TRANSMISSION VIRUS VIA PLANT MATERIAL

A. Mechanical Transmission
Mechanical inoculation involves the introduction of infective virus or viral RNA into a wound on the plant’s surface. When virus establishes itself successfully in the cell, infection occurs. This form of transmission occurs naturally with a few viruses such as Tobacco mosaic virus (TMV) and Potato virus X (PVX) that are very stable and reach high concentrations in the plant. TMV can readily contaminate hands clothing, and implements and can be spread by workers and, for instance, birds in tobacco and tomato crops. TMV may be spread mechanically by tobacco smokers because the virus is commonly present in processed tobacco leaf. For example, a survey showed that all 37 brands of
cigarette and 60 out of 64 smoking tobaccos contained infectious TMV.
Mechanical transmission is of great importance for many aspects of experimental plant virology, particularly for the assay of viruses, often by local lesion production; in the propagation of viruses for purification; in host range studies; in diagnosis; and in the study of the early events in the interaction between a virus and susceptible cells. Mechanical inoculation is usually done by grinding up infected leaf tissue in a buffer—usually a phosphate buffer that contains additives that control nucleases and polyphenols—incorporating an abrasive such as celite or carborundum, and then rubbing the extract gently on the leaves of the recipient plant. The gentle application wounds the leaf surface without causing cell death.

B. Seed Transmission
About one-seventh of the known plant viruses are transmitted through the seed of at least one of their infected host plants. Seed transmission provides a very effective means of introducing virus into a crop at an early stage, giving randomised foci of primary infection throughout the planting. Thus, when some other method of transmission can operate to spread the virus within the growing crop, seed transmission may be of very considerable economic importance. Viruses may persist in seed for long periods so commercial distribution of a seed-borne virus over long distances may occur. Seed transmission rates vary from less than 1 to 100 percent, depending on virus and host.
Two general types of seed transmission can be distinguished. With TMV in tomato, seed transmission is largely due to contamination of the seedling by mechanical means. The external virus can be readily inactivated by certain treatments eliminating all, or almost all, seed-borne infection. In the second and more common type of seed transmission, the virus is found within the tissues of the embryo. The developing embryo can become infected either prior to fertilisation by infection of the gametes (indirect embryo invasion or gametic transmission) or by direct invasion after fertilisation. Generally speaking, for infection of the embryo from the mother plant, the earlier the plant is infected, the higher the percentage of seed that will transmit the virus. Obviously, for indirect embryo invasion by pollen, the infection takes place at pollination. 

The direct route of seed infection from the mother plant poses problems in that symplastic connection is severed at meiosis. To infect the embryo, the virus has to reach either the floral meristems, which are beyond the limits of normal long-distance movement in the phloem, or the embryo itself. The route of direct embryo infection of peas by Pea seedborne mosaic virus has been examined in detail.

C. Pollen Transmission
Some viruses are transmitted from plant to plant via pollen. As with seed transmission, two mechanisms appear to operate in pollen transmission: gametic infection of the embryo and direct infection of the mother plant.

D. Vegetative Propagation
Vegetative propagation is an important horticultural practice, but it is also, unfortunately, a very effective method for perpetuating and spreading viruses. Economically important viruses spread systemically through most vegetative parts of the plant. A plant once systemically infected with a virus usually remains infected for its lifetime. Thus, any vegetative parts taken for propagation, such as tubers, bulbs, corms, runners, and cuttings, will normally be infected.

E. Grafting
Grafting is essentially a form of vegetative propagation in which part of one plant (the scion) grows on the roots (the stock) of another individual. Once organic union has been established, the stock and scion become effectively a single plant. Where either the rootstock or the individual from which the scion is taken is infected systemically with a virus, the grafted plant as a whole will become infected if both partners in the graft are susceptible. Grafting may succeed in transmitting a virus where other methods fail.