Agriculture Reference
In-Depth Information
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Classical plant breeders have been able to force hybridization between different plant
species by making wide crosses and then using techniques such as embryo rescue,
protoplast fusion, and mutagenesis to generate diversity that would not normally
exist in nature. These plant-breeding techniques were not controversial and did not
get wide publicity but did allow plant breeders to develop food crops with important
traits. Wide crosses in wheat have been used for many decades to develop wheat
varieties with resistance to various pathogens that can cause enormous crop losses.
The first interspecific wheat crop developed by humans was named triticale and was
a cereal hybrid developed by mixing chromosomes of wheat and rye.
Molecular biology allows breeders to develop transgenic crops similar to wide
crosses, but with more control, through use of recombinant DNA. A piece of DNA
containing one or more specific genes from nearly any organism, including plants,
animals, bacteria, or viruses, is introduced into another crop species. This means that
genes can be transferred between any species as opposed to only within the same
species as normally occurs within plants during sexual replication. These resulting
transgenic
plants (the general public refer to these new plants as genetically modified
organisms or GMOs) are developed through a technique called
genetic engineering
.
The development of genetically engineered crops was made possible by the dis-
covery in the 1970s of restriction enzymes that can cut double-stranded DNA and
ligases that enable pieces of DNA to be spliced together. The first transgenic organ-
ism developed was a bacterium. The first transgenic plant was developed in 1983,
just 25 years ago. Today, the major transgenic crops grown commercially are either
herbicide tolerant or insect resistant. Scientists are also working on other biotic and
abiotic (mainly drought and salt tolerance) stress resistance and crops with improved
nutrition (e.g., golden rice).
Transgenic plants are generated in the laboratory by adding one or more genes
to a host plant's genome. This technique is called
transformation,
and it is usually
achieved by direct bombardment of plant cells with a gene gun or by horizontal gene
transfer with the aid of soil bacteria like
Agrobacterium tumefaciens.
The last bac-
teria acts as a vector and can carry the new genes attached to plasmids in their cells
into the host plant cell, where they hybridize with the host chromosomes. In the gene
gun system, DNA is impregnated onto gold particles that are then shot into plant
pieces and cells. The DNA of the new gene then integrates into the chromosome
of the host plant to produce transgenic plants with the new trait. In both systems,
an additional antibiotic marker gene is often attached to the DNA strand in close
proximity to the gene to be transferred and is used to enable easier selection of cells
that have successfully received the gene of interest. Tissue culture is then used to
grow the infected cells. The medium on the agar plate contains an antibiotic for the
inserted marker gene, and only cells that have the antibiotic marker closely attached
to the new gene survive for selection. These cells are then transformed into plantlets
and grown out as crop plants containing the genetic trait somewhere in their chro-
mosomes. This system facilitates genetic exchange in crops. It is far more precise
because only a single (or just a few) gene is transferred with a specific trait. As a
result, ancillary, unwanted traits that can accompany normal crosses are avoided.
