THE PHYSIOLOGY OF VIRUS DISEASES IN PLANTS

In this paper the movement in the plant of the causative agent of virus disease is discussed. The relevant data in the literature are summarised.
A method is described whereby a portion of the stem in the middle of a tomato plant was killed either by chloroform or by steam. In this way the living upper and lower portions of the plant were connected by a bridge of dead tissue. It is shown that the symptoms appeared in that part of the plant in which the inoculation was made. The virus agent did not travel across the dead region.
The xylem tracts were not materially affected by this treatment, and water travelled across the region. Evidence of this is the fact that the distal portion remained turgid and sometimes continued growth for a considerable time. If the stem were removed above the ground level and put into eosin solution, this travelled readily over the dead tissue. That the vessels were not occluded by protein plugs is shown by the fact that particulate substances were carried up the xylem tracts past the dead region.
No evidence of adsorption of the virus agent to the cell remains could be adduced, so it is assumed that it was not travelling in the xylem stream.     

TISSUE CULTURE IN THE PRODUCTION OF NOVEL DISEASE-RESISTANT CROP PLANTS

1. Plant tissue cultures form the basis of a number of techniques which have been developed to effect genetic changes in plants. Progress is being made in the application of these techniques in breeding new, disease-resistant cultivars. 2. It is possible to induce and select for mutants among populations of cultured plant cells. Novel disease-resistant plants of a small number of species have been regenerated from cells selected in culture for their resistance to toxins produced by pathogens, both with and without prior exposure to mutagens. It is not known whether such procedures are widely applicable, and the nature of the genetic changes involved has not yet been determined. 3. The tissues of plant species which are propagated vegetatively are normally genetic mosaics with regard to many characteristics, including resistance to disease. Thus, some of the plants regenerated from cultured cells of such species are more resistant to pathogens than the parent plants. Novel plants produced in this way are already being used in some breeding programmes. 4. Many attempts have been made to modify the genomes of cultured plant cells by means of exogenous nucleic acids. The evidence for integration and replication of this genetic material is equivocal. The technique, therefore, offers no immediate prospects for the development of novel disease-resistant plants, but may be important in the long term as methods are perfected for using plasmids and other agents as carriers of useful genes. 5. Steady advances are being made in producing somatic hybrids of crop plants by fusion of isolated protoplasts. In the long term it may be possible to use protoplast fusion to transfer desirable disease-resistance traits between related species which cannot be hybridized by conventional breeding methods. 6. The culture of excised embryos may be used to grow interspecific and inter-generic hybrid plants in cases where incompatibility occurs after normal fertilization. The technique is already being used by breeders in the production of disease-resistant hybrids of crop species. 7. It is concluded that tissue culture has a limited but useful role to play in the development of novel disease-resistant crop plants.