Canola study solves seed oil mystery

12/06/04

EAST LANSING, Mich. – Scientists from Michigan State University have uncovered a previously unknown metabolic mechanism used by plants to create seed oil.

The results, described Wednesday in the British journal Nature, address a longstanding question in plant biology – why do oilseed plants rely on a seemingly inefficient metabolic process to produce such prodigious amount of energy-rich oil? The answer, according to the MSU team, is that plant seeds are more efficient than anyone thought.

"Seeds achieve this high efficiency by using long-known biochemical reactions that are combined in an unconventional way, which had not been expected by biochemists," said Jörg Schwender, MSU plant biology professor and lead author of the study.

The researchers studied canola (or rapeseed), an annual crop in the mustard family that is widely cultivated throughout the upper Midwest, Canada, Europe and Asia. The oil extracted from the seeds of this plant is used to make everything from margarine to industrial lubricants.

Seeds store large oil reserves to use as energy to germinate and grow. In canola, for example, oil can comprise half of the seed's weight.

The rise of modern biochemistry over the last few decades has increased interest in making quantitative descriptions of plants and animals' biochemical reactions.

When it came to canola, the biochemical balance sheet just didn't add up. As far as researchers could tell, the seeds were relying on a creaky and inefficient pathway to produce their sought-after oil.

All plants employ carbon from carbon dioxide to make organic biomass compounds such as sugars, oils and proteins in stems, leaves and flowers.

To harvest carbon from the air, plants go to lots of trouble to convert carbon dioxide into simple sugars. When canola subsequently transformed these sugars into oils, the plants appeared to cough up lots of the carbon dioxide back into the atmosphere.

The chemical reaction appeared to follow the same backwards logic as a person who toils all day on the job to earn $100, only to buy a $5 sandwich and give the remainder of his paycheck back to his employer.

In its experiment, the MSU team tagged carbon atoms and tracked how they were processed by developing canola seeds.

During the conversion of sugars to oils, researchers expected to see the tagged carbon go through a step-by-step series of chemical reactions known as glycolysis, used by all plants and animals to turn sugar into energy and cellular building blocks. This energy, in turn, is used to link the carbon building blocks into molecules of oil.

Instead, the scientists observed an enzyme called Rubisco providing a more efficient pathway to convert sugar to carbon chains for oil. And the pathway involved lots less coughing up of carbon dioxide.

Scientists have long known that in the process of photosynthesis, Rubisco is the key enzyme that captures atmospheric carbon dioxide for conversion into sugars.

However, the MSU team was surprised to see Rubisco – the enzyme's shorthand stands for ribulose bisphosphate carboxylase/oxygenase – also acting as a key agent producing oil in the seed.

In fact, in terms of metabolic heavy-lifting, Rubisco appeared to be much more efficient than glycolysis. The newly uncovered Rubisco bypass pathway produced 20 percent more of the carbon-chain building blocks to make oil while losing 40 percent less carbon dioxide than is lost during glycolysis.

The results cast new light on the seemingly well-understood protein Rubisco, which accounts for 50 percent of a plant's total protein content and is likely the mostly abundant protein on Earth.

Through its role in the snatching carbon atoms from atmospheric carbon dioxide, Rubisco has been recognized as the main chemical gateway for carbon to enter the biosphere. The new findings suggest that Rubisco also gives plants a way to greatly reduce losses back to the atmosphere while they're synthesizing oil.

"Understanding the pathways plants use to make oil will help us to develop new crop varieties with greater oil content," said co-author John Ohlrogge, MSU distinguished professor of plant biology and Michigan Agricultural Experiment Station scientist. "And this becomes especially important as the world depletes its supplies of petroleum."

Source: Eurekalert & others

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