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2-Plants: Calcium-enhanced potatoes may soon offer a new healthy choice

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TITLE:  Calcium-enhanced potatoes offer a new healthy choice
SOURCE: by Katharina Schoebi, Checkbiotech, Switzerland
DATE:   30 Sep 2005

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Calcium-enhanced potatoes offer a new healthy choice

The majority of people do not consume enough calcium and are thus
suffering from health problems. Transgenic potato tubers containing up to
three-fold more calcium may soon be an edible tool for boosting calcium

The body needs calcium to reduce osteoporosis, a disease characterised by
reduced bone density and the primary cause of bone fragility. The
recommended amount of daily calcium intake depends on the person's age.
For example, nursing children need between 220 to 400 milligram of
calcium per day. The older a child becomes, the more calcium it needs. A
teenager should consumer around 1.2 gram calcium per day, whereas adults
should eat 1.5 gram of calcium per day.

Dairy products are a good source of calcium. Cheese is especially rich in
calcium. A 100 gram piece of Parmesan contains 1,180 milligram of
calcium. Thus, the daily calcium need would already be met by eating a
piece of cheese and drinking a glass of milk.

However, some individuals suffer from lactose intolerance and thus limit
their consumption of dairy products or even avoid it totally, while some
ethnic groups do not use dairy products in their diet, or restrict them
considerably. These individuals have to meet their daily calcium needs by
eating vegetables and fruit. Unfortunately, widely consumed vegetables,
such as potatoes, grains and rice, do not contain enough calcium. For
example, an average tuber of boiled potato contains only 20 milligram of
calcium. Given that an average person would eat one boiled potato per
day, one would consume 7.3 gram calcium in a year - the amount
recommended for only one week!

"If we double or triple the amount of calcium in a tuber, we could
provide several weeks worth of the dietary reference intakes of calcium,"
said Dr. Kendal Hirschi from the Baylor College of Medicine in Houston,
Texas, in his publication in the Journal of Agricultural and Food Chemistry.

The reason for Dr. Hirschi's excitement was that his publication dealt
with transferring a gene that encodes the calcium transporter CAX1 to a
potato variety. CAX1 is a calcium transporter in the vacuole membrane of
plants. The researcher's hypothesis was that increased activity of CAX1
in potatoes should result in an increase in total calcium level. This
hypothesis was based on his past research that CAX1 increased calcium
levels in tobacco (Plant Cell, 1999).

Analyzing their data, the researchers found out that the transgenic
potato tubers contained 1.5- to 3-fold more calcium than natural tubers,
and in transgenic leaves, the calcium levels increased up to 1.7-fold.
Since even the third generation of potato plants showed an increase in
calcium content, the researchers believe that this trait is stable
through successive generations.

Overall, comparing the mineral content and consistency of common potato
varieties and calcium-enhanced potatoes, Dr. Hirschi's group was not able
to find any differences.

The researchers underline, however, that no single food source will
rectify calcium intake deficiencies alone and the genetically enhanced
potatoes follows suite. Thus, transgenic potatoes will not cure
osteoporosis, but they rather will provide additional calcium and will be
a model for the modification of the calcium content in many more food
crops, Dr. Hirschi and his colleagues argue.

To assess the so called bioavailability of calcium (the amount of calcium
that can be digested, absorbed and used by the body) further studies are
needed. Right now, the researchers are undertaking feeding studies in
mice to see if the transgenic potatoes are actually more nutritious. In
addition, some experiments should assess if calcium-enhanced potatoes
still taste like common commercial varieties.

Since there is some evidence that genetic transformation does not alter
either the ethylene levels or the sugar content of tomatoes, Dr. Hirschi
suggests that transgenic potatoes will also taste like common varieties.

The research group has carried out some experiments to determine, whether
a higher calcium amount in the tuber affects the vitamin C content of the
potatoes. With greenhouse grown potatoes, they did not see any
difference. In addition, the consistency appears to be similar, but at
this point the researchers have not finished their work in this area. Dr.
Hirschi told Checkbiotech.

Future studies will also be done to find out if increasing calcium levels
will decrease the incidence of pathogen infection and post harvest decay,
two major problems worldwide. "Our hope is that the modifications will
lower the incidence of postharvest infection and will not alter the
insect attacks on the plants," Dr. Hirschi explained.

Plants need calcium for normal growth and development, and it is
especially critical for correct cell wall and cell membrane formation. It
is known that potatoes grown in calcium enriched soils have an increased
tuber quality, whereas calcium deficiencies in potatoes result in
internal disorders such as: hollow-heart, internal brown spot, brown
center and reduced storage life. Thus, transgenic potatoes containing
higher calcium levels could lead to better yields for farmers and a more
nutritious product for consumers.

Dr. Hirsci looks forward to working with the Vegetable and Fruit
Improvement Center at Texas A&M. This center specializes in working with
growers to bring new produce options to consumers.

Dr. Hirschi and his team have recently carried out similar studies with
tomatoes and carrots, and he noted, "We should try as many plants as
possible." The results of his study are encouraging, Dr. Hirschi said.
However, a lot more work needs to be done to show these plants are
durable and safe. "Working with plants is fun, because you can try to
help people - but it is a slow process."

Katharina Schoebi is a biologist and Chief Science Writer for
Checkbiotech. Contact her at

Kendal D. Hirschi et al. Genetic Manipulation for Enhancing Calcium
Content in Potato Tuber. Journal of Agricultural and Food Chemistry.
(2005) 53, pp. 5598-5603

Link to the abstract:

Kendal D. Hirschi et al. Expression of Arabidopsis CAX1 in Tobacco:
Altered Calcium Homeostasis and Increased Stress Sensitivity. Plant Cell.
(1999) 11, pp. 2113-2122

Kendal Hirschi
Baylor College of Medicine
1100 Bates Street
Texas 77030
Phone: 713-798-7011
Fax: 713-798-7078


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