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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.

phytochrome moleculeA 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 algaeCulturing
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]
Cyanophora paradoxa and algal phytochrome spectra
(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:

  1. 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.
  2. 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.
  3. Andy Fell, "Algae 'see' a wide spectrum of light," University of California, Davis, Egghead Blog, April 30, 2014
  4. Clark Lagarias talks about phytochromes, algae and light detection, YouTube Video, April 30, 2014.

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