Production of wheat lacking B-type starch granules

Starch is a major component of cereal grains and its functional properties have a significant impact on grain utilisation. Of considerable importance is the size and shape of the starch granules. In wheat, barley, rye and most of their wild grass relatives, there are two types of starch granules, called A- and B-type. These differ in size, leading to a bimodal granule-size distribution that is unusual amongst plant starches and not found in other grasses, including Brachypodium, oats, rice and maize.
The smaller, B-type starch granules have negative impacts on many end-uses of wheat and barley. So far, attempts to reduce or remove B-granules from these crops by breeding have failed. The reason for this is the lack of genetic variation in B-granule content between cultivars. However, there is much greater variation for this character between species of Aegilops (Goat Grass), wild grasses that include the ancestors of bread wheat. The existence of Aegilops species lacking B-granules suggests that it should be possible to introduce variation for B-granule content into the closely-related crop species.
The project builds on previous work in which we identified a major QTL controlling the content of B-type starch granules in Aegilops. Our ultimate goal is to identify and manipulate the gene responsible for the control of B-granule content in wheat and barley, Bgc-1. In this project, we will investigate gene order in the region of the genome harbouring Bgc-1 and compare it with that in other grasses. If the opportunity arises within our project's timeline, we will be ready to use the latest developments from ongoing genomics projects in other labs to identify the orthologs of Bgc-1 in wheat and barley and we will begin to manipulate Bgc-1 in these crops using RNAi and TILLING technologies.
Prior to the identification of Bgc-1, we will start to produce mutant wheat plants lacking B-granules using a pre-existing collection of deletion lines of a breadmaking wheat cultivar Paragon. Previously, this population has been successfully used to generate wheat mutants with novel phenotypes by stacking deletions of genes. By screening for deletions spanning the Bgc-1 region, we can select lines likely to lack Bgc-1 in each of the three genomes of wheat and then stack these into a single plant by repeated rounds of crossing and selection.
This project will produce: 1) a fine map of the Bgc-1 region and possibly identification of the Bgc-1 gene in Aegilops and a comparision of this region/gene with those in other grasses. 2) near-isogenic lines of wheat and Aegilops that will allow functionality testing to determine the utility of B-granule-less grains. In addition, if Bgc-1 is identified, the production of genetically manipulated lines of wheat and barley and/or TILLING mutants lacking B-granules will be underway by the end of the project.

We recently made a significant breakthrough in our understanding of the control of B-granule content in the Triticeae: a population of Aegilops segregating for B-granule content was used to identify a major QTL that is responsible for B-granule content. Subsequently, we have made progress towards fine mapping the Bgc-1 gene using the same Aegilops population. Refinement of the phenotyping procedure together with the use of homozygous recombinant lines has enabled conversion of the trait from a QTL to a simple qualitative (Mendelian) trait. Within this project we will extend the fine mapping to define a sub-cM interval encompassing the Bgc-1 locus. As the genome sequence of Aegilops (or any other Triticeae) is unknown at present, we cannot be certain of identifying Bgc-1 by fine-mapping alone. However, we are optimistic that ongoing genomics projects in wheat, Aegilops and other cereals are generating tools and resources that will facilitate its identification.
Prior to the identification of Bgc-1, we will begin to select lines likely to lack this gene by screening a wheat deletion-mutant population for deletions spanning the Bgc-1 region. In this Paragon population, the deletions vary in size but include ones of ~10cM that eliminate multiple contiguous genes. The high level of conserved gene order observed between grass species in the region containing Bgc-1 and the close taxonomic relationship between wheat and Aegilops makes it highly likely that the wheat homoeologs of Bgc-1 will be in syntenous positions. The B-granule content and functional properties of the grains and starches isolated from the wheat deletion mutants and from recombinant inbred lines of Aegilops will be tested. The improved lines with the most useful characteristics will be bulked up and grain will be made available at the end of the project for end-user trials e.g. bread and baking trials, mashing and alcohol yield trials.

Government policies recognise the need to improve grain quality whilst maintaining or enhancing yield. Thus, genetic improvements affecting critical processing attributes, such as starch functionality, are encouraged. The smaller, B-type starch granules present in the grains of wheat and barley are known to have negative impacts on many end-uses. However, efforts to breed wheat and barley cultivars with altered B-granule content have failed because variation in granule-size distribution among elite wheat and barley cultivars is almost non-existent. In contrast, amongst the closely-related wild grasses (Aegilops), there are some species which lack B-granules. This project is designed to provide a deeper understanding of the control of B-granule content in Aegilops, thus allowing the introduction of novel variation for this trait into the related crop species. It has been submitted to the CIRC initiative as it is clear the project is directly relevant to the objectives of many stakeholders, providing as it does the means to achieve a step-change in grain quality. The proposed work is directly relevant to the BBSRC key strategic aims of 'advancing fundamental understanding of complex biological processes' and of 'helping to provide the skilled researchers needed for industrial R&D and academic research'.

Who will benefit from this research?

Plant breeders, millers/bakers, brewers/distillers, farmers

How will they benefit from this research?

UK breeders will benefit from the introduction of novel genetic variation for a key parameter affecting starch functionality that is at present not available in elite wheat and barley cultivars and will provide UK growers with new market opportunities. Scientists in both the academic and commercial private sectors will benefit from the Aegilops grain and leaf RNA sequence database, which is an open-ended resource. The lack of B-granules in wheat is predicted to improve milling and baking functionality and reduce the waste associated with wet processing procedures. In barley, the B-granule-less grains are predicted to enhance processing and the quality and alcohol yield of the final product. Importantly, the introduction of novel grain quality should be achieved in both crops without major impacts on yield.

Our data and germplasm, after appropriate protection of IP in accordance with the special conditions relating to CIRC, will be released into the public domain and will be available to industrialists and academics worldwide. It will contribute directly to wheat (and ultimately to barley) breeding in the UK by producing breeding lines that better meet the needs of end-users. The use of genetic/genomic information from /other cereal systems to help define candidate genes will provide a paradigm for other similar work. The development and release of gene-based markers will aid breeders in manipulating the B-granule trait in elite wheat varieties.