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| Introduction:
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Soybeans are an important agricultural crop because of the oil and protein
contained in the seed. The seed contains about 20% oil and 40% protein. The
oil is used mainly for human consumption in products as cooking oil,
margarine, and salad dressings. After the oil is extracted from the seed the
material that is left is the soybean meal. The soybean meal is an excellent
protein source for animal husbandry and is an important component in the
feeding ration in the poultry and hog industries. In many countries, the
soybean is an important component of the human diet.
Soybeans are a leguminous plant. What this means is that through a
companionship of the plant roots and beneficial bacteria they fix atmospheric
nitrogen to supply some of the plants nitrogen needs. Other common plants that
are also legumes are the bean, pea, and peanut. All of these plants can fix
atmospheric nitrogen to supply some of their nitrogen needs. Nitrogen is used
by plants and animals to form different nitrogen containing compounds found in
all living organisms. One group of these compounds are called amino acids that
are used by biological organisms to make proteins. There are different types
of proteins in living organisms that have different functions. My research
deals with proteins that are found in seeds and specifically seed storage
proteins. |
| Soybean seed
proteins: |
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The soybean seed contains special cells that make and store the seed storage
proteins. Once the proteins are formed they are stored in special structures
within the cells that are called protein storage vacuoles (Figure 1). The
pictures in Figure 1 are called micrographs and were obtained by viewing very
thin sections of soybean seed in a transmission electron micrograph. The
protein storage vacuoles differ in size and the protein is made and stored
during the time the seed is growing on the plant. The top picture is from a
young seed and the dark portion represents protein that is being stored in the
protein storage vacuole. As the seed grows the vacuoles fill with more storage
protein and become full as shown in the bottom picture. |
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| Figure 1. Transmission electron
micrographs of protein storage vacuoles in developing soybean cotyledon parenchyma cells.
(Top) Parenchyma cells observed in a young developing seed. (Bottom) Parenchyma cells
observed in a mature developing seed. |
Figure 2. Low temperature
scanning electron micrographs of protein storage vacuoles in developing soybean cotyledon
parenchyma cells. (Top) Parenchyma cells observed in a young developing seed. (Bottom)
Parenchyma cells observed in a mature developing seed. |
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An example of how the
cells and protein storage vacuoles are arranged is shown in Figure 2. These
are pictures of developing soybean seed that were observed with another type
of electron microscopy. This electron microscope is a scanning electron
microscope and is used to look at the surface structure. These pictures were
taken with a special instrument because it looks at frozen tissue and is
called low temperature scanning electron microscopy. There are many cells in
the soybean seed and the round structures are protein storage vacuoles. The
picture shows the protein storage vacuoles in a young developing seed (top)
and another (bottom) that is close to maturity. The seed makes thousands of
these protein storage vacuoles to store the seed storage protein.
Luckily, there are only a few major seed storage proteins in soybean. Two
storage proteins, B-conglycinin and glycinin, account for approximately 70% of
the protein stored in the protein storage vacuoles. Generally, glycinin is
the more abundant of the two storage proteins.The amount of these two storage
proteins will differ in soybean seeds depending on their genetics and where
they are grown. Where the application of a protein product calls for more
sulfur amino acids then a higher glycinin content is preferred. The ratio of
these two storage proteins is also important in the manufacture of food
products. Changes in the B-conglycinin:glycinin ratio result in differences in
the physical properties of tofu. Tofu containing more B-conglycinin is softer
than tofu from soybean containing more glycinin.
Recently, the protein fraction of soybean seeds has become more important than
that of the oil. This is mainly due to an increase in the standard of living
throughout the world and the need for more protein for animal rations. Soybean
breeders have made new protein lines that exceed 50% protein. This has been
relatively easy to do and also means that there may be some simple factor that
allows the soybean seed to make and store more protein in the protein storage
vacuoles. The kind and amount of storage protein increased in these new
soybean lines is not known. The previous two examples demonstrate that knowing
the storage protein content is important. We are attempting to determine
which storage proteins are increased by looking at the quantity of B-
conglycinin and glycinin in these lines. This is being done by analyzing their
subunit structure by gel electrophoresis (Figure 3). |
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| Figure 3.
Polyacrylamide gel of some high-protein soybean lines. The principal
protein subunits of B-conglycinin are the a', a, and B subunits and the
principal polypeptides of glycinin are the acidic A3, A1,2,4 and basic
polypeptides. |
| In gel
electrophoresis,
proteins are separated by their size and charge. The protein bands are
stained to make them visible. The proteins at the top of the gel are larger
than those at the bottom. The gel shows the a', a, and b subunit protein bands
that make up the B-conglycinin storage protein. The gel also shows the acidic
and basic polypeptides that make up the glycinin storage protein. We have
found differences in the content of the components of B- conglycinin and
glycinin in the high protein lines. This means there may be several mechanisms
regulating the amount of these two storage proteins that are synthesized and
stored in the high protein lines. This information may be used to selectively
increase the type of storage protein for specific uses. Soybeans used in
feeding rations for the hog industry would be better served with soybeans
containing more B-conglycinin and the soybeans used in the poultry industry
would be better served with soybeans containing more glycinin. Future research
will be directed towards regulation of the quantity of B-conglycinin and
glycinin in the soybean seed. |