Purdue University scientists have defined a hidden option that plants have for making an essential amino acid, which could be a step toward boosting plants' nutritional value and improving biofuel production potential.

The amino acid phenylalanine is required to build proteins and is a precursor for more than 8,000 other compounds essential to plants, including lignin, which allows plants to stand upright but acts as a barrier to cellulosic ethanol production.

It was believed that plants could use two pathways to create phenylalanine. Natalia Dudareva, a Purdue horticulture professor, and Hiroshi Maeda, a postdoctoral researcher, have confirmed that while plants predominantly use one pathway, they have another at their disposal. This second pathway might one day allow scientists to increase a plant's production of the essential amino acid.

"That would allow us to increase the nutritional value of some food," Maeda notes. "But also by increasing these compounds, the plants would be better able to protect themselves from changes in the environment."

Maeda adds that reducing phenylalanine could lead to reduced lignin in plants, which would improve cellulosic digestibility for ethanol production.

Phenylalanine is one of the few essential amino acids that humans and animals cannot synthesize, so it must come from plants. It is produced when sugars enter a plant's shikimate pathway, which creates a link between processing sugars and generating aromatic compounds. The next steps had not been known until now, and were thought to involve one of two proposed routes - the phenylpyruvate or arogenate pathways.

Dudareva and Maeda found a gene responsible for phenylalanine production, and suppressing it knocked out 80 percent of the phenylalanine content in petunias. The hypothesis was that the gene suppression would act like a clogged pipe, creating an abundance of compounds that would have later become phenylalanine in a normal plant.

But that's not what happened. "These plants knew that the last step of phenylalanine production was down and slowed the first steps," Dudareva says. The plant created some sort of feedback mechanism that slowed down the entry point of the shikimate pathway.

Dudareva and Maeda wanted to see what would happen if they forced the shikimate pathway to function, so they gave the petunias shikimic acid. The plants were flooded with the upstream compounds as expected, but since they could not use the usual arogenate pathway to convert them to phenylalanine, they used another path that scientists had only theorized existed.

"What this tells us is this other pathway could be active under certain conditions," Dudareva adds.

Understanding how the pathways work is a first step in finding ways to increase phenylalanine to increase or decrease foods’ nutritional values, which may help in biofuel production.

Dudareva and Maeda will next try to determine how the plant creates feedback to the shikimate pathway. Disrupting that could lead to an abundant production of phenylalanine in plants.

Source: Purdue University