Susceptibility of oats to victoria blight, caused by the fungus Cochliobolus victoriae, and sensitivity to the host-specific toxin victorin, produced by the fungus, are controlled by the dominant allele at the Vb locus. It has been postulated that the Vb locus encodes a toxin receptor, although direct evidence for such a receptor is not available. Our recent studies on structure-activity relationships of the toxin established a methodology for producing '25I-labeled victorin. Electrophoretic analysis of proteins from isogenic susceptible and resistant oat genotypes following treatment of leaves with radiolabeled victorin showed that victorin binds in a covalent and a genotype-specific manner to a 100-kDa protein only in susceptible oat leaf slices. This in vivo binding was competitively displaced by reduced victorin, a nontoxic protective compound, and appeared to be correlated with biological activity. In vitro binding to the 100-kDa protein in leaf extracts showed several differences from in vivo binding. Binding was not genotype specific and required a reducing agent that was not required for in vivo binding. Differential centrifugation showed that the 100-kDa victorin binding protein was not a cytosolic protein but was enriched in a high-speed particulate fraction. The data support the hypothesis that the 100-kDa protein is the victorin receptor.A major objective of contemporary plant pathology is to describe the conditions that define specificity and determine the molecular mechanisms of disease resistance and susceptibility in plant host-parasite interactions. Classical genetical approaches have identified genes that determine host resistance or susceptibility and genes that determine pathogen virulence or avirulence. The genetics of some host-parasite interactions conform to the gene-for-gene relationship as classically defined by Flor (1). In these types of interactions, disease resistance (incompatibility) is determined by a dominant gene for resistance in the host and a corresponding dominant gene for avirulence in the pathogen. The absence of a dominant allele at either gene locus results in a susceptible (compatible) interaction (2).Alternatively, many diseases involving host-selective (host-specific) toxins (HST) are best described by the genetical inverse of the gene-for-gene relationship. In these plant diseases, a dominant gene in the host confers susceptibility to the pathogen and sensitivity to its toxin (compatibility) and a corresponding gene in the pathogen confers toxin production (virulence) (3-5).HSTs produced by certain pathogenic fungi are directly involved in pathogenesis and characteristically reproduce the visible and biochemical symptoms of the disease caused by the toxin-producing pathogen. Furthermore, the toxin is active only against genotypes of the host that are susceptible to the pathogen.We have been investigating victoria blight of oats caused by the fungus Cochliobolus (Helminthosporium) victoriae, which produces the HST victorin (6-8). Only oat genotypes carrying t...

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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.