Agriculture Reference
In-Depth Information
Enzyme Linked Immunosorbent Assay (ELISA)
ELISA is a sensitive analysis method that allows for the rapid and simultaneous testing of a large number
of samples. Although ELISA techniques have been used for many years, it has only been recently that such
techniques have become useful for distinguishing the presence or absence of genetically modiied charac-
teristics of varieties. Antibodies are substances that recognize the unique molecular structure of proteins
or antigens and then bind to them. These antibodies are very speciic and bind only to speciic types of
proteins. Thus, if a corn seed has the Bt gene for insect resistance, it will produce a speciic Bt protein that
an appropriate antibody binds to. However, since the bound antibody and antigen are too small to visualize,
an enzyme label is added to the mixture that generates a visible color change in a substrate if the enzyme
successfully combines with the antibody/antigen complex (Fig. 9.4). The color reaction for each seed can
be read by eye in 96-well microtiter plates. Commercially available ELISA kits and reagents are available
for genetically modiied crops such as Bt corn and Roundup Ready
TM
soybeans.
Figure 9.4. Diagram of a double antibody sandwich ELISA test (courtesy of BioDiagnostics, Inc.)
Lateral flow Strips
Lateral low strips are immunoassay products. A typical strip has (1) a test line (coated with the antibodies
of the trait being tested, and (2) a control line which appears if the strip is working properly. The test line
appears if the proteins (antigen) of the trait are present in the seeds that are being tested. This method is easy
to use and does not require specialized equipment.
bIoASSAy TESTS To dETECT GEnETIC TrAITS
This category of tests utilizes less sophisticated techniques to easily distinguish between conventional and
genetically modiied cultivars. It is mostly used for detecting herbicide tolerance traits. It is accomplished
by exposing seeds or seedlings to a particular herbicide to determine whether such seeds or seedlings carry
the gene(s) for certain herbicide tolerance.
Herbicide or Insect Tolerance Tests
The irst herbicide tolerance test was a seed soak method for Sulfonylurea tolerant soybean (STS) seeds
(Sebastion and Chaleff, 1987). However, following the introduction of Roundup Ready
TM
soybean seeds
in the 1990s, an increased emphasis on detection of those soybean seeds that possessed the herbicide toler-
ance trait was essential. For example, should a farmer believe that Roundup Ready soybean seeds had been
planted, but were not, subsequent spraying of the nonselective herbicide would result in a complete crop
failure. Seed technologists quickly identiied three approaches to successfully determine herbicide toler-
ance. These included presoak, substrate imbibition and seedling spray tests.
