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Cytology and Histology

Effects of Passaging a Defective Isolate of Impatiens Necrotic Spot Virus at Different Temperatures. R. H. Lawson, U. S. Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705-2350; M. M. Dienelt, and H. T. Hsu. U. S. Department of Agriculture, Agricultural Research Service, Florist and Nursery Crops Laboratory, Beltsville, MD 20705-2350. Phytopathology 83:662-670. Accepted for publication 20 February 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/Phyto-83-662.

The effects of two temperatures on the cytopathology of a defective isolate of impatiens necrotic spot virus (INSV-Igg) were compared with the effects on two normal isolates of tomato spotted wilt virus (TSWV-NC4 and TSWV-D). INSV-Igg produced very few enveloped virions in plants grown at approximately 20 C. Instead, masses of nucleocapsid (N) protein appeared in characteristic chainlike formations. The N protein was serologically distinct from that of TSWV. In five experiments with eight to 10 mechanical transfers, each at elevated temperatures, one experiment in a greenhouse during summer and the other four in controlled environment chambers (27/24 C, light/dark), virions appeared in Nicotiana benthamiana plants infected with INSV-Igg between the first and fourth passages and thereafter, were consistently produced. In two out of four growth-chamber experiments, extracts from infected plants grown at 27/24 C (light/dark) did not react in enzyme-linked immunosorbent assay (ELISA) with antisera to INSV N protein after the third passage. This serological change was correlated in one experiment with a change in the appearance of the N protein: Characteristic chains were no longer observed in infected cells, but large, amorphous, electron-dense masses appeared in which virions were visible. These masses failed to react with antisera to INSV N protein in immunogold-labeling experiments. In parallel serial passages at 21/18 C (light/dark), virions were not observed in infected plants. Cytopathology and serological reactivity of the N protein remained unaltered. Passaging at different temperatures did not alter the cytopathology of plants infected with TSWV-NC4 or TSWV-D. The high temperature triggered an increase in production of virions in the INSV-Igg culture and was sometimes accompanied by an antigenic change in the N protein.