The following points highlight the two main steps involved in the process of reproduction in plant viruses. The steps are: 1. Entry of Virions Into Host 2. Replication.
Step # 1. Entry of Virions Into Host:
Plant viruses, in general, do not have any specific mechanism for attachment to host cells. They are transmitted passively by several means. Tobacco mosaic virus (TMV) and some other plant viruses are carried by wind, rain drops, gardening equipment and grazing animals to host plants. They enter into leaves damaged by mechanical injuries, like abrasion.
Some other plant viruses are transmitted to healthy hosts through seeds, tubers and other propagules. The tobacco ring spot virus is carried by nematodes infecting roots. Tobacco necrosis virus is transmitted through parasitic fungi. Potato X virus enters healthy host through Cuscuta, a parasitic angiosperm drawing nourishment from the host plant through haustoria.
However, the most important agent of transmission of plant viruses is the insects feeding on plant cell sap. Among these insect vectors, the most important ones belong to the Hemiptera e.g. the aphids and the orthoptera e.g. the grasshoppers. Some beetles belonging to the Arthropoda can also propagate plant viruses. Some common insect vectors include Myzus persicae, Macrosiphum euphorbiae, Thrips tabaci etc.
The most common mode of entry of plant viruses is that the virus particles (virions) adhering to the feeding organ of insect vectors are introduced passively when they feed on leaves or stem of healthy plants. In this way, viruses are transmitted from diseased plants to healthy ones where the vectors act as mere carriers. Some plant viruses, like the wound-tumour virus, can actively grow in the tissues of insect vectors. They multiply in the insect tissue and are carried to the salivary gland before being introduced into the host plant.
Step # 2. Replication:
The replication of plant viruses in host cells is not so well-studied, mainly because they have not been cultivated in artificial conditions. One of the best studied plant viruses is TMV. It has been cultivated in plant protoplast cultures. Some viruses, like the wound-tumour virus can also be grown in cell cultures of insect vectors.
TMV has a ss-RNA(+) genome which after entry and un-coating can function as m-RNA directing synthesis of viral proteins. Four viral proteins are known to be synthesized including the single capsid protein. TMV RNA is possibly replicated by a host RNA-polymerase, because unlike animal hosts, most plants possess an RNA-dependent RNA polymerase.
However, some other plant viruses, like turnip-yellow mosaic virus (Tymovirus group) and cowpea mosaic virus (Comovirus) probably replicate their ss-RNA genome with the help of a virus-specific RNA-polymerase.
Assembly of virions of TMV occurs in the cytoplasm of host cells. When large number of capsomer proteins and ss-RNA have accumulated in the infected cell, the virions are assembled spontaneously in a highly organized manner. The capsomers are assembled to form disks made up of two layers of helically arranged protomers.
The TMV RNA is associated with it near the 3′-end of the molecule. Addition of capsomer disks gradually lengthens the size of the virion. As it grows in length, the RNA molecule passes through the central hollow channel forming a loop at the growing end (Fig. 6.15). The assembled virions of TMV can produce microscopically visible intracellular accumulations of viral aggregates often in crystalline form.
TMV can spread in the plant body through the vascular system, usually through phloem, cell-to- cell transmission takes place through plasmodesmata which are the cytoplasmic connections between cells. Involvement of viral proteins in such transmission has been suggested, though their exact role is not known. Chlorophyll synthesis in infected cells is inhibited, giving rise to the characteristic mosaic symptom of the infected leaves having yellow and green patches.