Plants keep one foot on the brakes

November 02, 2015

Pressing on the gas and the brakes at the same time hardly sounds like good driving technique, but Weizmann Institute scientists have discovered that plants drive some of their key processes in precisely such manner.

A research team headed by Prof. Avihai Danon of the Plant and Environmental Sciences Department - postdoctoral fellow Dr. Erez Eliyahu and then graduate students Ido Rog and Inbal Dangoor - studied the mechanisms controlling the plant production of starch, which is the most common carbohydrate in the human diet and present in large amounts in such staple crops as rice, potatoes and maize. The plant starts making starch as soon as the morning light turns on its photosynthesis and stops when photosynthesis subsides at night.

About half a century ago, scientists discovered the "on" switch for starch production: a launch enzyme that is activated, via a series of regulatory proteins, by the flow of electrons generated in photosynthesis. Now in the new study, reported in the Proceedings of the National Academy of Sciences, USA, (PNAS), Weizmann Institute scientists have discovered the "off" switch for starch production. Working with a mustard-like plant called Arabidopsis, they figured out the chain of biochemical events leading to its shutdown at night. The decrease in light causes a small signaling protein called ACTH4 to lose electrons and become oxidized, which, in turn, quickly prompts it to transmit the "take a break" message to the starch production enzyme.

The discovery of the "off" switch is significant in itself because the ability to rapidly shut down starch production on demand is an essential attribute of plant life. At night, the plant breaks down the starch - its major energy store - using it for growth and development. Should production fail to stop, the plant would enter a futile, energy-wasting cycle, in which it would mistakenly keep making new starch from the breakdown products.

In further investigating the shutdown mechanism, the scientists made yet another, entirely unexpected revelation. They realized that this mechanism remains active at a low level throughout the day. It's as if the plant drives its starch production by pressing on the gas pedal and the brakes simultaneously: turning the production on while at the same time keeping it in check. When the scientists genetically engineered the plants to eliminate the "brakes," starch production shot up by nearly 20 percent. This suggests that generally, production efficiency stands at only about 80 percent because the brake pedal is on all the time. In the future, this research may make it possible to increase starch production in agricultural crops.

Why do plants naturally produce starch in such an inefficient manner? The scientists believe that this seemingly wasteful approach in fact holds the key to a clever adaptation strategy.

Since light intensity, and with it the rate of photosynthesis, often fluctuates rapidly throughout the day, the plant needs to adjust its metabolism on an ongoing basis. In particular, starch synthesis needs to be closely attuned to photosynthesis so that the compounds created in photosynthetic reactions are promptly taken up - otherwise reactive oxygen molecules called free radicals, generated as a byproduct of photosynthesis, can build up to excessive levels, harming the plant.

Pressing their gas and the brake pedals simultaneously enables plants to control their starch production rapidly and effectively - by adjusting the relative strengths of the two pedals. Moreover, keeping the brake pedal lightly pressed all the time leaves room for a potential increase should sunlight suddenly become extremely intense. Indeed, the scientists found that when light intensity increased abruptly in their experiments, starch synthesis shot up transiently as well.

If plants were already making starch at full capacity, they couldn't increase its production any further - and would perish from the resulting excess of free radicals generated in the soaring photosynthesis. Keeping "one foot on the brakes" is therefore part of the sophisticated set of control mechanisms that has helped plants survive over hundreds of millions of years in an abruptly-changing and often unpredictable environment.
Prof. Avihai Danon's research is supported by the Raymond Burton Plant Genome Research Fund; the Lerner Family Plant Science Research Fund; the Leona M. and Harry B. Helmsley Charitable Trust; the Jacob and Charlotte Lehrman Foundation; Mr. Jack N. Halpern, New York, NY; and Adolfo Eric Labi, Italy. Prof. Danon is the incumbent of the Henry and Bertha Benson Professorial Chair.

The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.

Weizmann Institute of Science

Related Photosynthesis Articles from Brightsurf:

During COVID, scientists turn to computers to understand C4 photosynthesis
When COVID closed down their lab, a team from the University of Essex turned to computational approaches to understand what makes some plants better adapted to transform light and carbon dioxide into yield through photosynthesis.

E. coli bacteria offer path to improving photosynthesis
Cornell University scientists have engineered a key plant enzyme and introduced it in Escherichia coli bacteria in order to create an optimal experimental environment for studying how to speed up photosynthesis, a holy grail for improving crop yields.

Showtime for photosynthesis
Using a unique combination of nanoscale imaging and chemical analysis, an international team of researchers has revealed a key step in the molecular mechanism behind the water splitting reaction of photosynthesis, a finding that could help inform the design of renewable energy technology.

Photosynthesis in a droplet
Researchers develop an artificial chloroplast.

Even bacteria need their space: Squished cells may shut down photosynthesis
Introverts take heart: When cells, like some people, get too squished, they can go into defense mode, even shutting down photosynthesis.

Marine cyanobacteria do not survive solely on photosynthesis
The University of Cordoba published a study in a journal from the Nature group that supports the idea that marine cyanobacteria also incorporate organic compounds from the environment.

Photosynthesis -- living laboratories
Ludwig-Maximilians-Universitaet (LMU) in Munich biologists Marcel Dann and Dario Leister have demonstrated for the first time that cyanobacteria and plants employ similar mechanisms and key proteins to regulate cyclic electron flow during photosynthesis.

Photosynthesis seen in a new light by rapid X-ray pulses
In a new study, led by Petra Fromme and Nadia Zatsepin at the Biodesign Center for Applied Structural Discovery, the School of Molecular Sciences and the Department of Physics at ASU, researchers investigated the structure of Photosystem I (PSI) with ultrashort X-ray pulses at the European X-ray Free Electron Laser (EuXFEL), located in Hamburg, Germany.

Photosynthesis olympics: can the best wheat varieties be even better?
Scientists have put elite wheat varieties through a sort of 'Photosynthesis Olympics' to find which varieties have the best performing photosynthesis.

Strange bacteria hint at ancient origin of photosynthesis
Structures inside rare bacteria are similar to those that power photosynthesis in plants today, suggesting the process is older than assumed.

Read More: Photosynthesis News and Photosynthesis Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to