Ch. 8— The Application of Genetics to Plants • 141 
A'eif' frenetic technologies for 
plant breeding 
The recent breaklliroughs in genetic engi- 
neering permit the plant breeder to bypass tbt' 
various natural breeding barriers that have 
limited control ot the transfer of genetic in- 
j formation. \\ bile the new technologies do not 
' necessai'ily offer the plant bi'eeder the radical 
I changes that recombinant 1)\'.\ (rl)\'.\) technol- 
! ogv provides the microbiologist, they will, in 
theory, s[)eedup and perfect the process of ge- 
netic refinement. 
The new technologies fall into two catego- 
ries: those involving genetic transformations 
through cell fusion, and those involv ing the in- 
sertion or modification of genetic information 
through the cloning (e.xactly copving) ot t)\'.\ 
and DN,A vectors (transfer DNA). Most genetic 
transformations require that enzymes digest 
the plant's impermeable cell wall, a process that 
leaves behind a cell without a wall, or a proto- 
plast. f’rotoplasts can fuse w ith each other, as 
well as with other components of cells. In 
theory, their ability to do this permits a wider 
e.xchange of genetic information. 
The approach e.xploiting the new technol- 
ogies is usually a three-phase program. 
Phase I. Isolated cells from a plant are estab- 
lished in tissue culture and kept aliv e. 
Phase If. Genetic changes are engineered in 
those cells to alter the genetic makeup 
of the plant; and desired traits are 
selected at this stage, if possible. 
Phase III. The regeneration of the altered single 
cells is initiated so that they grow into 
entire plants. 
This approach contains similarities to the genet- 
ic manipulation of micro-organisms. However, 
there is one major conceptual difference. In 
micro-organisms, the changes made on the cel- 
lular level are the goals of the manipulation. 
W ith crops, changes made on the cellular level 
are meaningless unless they can be reproduced 
in the entire plant. Therefore, unless single cells 
in culture can be grown into mature plants that 
have the new, desired characteristics— a proce- 
dure which, at this time, has had limited suc- 
cess— the benefits of genetic engineering will 
not be widespread. If the harriers can be over- 
come, the new technologies will offer a new 
way to control and direct the genetic character- 
istics of plants. 
PHASE I: TISSPE CULTURE TO CLONE PLANTS 
Tissue culture involves gi'ovving cells from a 
plant in a culture or medium that will support 
them and keep them viable. It can be started at 
three diffei'ent levels of biological organization: 
with plant organs (functional units such as 
leaves or i-oots):* with tissues (functioning ag- 
gregates of one type of cell, such as epidermal 
cells (outermost layer) in a leaf; and with single 
cells, rissue cultures by themselves offer spe- 
cific benefits to plant breeders; just as fermenta- 
tion is crucial to microliial genetic technologies, 
tissue culture is basic to the application of the 
other new genetic technologies for plants. 
The idea of growing cells from higher plants 
or animals and then regenerating entire plants 
from these laboratory-grown cells is not new. 
However, a better scientific understanding now 
exists of what is needed to keep the plant parts 
alive. 
In tissue culture, isolated single plant cells are 
typically induced to undergo repeated cell divi- 
sions in a broth or gel, the resulting amorphous 
cell clump Is known as a callus. It culture condi- 
tions are readjusted when the callus appears, its 
cells can undergo further proliferation. As the 
resulting cells differentiate (become special- 
ized), they can grow into the well-organized 
tissues and organs of a complete normal plant. 
The callus can be further subcultured, allowing 
mass propagation of a desired plant. 
At this time, it is not uncommon to produce as 
many as a thousand plants from each gram of 
starting cells; 1 g of starting carrot callus rou- 
tinely produces 500 plants. The ultimate goal of 
tissue culturing is to havm these plantlets placed 
in regular soil so that they can grow and devel- 
op into fully functional mature plants. The com- 
plete cycle (from plant to cell to plant) permits 
production of plants on a far more massive 
scale, and in a far shorter period, than is possi- 
ble by conventional means. (See table 25 for a 
‘Also referred to as organ culture. 
