Why Plants Bend Towards Blue Light
4/17/98
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Title: Why Plants Bend Towards Blue Light
Source: United Press International
Status: Copyrighted, contact source to reprint
Date: 4/17/98
Byline: Lidia Wasowicz, UPI Science Writer
SAN FRANCISCO (UPI) -- More than a century after Charles Darwin and others
noted plants bend specifically towards blue light, biologists have found out
why -- and the finding could prove of great interest to farmers.
Somewhere down the road, the scientists speculate, the research could lead to
developments that would allow optimal crop production even where the sun
doesn't shine.
Lead study author Margaret Ahmad and colleagues at the University of
Pennsylvania say there appear to be two proteins working together to mediate a
plant's classic phototropic response. And there likely is even more to the
story.
In 1993 scientists found a protein in the plant Arabidopsis -- which they
dubbed cryptochrome 1, or CRY1 -- seemed involved in the way plants react to
light. The mystery remained of why plants lacking this protein continue to
respond the same way.
In the new report in the British journal Nature, the investigators identify a
second protein, called CRY2, which shares some of CRY1's chores. The research
shows plants with an extra hit of both proteins have an enhanced reaction, yet
some of those missing both can still bend toward the light.
``Cryptochrome photoreceptors are involved in almost all aspects of a plant's
responses to blue light, especially in cases where high light intensities are
required for the response,'' Ahmad tells UPI.
``Thus, increasing the levels of blue-light photoreceptor can make plants grown
in dim light -- for example, house plants -- look more similar -- thicker,
shorter stems, more expanded leaves, darker green, more lush -- to plants grown
under high light intensities. Increasing the levels of CRY photoreceptors in
transgenic plants could lead to the development of more compact, higher-mass
crop cultivars as well, especially for plants grown in dim light or shade
conditions, which could be of great interest to farmers.''
Phototropism plays an especially important part in the germination of
seedlings: The emerging shoot must grow towards the light source to survive. In
higher plants, this bending response occurs almost exclusively in response to
blue light and not to longer wavelengths such as red and far-red light, a fact
first documented by German scientists in 1864 and reiterated by the English
naturalist Darwin in his 1891 book, ``The Power of Movement in Plants.''
The inference drawn was of the presence of some specific blue-light receptor,
but its precise nature remained a mystery.
Over the years, Winslow Briggs at the Carnegie Institute and others shed
considerable light on the physiology of the phototropic response, identifying
the implicated components at the molecular level.
The studies showed there, indeed, is a ``photoreceptor,'' and it is a complex
molecule. There are either multiple light-sensing pigments bound to the same
molecule or, more likely, a number of light-absorbing molecules working
together to elicit the response.
CRY1 was the first bona-fide blue-light photoreceptor identified in any
organism, Ahmad says. While the full extent and nature of its function have not
yet been unmasked, studies show Arabidopsis plants lacking CRY1 protein are
deficient in stem growth, leaf expansion, flowering time and gene expression --
all of which are linked to the blue-light response.
``The exciting feature of the CRY1 photoreceptor is that it is very similar to
photolyases -- which are DNA repair enzymes found in all organisms, including
man,'' Ahmad says. ``Photolyases repair DNA specifically in response to blue
light. The blue-light receptor CRY1 no longer repairs DNA. Since it obviously
evolved directly from such DNA repair enzymes, it must have acquired a novel
function as photoreceptor in plants.''
Cryptochrome-like genes have been identified in all plant species studied,
including algae, but not in fungi or other eukaryotes. This, says Ahmad,
suggests the photoreceptors entered the scene early on in plant evolution.
``Like many plant proteins, the cryptochrome photoreceptors are encoded by a
gene family with considerable homology,'' Ahmad says. ``It would not surprise
me if more members of the gene family (CRY3, etc.) were identified in other
higher plants or that some plant species may be lacking in one or the other CRY
receptor.''
What's important, she says, is that these cryptochromes all pretty much share
their function and can fill in for one another if one is lacking. Only if all
of them are missing (as in the case of Arabidopsis plants lacking both CRY1 and
CRY2) do they have an observable impact on the plant's response to light.
Ahmed says, ``It is most important to add that CRY1 and CRY2 do not mediate the
phototropic response exclusively; plants still bend in response to blue light
if they lack both cryptochromes.''
``Our data suggest that CRY1 and CRY2 may be acting in association with, or
upstream of, additional light-harvesting photoreceptors implicated in
phototropism,'' she concludes. ``A possible scenario would include the
cryptochromes passing light energy (via electron transfer) to another pigment-
containing molecule implicated in phototropism.''