Understanding starch granule formation using wild wheat relatives

This project aims to harness wild wheat relatives to discover new mechanisms of starch biosynthesis, providing novel targets for breeding wheat varieties with enhanced quality and health benefits. Wheat is a foundation for global nutrition that provides ~20% of human caloric intake, and these calories come primarily from starch. Starch is synthesised in the endosperm as semi-crystalline starch granules, and grains of the Triticeae (including wheat, barley and their wild relatives), contain two types of granules: large A-type granules and small B-type granules. This bimodal distribution of starch granules impacts bread and pasta quality, as well as nutritional properties. Larger starch granules tend to digest more slowly in the upper gut, and can act as resistant starch that is beneficial for gut microbiota and health. However, we are only beginning to understand the mechanisms underpinning A- and B-type granule formation. Fully understanding this process is important for developing next-generation wheat varieties with optimised starch granule size distributions for improved end-use quality and health benefits. Our preliminary work uncovered extensive natural variation in starch granule size distribution among wild wheat species, far exceeding that found in modern varieties. This provides a timely opportunity to leverage this variation to identify novel genes and mechanisms involved in starch granule formation. Our first three objectives centre on Aegilops tauschii, the wild species that is the progenitor of the D-genome in modern bread wheat. In a Genome Wide Association Study (GWAS), we discovered that variation in starch granule size distribution was associated with polymorphisms in the Limit Dextrinase (LDA) gene, encoding an enzyme that can release linear malto-oligosaccharides (MOS) from starch. In Objective 1, we will test the hypothesis that LDA plays a critical role in B-type granule initiation by influencing the availability of MOS substrates. The role of LDA in developing wheat grains will be investigated using a combination genetic and biochemical approaches. In Objective 2, we will identify a novel gene from the GWAS by elucidating the causative gene under a major peak on Chromosome 3 that associated with the number of B-type granules. We will use transcriptomics and fine mapping approaches to narrow down the candidate genes under the peak, and functional validation will be performed using wheat mutants. In Objective 3, we will transfer the valuable variation in granule size distributions from Aegilops to modern wheat breeding by creating synthetic hexaploid wheat lines, engaging closely with wheat breeders and the general public to maximise uptake of this approach. In the final objective, we will explore mechanisms underpinning broader interspecies variation in granule size distributions by studying three examples of Triticeae species that have naturally lost B-type granules across three genera (Aegilops, Elymus, and Agropyron). We will use RNA sequencing to look for variation in known genes involved in B-type granule formation, and identify differentially expressed genes that could be candidate genes involved B-type granule formation, which we will validate using wheat mutants. Overall, this project will advance our understanding of A- and B-type granule formation in wheat and other Triticeae. It will create new genetic resources for improving functional and nutritional quality, which can have long term impact on sustainable agriculture, food processing and improved public health outcomes.