Scientists ID sterol essential for oil accumulation in plants

Date:
September 22, 2021
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists seeking to unravel the details of how plants produce and
accumulate oil have identified a new essential component of the
assembly line. They discovered a particular sterol -- a molecule
related to cholesterol -- that plays a key role in the formation
of oil droplets.

The findings may suggest new ways to engineer the oil content of a
variety of plant tissues for potential applications in bioenergy,
chemical engineering, and nutrition.



FULL STORY ========================================================================== Scientists seeking to unravel the details of how plants produce and
accumulate oil have identified a new essential component of the assembly
line. They discovered a particular sterol -- a molecule related to
cholesterol -- that plays a key role in the formation of oil droplets.


========================================================================== "This research greatly extends our understanding of the molecular
factors that govern the formation of lipid droplets, which are vital
organelles for oil storage and metabolism in all eukaryotic organisms,"
said Changcheng Xu, a biologist at the U.S. Department of Energy's
Brookhaven National Laboratory, who led the study. The findings,
published in Nature Communications, may suggest new ways to engineer
the oil content of a variety of plant tissues.

The work may be particularly important for informing genetic engineering strategies aimed at boosting the oil content of leaves and stems. These
plant tissues usually do not accumulate oil, but they could be engineered
as an abundant source of sustainable oils for making biofuels and other commodity products, the scientists say.

The findings also apply to the accumulation of oil in plant seeds, the
main place oils naturally accumulate in plants. These natural reservoirs
of plant oils provide nutrition for plant embryos and seedlings --
as well as animals and humans.

"We found that a deficiency in a particular type of sterol causes a drop
in oil accumulation in seeds and leaves," said Xu.

Green light for oil production Xu and his team have been working for
years to increase oil accumulation in plant leaves and stems.



========================================================================== "Leaves compared with seeds are much more abundant as a possible
bioenergy material," he noted. "Also, because the oil in seeds is
used for food, we're working to accumulate oil and other commodity
bioproducts in non-seed parts of plants -- like leaves and stems --
to avoid competition between food and fuel." The team has made some
strides in getting leaves to accumulate substantial quantities of oil,
using the common laboratory plant Arabidopsis.

They developed a neat way to track oil accumulation. Through genetic engineering, they created Arabidopsis plants in which a green fluorescent protein is always attached to a protein called oleosin. Oleosin only accumulates on the surface of lipid droplets. It forms part of the
membrane surrounding these oil-storage compartments within cells to help stabilize them.

If a sample of plant tissue -- leaf, stem, or seed -- contains lipid
droplets, they stand out as little green dots under a fluorescence
microscope.

"We treated our Arabidopsis plants with a mutagen to try to trigger
mutations that would increase oil accumulation," Xu said, using the fluorescence technique to identify strains with more and/or bigger
green dots.

Ironically, they made their discovery about sterol in an Arabidopsis
strain that accumulated almost no oil.



==========================================================================
"The main purpose of the current work was to figure out which genetic modification caused this dramatic drop in oil accumulation," Xu said. "We thought tracking down this gene might give us some new genes/proteins
that are important in lipid droplet formation or accumulation." Clues to interior assembly At the microscopic scale, scientists know that lipid
droplets form in the "endoplasmic reticulum," or ER, of cells. That's an internal network of membranes within cells (not the membrane surrounding
the cell) that acts like a sort of factory -- assembling and packaging
various materials such as proteins and lipids.

Lipid storage droplets form when oil begins to accumulate between
the two layers of the ER membrane, but only in certain regions of the
ER. Eventually, when enough oil is present, the little membrane bits
pinch off leaving the oil encapsulated in self-contained compartments.

As the study at Brookhaven shows, studying a plant that doesn't accumulate these lipid droplets can offer clues to the biochemical factors that
drive the process -- and what's unique about the particular ER domains
where it occurs.

Zeroing in on the gene To figure out which mutation triggered the dramatic
drop in oil accumulation, the Brookhaven team used a technique known as positional cloning -- a way of searching every region of chromosomes to pinpoint a particular gene that is responsible for a characteristic of interest. The technique narrowed the search to a specific region in one
of the plant's chromosomes.

"This region still contains hundreds of candidate genes," Xu said.

After using whole-genome sequencing to search for any mutation in this
region, the team identified a gene they suspected was involved. The
gene codes for an enzyme responsible for one biochemical step in the
multi-step synthesis of sterol, a molecule related to cholesterol found
in ER and other cellular membranes.

By selectively "knocking out" the normal (unmutated) version of
this gene, the scientists were able to duplicate the effect of the
mutation. That is, plants with the gene knocked out accumulated no
lipid droplets. In addition, adding back the unmutated gene restored
oil droplet accumulation.

"This experiment provided clear evidence that sterol plays an essential
role in forming oil droplets," Xu said.

But the scientists went further. They also studied what would happen if
they mutated genes for enzymes "upstream" from this particular enzyme
in the multi- step sterol synthesis pathway. And they measured sterol
levels in these mutants.

The detailed studies allowed them to zero in on the specific type of
sterol that, when deficient, results in low oil accumulation.

Mutations in the same genes resulted in reduced oil accumulation in
leaves and seeds. In seeds, where lipid droplets are easier to see, the scientists also conducted quantitative studies of their shapes and sizes.

Together the results provide evidence of a universal role for this
particular sterol in lipid droplet formation.

"We believe this sterol is vital for the formation of a microdomain in
the ER membrane that is involved in the formation of lipid droplets,"
Xu said. "The deficiency in sterol results in a defect in the formation
of such a microdomain." Now that they know what happens when these
genes are turned off, the scientists suggest that strategies to turn
them on and ramp up their expression could be one way to increase oil accumulation in leaves, stems, or seeds.

The team will be exploring these strategies in future experiments.

This work was funded by the DOE Office of Science (BES).

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Linhui Yu, Jilian Fan, Chao Zhou, Changcheng Xu. Sterols are
required for
the coordinated assembly of lipid droplets in developing
seeds. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021-25908-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210922090838.htm

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