Plant Virus-Host Cell Relations

Abstract

Multiplication of a virus results from the interaction of two independent genomes: that of the virus with that of the host. The host cell cannot manu­ facture virus unless it has been infected, and the virus will not multiply un­ less the genome of the host somehow fits this purpose. Whether the combination of a given virus particle with a particular host cell will result in the formation of a virus reproducing system depends on many factors. An important one of these is, that the virus must provide in­ fectious information, i.e. information sufficient to trigger full reproductive cycles for the virus in the particular host. Another group of factors is that concerned with the infectibility of the host. Virus nucleic acids, known to be the carriers of viral genetic informa­ tion, are generally enveloped by a coat consisting mainly or solely of protein. In the case of bacteriophages, this coat provides specific tools which, when triggered by specific receptors of the bacterial cell wall, enable the phage to inject its nucleic acid into the host cell. Plant viruses seem unable to function similarly. Thus they need external agents for establishing the necessary close contact with the living protoplasm of the host cell. Since plant cells, and particularly epidermis cells, are very well sealed off from the environment, this barrier has to be overcome. In the synthesis of viruses two kinds of fundamentally different proc­ esses are involved: (a) reading the viral genetic information and carrying into effect the messages which it contains, and (b) copying this information in order to reproduce identically the nucleic acid moiety of the infecting particle. These processes can be accomplished only if the coat protein is re­ moved from the nucleic acid. In the case of plant viruses this must be also done by the host cell. As will be indicated later, viruses multiply in particular regions or at par­ ticular sites within the plant cell. Unless the primary point of their contact with the cell is already identical with such region or site, the virus particle, or at least its nucleic acid, must be transported within the cell to such sites. Then the nucleic acid has to be reproduced, and all the specific viral com-