Ch.8 — The Application of Genetics to Plants • 149 
Figure 29.— A Model for Genetic Engineering 
I of Forest Trees 
1 
- a. Selection of genetic material from germplasm bank 
^ b. Insertion of selected genes into protoplasts 
c. Regeneration of cells from protoplasts and 
multiplication of cell clones 
d. Mass production of embryos from cells 
e. Encapsulation to form ‘seeds’ 
I f. Field germination of ‘seeds' 
g. Forests of new trees 
I 
SOURCE; Office of Technology Assessment. 
Creation and Maintenance of Disease- 
; Free Plants.— Cultixars maintained through 
i standard asexual propagation over long periods 
f often pick up viruses or other harmful path- 
I ogens, which while they might not necessarily 
kill the plants, may cause less healthy growth. A 
plant’s true economic potential may be reached 
I only if these pathogens are removed— a task 
, which culturing of a plant’s meristem (growing 
I point) and subsequent heat therapy can per- 
‘ form. Not all plants produced through these 
methods are \ irus-free, so screening cells for 
viruses must be done to ensure a pathogen-free 
plant. In horticultural species, the adv antages of 
vii'us-free stock often appear as larger flowers, 
moi'e \ igorous growth, and improved foliage 
(|uality. 
T oday, \ irus-free fruit plants are maintained 
and distributed from both pi'ivate and public 
re|)ositories. Work of commercial importance 
has been done with such plants as sti'awherries, 
sweet [)otatoes, citrus, freesias, irises, rhuharhs, 
gooseberries, lilies, hops, gladiolus, geraniums, 
and chrysanthemums.'-* Over 134 \ irus-free 
potato cultures have also been developed by tis- 
sue culture.'® 
Constraints on the neiv genetic 
technologies 
Although genetic information has been trans- 
ferred by vectors and proto|)last fusion, iio DNA 
transfoi’iiiations of commercial value have yet 
been performed. The constraints on the suc- 
cessful application of molecular genetic technol- 
ogies are both technical and institutional. 
TECHMCAL CONSTRAINTS 
Molecular engineering has been impeded by a 
lack of understanding about which genes would 
he useful for plant breeding purposes, as well as 
by insufficient knowledge about cytogenetics. 
In addition, the available tools— vectors and 
mutants— and methods for transforming plant 
cells using purified DNA are still limited. 
Cells carrying traits important to crop pro- 
ductiv’ity must be identified after they have 
been genetically altered. Even if selection for an 
identified trait is successful, it must be dem- 
onstrated that cells with altered properties con- 
fer similar properties on tissues, organs, and, 
ultimately on the whole plant, and that the 
genetic change does not adversely affect yield 
or other desired characteristics. Finally, only 
limited success has been achieved in regenerat- 
ing whole plants from individual cells. While the 
list of plant species that can be regenerated 
from tissue culture has increased over the last 5 
years, it includes mostly vegetables, fruit and 
'“M. Misawa, K. Sakato, M. Tanaka, M. Havashi, and H. Same- 
jima, "Production of Physiologically Active Substances by Plant 
Cell Suspension Cultures," H, E. Street (ed ), Tissue Culture and 
Plant Science (New York: Academic Press, 1974). 
'^Murashige, op. cit. 
