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Algal Phytochromes
May 23, 2014
Many words of the
English language are built from parts of words from other languages, notably
Latin and
Greek. Just as
parents decide to give their
Children traditional names,
scientists decide that when a new word is needed, it's easiest just to mine the known
lexicons. The
electron got its name from the Greek word for
amber, ηλεκτρον (elektron), since rubbing amber produces
static electricity.
Many words use the Greek word for
color (chroma, χρωμα), including the
trademarked photographic film,
Kodachrome, immortalized in the 1973
Paul Simon song by the same name. If we combine chrome with the Greek word for
plant (phyton, φυτον), we obtain
phytochrome, the
photoreceptor pigment used by plants to detect
light.
Since most plant
leaves are
green, it's apparent that they reflect green light; so, they
absorb sunlight in the
red and
infrared. Phytochromes are used by plants in several ways.
Angiosperms (flowering plants), detect light to regulate their flowering. They're used, also, to regulate the
germination of
seeds and the size and number of leaves.
A phytochrome molecule.
Phytochromes are proteins containing a bilin chromophore. A chromophore is the part of a molecule responsible for its color.
(Via Wikimedia Commons.)
Phytochrome research is nearly a hundred years old. It began with studies in 1918 by
botanist Harry A. Allard and
physiologist Wightman W. Garner, both employees of the
United States Department of Agriculture (USDA), who were working on a problem with
soybeans.
farmers, who tried to spread-out their soybean
harvest by planting over a two week period, found that the plants would all flower at the same time.[1]
In a simple
experiment, Allard and Garner grew some
Biloxi soybeans in
pots, leaving some outside all day long, but the others they placed in a dark
shed every
afternoon, bringing them back outdoors every
morning. The soybeans, exposed to apparently shorter days, flowered five
weeks earlier.
This research had an immediate benefit to
florists, who were then able to get their flowers to bloom yearlong.[1] And there were still surprises. It was found that a single, thirty second burst of light in the extended period of darkness would prevent flowering.[1]
As an
experimentalist, I always enjoy reading about simple experiments that have had a big impact on
technology. One of these was done by
Sterling B. Hendricks, a
chemist who was
Linus Pauling's first
graduate student. Working with USDA botanist Harry A. Borthwick and USDA physiologist Marion W. Parker, he passed the intense light of a ten
kilowatt carbon arc lamp through large
prisms so that its swath of colored light fell across fourteen soybean plants 42
feet away.[1]
The plants were conditioned by first being grown at 16-hour days, followed by ten hour days to induce flowering. The plants were then briefly exposed to portions of the light
spectrum in the middle of the dark period for six days. After a week of a regular long night cycle, they found that exposure to
yellow and red light had the greatest affect on flowering.[1]
In 1959, the phytochrome pigment was identified
spectrophotometrically by
biochemist Harold Siegelman, and
biophysicist Warren Butler who coined the term, phytochrome.[1] In recent years,
genetic engineering has allowed control of phytochrome
expression, allowing alteration of the
shade-avoidance response. This allows control of the height to which a plant will grow, so you can have taller food crops, but shorter
lawn grass.
The world's
waters are filled with
algae, but water blocks red light, so their phytochromes have
evolved differently. The
chemistry of algal phytochromes has been investigated by a
research team from the
University of California, Davis, the
Monterey Bay Aquarium Research Institute (Moss Landing, CA),
Rutgers University (New Brunswick, NJ), and the
Canadian Institute for Advanced Research (Toronto, ON, Canada).[2-5] Their findings have been reported in the
Proceedings of the National Academy of Sciences.[2]
Land plant phytochromes detect shading by neighboring plants by the
ratio of red to far-red light, and they change their development accordingly. About 20% of a plant's
genes are regulated by phytochromes[2-3]. Says UC-Davis
professor and senior
author of the study,
Clark Lagarias, "They control all aspects of a plant's life."[3] The phytochrome
molecules are structurally related to
chlorophyll.[2]
Culturing
Cyanophora algae.
(UC-Davis image.)
In water, where red and far-red light are rapidly
attenuated with
depth, plants must use shorter
wavelengths of light, also, for
photosynthesis.[2] The research team studied the phytochromes of
taxonomically diverse
eukaryotic algae considered to be important for
coastal ecosystems and for the
global carbon cycle.[2] They characterized the photosensory properties of seven phytochromes from green algal (
prasinophyceae), brown algal (
heterokont),
Ectocarpus siliculosus, and
glaucophyte species.[2]
As shown in the figure, they found that these phytochromes detect light throughout the
visible spectrum, sensing yellow,
orange, green, and even
blue light.[2] Since these different colors penetrate to different depths in water, such diversity offers an
evolutionary advantage in that the algae can use whatever light is available. It appears that the ancestral form of phytochrome was sensitive to red light, and its structure evolved to accommodate the additional wavelengths of light.[3]
(Left image, Cyanophora paradoxa algae. Right image, spectra of phytochrome pigments found in nature. Both images from UC-Davis.)
This discovery could help in the development of aquatic crops, including those used for
algal biofuels. This research was supported by the
National Institutes of Health, the
National Science Foundation, the US Department of Agriculture, the
US Department of Defense, the
David and Lucile Packard Foundation, and the
Gordon and Betty Moore Foundation.[2]
References:
- Jim De Quattro, "Tripping the Light Switch Fantastic - History of Research at the U.S. Department of Agriculture and Agricultural Research Service," Agricultural Research, September, 1991.
- Nathan C. Rockwell, Deqiang Duanmu, Shelley S. Martin, Charles Bachy, Dana C. Price, Debashish Bhattacharya, Alexandra Z. Worden and J. Clark Lagariasa, "Eukaryotic algal phytochromes span the visible spectrum," Proc. Natl. Acad. Sci., vol. 111 no. 10 (March 11, 2014), pp. 3871-3876.
- Andy Fell, "Algae 'see' a wide spectrum of light," University of California, Davis, Egghead Blog, April 30, 2014
- Clark Lagarias talks about phytochromes, algae and light detection, YouTube Video, April 30, 2014.
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