Four metabolites named peritoxins A and B and periconins A and B have been isolated together with the known metabolite circinatin from culture filtrates of the fungal pathogen Periconia circinata. Peritoxins A and B, which correspond to the P. circinata toxins Ia and Ha partially characterized in previous work, are selectively toxic to genotypes of Sorghum bicolor susceptible to the pathogen, whereas periconins A and B are biologically inactive. Combination of instrumental analysis and chemical degradation has led to structural assignments for each of the four compounds; only the configuration at some of the chiral centers remains undefined. Structural comparison suggests a precursor role for circinatin in the formation of the peritoxins and the periconins.The soil-borne fungus Periconia circinata (Mangin) Sacc. causes milo disease, a root and crown rot of the grain sorghum Sorghum bicolor (L.) (1). Early studies by Leukel (2) identified P. circinata as the causal agent and suggested that a toxic metabolite was involved in the milo disease syndrome. Scheffer and Pringle (3) established that only pathogenic isolates of the fungus produce a toxin with activity against sorghum and documented its involvement in disease symptom development. The toxin, designated PC toxin, elicits the disease symptoms in genotypes of sorghum that are susceptible to the pathogen, and toxin production is essential for pathogenicity of the fungus. Thus, PC toxin is one of a group of phytotoxic metabolites known as hostspecific toxins (4). Because milo disease is controlled by resistant genotypes with the homozygous recessive alleles at the pc locus (5), P. circinata no longer poses a threat to sorghum production. However, this host-pathogen interaction affords several advantages in biochemical and molecular studies of disease development: near-isogenic resistant (pcpc) and susceptible (PcPc) genotypes of sorghum are available (5), several reliable bioassays for toxin activity have been developed (6), and two preparations with toxic activity have been isolated and partially characterized (7). Therefore, the determinative events of the host-pathogen interaction can be reduced to the interaction of the toxin with a single gene-determined site or product.In a previous paper (8) it was shown that when the fungus was grown under conditions that suppressed toxin production it accumulated a nontoxic compound, circinatin (structure 1), ¶ which was suspected to act as a biosynthetic precursor of the unknown toxins.Analysis of culture filtrates obtained under conditions known to maximize toxin production (7) has now yielded, in addition to circinatin, two toxic compounds, the peritoxins A and B, and two biologically inactive congeners, the periconins A and B. Here we provide evidence that establishes 2-5 as the structures of the four metabolites; these structures are in keeping with the hypothesis oftheir filiation from circinatin (1) as a common precursor. MATERIALS AND METHODSA toxin-producing strain of P. circinata was grown for 21 days ...
The structures of the toxins produced by Cockliobolus victoriae, victorin B, C, D, E, and victoricine, have recently been established. These toxins and modified forms of victorin C were tested for their effect on dark CO2 fixation in susceptible oat (Avena sativa) leaf slices. Halfmaximal inhibition of dark CO2 fLxation occurred with the native toxins in the range of 0.004 to 0.546 micromolar. An essential component for the inhibitory activity of victorin is the glyoxylic acid residue, particularly its hydrated aldehyde group. Removal of glyoxylic acid completely abolished the inhibitory activity of victorin, and the reduction of the aldehydo group transformed the toxin into a protectant. Conversion of victorin to its methyl ester resulted in diminution of inhibitory activity to 10% of the original activity of the toxin, whereas derivatization of the eamino group of the ft-hydroxylysine moiety resulted in a decrease of inhibitory activity to 1% of that of victorin C. However, the derivatized toxin retained its host selectivity. In addition, the opening of the macrocyclic ring of the toxin drastically reduced the inhibitory activity.
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