152 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
Examples of netv genetic approaches 
The ways in which the new genetic ap- 
proaches could aid modern agriculture are 
described in the following two examples: 
SELECTION OF PLANTS FOR 
METABOLIC EFFICIENCY 
Because terrestrial plants are immobile, they live and die 
according to the dictates of the soil and weather conditions 
in which they are planted; any environmental stress can 
greatly reduce their yield. The major soil stresses faced by 
plants include insufficient soil nutrients and water or toxic 
excesses of minerals and salts. The total land area with 
these conditions approaches 4 billion hectares (ha), or 
about 30 percent of the land area of the Earth. 
Traditionally, through the use of fertilizers, lime, 
drainage, or freshwater irrigation, environments have 
been manipulated to suit the plant. Modern genetic tech- 
nologies might make it easier to modify the plant to suit the 
environment. 
Many micro-organisms and some higher plants can tol- 
erate salt levels equal to or greater than those of sea water. 
While salt tolerance has been achieved in some varieties of 
plants, the classical breeding process is arduous and lim- 
ited. If the genes can be identified, the possibility of actual- 
ly transferring those for salt tolerance into plants makes 
the adaptation of plants to high salt, semiarid regions with 
high mineral toxicities or deficiencies a more feasible pros- 
pect. In the future, selecting among tissue cultures for 
metabolic efficiency could become important. Tissue 
culture systems could be used to select cell lines for 
resistance to salts and for responsiveness to low-nutrient 
levels or less fertilizer. However, too little is known about 
the biochemistry and physiology of plants to allow a more 
directed approach at this time. Chances for success would 
be increased with a better understanding of plant cell 
biology. 
Such techniques could be applied to agricultural pro- 
grams in less developed countries, where, commonly, sup- 
plies of fertilizers and lime are scarce, the potential for ir- 
rigation is small, and adequate support for technological 
innovation is limited. In addition, the United States itself 
contains marginal land that could be exploited for forest 
products and biomass. The semiarid lands of the South- 
west, impoverished land in the Lake States, and reclaimed 
mining lands could become cost-effective areas for produc- 
tion. 
NITROGEN FIXATION 
It has been known since the early 1800’s that biological 
fixation of nitrogen is important to soil fertility. In fixation, 
micro-organisms, such as the bacterium Rhizobium, 
transform atmospheric nitrogen into a form that plants 
can use. In some cases— e.g., with legumes this process oc- 
curs through a symbiotic relationship between the micro- 
organism and the plant in specialized nodules on the plant 
roots. Unfortunately, the major cereal crops such as 
wheat, corn, rice, and forage grasses do not have the 
capacity to fix atmospheric nitrogen, thus are largely 
dependent on chemically produced nitrogen fertilizers. 
Because of these crops, it has been estimated that the 
world demand for nitrogen fertilizers will grow from 51.4 
million metric tonnes (1979 estimate) to 144 million to 180 
million tonnes by the year 2000.’* Therefore, geneticists 
are looking into the possibility that the genes for nitrogen 
fixation present in certain bacteria (called "nif genes") can 
be transferred to the major crops. 
Laboratory investigation has focused on the molecular 
biology of nitrogen fixation in the free living bacterium, 
Klebsiella pneumoniae. A cluster of 15 nif genes has been 
successfully cloned onto bacterial plasmids using rllN.A 
technology. These clones are being used to study the 
molecular regulation of nif gene expression and the 
physical organization of the nif genes on the Klebsiella 
chromosome. In addition they have aided the search for 
nitrogen fixation genes in other bacteria. 
It is thought that a self-sufficient package of nitrogen- 
fixing genes evolved during the course of plant adaptation, 
and that this unit has been transferred in a functional 
form to a variety of different bacterial spt^cies, including 
Klebsiella and Rhizobium. If the right IlNA \ector can he 
found, the nif genes might he transferred from bacteria to 
plants. The chloroplasts, the cauliflower mosaic \ irus, and 
the Agrobacterium Ti-plasmid are being in\estigated as 
possible vectors. 
The way that Agrubacteria, in particular, infect cells is 
similar to the way Rhizobia infect plants and form 
nitrogen-fixing nodules. In both cases, the |)hysical attach- 
ment between bacterium and plant tissue is necessary for 
successful infectioti. In the case of Agrobarleria, tumors 
form when a segment of the Ti-plasmid is ins«*rted into the 
nuclear genome of the |)lant cell. Scientists do not yet 
know exactly how a segment of tin? rhizohi.il genome is 
transferred into the root tissue to induce the formation of 
nodules; nevertheless, it is ho|)ed that Agrobarleria u ill act 
as vectors for the introduction and expression of toriMgn 
genes into plant cells, just as Rhizobia do naturalK 
Other researchers ha\c hec'ii iincstigating the re- 
quirements for getting nif geni's to exjiress themselves m 
plants. Nif genes from Klebsiella have alie.idy been 
transferred into common yeast, an organism that can he 
grown in environments without o.xvgen Unfortun.itelv 
the [jresence of oxygen destroys a majoi' enzyme lor 
nitrogen fixation and sevei'ely limits the potential .ipplic .i 
tions in higher plants. Nevei theless, it is hoped that ml 
gene expression in yeast will he applicable to higher pl.iots 
An approach that does not invoKi' genetu engineering 
uses improved Rhizobia strains that .ire sviiihiotu with 
.soybeans. I hrough selection, Rhizobia imit.inis .ire being 
found that out perform the original wild strains I uriher 
'“F. Aiisulief "Biological ,\ilrogen I iv.iliotr .Sii/i/Hirfaig /’.i/w'n 
World Food and Kulrilion Sludv (U aNhington. I) ( \alional \i .ul 
emv of Sciences, l!)77l. 
