Peritoxins and their effects on sorghum

Abstract

Peritoxins are low molecular weight, chlorinated peptides produced only by pathogenic strains of the sorghum root rot fungus, Periconia circinata. Genetic data relating sensitivity of sorghum (Sorghum bicolor (L.) Moench) to these toxins and susceptibility to the pathogen are consistent with the hypothesis that both traits are controlled by a single semidominant gene (Pc). At low concentrations (5–500 nM), the toxin selectively reproduces disease symptoms, inhibits growth of primary roots, stops mitosis, induces electrolyte leakage, and enhances the synthesis of a group of 16-kDa proteins and the corresponding mRNAs. Enhanced expression of these 16-kDa proteins and disease symptoms are also induced in a nongenotype-specific manner by treatment with mercury, suggesting that the two events are causally related. However, the upregulation of 16-kDa protein expression induced by a fungal elicitor is independent of visible disease symptoms and is, therefore, not a direct cause of damage. Results of experiments with treatments that protect seedlings against the effects of toxin (e.g., proteinase, heat shock, inhibitors of protein or RNA synthesis, inhibitors of protein kinase C activity, and biotinylation of membrane proteins) suggest that disease symptoms result from an interaction of peritoxin with a proteinaceous receptor on or near the cell surface and interference with the normal function of a signal transduction pathway. Key words: peritoxin, sorghum, Periconia circinata, host-selective toxin, milo disease, Periconia root rot.