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Abstract
Nicotiana edwardsonii plants were examined by electron microscopy 5 months after inoculation with Sonchus yellow net virus (SYNV). No virions were observed in leaf or root cells, but cells in sections of calyx tissue contained large numbers of virus particles. Most of these particles were only 73 to 86% as long as particles of standard SYNV. Nicotiana edwardsonii inoculated with sap extracted from chronically infected calyx became systemically infected but exhibited chlorotic mottling, instead of the normal vein-clearing symptoms. Most virus particles in these plants were short, and when purified, sedimented more slowly than standard SYNV. Purified short particles were not infective, but plants inoculated with a mixture of short and standard particles developed mottling symptoms and yielded predominantly short particles. Proteins from short particles were electrophoretically and antigenically identical to those from standard virus. RNA from short particles was 77% the size of RNA from standard SYNV and hybridized to cloned SYNV cDNA. These short particles have all the characteristics of defective interfering particles.
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