A Place in the Sun

Apr 03, 2008
A place in the sun
Plants can tell the difference between the shade of an inanimate object and the shade of another plant. When a plant detects competition from neighboring plants, it initiates a set of responses, called collectively the shade avoidance syndrome, that alter its growth and physiology. A rapid and transient increase of newly synthesized auxin via a newly discovered auxin synthesis pathway allows plants to elongate and grow toward the sun. Courtesy of Dr. Jean-Luc Ferrer, Salk Institute for Biological Studies

Those spindly plants that desperately try to reach for a break in the canopy formed by larger plants all suffer from the same affliction: Shade avoidance syndrome or SAS. Now, the molecular details of SAS have been brought to light by researchers at the Salk Institute for Biological Studies.

To step out of their neighbors’ shade, plants switch on a natural chemical factory for the synthesis of the plant growth hormone auxin that lets a plant grow and ultimately stretch toward the sun, the Salk researchers report in an article published in the April 4, 2008 issue of the journal Cell. Understanding this response at a molecular level will allow scientists to naturally manipulate this response to increase yield in crops ranging from rice to wheat.

“Plants compete with each other for light, and shade avoidance syndrome has a big ecological and economic impact, especially in the high density plantings typical of modern agriculture,” says Howard Hughes Medical Institute investigator Joanne Chory, Ph.D., a professor in the Plant Biology, who led the study. “Suppressing the shade avoidance reaction in crops may allow us to increase biomass and seed yield.”

Plants can sense and respond to the presence of other plants in their neighborhood by the relative increase in incoming far-red light resulting from absorption of red light by canopy leaves and reflection of far-red light from neighboring plants.

To secure their place in the sun, plants direct their growth resources toward stem elongation and away from bulking up harvestable portions such as leaves and seeds. “If all else fails, the plants put out what I like to call a premature ‘desperation flower’ to produce at least a couple of seeds that might find better growing conditions during the next season,” explains Chory.

In an earlier study, Chory had confirmed the existence of a separate molecular pathway that plants use to adjust their growth and flowering time to shade. But the molecular events linking the detection of changes in light quality to changes in growth patterns were still poorly understood.

To identify genes that are involved in the shade avoidance syndrome, first author Yi Tao, a postdoctoral researcher in Chory’s lab, searched a collection of mutated Arabidopsis thaliana seedlings for plants that no longer responded to crowded growth conditions. Like many commercially grown crops, Arabidopsis — the lab rat of plant biologists — doesn’t tolerate shade well.

She identified a handful of genes that play a role in the shade response, one of which encoded an enzyme similar to alliinase, the enzyme that produces the characteristic flavor of onion, garlic and other members of the Alliaceae plant family. To predict the function of the newly identified enzyme, Chory turned to her Salk colleague Howard Hughes Medical Institute investigator Joseph P. Noel, Ph.D, director of the Jack H. Skirball Center for Chemical Biology and Proteomics.

Although Arabidopsis lacks garlic’s pungency, Noel could model the newly discovered enzyme’s structure based on the already-known, three-dimensional structure of alliinase. “The active site chemically resembled a nook and cranny likely to bind the amino acid tryptophan,” says Noel. “That’s when it became really exciting since we knew that plants can use tryptophan to synthesize auxin.”

After virtual biochemistry led the way, real-life biochemistry confirmed that the enzyme indeed uses tryptophan to catalyze the first reaction in a three-step auxin-synthesis pathway and the new enzyme became known as tryptophan aminotransferase of Arabidopsis, or TAA1 for short.

Despite the importance of auxin for plant growth and development, the details of how auxin is synthesized continue to puzzle plant biologists. Multiple biochemical pathways for the production of auxin have been identified or proposed but the specific function of each pathway and how they intersect is not known. Now, the role of at least one pathway has become clearer.

“When the major photoreceptor for shade avoidance detects neighbors, it triggers the TAA1 pathway resulting in a rapid increase in free auxin, which is transported to sites in the stem where it can participate in the growth response,” explains Chory. “Although we showed earlier that at least two additional biosynthetic routes to auxin exist in Arabidopsis, these other pathways are unable to compensate for the loss of the TAA1-dependent pathway.”

Source: Salk Institute

Explore further: Researchers discover new mechanism of DNA repair

Related Stories

Restored streams take 25 years or longer to recover

Jun 30, 2015

New research has found that the number of plant species growing just next to restored streams can take up to 25 years to increase above those channelized during the timber floating era. This is according ...

States agree to cut pollutants behind Lake Erie algae

Jun 12, 2015

Ohio and Michigan have agreed to sharply reduce phosphorus runoff blamed for a rash of harmful algae blooms on Lake Erie that have contaminated drinking water supplies and contributed to oxygen-deprived dead zones where fish ...

Recommended for you

Researchers discover new mechanism of DNA repair

Jul 03, 2015

The DNA molecule is chemically unstable giving rise to DNA lesions of different nature. That is why DNA damage detection, signaling and repair, collectively known as the DNA damage response, are needed.

The math of shark skin

Jul 03, 2015

"Sharks are almost perfectly evolved animals. We can learn a lot from studying them," says Emory mathematician Alessandro Veneziani.

Cuban, US scientists bond over big sharks

Jul 03, 2015

Somewhere in the North Atlantic right now, a longfin mako shark—a cousin of the storied great white—is cruising around, oblivious to the yellow satellite tag on its dorsal fin.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

DGBEACH
not rated yet Apr 04, 2008
If all else fails, the plants put out what I like to call a premature %u2018desperation flower%u2019 to produce at least a couple of seeds that might find better growing conditions during the next season,%u201D explains Chory


If they suppress the shade avoidance reaction, might they not reduce the numbers of flowers, and subsequently pollen, produced by the plants?
Are these flowers the same types normally produced?
Isn't pollenation a major part of the growing process?
Does anyone here know the answer to this?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.