The Relation of Polyphenoloxidase in Leaf Extracts to the Instability of Cucumber Mosaic and Other Plant Viruses

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Abstract
Extracts made with buffered sodium diethyldithiocarbamate (DIECA) or potassium ethylxanthate from tobacco leaves infected with cucumber mosaic virus (CMV) were 5-500 times more infective than those made in buffer alone, or in buffer containing three other metal chelators which did not prevent the extracts going brown. DIECA preserved infectivity slightly better than did potassium ethylxanthate; both prevented browning equally. With DIECA in the extraction fluid, infectivity was not increased by other substances that enabled mitochondrial enzyme systems to be removed intact. CMV was inactivated by leaf polyphenols only when these were being oxidized. Oxidized polyphenols from virus-free leaves did not inactivate CMV when added together with copper, whereas deproteinized extracts of leaves crushed in an atmosphere of nitrogen did. When copper and chlorogenic acid, the main polyphenol in tobacco leaves, were added to infective extracts that contained polyphenoloxidase, the chlorogenic acid was oxidized and CMV was inactivated. A tobacco necrosis virus was slightly inactivated by incubating leaf extracts with chlorogenic acid and copper, but four other viruses were not. The tobacco necrosis virus was also the only one of these five to be at all stabilized by DIECA in the extraction fluid. The reported instability of some other plant viruses in leaf sap may mean that they also are susceptible to inactivation by polyphenoloxidase systems.