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A new chloroplast 'app' for making biofuels

Seed oil, aka vegetable oil, is a basic part of food. Scientists have been experimenting with harvesting that oil to make biofuels that could someday power our jets and cars.

But seed oil production is complicated, and we still have a lot to know about how the oil is produced and managed before we can reap the benefits.

What we do know is that the precursors for seed oil come from many sources in a plant cell. And Kun (Kenny) Wang and the Benning lab have recently identified a new, potentially significant one.

The study is published in the journal The Plant Cell.

Dial phones and Smartphones

In middle school, we learn that different parts of the cell do specific things. Mitochondria make energy. The cell wall protects the cell. And so on.

These definitions hold true, but they are a bit like describing this:

An old dial phone
Defining cell parts simply is a bit like describing old dial phones that can only do one thing. In reality, cell parts are more like smartphones.
CC0/Public Domain

Old dial phones can do only one thing: phone calls.

Cell components are like smartphones, more complex. Although they keep their original purpose (phone calls), they have a lot of apps that do useful things unrelated to making calls.

That’s true for the chloroplast. It is mainly known as the source of photosynthesis, the process that sustains life on Earth. Now, we are finding that it also has an app to help produce seed oil.

“Previously, people thought that oil production was largely based in one cellular component, the massive cellular factory known as the endoplasmic reticulum,” Kenny, a graduate student in the Department of Biochemistry & Molecular Biology, says.

“We are finding that the chloroplast also helps in ways we did not think of before.”

The chloroplast’s oil app

Seed oil is made out of lipids: small molecules found in fats, oils, and waxes, and that make up the boundaries of all living cell components. The main reason they are interesting for biofuels is that they store energy.

“We identified a new enzyme – which we call PLIP1 (PLASTID LIPASE 1) – that breaks down lipids that make up the chloroplast’s internal membranes – the thylakoids, to be precise,” Kenny says.

Left over lipid products are then transported to the endoplasmic reticulum, where they become building blocks for seed oil.

A chloroplast
PLIP1 degrades internal chloroplast membranes surrounding the thylakoids. The left over lipid products are transported to the endoplasmic reticulum, where they eventually turn into seed oil.
Source: Christoph Benning. Oil drop by User:Slashme (Own work) [Public domain], via Wikimedia Commons

“This use and recycling of lipids is part of a process that keeps chloroplast membranes finely tuned to any developmental or environmental changes. The interesting part, though, is how this seemingly unrelated process connects to making seed oil.”

To confirm the function, Kenny found a plant that had much less PLIP1 - basically, a buggy app - meaning that lipid breakdown products couldn’t move as much out of the chloroplast.

“With less PLIP1, young embryos developed slower than normal. Seed weight and seed oil content were also reduced. We saw PLIP1 mostly active in the younger stages of a plant’s life cycle, during seed production.”

All roads lead to Rome: PLIP1 and biofuel production

Kenny wants to increase the number of PLIP1s in plants targeted for biofuels. The idea goes that more of them leads to more lipid products for more seed oil harvested.

Cell components are like smartphones, more complex... They have a lot of apps that do useful things unrelated to making calls.

One advantage with PLIP1 is that it is found in most land plants, which makes it easy to experiment on different species, including crops like sorghum or switchgrass.

PLIP1 also seems responsible for 10% of the lipid precursors that end up as seed oil.

“That might seem like a low number. However, seed oil is made from many sources, and the main one is responsible for 20-40% of final product. In that light, 10% is significant.”

For context, the USDA reports that US soybean (another common oil crop) production was estimated at over 4 million bushels, for a forecasted value of $38 billion in 2016

Kenny’s mentor, Christoph Benning, says that 10% of soybean yield could amount to a humungous contribution.

But the road ahead is tricky. Kenny has already tried dialing up the PLIP1 app in one plant, but that somehow turned on another app that does plant defense. The result: a smaller plant with less seed oil.

“Oil production and defense functions don’t co-exist well. We have a few ideas to bypass this caveat, and we’ve already filled out a patent application to try this strategy in a new plant. We still have a lot to learn about how lipids are made and moved.”

Banner image of smartphone, CC0/Public Domain. This work was primarily funded by the US Department of Energy, Office of Science. The authors would also like to thank John Froehlich for his contribution.

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