What do poplars, sharks, and biofuels have in common? While it may sound like a bit of a mystery, a team led by biochemists from Michigan State University has reported exciting findings about all three in the quest for cleaner energy.

Published in the Plant Biotechnology JournalIn their latest paper, the team investigates how poplar trees can be engineered to produce an extremely valuable chemical typically obtained from shark livers.

By engineering poplars to produce this chemical, they would become much more economical. They are already a promising source of biofuels and could also reduce harmful shark hunting.

“I think this project really shows how we can use industrial crops in new ways,” said Jake Bibik, first author of the paper and a former doctoral student in the lab of Michigan State researcher Björn Hamberger.

“The use of genetically modified, non-edible crops such as poplar could provide a more sustainable alternative to the production of chemicals normally derived from fossil fuels, or even entirely new specialty chemicals.”

More value for your money

Poplars themselves meet a number of criteria for success as a raw material for biofuel.

They grow quickly on land not used for agriculture and their biomass (the organic material that stores energy) can be broken down and fermented to produce biofuel.

One of the biggest challenges comes down to simple economics. “Biofuels are still not competitive with the cheap petrochemicals that are out there,” said Hamberger, a James K. Billman Endowed Professor in the Department of Biochemistry and Molecular Biology in the College of Natural Science.

Hamberger is also a co-investigator at the Great Lakes Bioenergy Research Center (GLBRC), a bioenergy research center led by the University of Wisconsin-Madison.

GLBRC scientists have long sought ways to extract other high-value products from biomass. Research has shown that poplar can be a viable source of p-aminophenol, which is used to make dyes, adhesives and other polymers, as well as acetaminophen, the active ingredient in Tylenol.

When Hamberger joined the center in 2015, he proposed researching terpenes, a group of chemical compounds that plants use in unique interactions with the environment, such as attracting pollinators or defending against pests.

“Terpenes are the oldest and largest class of specialized metabolites on the planet,” said Hamberger, who is also a professor in MSU’s Molecular Plant Sciences, Genetics and Genome Sciences, and Cell and Molecular Biology programs.

“Because they are important for all kinds of interactions, their diversity has increased to a spectacular point. The chemistry between them is simply amazing.”

Terpenes have been used by humans for thousands of years and have been shown to have anti-inflammatory, antimicrobial, anticancer, and antibacterial properties, making them important components in flavorings, cosmetics, and perfumes, to name a few.

Bibik, Hamberger and their collaborators focused on a terpene called squalene, an organic compound widely used in cosmetic products and a key ingredient in vaccines.

Today, most squalene comes from shark liver. The chemical’s name is even derived from the Latin word for shark: squalus.

During the project, the team engineered poplar trees to produce squalene via two different chemical pathways.

One pathway utilized the gel-like substance cytosol found in the center of cells, while another pathway targeted the production of squalene in chloroplasts, the organelles responsible for photosynthesis.

“By taking carbon away from normal metabolism to make specialized chemicals in unique poplar tissues and droplets, Hamberger and his team are using a highly innovative strategy to turn trees into biological factories,” said University of British Columbia professor Shawn Mansfield, an expert on poplar transformation and a collaborator on the latest paper.

“It is exciting to be part of this very innovative and progressive project.”

While the cytosolic pathway was found to disrupt poplar root formation, the chloroplast pathway resulted in the production of 0.63 milligrams of squalene per gram in leaves.

With this promising result, it was time for what Hamberger called a “reality check.”

Looking for an upgrade

Working with Christos Maravelias, a professor of chemical and biological engineering at Princeton University, the team then conducted an analysis to determine the minimum retail price at which their poplar-derived squalene would need to be sold to break even.

The researchers found that amount to be $144 per kilogram. Shark squalene costs $40 per kilogram.

“If you want to sell a green product to a customer, it not only has to be green, but it also has to be affordable,” Hamberger said.

“Fortunately, there are several ways to increase the value. One way is to increase the total production, and the other takes us into the cool world of perfumes and another marine product: ambergris.”

Ambergris is produced in the digestive tract of sperm whales and is used in perfumes to preserve scents.

Hamberger said it should be possible to “upgrade” squalene to ambrein, another valuable terpene that forms ambergris. Scientists have already shown that bacteria can be engineered to produce ambrein, paving the way for further research into how poplar trees might do the same.

Additionally, Tom Sharkey, another MSU collaborator who worked on the paper, showed that poplar trees engineered to produce squalene emitted less isoprene gas, which indirectly contributes to global warming.

“Jake, Björn and their colleagues are working on what may be the most promising approach to making specialty chemicals and fuels for when only liquid fuel will do, such as jet travel,” said Sharkey, a University Distinguished Professor in BMB. Sharkey also is affiliated with MSU’s Department of Energy Plant Research Laboratory and the Plant Resilience Institute.

With these joint findings, the researchers have broken new ground in the quest to transform poplars into an even more attractive source of biofuels and valuable substances.

For Bibik, who is now a senior scientist at the biotechnology company MelaTech, the team’s findings ultimately represent another step in harnessing our planet’s biochemical diversity to tackle some of its biggest challenges.

“I believe this work contributes to a growing foundation needed to translate plant engineering and terpenoid research into meaningful biotechnologies.”