www.checkbiotech.org ; www.raupp.info ; www.czu.cz

For plants, the ability to accurately sense light governs everything from
seed germination, photosynthesis and pigmentation to patterns of growth and
flowering, November 2005 by Terry Devitt.

Now, for the first time, scientists have obtained a detailed map of one of
biology's most important light detectors, a protein found in many species
across life's plant, fungal, and bacterial kingdoms.

By resolving the three-dimensional structure of the protein known as
phytochrome, scientists can now tease out the secrets of how plants, in
particular, react to light, opening the door for a host of manipulations
that could have sweeping implications for agriculture.

Writing in the Nov. 17 issue of the journal Nature, a team of scientists
from the University of Wisconsin-Madison report that they have obtained the
crystal structure of a phytochrome from a bacterium, the first such
light-gathering structure depicted for all of biology. The structure of the
bacterial phytochrome, according to the report, suggests its architecture
first arose a billion or so years ago in a common ancestor and is shared
among not only bacteria, but also by plants and fungi.

"This is probably the most important light regulator in agriculture," says
Richard Vierstra, a UW-Madison plant geneticist and one of two collaborating
senior authors of the Nature paper.

"It tells plants when to germinate. It tells them where to grow to absorb
the most light and to avoid competition. It tells them when to flower. It
tells them when to die at the end of the growing season."

The accomplishment of the Wisconsin researchers, including first author
graduate student Jeremiah Wagner, caps a 30-year quest by biologists to
drill down to the inner workings of how plants, fungi and bacteria use light
to guide their development. It will likely spur a rush by scientists to
capitalize on the new knowledge and may one day lead to such things as
plants whose growth, flowering and death can be precisely manipulated.

"We can now start changing how phytochromes work in a rational way to
improve how plants respond to light," says Wagner. "People have been trying
to do this for a long time. Practically speaking, we can now try to
re-engineer the vision system of a plant."

According to Vierstra, there are many kinds of phytochromes found in every
plant, and they exist in virtually all cells. They occur in greater
concentrations in cells that respond directly to light, such as in root tips
and new shoots.

The phytochrome revealed by the Wisconsin team was derived from a microbe
known as Deinococcus radiodurans, a bacterium renowned for its tolerance to
ionizing radiation. It was only within the last eight years that scientists
from Vierstra's and other labs discovered that, like plants, some bacteria
harbor phytochromes. That finding opened the way for the Wisconsin team to
define the structure of a phytochrome, as bacteria are easy to grow in the
lab and their proteins are easier to purify and manipulate than plant
proteins.

Once isolated, the phytochrome was crystallized and its molecular structure
was mapped using a beam of X-rays to develop a three-dimensional picture of
the protein. That three-dimensional portrait, which reveals the atom-by-atom
configuration of the molecule, is the key to understanding the "nuts and
bolts" of how the photoreceptor senses light and triggers a series of
downstream events that control growth and development, according to Katrina
Forest, the other senior member of the team and a UW-Madison professor of
bacteriology.

"There were some surprises," says Forest of the ribbon-like protein. "This
protein has a knot."

That is a startling feature, she notes, observed in only a handful of
proteins out of tens of thousands whose structures are known. The group
speculates the knot may help stabilize the protein so it can do its job of
capturing light and triggering the cascade of downstream events under its
control.

"We think that without the knot, the protein conformational changes
(prompted by light) would be too floppy to be efficiently channeled to
downstream proteins," Forest explains.

Phytochromes have unique properties that enable them to switch between two
stable states that sense red and far-red light. The light is actually
detected by a specialized pigment that sits within a pocket on the protein.
Red and far-red light, says Forest, have the effect of reversibly changing
the structure of the pigment in the pocket, which then flips a switch on the
protein to trigger the events of growth and development.

Intriguingly, the phytochrome has the ability to store the light it has
detected, initiating a response days after it is sensed, Vierstra says.
"This memory allows the plant to predict where the light will come from each
day and measure the length of daylight so that they flower in the correct
season."

By deducing the architecture of the phytochrome protein, according to
Vierstra and Forest, it may be possible to engineer and introduce into crops
phytochromes that respond to different wavelengths of light, or are more or
less active. These changes, in turn, could allow plants to grow under
different climate regimes or flower at different times of the year, for
example.

Gaining precise control over flowering events, says Vierstra, is a key to
the success or failure of most crops, as most of what we eat comes from
seeds and fruits produced by flowers. In another scenario, it may be
possible to dampen the role of phytochromes in crop plants to avoid having
them compete with each other for light when grown close together in a field.
In addition to Wagner, Vierstra and Forest, the paper was authored by Joseph
S. Brunzelle of Northwestern University. The work was funded primarily by
the National Science Foundation. Funding was also provided by the U.S.
Department of Energy and the W.M. Keck Foundation.

[www.wisc.edu]

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