Unravelling novel mechanisms of starch granule biogenesis in potato

This project will develop a mechanistic understanding of starch granule formation in developing potato tubers. Starch is the major form in which plants store carbohydrates, and is vitally important to humankind as a major source of calories in our diet. Despite its importance, we still do not fully understand how plants produce starch. In its native form, starch forms insoluble granules that are composed of glucose polymers. The size of the granules is highly variable between species, and influence the physical and chemical behaviour of starch during cooking, as well as on the texture of the final food products.

Recently, we have made breakthroughs in understanding the mechanism by which starch granules are initiated (or primed) in leaves of the model plant, Arabidopsis thaliana, and in cereal grains. In these plants, granule size is influenced by the number of granules initiated per plastid. However, granule initiation has not been investigated in potato, which contains the largest granule sizes among starch crops. The knowledge gained from other species make it particularly timely to discover the control of granule size and number in potato. Further, potato starch granules have a distinct ellipsoid shape, but the processes that determine the morphology of the granules are not understood in any species.

This project aims to discover molecular mechanisms of starch granule initiation and morphogenesis. Firstly, we will investigate which of the granule initiation proteins establish correct granule number and size in developing tubers, by creating and studying a series of mutants defective in each initiation protein. We will then test the hypothesis that there is active granule morphogenesis in potato, based on preliminary data that this process requires a specific initiation protein isoform. State-of-the-art 3D electron microscopy and fluorescence microscopy will be used to study the structure of amyloplasts (the plastid organelle in which starch is synthesised and stored) in detail through tuber development - showing how many granules are typically initiated per amyloplast, the number of amyloplasts per cell, and how new granule formations are coordinated with new amyloplast formation.

As a final objective to facilitate further impact, we will use the unique materials generated through the course of this project to assess the effects of granule size and morphology on the physical and chemical behaviour of starch during cooking, starch digestibility, as well as textural and sensory qualities in a variety of prepared potato food products.

Overall, this project will greatly advance our understanding of fundamental mechanisms of starch granule biogenesis in potato.

Starch is a unique biopolymer in that it forms semi-crystalline starch granules. The aim of this work is to investigate mechanisms of starch granule formation in potato tubers. Potato starch is widely consumed as a dietary carbohydrate in many parts of the world and consists of distinctive large ellipsoid starch granules. However, little is known about how these granules are formed during tuber development, and the mechanisms underpinning their unique size and shape. In this project, we will explore starch granule initiation and morphogenesis in developing potato tubers. Using a series of gene-edited potato mutants, we will study how the unique complement of granule initiation proteins expressed in the tuber influences the number and size of starch granules. We will also explore the process of granule morphogenesis, based on exciting preliminary data that this could be an active process requiring a specific initiation protein homolog. We will examine the links between new granule formation and new amyloplast formation during potato tuber development, and study the structure of amyloplasts in unprecedented detail using 3D electron microscopy. Since starch granule morphology is thought to be a major determinant of starch quality in foods, we will also use the unique material generated within this project to test the impact of granule size and shape on starch functionality, digestibility and potato food quality, together with the industrial partner, Simplot. This work will lead to exciting fundamental mechanisms of granule biogenesis in an important crop, which will have major implications on potato quality.