Cytopathological changes induced by carrot mottle virus in systemically infected cells of plants grown at 17 °C and illuminated at 4000 lux for 8 h/day, were studied in relation to the infectivity of buffer and phenoltreated extracts. Changes were most obvious in palisade cells. About six days after inoculation, tubules appeared in the cytoplasm, associated with the plasmodesmata. Later the tubules, some of which became sheathed by cell wall material forming plasmodesmatal outgrowths, extended towards the vacuole, others towards the nucleus, causing invaginations in it. Endoplasmic reticulum and Golgi bodies increased, especially in areas adjacent to the nuclei, and complexes of cytoplasmic tubules with endoplasmic reticulum were observed. Membrane-bound particles, some with densely-staining central spots, appeared in the vacuole close to the tonoplast and reached their maximum number after 8 to 9 days. Sometimes they were clustered where a plasmodesmatal tubule or outgrowth came close to the tonoplast.

Tests with the phenol-treated and buffer extracts showed that the cells contained both labile and stable forms of infectivity. The former predominated early in systemic infection (about 5 to 7 days after inoculation) and then declined, while the amount of stable infectivity increased, reaching a maximum after 8 to 9 days. It is suggested that the labile form of infectivity consists of RNase-sensitive material, probably nucleocapsids, which become stable (RNase-resistant) when they receive a protective envelope on entry into the vacuole.


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