Ch. 8— The Application of Genetics to Plants • 145 
pi imarily experimental, sueeesst’ul plant sex-cell 
cultures ha\e l)een achie\ecl lor a \arietv of 
im[)oi’tant culti\ars, including rice, tobacco, 
wheat, hai'lev, oats, sorghum, and tomato, llou- 
e\ei', because th(> t(‘chni(|ue can lead to hizai'i'e 
unstable chromosomal ar’rangements, it has had 
few applications. 
Kmhryo culturi's ha\ c been used to g(M'mi- 
nate, in \itro, those iMiihryos that might not 
otherwise sur\ i\ e because of basic incom[)atihil- 
ities, especialh wlien plants from different 
genei'a are crossed. Kmhi'yos may function as 
starting material in tissue cultuia' s\stems re- 
(|uiring jmcnile material. They are being used 
to speed up germination in such sjjecies as oil 
[)alms. v\hich take u[) to 2 years to g(>rminate 
under natural coiulitions. 
Protoplast I'usioii.— In proto[)last fusion, 
either two entire protoplasts are brought to- 
gether, or a single protoplast is joined to cell 
components— or organelles— from a second pro- 
toplast. When the com[)onents are mixed under 
the right conditions, they fuse to form a single 
hybrid cell. I'he hv hrids can he induced to pro- 
liferate and to regenerate cell walls. The func- 
tional plant cell that results may often he 
cultured fui ther and regenerated into an entire 
plant— one that contains a combination of genet- 
ic material from both starting plant cell progeni- 
tors. \\ hen protoplasts are induced to fuse, they 
can, in theor\', exchange genetic information 
w ithout the restriction of natural breeding har- 
riers. ,At present, protoplast fusion still has 
many limitations, mainly due to the instability of 
chromosome pairing. 
Organelles are small, specialized components 
within the cell, such as chloroplasts and mito- 
chondria. Some organelles, called plastids, carry 
their own autonomously replicating genes, as a 
result, they may hold promise for gene transfer 
and for carrying new genetic information into 
protoplasts in cultures, or possibly for influenc- 
ing the functions of genes in the cell nucleus. 
(See Tech. Note 8, p. 163.) 
The feasibiliU' of protoplast fusion has been 
borne out in recent work with tobacco — a plant 
that seems particularly amenable to manipula- 
tion in culture. ,An albino mutant of Nicotiana 
tahacum was fused with a \arietv of a sexually 
incompatible Nicotiana species. The resultant 
hybrids were easily recognized by their inter- 
mediate light green color. They ha\ e now been 
rt^genei'ated into adult plants, and are currently 
being used as a promising source of hornvvorm 
resistance in tobacco plants. 
Iranslfer by DIVA Clontis and Foreign 
A'eetors.— Hecomhinant DNA technology 
makes possible the selection and production of 
moi-e copies (amplification) of specific DNA 
segments. Se\ eral basic approaches exist. In the 
"shotgun” appi'oach, the whole plant genome is 
cut by one or moi'e of the commercially avail- 
able restriction enzymes. The DNA to he trans- 
ferred is then attached to a plasmid or phage, 
w hich carries genetic infoi-mation into the plant 
cell.— E.g., a gene coding for a protein (zein) that 
is a major component of corn seeds has been 
spliced into plasmids and cloned in micro-orga- 
tiisms. It is hoped that the zein-gene sequence 
can he modified through this approach to in- 
crease the nutritional quality of corn protein 
before it is reintroduced into the corn plant. 
f’oreign \ectors are nonplant materials (vi- 
ruses and bacterial plasmids) that can he used to 
transfer DNA into higher plant cells. Trans- 
formation through foreign vectors might im- 
prove plant varieties or, by amplifying the de- 
sired DNA sequence, make it easier to recover a 
cell product from culture. In addition, methods 
have been discovered that eliminate the foreign 
DN,A from the transformed mixture, leaving 
only the desired gene in the transformed plant. 
The most promising vector so far seems to be 
the tumor-inducing (Ti) plasmid carried by 
Agrobacterium tumefaciens. This bacterium 
causes tumorous growths around the root 
crow ns of plants. It infects one major group of 
plants— the dicots (such as peas and beans), so- 
called because their germinating seeds initially 
sprout double leaves. Its virulence is due to the 
Ti plasmid, which, when it is transferred to 
plant cells, induces tumors. Once inside the cell, 
a smaller segment of the Ti plasmid, called T- 
DNA, is actually incorporated into the recipient 
plant cell’s chromosomes. It is carried in this 
form, replicating right along with the rest of the 
