Recreating the synthesis of starch granules in yeast

Most plants and algae produce a carbohydrate called starch, which provides the plant with a dense store of energy. Starch is also the main carbohydrate in our diet and its unusual physical properties mean that it has many industrial uses. It is made of two different sugar-based molecules known as glucans and forms large, partially crystalline granules inside plant cells. Several enzymes are known to be involved in making starch, yet it is not clear exactly how the process works. Animals and fungi cannot make starch but they do make another type of carbohydrate called glycogen, which is also a glucan. Yeast is a single-celled fungus that is often used in research because it is easy to genetically engineer and quick to grow. To study the plant enzymes that make starch in more detail, Pfister et al. aimed to genetically engineer yeast to make their own starch. For the experiments, different combinations of enzymes involved in starch production in a plant called Arabidopsis thaliana were inserted into mutant yeast cells that were unable to make glycogen. The experiments show that all the plant enzymes are active in yeast and retain the roles that they perform in plants. Some of the enzyme combinations yielded glucan granules that occupied a large part of the yeast cell. These granules had many of the physical characteristics of plant starch, showing that yeast can be used as a system to better understand how starch is made. Important next steps will be to insert more plant proteins into the yeast and to fine-tune the production of these proteins. This should help researchers to design starches with desired properties in yeast and ultimately engineer crop plants to produce them.