Cytological observations of cultured insusceptible host cells microinjected with maize dwarf mosaic virus

Abstract

The concept o f a plant's immunity to virus disease implies a lack of infection as well as in ability of the virus to replicate. Plants having subliminal or latent virus infections are often considered immune. Microscopic study of the individual cells of a supposed immune plant, into which the virus has been directly introduced, might reveal cytopathic effects or premature death if resistance mechanisms are operating. Suspension cultured cells and cell chains of Nicotiana tabacum 'Samsun NN' were microinjected with maize dwarf mosaic virus (MDMV) strain A, to which they are reportedly immune. The purity of the virus preparations was analyzed by polyacrylamide gel electrophoresis, and all samples tested contained a single protein band of 36,560 daltons. This protein, which represented a single type of polypeptide subunit in the capsid of the virus, was very similar in MW to that of strain B of MDMV, as well as to the capsid polypeptide of sugarcane mosaic virus, a closely related pathogen. Nine of thirty-four tobacco cells successfully microinjected with infectious virus survived the shock of that puncture so that observations could be made. Of 7 cells injected with heat-inactivated virus and 7 with "purified sap" from healthy plants, 2 each survived. Immediate wound response reactions were noted in all cells, including accumulation of cytoplasm around the needle puncture, increased speed of cyclosis, temporary migration of the nucleus toward the injection site, formation of major cytoplasmic strands between the nucleus and the puncture, and deposition of callose as a plug around the needle tip in the cell. There was no evidence of infection or of cellular reaction to the virus, since no inclusions were produced, no effect on organelles was noted, and the longevity of the cells was not reduced. One cell divided three times within 36 hr, beginning 46 hr after receiving an injection of infectious virus, a phenomenon typical of healthy cells. Thus, if viral replication was occurring, it was not at the expense of the normal, replicative function of the host genome. ...