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OCR for page 5
Crop Improvement
Crop improvement, the engineering of plants for the benefit of hu-
manity, is as old as agriculture itself. Some 10,000 years ago, primitive
people made the transition from hunting and foraging to cultivating
crops. With that switch began the continuous process of improving the
plants on which we depend for food, fiber, and feed.
Throughout the milennia, two techniques have been used to improve
crops, according to Lawrence Bogorad, a plant molecular biologist at
Harvard University. The first is selection, which draws on the genetic
variation inherent in plants. The earliest farmers selected plants having
advantageous traits, such as those that bore the largest fruit or were the
easiest to harvest. Perhaps through some rudimentary awareness that
traits were passed from one generation to the next, the choicest plants
and seeds were used to establish the next year's crop. Natural selection,
which determines the survival of species, was now augmented by ar-
tificial selection. By selecting and isolating choice plants for cultivation,
the early farmers were in essence influencing which plants would cross-
pollinate. Through selection and isolation, they were narrowing, yet
controlling, the available gene pool for each crop.
Plant remains found in ancient Egypt and Mesopotamia indicate that
plant cultivation was already widespread by that time. In earlier ruins
of pre-Incan Indian villages in Peru, archeologists have uncovered Lima
beans that have seeds nearly 100 times larger than those of wild Limas
in the area. This suggests that the Incans obtained their beans from still
earlier plant breeders who left no record.
The second technique was breeding. The farmers selected two plants
and then crossed them to produce offspring having the desirer! traits
of both parents. The process was hit or miss, however, since early plant
breeders did not understand the genetic transmission of traits and could
OCR for page 6
6
GENETIC ENGINEERING OF PLANTS
not precTict the likely outcome of a particular cross. Nonetheless, valuable
traits did arise that could be selected and maintained in the population.
The physical basis of inheritance or what actually happens when
two plants are crossed was not understood until the early 19OOs. The
key was Gregor Mendel's breeding experiments in the IS60s, though
the importance of his work was not recognized until after his death.
Working with peas in his monastery garden in Austria, Mende1 deduced
that hereditary information is stored in discrete units that we now call
genes. Moreover,, he reasoned that each trait, such as color, is controlled
by two genes, one from the male parent and one from the female parent.
Soon after, other researchers found that genes are transmitted in
blocks of 5,000 or so, rather than independently as Mendel had sur-
mised. What Mende} did not know was that genes do not exist separately
in the cell; rather, they are linked together on long chromosomes in the
cell nucleus. Thus, while the gene is the unit of heredity, the chromo-
some is the unit of transmission. Each parent contributes half of the
chromosome complement to the offspring; in humans, for instance, each
parent contributes 23 chromosomes.
In the early 1900s, biologists learned how chromosomes are assorted
during cell division and how that determines the properties of the
offspring. They learned how to locate genes on chromosomes, because
chromosomes break and rejoin, or cross over, fairly regularly during
cell division, leading to new genetic combinations. They also learned
that sometimes chromosomes are present in multiple copies, or in re-
duced number, anct that this particular closage affects gene expression.
The First Biological Revolution
The foundation of Mendelian genetics enabled plant breeders to cross
plants with new precision, carefully manipulating the plant genome to
produce new, improved varieties. These breeding techniques have been
used to develop higher-yielding varieties, including plants resistant to
pests or disease. These improved varieties have contributed to a dramatic
explosion in agricultural output. In the past 50 years in the United States,
farm productivity has increased two-and-a-half times, while farm acreage
has declined 6 percent. One of the most spectacular successes was the
development of hybrid corn in the 1930s, which quickly doubled corn
yields.
Breeding advances have also meant more food for the rest of the
world. In the 1950s and 1960s, Norman BorIaug at the Center for Maize
and Wheat Improvement in Mexico developed semidwarf wheat vari-
eties, and the International Rice Research Institute in the Philippines
OCR for page 7
CROP IMPROVEMENT
7
developed similar improved rice varieties. When introduced in the 1960s
in India and later in China, the wheat and rice varieties became the
basis of the "Green Revolution," in which crop yields increased an
estimated four to seven times. For these reasons, the introduction of
applied genetics to agriculture is sometimes called the first biological
revolution.
Nonetheless, these productivity gains are not due to genetic advances
alone. In the United States, half of this gain is generally attributed to
the simultaneous improvements in farm management in cropping
practices, in farm machinery, and especially in the development of new
agricultural chemicals such as pesticides, fertilizers, and herbicides. Sim-
ilarly, the introduction of improved wheat and rice varieties in South
Asia was accompanied by a heavy investment in irrigation and agricul-
tural chemicals.
Though agriculture has profited immensely from the improved breed-
ing practices developed from Mendelian genetics, the technology does
have its limitations. One problem, as Bogorad described at the convo-
cation, is time. It may take generations and generations to develop a
desired strain through selection and breeding. The greatest limitation,
however, is simply the available supply of genetic diversity.
OCR for page 8
8
GENETIC ENGINEERING OF PLANTS
As Darwin discovered more than 100 years ago, new species evolve
through natural selection. If part of a breeding population becomes
isolated, its gene pool becomes more and more distinct from that of the
parent population. Often, biological barriers arise that prevent the two
populations from interbreeding. Consequently, within each distinct spe-
cies, genetic variation among individuals decreases. Because of such
natural breeding barriers, the plant breeder in search of useful new
variants is confined to members of the same species or closely related
species. Compounding the problem, many major crops have been under
cultivation for thousands of years, which has led to an increasingly
homogeneous gene pool. In some cases, the desired trait is simply not
available in the breeding population. "You can breed and breed and
breed, and never get the trait you are looking for," Bogorad said.
For this reason, in many essential crops we may be reaching what
Lowell N. Lewis, director of the California Agricultural Experiment Sta-
tion, called a "biological roadblock" in the drive for greater productivity.
"Yields have started to level off, and in some cases are declining. For
these crops, it is no longer simply a matter of sprinkling on a little more
fertilizer."
To Vernon Ruttan, an agricultural economist at the University of Min-
nesota, the closest analogy to the current situation is the closing of the
land frontier in the Unitec! States in the IS90s. As land became scarce
and expensive, farmers could no longer increase output by simply ex-
tending their existing techniques to new land. Instead, increased agri-
cultural output became dependent upon improved varieties and agri-
cultural chemicals, which came into use over the years through the work
of plant breeders and agricultural scientists. In essence, Ruttan said,
these agricultural chemicals became a substitute for land. Now farmers
in the more developed countries are beginning to exhaust the potential
of these chemical technologies as well. For instance, the application of
nitrogen fertilizer once assured a sizeable boost in yield. Now the gains
come harder. Corn is one of many examples. From 1954 to 1960, the
use of nitrogen fertilizer increased corn yields by two bushels per acre
per year. From 1971 to 1980, fertilizer added only half a bushel.
Burgeoning Demand for Food
Meanwhile, as the increase in agricultural productivity slows, the
demand for food continues to rise. The Unitecl States still produces a
surplus of grain, but as Orville Bentley, assistant secretary for science
and education of the U.S. Department of Agriculture, stated, "for coun-
tries that can produce in excess of their needs, there are many more
OCR for page 9
CROP IMPROVEMENT
2.0
1.5
LIJ
1
LL
CD
0.5
m
o
9
1954 1960 1970 1980
YEAR
The contribution of nitrogen fertilizer to U.S. corn yields during the periods 1954-1960,
1961-1970, and 1971-1980. From W. B. Sundquist et al., "A Technology Assessment of
Commercial Corn Production in the United States," Minnesota Agricultural Experiment
Station Bulletin 546, 1981.
that are experiencing food shortages." According to W. David Hopper,
vice president of the South Asia Region of the World Bank, although
there has not been as massive a famine as the one in India in 1943,
hunger and maInourishment are still pervasive worldwide. The Food
and Agriculture Organization of the United Nations estimates that some
500 million people are severely undernourished.
If worIcl population continues to grow at 1.S percent annually, food
production will have to at least double in the next 40 years to keep pace
with demand. Hopper suspects that the demand for food will double
in 30 years; as people become more affluent, they will seek a greater
and more varied cliet. Moreover, said Bentley, "not only will an increas-
ing number of people need to be fed, but that food must be produced
from inferior soil under poor climatic and deteriorating biological con-
ditions." Existing biological and chemical technologies may not be ad-
equate for the task.
In the short term, for the next 10 years, Hopper predicted that worIct
fooct production can keep pace with demand if there is a substantial
investment in these biological and chemical technologies. In India and
China, for instance, improved varieties of wheat and rice are fairly well
distributed. These nations, like other developing nations, now need
what Hopper called a steady accumulation of the "betters"—better use
OCR for page 10
10
GENETIC ENGINEERING OF PLANTS
of irrigation, better use of pesticides and fertilizer, better agronomic
practices. If both improver! biological varieties and supporting technol-
ogies are provided, these nations should experience a surge in produc-
tivity. But within 20 to 30 years, Hopper warned, the cleveloping nations
will also begin to exhaust the potential of these technologies. Unless
new biological materials—new varieties are introduced, there will not
be enough food for the worId's population.
THE THIRD WORLD
Faced with growing populations, many Third World countries ur-
gently need to increase their agricultural output. Yet, introducing new
agricultural practices is not a simple task, according to W. David Hop-
per, vice president of the South Asia Region of the World Bank. More-
over, increased yields will not come simply from introducing improved
varieties or cropping practices. There must also be social and economic
incentives for the farmer to adopt these new agricultural technologies.
In short, the new agricultural technologies must be profitable, even for
farmers who practice collective agriculture. There must also be an or-
ganizational structure in the country that will support the adoption of
new practices. For example, there must be a source of fertilizers, pes-
ticides, and farm equipment, and irrigation must be available. The
farmer must also have a market where he can sell his product. And
there must be a transportation system linking all of these.
In the 1 950s, international development agencies tended to neglect
one or more of these components, Hopper said. Yet all three the
agricultural technologies, economic incentives, and infrastructure- co-
alesced in the 1 960s in India and China, Hopper said. Norman BorIaug's
improved wheat varieties were introduced, as were the advanced rice
varieties of the International Rice Research Institute. These were ac-
companied by a major investment by the development agencies that
allowed the expansion of irrigation systems and the widespread use of
agricultural chemicals. The result was the "Green Revolution."
The rest of the world has not been so fortunate, Hopper said. Many
African nations, for example, are still "desperately short" of techniques
for working with their soils. Genetic engineering can play a major role
in developing new varieties suited for these conditions and could direct
the future course of agriculture in developing nations. Yet attention must
also be given to the supporting technologies that will make these new
varieties more productive than the traditional techniques.
OCR for page 11
CROP IMPROVEMENT
1 1
OCR for page 12
12
GENETIC ENGINEERING OF PLANTS
Working with existing gene pools, plant breeders must somehow
develop varieties that are higher yielding, more nutritious, adapted to
harsh environments, less costly to farm, and perhaps resistant to pests
and disease. That is where molecular biology and genetic engineering
hold great promise.
Molecular Genetics
Genetic engineering enables molecular biologists to reshuffle genes in
combinations not possible in nature, opening up a vast new source of
genetic diversity for crop improvement. "One of the most remarkable
achievements of genetic engineering and molecular biolov~,v is that we
now operationally have a kind of world gene pool," Bogorad explained.
"Darwin aside, speciation aside, we can now envision moving any gene,
in principle at least, out of any organism and into any organism."
In some cases, gene transfer will entail combining the genes of two
plants, as do today's plant breeders—but without the limitations of
working with the whole plant. Although Mendelian genetics eliminated
much of the guesswork in classical breeding, there is still an element
of trial and error: when two entire genomes are combined in a sexual
cross, the breeder cannot be certain of the outcome. He may be breeding
for one trait, controlled by one gene, but the hundreds of thousands of
other genes in each plant complicate the task. By contrast, the molecular
genetic engineer can pluck that single gene from the donor plant and
insert it into the recipient, leaving the extraneous genes behind. That
specificity also brings a saving in time. Through gene transfer an im-
. . . . . . .
. . . . .
v v
project variety can ne created in a single experiment, in one generation.
Yet, using conventional techniques, it takes repeated back crosses to
eliminate the unnecessary genes and thus many generations and several
years to create an improved variety.
Moreover, the genetic engineer in search of a gene for pest resistance,
heat tolerance, or another trait is no longer constrained by the natural
breeding barriers—he can select from anv species. Eventually, the ge-
v , ~
netic engineer may also select from outside the plant kingdom, borrow-
ing genes from animals or bacteria. A recent experiment demonstrated
that such transfers are indeed possible: a gene for antibiotic resistance
was transferred from a bacterium into a petunia plant, where it conferred
resistance on the plant. Eventually, it may be possible to transfer the
genes for nitrogen fixation from bacteria to plants, thereby reducing the
need for fertilizer.
OCR for page 13
CROP IMPROVEMENT
Uncertain Impact
13
Molecular genetic engineering is still in its infancy. It is too early to
gauge the impact it will have on agriculture and crop improvement. As
Ruttan explained, "It took 30 years to make the transition from getting
most of our productivity growth by bringing new land into production
to beginning to get it from the old biological technology the first bi-
olog~cal revolution. The question of whether and when the second bi-
olog~cal or biotechnological revolution will reverse the current produc-
tivity decline is still unanswered."
Some of the simpler new techniques, based on the ability to regenerate
plants from cells in culture, are already offering a shortcut in selection
and breeding for some plants. These techniques are generally known
as somatic cell genetics, as they involve the manipulation of cells, as
opposed to genes or whole plants, for crop improvement.
Gene-transfer techniques are far less accessible than somatic cell ge-
netics. Their successful application will depend upon breakthroughs in
the understanding of gene expression and regulation, as well as in-
creased knowledge of plant physiology, biochemistry, and development.
It is also too early to judge how plants will responc! to such manipulation.
For those reasons, Ruttan and others have predicted that, even with
the much-needed increase in research, the impact of biotechnology will
be small until the late 199Os.
Though their specific applications cannot be predicted, these new
genetic engineering techniques seem likely to become powerful adjuncts
to conventional breeding practices. Ultimately, their success will depend
on how well they can be integrated with conventional technologies.
Molecular biologists will need to work closely with plant breeders to
identify promising projects for genetic engineering. When a new variety
is developed in the laboratory, it will face the same scrutiny as does
any new variety; it will need to undergo lengthy evaluation in the field.
It must perform, offering an advantage in quality, yield, time, or cost,
if the farmer is to adopt it. For many major crops, sophisticated and
effective breeding strategies already exist; it is unlikely that these new
gene-transfer techniques will supplant them. Instead, they may offer
the greatest advantage in engineering of crops that are difficult to ma-
nipulate by conventional techniques.
The new genetic technologies will undoubtedly aid agriculture in ways
that cannot be anticipated now. Cell culture techniques, for instance,
are providing a valuable supply of genetic diversity that was totally
unexpected when work began a few years ago.
The greatest impact of these new technologies, however, may be in
OCR for page 14
14
GENETIC ENGINEERING OF PLANTS
elucidating the basic biology of plants. Though this work is just begin-
ning, gene-transfer techniques are already proving an invaluable too}
for exploring the structure, function, and control of genes. This new
knowledge can then be used to devise more ctirect, and thus quicker,
breeding strategies either at the whole plant, cellular, or molecular
level.
The vast potential of genetic engineering does not diminish the need
for other acivanced research. As Lowell N. Lewis pointed out, to increase
agricultural output in developing nations will require more research on
the biology and ecology of tropical food plants, as well as the pests and
diseases that plague them. It will entail bringing underutilized plants
into production and a continued search for new, valuable germ plasm.
Representative terms from entire chapter:
crop improvement
