146 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animais 
chromosomal DNA as plant cell proliferation 
proceeds. Researchers have been wondering 
whether new genetic material for plant im- 
provement can be inserted into the T-DNA 
region and carried into plant cell chromosomes 
in functional form. 
Adding foreign genetic material to the T-DNA 
region has proved successful in several ex- 
periments. Furthermore, it has been found that 
one type of plant tumor cell that contains 
mutagenized T-DNA can be regenerated into a 
complete plant. This new discovery supports 
the use of the Agrobacterium system as a model 
for the introduction of foreign genes into the 
single cells of higher plants. 
Many unanswered questions remain before 
Agrobacterium becomes a useful vector for 
plant breeding. Considerable controversy exists 
about exactly where the Ti plasmid integrates 
into the host plant chromosomes; some inser- 
tions might disrupt plant genes required for 
growth. In addition, these transformations may 
not be genetically stable in recipient plants; 
there is evidence that the progeny of Ti-plasmid- 
containing plants do not retain copies of the Ti 
sequence. Finally, Agrobacterium does not read- 
ily infect monocots (a second group of plants), 
which limits its use for major grain crops. 
Another promising vector is the cauliflower 
mosaic virus (CaMV). Since none of the known 
plant DNA viruses has ever been found in plant 
nuclear DNA, CaMV may be used as a vector for 
introducing genetic information into plant 
cytoplasm. Although studies of the structural 
organization, transcription, and translation of 
the CaMV are being undertaken, information 
available today suggests that the system needs 
further evaluation before it can be considered 
an alternative to the Agrobacterium system. 
Although work remains to be done on Ti- 
plasmid and CaMV genetic mechanisms, these 
systems have enormous potential. Most immedi- 
ately, they offer ways of examining basic mech- 
anisms of differentiation and genetic regulation 
and of delineating the organization of the 
genome within the higher plant cell. If this can 
be accomplished, the systems may provide a 
way of incorporating complex genetic traits into 
whole plants in stable and lasting form. 
Screening for Desired Traits.— The bene- 
fits of any genetic alteration will be realized 
only if they are combined with an adequate svs- 
tem of selection to recover the desired traits. In 
some cases, selection pressures can be useful in 
recovery.® The toxin from plant pathogens, foi’ 
example, can help to identify disease resistance 
in plants by killing those that are not resistant. 
So far, this method has been limited to identify- 
ing toxins excreted by bacteria or fungi and 
their analog; after sugarcane calluses were ex- 
posed to toxins of leaf blight, the resistant lines 
that survived were then used to dexelop new 
commercial varieties. In theory, however, it is 
possible to select for many important traits. 
Tissue culture breeding for resistance to salts, 
herbicides, high or low temperatures, drought, 
and new varieties that are more responsive to 
fertilizers is currently under study. 
Five basic problems must he overcome hefor(> 
any selected trait can he considei’ed beneficial 
(see figure 28): 
• the trait itself must he identifieil: 
• a selection scheme must he found to iden- 
tify cells with altered prop(M’ti('s: 
• the properties must |)rove to he du(* to ge- 
netic changes; 
• cells with altered properties must cotif(*r 
similar properties on the vv hole plant: and 
• the alteration must not adwr.selv affe('t 
such commerciallv important charactei is- 
tics as yield. 
While initial scr(!ens inv olv ing cells are easier 
to carry out than sci’eening tests of entire 
plants, tolerance? at the? ('(‘Ilular level must he 
confirmed by inoculations of the mature plants 
with the actual pathog(*n under field conditions 
PHASE III: HE«E\EKATI\(; UHOI.E PEAMS 
FROM CELLS I\ TISSl'E Cl I-'HIRI: 
New methods are hiMiig develo|)ed to: 
• increase the speed with which (Tops are 
multiplied through mass propagation, and 
• create and maintain disease-tree plants 
Mass Propagalioii.— The greatest single 
use of tissue culture systems to date has heen 
for mass propagation, to (*stahlish selected 
®J. K. Shepard, I). Hidiii-v, atui I Shahiii I’dlaln I’nilupl.i i i' 
Crop Improvi^menl. " .Science 20K 17 1!1K(I 
