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Physiology and Biochemistry

Enzymatic Release and Metabolism of Hydrogen Cyanide in Sorghum Infected by Gloeocercospora sorghi. D. F. Myers, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address of senior author: Department of Plant Pathology, Montana State University, Bozeman, MT 59717; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 68:1717-1722. Accepted for publication 22 June 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1717.

The activities of enzymes that may be important in the release or metabolism of hydrogen cyanide (HCN) during disease development were determined in extracts of sorghum infected by Gloeocercospora sorghi. The activity of β-glucosidase increased 20-fold during the first 24 hr after inoculation and then remained at that level. This enzyme hydrolyzes dhurrin, the cyanogenic glucoside in sorghum, to glucose and p-hydroxymandelonitrile. The increase in β-glucosidase activity preceded the first detectable decline in HCN potential of infected leaves. The activity of oxynitrilase, an enzyme that releases HCN from p-hydroxymandelonitrile, remained constant. Of the HCN-metabolizing enzymes examined, activity of formamide hydro-lyase was the highest. No formamide hydro-lyase activity was detected earlier than 12 hr after inoculation, but activity increased at least 200-fold 1848 hr after inoculation. This increase preceded the decline in HCN potential in diseased leaves. The product of formamide hydro-lyase activity, formamide, did not accumulate in diseased sorghum. No formamide hydro-lyase was detected in extracts of diseased older leaves in which no HCN was detected. Activities of β-cyanoalanine synthase and rhodanese (two other HCN-metabolizing enzymes) were detected at low levels in healthy sorghum but were not detected in the pathogen. These activities did not increase during disease development. These data are consistent with the hypotheses that G. sorghi is exposed to HCN upon pathogenesis of sorghum leaves and that formamide hydro-lyase may be involved in HCN detoxification.