CELL AND ORGAN CULTURE METHODS IN VIRUS DISEASE THERAPY

Any virus-susceptible cultivar that is propagated for long periods by conventional asexual methods is probably uniformly infected if the virus is unchecked and propagation is continued indefinitely. The exocortis infection of Temple orange is a good case in point. Every Temple orange tree in the world is probably infected with the pathogen. The presence of virus may not be readily detected. Symptoms of infection could vary from quick decline and death to one of reduced vigor, yield and quality. Some viruses may show no measurable or otherwise noticeable effects, and in rare cases the infection may even be beneficial.

Unlike bacterial and fungal infections, it has not been possible to overcome plant virus infection through chemotherapeutic methods. Chemicals which repress virus multiplication generally also affect the host plant adversely. Elimination of some viruses is achieved by exposing infected plants to relatively high temperatures (35–40°C) continuously for prolonged periods. Obviously, this method has only limited application.

A more practical procedure is to restrict asexual propagation to plant parts which are not infected with virus. A number of investigators have shown that virus concentration is not uniform throughout an infected plant (Limasset et al., 1949). Some tissues and organs, e.g., the tip of a growing stem, are frequently free of virus. Plants originating from such tissues are expectedly virus-free. Furthermore, exposure of the infected plant to relatively high temperature for suitable periods, 35–40° C continuously for a few days or as much as several months, increases the probability that cuttings, budwood and other propagules obtained from such tissues and organs are virus-free. Indeed, many cultivars have been recovered free of some viruses by taking cuttings, divisions and scion material from infected plants which have been exposed to the high temperature for prolonged periods (Hollings, 1965; Kassanis, 1954, 1957a; Mellor and Stace-Smith, 1967; Raychaudhuri, 1966). The procedure has been referred to popularly as thermotherapy or heat therapy.

Nevertheless, thermotherapy has not been successful with many viruses because the size of the plant part necessary for conventional methods of propagation is too large to exclude all infected cells and tissues. Rooted plants of Tropaeolum majus and Lupinus albus could be obtained in sterile culture and in an artificial nutrient by starting with excised tissues composed of the shoot apical meristem and 2–3 leaf primordia (Ball, 1946). Morel and Martin (1952) subsequently applied this procedure to virus infected dahlias and were able to recover disease-free plants by culturing 0.25 mm long stem tip explants. The procedure of recovering virus-free plants through in vitro culture of shoot tip explants has now been employed for twenty years with varying