History of Research at the
U.S. Department of Agriculture and Agricultural Research Service
Light Switch Fantastic
ARS botanist Harry A. Borthwick helped discover the plant photoreceptor protein
In 1918, a
pair of U.S. Department of Agriculture scientists in northern Virginia asked
two simple questions, launching an intellectual voyage still far from over.
Their discoveries were the prelude to the search for what would prove to be one
of nature's most important light switches. During the years 1936 to 1959, a
dozen scientists at USDA's research center in Beltsville, Maryland, pursued and
finally forced their quarry, phytochrome, into the daylight.
Today we know that phytochrome is a
dual-form plant protein. It is switched back and forth by red light and by
far-red, a zone at the horizon of our eyes' visual limits. By this
transformation, phytochrome ordains whether a plant will start, or put off,
making flowers. And in the next season, phytochrome wakens seeds to germinate
or prolongs their sleep.
Skeptics, who reasonably believed
phytochrome an illusion, dubbed it a pigment of the imagination.
Ironically, imagination was the key to unveiling phytochrome. It showed itself
in clues the scientists saw in phytochrome's reflected realitythe
measured life cycle of plants.
Plants UV Protection
New Clue About
Plants' Sunlight Sensors Revealed
Tobacco, Bullheaded Soybeans
The search began in 1918. Why, wondered
botanist Harry A. Allard and physiologist Wightman W. Garner, did Maryland
Mammoth tobacco not know when to stop making leaves and start making flowers
The tobacco, first noticed in 1906, seemed a
boon to growers. It grew as tall as 15 feet and put out nearly 100 leaves until
frost would kill it. But what good was this oddity? It rarely flowered in
Maryland, and it never produced seed in the field that could be used to plant
the next crop. Its tantalizing ability to produce leaves seemed a
And why, Garner and Allard puzzled, were
soybean farmers being frustrated at spreading out their harvest time? They
would plant the crop 2 weeks apart, but the plants would all set flowers at the
The two researchers solved both mysteries
with the same experiment. In retrospect, the test seems so simple it might have
sprung from the naive fancy of a childexcept no one had tried it
In July 1918, Allard and Garner grew some
Biloxi soybeans and Maryland Mammoth in pots. Some plants they left outside all
day long. But every afternoon, they placed one group in a shed without windows,
returning them outdoors the next morning.
The tobacco flowered 3 months earlier; the
soybeans, 5 weeks earlier. Thus was born the concept of photoperiodisman
organism's response to the relative lengths of night and day.
At the time, the light/dark ratio lay mostly
unexplored, an oddity in the standard explanation that plants grew and
developed by laws of soil, climate, and the total amount of light they
The two USDA scientists soon found that
different plants had different kinds of photoperiods. Maryland Mammoth and
Biloxi soybeans, as well as chrysanthemums and others, flower in reply to the
shortening days of late summer.
Lettuce, spinach, and the like are long-day
early summerflowerers. And day-neutral plants like tomatoes and
dandelions flower right up until frost.
People didn't wait to learn what made
photoperiodism work; they used it. Florists, no longer chained to a plant's
outdoor season, began growing flowers indoors year round, serving each type the
ration of light it needed to yield flowers. Plant breeders came out with crop
varieties suited to day length, as fixed by the angle that a given latitude
presented to the sun during a given season. Seed of Maryland Mammoth was
successfully grown in Florida, where the plant flowered and set seed in the
state's brief, mild days of winter.
It would take 41 years to isolate
phytochrometo get, in the words of Sterling B. Hendricks, a bottle
of the stuff that was the trigger for photoperiodic response.
Continuing phytochrome studies at the Beltsville (Maryland) Agricultural
Research Center, plant physiologist Steve Britz measures horizontal light flux
in a soybean canopy with a quantum sensor to assess the effects of shading.
In l936, with little fanfare, USDA
established a tiny research project on photoperiodism at its sprawling research
center in Beltsville.
Botanist Harry A. Borthwickheading the
projectand physiologist Marion W. Parker were unsatisfied with using the
conspicuous arrival of flowers as a meter of photoperiodism. Instead they
examined primordia. These microscopic bodies emerge as the first
visible signs of flowers.
They quickly discovered that only 2
short dayswhen nights were as long as 10-1/2 hoursmade
Biloxi soybeans produce primordia 5 days later.
Shortly thereafter, they found that this
flowering could be prevented if a single, 30-second burst of light butted in on
the long night. How could something so seemingly trivialhardly enough
light to wake a sleeping childhalt a plant's charge toward
Again there was no answer, but breeders and
horticulturists quickly enjoyed savings on light bills for their greenhouses.
Instead of leaving the lights on for several hours each night to create a short
night, they used a few minutes of light to get the same result.
At the time, the major focus of
photoperiodism studies was to look for some kind of signalperhaps a
hormonethat travels from a plant's leaves to its fast-growing tips. But
about 1940, Borthwick and Parker saw that they should home in on the leaf's
interplay with light itself. About leaves, they knew much; they needed someone
who knew about light.
Engineer Karl Norris displays the dual monochromator spectrophotometer which
made history with its rapid and precise assays of phytochrome.
Takes on Color
They sought out Hendricks, a Beltsville
colleague. An expert mountain climber, Linus Pauling's first grad student, and
a brilliant chemist, Hendricks was one of the first Americans to use x-rays to
study molecular structure.
The trio knew their quarry had to be a
substance that could detect whether light was present. So it must be a pigment.
But sunlight comes in a spectral stew of visible and invisible tints. Which
color ingredient energized their phantom pigment? And how could they find
Theoretically, the solution was simple. Just
as water droplets carve the sun's rays into an arching rainbow, the scientists
used a prism to filter light into its separate facets.
Hendricks knew how spectrographs worked and
how to tinker with them. But most spectrographic work dealt with purified
material, not the queer confederacy of solids and liquids that makes up leaves.
Further, the light intensities used in spectrographs were smalldesigned
for the needs of photographic film, not living plants. Finally, the spectra
would have to be spread over a wide area; the researchers would gain nothing if
several colors fell on the same leaf.
The solution was a $50 experiment so elegant
no large-scale research effort would ever have devised it. Their huge,
lO-kilowatt carbon arc-light was cadged from a Baltimore movie theater,
with memories of pulchritude, Hendricks reported later.
And somehow he obtained two large prisms,
which were already historic. They'd been used by Samuel Pierpont Langley.
Astronomer, physicist, and aeronautics pioneer, Langley had died in
For their test, light passed through the
prisms to cast its spectrum 42 feet away, in a 7-foot swath, across 14 soybean
plants at a time.
Several groups of plants had been grown on
16-hour days to prevent flowering; each plant had been stripped of all but one
leafthe youngest mature one. When the test began, the scientists cut the
plants' ordinary light exposure to 10 hours a day, which normally would induce
flowering. But in the middle of the dark period, they turned on the spectral
array for periods of 1 to 25 minutes.
After 6 days, they returned all the plants
to long photoperiods. A week later they checked for primordia. The fewer the
primordia, the greater the effect of specific light spectra on the
With this test, the pigment surrendered the
first solid clue to its color. Since the plants responded most strongly to red
and yellow light, the pigment must be absorbing these hues. And that meant the
active form of the pigment had to be blue or green.
This conclusion, obvious to any student of
light, may not be clear to a lay reader. Any color that we see is made up of
some combination of any or all of the red, yellow, green, and blue zones of the
visible spectrum. Like a selective mirror, a colored substance reflects only
what it cannot absorb. So somethinglike phytochromethat absorbs
only reds and yellows will reflect blue and green.
In 1948, tests with barleya long-day
speciesproved that the same pigment governed flowering in long-as well as
short-day plants. This too was exciting, but a couple of years later the
outlook seemed clouded. The spectrograph tests were important, but also
awkward, imprecise, and time consuming.
Next to the spectrograph room was a doorway
that would lead Hendricks, Borwick, and Parker to faster progress.
Photosynthesis taking place in wheat plants can be measured in field chambers
like this one being adjusted by plant physiologist Richard Garcia.
The door belonged to the Seeds
Investigations Laboratory. There, the husband-and-wife team of Eben H. and
Vivian K. Toole had for years been looking into why some seeds, like lettuce,
need light to germinate.
Some 15 years earlier, physiologist Lewis H.
Flint, another researcher in the lab, and E.D. McAlister of the Smithsonian
Institution had set about finding that, indeed, germination was promoted most
by red light.
Maybe that would be useful, Eben Toole
mentioned to the photoperiodism group sometime in 1951 or 1952.
Would it ever! Experiments would take days
instead of weeks. Moisten the seed for 16 hours, hit it with red light, then
watch for germination in a few days.
Further, Flint and McAlister had seen
something intriguing about far-red light and germination. Far-red light not
only failed to promote germination; it stalled it. Hendricks, Borthwick, and
Parker hadn't seen thisbut it would also turn out to be truein
flowering. Discovering that far-red and red somehow foiled one another was
critical in all further work to get a grip on the pigment.
On April 9, 1952, the loose-knit team of
scientists came up with another magnificently simple find. Seed hit with red
light germinated unless it was then hit with far-red; but if red again ensued,
it would germinate. Incredibly, all that mattered was which color came last
even if the seed was struck by 100 alternating cycles of red and
That summer, the researchers confirmed the
same switchability in flowering. Test plants flowered only if far-red light
ended the sequence.
While fascinating, these discoveries gave
the team no hard facts about the pigment's chemistry or its levels inside
leaves. But they had reasoned this out: Their studies showed the pigment was
probably blue or green and far more strongly colored than chlorophyll.
There was another clue: albino barley
plants, while responsive to their light tests, showed no blue tinge. That meant
the pigment's concentration was minuscule. To cause such dramatic effects on
plants, the stuff had to be some kind of catalyst.
That, they surmised, meant it was an
enzymeand therefore a protein.
Still, though they could measure what the
receptor protein did in plants, they couldn't detect it in a test tube. And
virtually no one else was doing this kind of work; literature searches circled
them back to what they had published themselves.
By 1953, Flint and Parker had left the team.
But Borthwick, Hendricks, and the Tooles were soon joined by two physiologists,
Albert A. Piringer and Robert J. Downs.
Then, around I956, a third phase of informal
collaboration with colleagues led the team to one final breakthrough.
A Bottle of
Agricultural engineer Karl H. Norris, who at
the time headed up a USDA Agricultural Marketing Service lab, had worked for
about 10 years on nondestructive ways to gauge the quality of produce.
To do this, he'd adapted several
spectrophotometers so they would measure typical patterns of light absorption
by goods such as eggs and apples. Ordinarily, these gadgets required material
that was nearly transparent.
Joining Norris in l956 was biophysicist
Warren L. Butler; 1957 saw the final ingredient added, when Borthwick brought
aboard Harold W. Siegelman, horticulturist turned biochemist.
That same year, the project receive its
first-ever operating budget, when Borthwick's lab was named one of ARS' two
Pioneering Research Laboratoriesthe other being Hendricks' mineral
The basic idea was to use the
spectrophotometer to tally changes in light absorption by plant tissue that
held the pigment protein. The researchers knew that red changed the protein
into a form sensitive to far-red. So, that form (which turned out to be green
pigmented) should absorb more far-red light than red. And vice versa: Far-red
light should make the now red-sensitive (blue) form of the protein absorb more
red than far-red.
For 2 more years, the team tried tissues
from various plants: lettuce, cocklebur, albino barley, and the like. None
In mid-June 1959, Hendricks showed up in
Butler's lab with dark-grown turnip seedlings for the spectrophotometer.
To our amazement and delight, mixed with skepticism, Butler later
reported, we found that the difference spectrum between the red and
far-red irradiated sample was precisely that predicted for phytochrome by the
physiological action spectra the scientists had been charting for years
with Hendricks' large spectrograph.
Within 2 hours, Siegelman tried the
technique with a sample of ground-up turnip seedling. It retained the
He then boiled a sample and tested it: no
reversibility. This agreed with the prediction that the receptor was a protein,
now destroyed by boiling.
The following April, the stuff in the bottle
had its official name, a borrowed one combining the Greek words for
plant and color. Butler suggested it half-jokingly, according to
Borthwick. Phytochrome, which had once referred to all visible plant pigments,
now named a dual-form proteingreen in its red-sensitive form, blue when
sensitive to far-red.
Later that summer, Hendricks was invited to
speak in Montreal at the Ninth International Botanical Congress. He suggested
Butler run a demonstration after his talk. Norris produced a portable
photometer, rigged to a wall-clock-size meter.
In repeated trials in Beltsville, the
meter's dial swung reliably between its 9-o'clock and 3-o'clock positions each
time a sample was alternately beamed with red and far-red light.
With high confidence, Hendricks, Butler, and
Siegelman set off by car for Montreal. Unfortunately, they did not yet know
that the far-red form of phytochrome is unstable once red light produces it in
a seedling. This insight came too late to prevent their Montreal demonstration
from becoming a dud.
Whenever the three scientists stopped for
gas during the drive to Canada, someone would check the trunk to make sure the
corn seedling samples were okay. But opening the trunk let in huge doses of red
light, converting the red-absorbing form of the protein into the wobbly,
far-red form. By the time they got to Montreal, the seedlings didn't hold
enough phytochrome to nudge the photometer dial by one whit.
However, Butler remembered
later, in spite of the failure, the audience appeared to be kind and
accepting and even to believe that we probably had achieved what we
Since 1959, phytochrome has continued to
resist scientific prying. Not until 1983 was a reliable means developed to
purify the protein in a fully intact form.
It's still unclear where phytochrome resides
in cells and exactly how it throws its genetic and behavioral switches in a
It's not even absolutely settled whether the
far-red-sensitive form of phytochrome is an enzymatic protein. A catalyst, yes,
but not all catalysts are enzymes. Phytochrome, like some other plant proteins,
may have a different way of filling its role as a biochemical amplifier.
In addition to phytochrome, scientists have
uncovered two or three other classes of photoreceptors, such as those that
absorb blue light and possibly even ultraviolet light.
Researchers today continue gathering and
sifting clues that may open up secrets of phytochrome and other photoreceptors
for human advantage. What may emerge could be new strategies to control weeds,
to make better use of a crop plant's preference for certain shades of light,
and to use biotechnologies to improve one plant by borrowing the genetic light
switch from another.
Such stories cannot yet be written in full,
but their outlines are being imagined.By Jim De Quattro, ARS.
"Tripping the Light
Switch Fantastic" was published
in the September 1991 issue of Agricultural Research magazine.
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