Michigan State University

Metabolism of the host-selective phytotoxin HC-toxin

--Robert B. Meeley and Jonathan D. Walton

The maize pathogen Cochliobolus (Helminthosporium) carbonum race 1 is highly virulent on maize that is homozygous recessive at the Hm1 locus, which is on the long arm of chromosome 1. This virulence is due to production by race 1 (but not other races) of a cyclic tetrapeptide known as HC-toxin (MNL 57:53-54). HC-toxin contains an unusual amino acid, L-2-amino-8-oxo-9,10-epoxidecanoic acid; both the epoxide and the vicinal ketone are required for activity. To test the hypothesis that the dominant resistance to HC-toxin controlled by Hm1 is due to metabolism of the toxin, especially at the labile epoxide group, we prepared radiolabelled HC-toxin by feeding the fungus tritiated D-alanine. In leaf uptake studies, HC-toxin is metabolized to a single product, which was purified and analyzed by HPLC, TLC, and mass spectrometry. Surprisingly, the metabolite retains an intact epoxide but the vicinal ketone is reduced to the corresponding alcohol. In a time course study, no detectable difference between the ability of resistant (Hm1/hm1) and susceptible (hm1/hm1) maize leaves to metabolize the toxin was found. The conversion of HC-toxin to the 8-alcohol occurs also in vitro: the activity is sensitive to boiling and protease treatment and uses NADPH as co-substrate. The enzyme has an apparent molecular weight of 42,000 by gel filtration, similar to carbonyl reductases found in other plants. Studies in progress on the kinetic characteristics of this enzyme are addressing its role in host-selective reaction to HC-toxin and its relation to the Hm1 locus.

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