain insights into the biology of VL-plant interaction in the apoplast, the secretome consisting of the extracellular proteome and metabolome as well as cell wall properties were studied in the model Brassicaceae, Arabidopsis thaliana. VL infection resulted in increased production of cell wall material with an altered composition of carbohydrate polymers and increased lignification. The abundance of several hundred soluble metabolites changed in the apoplast of VL-infected plants including signalling and defence compounds such as glycosides of salicylic acid, lignans and dihydroxybenzoic acid as well as oxylipins. The extracellular proteome of healthy leaves was enriched in antifungal proteins. VL caused specific increases in six apoplast proteins (three peroxidases PRX52, PRX34, P37, serine carboxypeptidase SCPL20, α-galactosidase AGAL2 and a germin-like protein GLP3), which have functions in defence and cell wall modification. The abundance of a lectin-like, chitin-inducible protein (CILLP) was reduced. Since the transcript levels of most of the induced proteins were not elevated until late infection time points (>20 dpi), whereas those of CILLP and GLP3 were reduced at earlier time points, our results may suggest that VL enhances its virulence by rapid down-regulation and delay of induction of plant defence genes.