Brassica napus seeds—the oil accumulating tissue of canola. The authors characterized a rapid and reversible feedback inhibition of fatty acid biosynthesis that occurs when the end product inhibits the first committed step in the pathway.
Scientists at the U.S. Department of Energy’s Brookhaven
National Laboratory have identified key elements in the biochemical mechanism
plants use to limit the production of fatty acids. The results suggest ways
scientists might target those biochemical pathways to increase the production
of plant oils as a renewable resource for biofuels and industrial processes.
“Now that we understand how this system operates—how plants ‘know’ when they’ve made enough oil and how they slow down production—we can look
for ways to break the feedback loop so they keep making more oil,” said
Brookhaven biochemist John Shanklin, leader of the group publishing the work in
of the National Academy of Sciences.
Similar biochemical feedback loops regulate a wide range of
metabolic processes in living things. They work similar to the way a thermostat
maintains a relatively constant temperature in your home: When it gets too
warm, the heating system turns off until the temperature falls to the set
point, at which time it turns on again.
“There were hints that such a feedback system might exist for
plant oil production,” said Shanklin, who credits Carl Andre—a former
postdoctoral research fellow now working at BASF Plant Science in North
Carolina—with designing and carrying out the intricate biochemical detective
work that uncovered the details.
“It’s very difficult to work on developing oil seeds because
they are very tiny,” Shanklin explained. So the scientists performed their
biochemical tests using a plant embryo cell culture to simulate what goes on in
With assistance from the Radiotracer Chemistry and Biological
Imaging group at Brookhaven, Andre synthesized “labeled” forms of the fatty
acids that occur as intermediates along the metabolic pathway that leads to oil
production. He fed these, one at a time, to the plant cell cultures, measuring
the labeled metabolites with the help of analytic chemist Richard Haslam of
Rothamsted Research in the U.K.
They also looked to see which added intermediates would inhibit oil production.
Andre’s work pointed to a fatty acid that occurs fairly late in
the production process, coupled to a carrier protein (essential to making the
oily substance soluble in water), as the key intermediate that puts the brakes
on oil production. It’s the first “desaturated” fatty acid—the first one with a
double bond between two of the carbon atoms, formed after all 18 carbon atoms
are added to the chain.
Then, knowing that this intermediate somehow sent the “slow-down” signal, the team sought to determine its “target”—how it actually
inhibits oil synthesis. They knew from other biochemical feedback loops that
the likely target would be an enzyme early in the synthesis pathway. But they
wanted to figure out exactly which one.
To do this, they monitored the production process by labeling
the intermediates one at a time with a radioactive form of carbon while also
feeding the cells an excess of the “slow-down” signaling fatty acid. If the
label from the intermediate ended up in the oil product, the “slow-down” signal
had to have its effect prior to that step.
The first two experiments gave them the answer: When they
labeled the first compound in the synthesis pathway, which is acetate, very
little labeled carbon ended up in the oil and oil production was strongly
reduced. But if they fed the second compound, labeled malonate, the labeled
carbon quickly entered the oil.
“From these findings we concluded that the accumulation of the
first desaturated fatty acid in the synthesis process inhibits the enzyme that
operates at the first step, which converts acetate to malonate,” Shanklin said. “That enzyme is Acetyl-CoA carboxylase, or ACCase.”
The next step was to make extracts of the tissue culture and
directly measure ACCase activity in the test tube. Addition of the suspected
slow-down signal provided independent proof that both the signal and target of
the signal were correctly identified.
To establish that this feedback mechanism operates in seeds and
is not just a weird quirk of the cell-culture setup, the scientists isolated
developing canola embryos to test the process. “It worked precisely the same
way,” Shanklin said.
With the details of the oil production feedback mechanism in
hand, Shanklin’s team is now exploring how they might interfere with the
process, including biochemical schemes to keep the “slow-down” signaling
metabolite from accumulating, ways to block its effects on ACCase, and more.
“If we can interrupt this process, we hope to fool the cells so
they won’t be able to gauge how much oil they have made and will make more,”
Source: Brookhaven National Laboratory