Making yeast synthesise starch

Starch is only produced by plants and algae, but now, researchers at ETH Zurich have produced starch in yeast — the first time this has been achieved in a non-plant organism.

A group led by Samuel Zeeman, Professor of Plant Biochemistry at ETH Zurich’s Institute of Agricultural Sciences, has succeeded in implanting yeast with the machinery that plants use to create this stored form of glucose.

Researchers took the blueprints for seven different enzymes involved in starch synthesis from the genome of thale cress (Arabidopsis thaliana). They then implanted these into the yeast’s genome, from which they also removed all enzymes involved in the synthesis of glycogen, the storage form of glucose in yeast, to prevent these enzymes from interfering with the synthesis of starch.

In total, the researchers generated over 200 strains of yeast, some of them with all seven enzymes and others with various reduced sets of them.

Strains containing all seven enzymes produced starch with only minimal differences from Arabidopsis starch. However, what was surprising were the products of strains in which one or more enzymes were missing: depending on the combination, some of these strains nevertheless produced some type of starch.

“Starch synthesis is not a linear process,” explained Zeeman. “If an enzyme is missing, the ones that are left keep working anyway and just build a slightly different product.” The researchers were able to show that, depending on the combination of the other enzymes, starch synthesis also works without debranching enzymes. These enzymes remove excess branching in the sugar chains produced during starch synthesis and were previously proposed to be indispensable to starch formation.

“At present, the yeast system is purely a research tool,” said the ETH professor. He explained that it allows starch synthesis to be simulated and influenced, as well as allowing more detailed investigation of the individual roles of participating enzymes and of the formation of starch’s complicated structure. “Doing these studies in yeast is far faster and simpler than in plants,” emphasised Zeeman. Asked about future applications, he added: “Of course, it would be possible to try out novel starch modifications in the yeast system to attempt to improve starch properties for certain areas of application.”

Starch is an important constituent of foodstuffs such as maize, rice or potatoes. It is of major interest for the manufacture of biodegradable materials and finds use in many unexpected places, such as in coating for paper. As such, starch is being constantly optimised for its various applications.