Biodegradation of the Polyketide Toxin Cercosporin

Abstract

Cercosporin is a non-host-specific polyketide toxin produced by many species of plant pathogens belonging to the genus Cercospora. This red-pigmented, light-activated toxin is an important pathogenicity determinant for Cercospora species. In this study, we screened 244 bacterial isolates representing 12 different genera for the ability to degrade cercosporin. Cercosporin degradation was determined by screening for the presence of cleared zones surrounding colonies on cercosporin-containing culture medium and was confirmed by assaying the kinetics of degradation in liquid medium. Bacteria belonging to four different genera exhibited the cercosporin-degrading phenotype. The isolates with the greatest cercosporin-degrading activity belonged to Xanthomonas campestris pv. zinniae and X. campestris pv. pruni. Isolates of these pathovars removed over 90% of the cercosporin from culture medium within 48 h. Bacterial degradation of red cercosporin was accompanied by a shift in the color of the growth medium to brown and then green. The disappearance of cercosporin was accompanied by the appearance of a transient green product, designated xanosporic acid. Xanosporic acid and its more stable lactone derivative, xanosporolactone, are nontoxic to cercosporin-sensitive fungi and to plant tissue and are labile in the presence of light. Detailed spectroscopic analysis (to be reported in a separate publication) of xanosporolactone revealed that cercosporin loses one methoxyl group and gains one oxygen atom in the bacterial conversion. The resulting chromophore (4,9-dihydroxy-3-oxaperlylen-10H-10-one) has never been reported before but is biosynthetically plausible via oxygen insertion by a cytochrome P-450 enzyme.

Potential environmental fate of elsinochrome A, a perylenequinone toxin produced in culture by bindweed biocontrol fungus Stagonospora convolvuli LA39

“…However, the ability of bacteria to biodegrade cercosporin has been proven (Mitchell et al, 2002). In absence of light, EA either in dry form or as solution in water may remain stable even at maximal air or soil temperatures that could nor- Fig.6 Effect of UV-A and UV-B on the stability of elsinochrome A in dry form mally be found in temperate regions, particularly.…”

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

“…X. campestris pv. zinniae isolates were shown to rapidly degrade cercosporin to produce the nontoxic breakdown product xanosporic acid (30). The degradation reaction was hypothesized to be caused by an oxygen insertion into one of the quinoid rings adjacent to the ketone carbonyl (29).…”

“…Screening for non-cercosporin-degrading mutants (Fig. 2) was carried out as previously described (30). For MutC and MutD, vector pLAFR6 was maintained through antibiotic selection until the mutants were discovered and then removed through plasmid curing.…”

“…1). Xanosporic acid was shown to be nontoxic to a cercosporin-sensitive mutant of C. nicotianae (30). The lactonized derivative of xanosporic acid, xanosporolactone, was used in a plant infiltration assay of tobacco and was also shown to be nontoxic (30).…”

“…Out of 244 isolates that were screened, 43 isolates that were able to degrade cercosporin were identified based on the production of a clear halo when bacteria were cultured on cercosporin-containing medium (30). Of the isolates tested, Xanthomonas species had the largest proportions of cercosporin degraders, and among those, all 32 isolates of Xanthomonas campestris pv.…”