Specific binding of victorin to a 100-kDa protein from oats

Abstract

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...

Identification of a high‐affinity binding protein for a hepta‐β‐glucoside phytoalexin elicitor in soybean

“…However, considering that treatment with Zwittergent 3-12 extracts about 50% of the binding activity, one would expect the 70-kDa band to account for at least 50"h of the specific ' ' ' 1 labeling in membranes, which is not the Besides providing clear identification of a membrane polypeptide with high affinity and specificity for a fungal signal molecule which triggers a plant defense response, this is one of the few reports for plant cells where a putative receptor for an external signal or effector has been identified by photoaffinity labeling and/or affinity chromatography. Previous reports have involved a binding protein for the growth regulator indoleacetic acid (Hicks et al, 1989a, b) and for the fungal phytotoxins fusicoccin (de Boer et al, 1989;Feyerabend and Weiler, 1989;Oecking and Weiler, 1991) and victorin (Wolpert and Macko, 1989). The mechanisms by which these proteins, including the heptaglucoside-binding protein, are linked to signal transduction in plant cells still have to be clarified before a clear receptor function can be assigned to them.…”

Electrical Membrane Properties of Leaves, Roots, and Single Root Cap Cells of Susceptible Avena sativa

“…It is known to increase the cell membrane permeability of susceptible oat plants (Avena sativa), containing the Victoria (Vb) gene, which also encodes for resistance (Pc-2) against Puccinia coronata (16,31). Changes in membrane permeability were most typically expressed in irreversible K+ efflux from leaves, roots, and leaf protoplasts (2,12,18,24,30).…”

Victorin's Secret

“…Whatever the explanation for the differences in the binding specificity seen in vivo and in vitro, the in vivo binding results strongly hinted that the 100-kD protein might be a biologically important target for victorin (a 15-kD VBP, by contrast, was found to bind victorin in both resistant and susceptible plants, both in vivo and in vitro; Wolpert and Macko, 1989). Wolpert et al (1994) therefore obtained and sequenced acDNA for the 100-kD VBP and found that it encodes the P-protein component of the GDC.…”

“…Many years of biochemical studies have shown that victorin binds strongly (and probably covalently) to severa1 oat proteins (e.g., Wolpert and Macko, 1989), but these analyses did not reveal how victorin might affect the metabolism of cells from susceptible plants. On pages 463-471 of this issue, Navarre and Wolpert now demonstrate a possible basis for victorin toxicity: it acts as a potent inhibitor of the glycine decarboxylase complex (GDC), a multienzyme complex found in the mitochondrial matrix.…”

“…This 100-kD protein was a particularly promising candidate to be a target for victorin because, unlike at least one other VBP, its ability to bind victorin in vivo is correlated with susceptibility to victoria blight (Wolpert and Macko, 1989). That is, when leaf slices of susceptible (Vb) plants are incubated with labeled victorin and the proteins are subsequently extracted, a 100-kD protein is found to be labeled.…”