Plant Transformation
The genetic manipulation of plants has been an ongoing science since
prehistoric times, when early farmers along the Euphrates began carefully
selecting and maintaining seed from their best crops to plant for the next
season. Early Americans also bred plants, and modern corn is a result of
thousands of years of genetic manipulation. 
With the advent of recombinant DNA technology in the 1970s, the genetic
manipulation of plants entered a new age. Genes and traits previously unavailable
through traditional breeding became available through DNA recombination,
and with greater specificity than ever before. Genes from sexually incompatible
plants, or from animals, bacteria or insects can now be introduced into
plants. Products of modern plant genetic engineering are already on the
market in various regions of the U.S. Examples include a slow-softening
tomato and cotton plants resistant to herbicides and insects. With many
more products in the pipeline, the genetic engineering of plants will have
a profound impact on the future of agriculture in America.
Modern plant genetic engineering involves
the transfer of the desired genes into the plant genome, and then regeneration
of a whole plant from the transformed tissue. Currently, the most widely
used method for transferring genes into plants is Agrobacterium-mediated
transformation. Agrobacterium is a naturally occurring pathogenic
bacteria in the soil that has the ability to transfer its DNA into a plant's
genome. Agrobacterium infection and gene transfer normally occurs
at the site of a wound in the plant, and causes a characteristic growth
referred to as a crown gall tumor. Scientists have taken advantage of this
naturally occurring transfer mechanism, and have designed DNA vectors from
the tumor-inducing plasmid DNA found in the bacteria that are capable of
carrying desired genes into the plant. The engineered or constructed genes
are inserted into the Agrobacterium vectors and enter the plant by
the bacteria's own internal transfer mechanisms. Transformation is typically
done on a small excised portion of a plant known as an explant. This small
piece of transformed plant tissue is then regenerated into a mature plant
through tissue culture techniques.
The following is a photographic overview of the process of plant transformation
and regeneration. The transformation of tomato is illustrated.
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Photographs and text by Brad Hall, University of California, Davis.
Additional photographs courtesy of Calgene, Inc., Davis, CA, and the UC Davis Biotechnology Program, Meyer Hall.
Introduction by Jeffery O'Neal
