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The natural genetic engineering of plants
Jean-Pierre Zryd, Institute of Ecology, University
of Lausanne, Switzerland
Genetically modified organisms are obtained by the introduction of
a foreign piece of DNA into their own genome. Does this process actually
differ from the natural processes occurring in plants and other living
organisms? According to some people, genetic engineering is a man made
tool that goes against natural laws. In fact during this century scientists
have discovered that nature does a lot of shuffling, rearranging and
transferring genetic material in bacteria, plants and animals: that
knowledge can now be used to speed up and improve the precision of classical
genetics and traditional breeding.
Gregor Mendel postulated, and demonstrated through clever experiments,
that the "hereditary units" (units we now call genes) which
determine the visible traits of an organism rearranged themselves in
a random manner during the sexual reproduction process. Since the early
twenties, it has been known that during the complex processes of reproduction,
rearrangement of genetic material is generated by the exchange of large
pieces of genetic material. Moreover, in the early thirties, Barbara
McClintock discovered that during the whole life of a maize plant, and
not only during the reproductive stages, genes frequently jump from
one place to another inside the genome, inducing visible mutations [5,
6]. The famous Indian corn kernels, which are so beautiful with their
patterns of red and yellow stripes and spots, are an example of this
phenomenon. Barbara McClintock got the Nobel prize in 1983, at the age
of 81, for her theory of transposable elements (jumping genes). The
fact that she received the prize as late as 50 years after her first
observations is due to the strong reluctance of the scientific establishment
to really accept the idea of a general genome instability. We now know
that transposons are present in all organisms and active all the time;
a very large portion (up to 50% and more) of the genetic material present
in plants and in animals is made of a special kind of transposons named
retrotransposons . People who are not familiar with biology tend
to think that living organisms are stable and that species are here
to indefinitely stay the same; but this is not the case, we are all
changing, and many species are evolving very fast. There is no doubt
that an efficient inbuilt system allows controlled mutations to occur
within living organisms genome; scientists think that such mutations
help organisms to cope with fast changing environmental conditions:
a must-equipment for plants, as they cannot move to escape.
Plant biologists are lucky people. They discovered that an ubiquitous
pathogenic soil bacteria, Agrobacterium tumefaciens, is able
to transfer a small but important part of its genetic material to plants
during the infectious process. This genetic material is transported
from the bacterial cell to the plant cell nucleus and integrates in
the plant genome, where it starts making its own products. The transferred
genes products induce tumour cell proliferation and the production of
highly nutritious metabolites which the bacteria can feed on; those
events are part of a process named genetic colonization. Scientists
discovered that the bacteria can be made non pathogenic (harmless) to
the plant, while remaining able to transfer a piece of DNA. From then
on, the exciting story began of plant genetic transformation methods
and of extensive plant genetic analysis. To transform a plant with the
help of Agrobacterium tumefaciens, all you have to do is to insert a
selected piece of DNA between two short specific DNA border-sequences;
this is enough to allow the integration of this foreign DNA in the plant
genome . Such an integration is random, but unique; this means that
the piece of DNA can land anywhere, but only in one place. It is then
up to the geneticist to check if the integration happened in a convenient
place of the genome. Today, most transgenic plants are produced by this
very efficient method.
The game of recombination
Adaptation processes and evolution are characteristics of life; they
result from genome modifications. We, human, are here because our primate
ancestors changed before us and it is almost inevitable that we will
also change with time. Genome modifications are generated, among others,
by shuffling elements in a process called genetic recombination. Recombination
occurs under different circumstances.
1) When a piece of DNA integrates anywhere into the genome, the process
is called illegitimate recombination. It is for instance what happens
when transposons move or when a plant is transformed via A. tumefaciens.
Almost all transgenic plants are obtained this way. Illegitimate recombination
is the predominant mechanism of recombination in most organisms.
2) When a piece of DNA integrates into a precise place of the genome
by recognizing a totally identical (homologous) piece of DNA, the process
is called homologous recombination. Recently it has been shown that
homologous recombination can be used to transform a plant by integrating
a piece of foreign DNA into a precise place of the plant genome (genes
targeting). So that the mechanism can take place, the foreign piece
of DNA has to contain a short sequence which is totally identical to
the landing site in the genome (target). As Didier Schaefer demonstrated
it in our group at the University of Lausanne, this method can be efficiently
applied to the moss Physcomitrella patens . Making transgenic plants
through homologous recombination of small and well chosen DNA stretches
promises well and should before very long become a high precision genetics
method making possible subtle modifications of very small and specific
part of the genome, at will .
Homologous recombination is a natural mechanism that occurs during the
production of gametes (sexual reproduction); in this case, recombination
events concern large pieces of the genome; they increase the genetic
diversity. While they chose the seeds for the next season, the very
early cultivators have benefited from this process (without being aware
of it!), and later the conventional breeding methods which have led
to our main agricultural crops took also advantage of it.
Toward increased diversity
When using conventional breeding methods, the presence of a significant
variability in the plant genome is a necessity. It can be achieved by
different means: the natural variability present in the extensive collection
of man made cultivars (traditional and indigenous agriculture) can be
used, as well as crossbreeding with identified closely related wild
species (botanical survey). In the twentieth century, variability have
been increased by submitting plants to ionising radiations. Radiations
induce mutations (small modifications inside the genes); they have also
the side effect of inducing a higher level of recombination. Most of
the main crops cultivars we use today are the offspring of such mutated
Many plant species, potato and orange tree for instance, spontaneously
propagate without sexual reproduction. Thus, propagation by taking cuttings,
layering, grafting and, more recently, by in vitro cultures has been
carried out. Such vegetative (non sexual) propagation methods are not
as precise however as it could be expected: sometimes, the plants you
get, instead of being the same as the mother plant, largely differ from
it. These differences are named somaclonal variations; they result from
significant modifications of the genome and can be used to produce new
Both methods, mutagenesis and the use of somaclonal variations, significantly
have increased the efficiency of conventional breeding.
Recently, DNA transfer methods (as a part of genetic engineering) have
been introduced, when available, into conventional breeding. With their
precision and their efficiency, these new techniques make possible to
control variability more safely; they are not intended to replace other
parts of the breeding technology, but to complement them.
DNA transfer: high precision genetics
DNA transfer is mainly intended for adding or modifying well known
and defined genes: this part of genetics could be named "high precision
genetics" in comparison with the traditional methods, which use
quite unpredictable recombinations of very large parts of the genome.
The introduction into a rice variety of three well known genes for the
synthesis of pro-vitamin A by research groups from Swiss and German
Universities is a masterly example of this strategy. Such a technical
achievement is of course just a part of the whole necessary work. These
high-tech rice plants are now used as parent plants in a more conventional
breeding programme of which aim is to introduce this valuable trait
in rice cultivars that could be used in local farming conditions. Variability,
whether artificial or natural, is a source of potential hazard: it can
have beneficial or neutral, but sometimes detrimental, effects; each
new variety has to be tested under natural conditions. The main advantage
of the new genetic of recombinant DNA (GMO) is to provide us with a
more precise monitoring of the breeding processes: this is a significant
benefit for all agricultural systems.
- Gelvin SB: Agrobacterium and plant genes involved in T-DNA transfer
and integration [Review]. Annual Review of Plant Physiology &
Plant Molecular Biology 51: 223-256 (2000).
- Kumar A, Bennetzen JL: Plant retrotransposons [Review]. Annual Review
of Genetics 33: 479-532 (1999).
- Puchta H, Hohn B: From centimorgan to base pairs - Homologous recombination
in plants [Review]. Trends in Plant Science 1: 340-348 (1996).
- Schaefer DG, Zryd J-P: Efficient gene targeting in the moss Physcomitrella
patens. The Plant Journal 11: 1195-1206 (1997).
- Wessler SR: Transposable elements and the evolution of gene expression.
[Review] [75 refs]. Symposia of the Society for Experimental Biology
51: 115-22 (1998).
- White SDJ: Of genes and genomes and the origin of maize. [Review]
[32 refs]. Trends in Genetics 14: 327-32 (1998).