Wheat floral organ size and its effects on grain size

Lead Research Organisation: Aberystwyth University
Department Name: IBERS


Wheat in its diverse forms (for example bread, pasta, and biscuits), accounts for over 20% of the calories consumed by humans globally. Wheat also provides over 20% of protein intake, more than all meat sources combined. This means that every person on the planet consumes the grains of almost 50 wheat plants per day, every day. There is an increased demand for wheat driven by the increase in (urban) population and the changes in dietary habits (more meat derived products which use wheat as animal feed). In the backdrop of a changing climate, it is imperative and urgent that we intensify global wheat production sustainably using science-based solutions. This proposal aims to generate scientific knowledge which will be used to deliver genetic solutions to improve yield in farmers' fields.

Crop yield is a complicated trait to study given that it is controlled by many genes and it is also affected by the environment, similar to intelligence in humans. To make progress towards understanding yield, we study its individual components, namely the number of wheat spikes in a given area, the number of grains in each spike, and the weight of each individual grain. We specifically focus on grain weight as it is the most stably inherited trait, meaning we can achieve greater improvements through genetic solutions.

Each wheat grain, within each spike, is surrounded by structures which affect its final size and are correlated with its final weight. Some wheat types, called Polish wheat that were first described by the famous botanist Linnaeus in 1762, have very long flower structures that lead to very long grains. We have shown that transferring this feature to UK varieties increases grain weight by over 6%.

We have recently identified a strong candidate for the gene controlling the long flower structure and grain size trait. This gene is a transcription factor, meaning that it turns other genes on and off. This suggests that it coordinates how flowers and grains develop. We compared the gene from Polish wheat with UK wheat varieties and found that the two versions of the gene were almost identical, apart from a small region which most likely affects how and where the transcription factor is active. We hypothesise that changing where and when this transcription factor is turned on affects the final grain weight in wheat.

In this project we will determine how the transcription factor affects flower structures and grain size. We will first determine precisely where and when the gene is expressed in UK and Polish wheat. We will use the latest imaging techniques, such as CT scans (like CAT scanners for humans), to look inside the spike and determine how the flower structures and the grains become bigger in Polish wheat (more cells, larger cells, etc). We will use innovative methods to precisely define if the larger grains are a direct effect of having larger flower structures, or if we can achieve larger grains independent of their size. We will characterise the small region of the gene which is distinct between UK and Polish wheat to define how this small difference leads to the dramatic effects on grain size. This will help identify other proteins that might turn the transcription factor on or off. Finally, we will use genomic technology to identify which genes are turned off and on by the transcription factor across flower structures and grains.

This in-depth understanding will allow us to come up with the most rationale approaches to improve yield, not only in wheat, but also in other crops such as rice whose grain size is restricted by the same floral organs. We will continue our dialogue with breeding companies to ensure this knowledge is taken up swiftly and transferred into UK varieties. We will also work with international partners to ensure that the knowledge is spread worldwide. In this way, we aim to deliver genetic solutions that will impact globally on humankind.

Technical Summary

Wheat is a vital energy and protein source for humans. It is estimated that wheat production must increase by 60% to meet demands in 2050. It is therefore necessary and urgent that we define science-based solutions to tackle this challenge. Our research focuses on understanding the genetics controlling grain size and weight in wheat. This project exploits recent developments in wheat genetics and genomics to advance our biological understanding of the mechanisms governing grain size in polyploid wheat.

We have recently identified a transcription factor as a strong candidate for a gene that affects the floral structures (glumes, lemmas, paleae) enveloping the grain, as well as grain weight itself. Near isogenic lines with the beneficial allele from Triticum polonicum have 30% longer glumes and 6% heavier grains. The only sequence variation between the T. polonicum and wild-type lines is non-coding. This sequence, which is absent in the long grain T. polonicum types, is highly conserved across cereals. We have shown ectopic expression of the T. polonicum allele in floral organs and grains.

Our aim is to determine the mechanisms by which the transcription factor affects floral organs and grain size in wheat. We hypothesise that modulation of its expression leads to changes in floral organ size which directly/indirectly affect grain size. We will use microscopy, CT scans, and expression analysis to define the role of the gene under wild-type and ectopic expression profiles. We will determine the causality between floral cavity volume and grain size using inducible expression systems. We will also characterise the polymorphic regulatory region and identify upstream regulatory proteins using this sequence as a Y1H bait. Finally, we will use RNA-Seq to define downstream genes and pathways that are affected by the ectopic expression of the gene in floral organs and grains. This understanding will inform strategies to best exploit this trait and deliver improved yield.

Planned Impact

We have identified a gene that increases the size of the floral organs surrounding the wheat grain. When transferred to UK adapted germplasm and grown in the field, we have seen an increase in grain weight of over 6%. If we can transfer this increase in grain weight into on-farm yield, it is the equivalent of 12 years of breeding efforts (given that globally wheat yields increase on average 0.5% per annum).

Importantly, by identifying individual genes that affect grain weight, we can determine strategies to best deploy them in combination. For example, combining this gene, which increases the length of the grain, with a gene we previously identified to increase grain width, we have observed concomitant increases in grain length and width. This wheat line with the two favourable genes leads to a 13% increase in grain weight in UK fields. We will work with UK breeding companies to transfer both genes into the latest UK adapted elite varieties within the Designing Future Wheat public pre-breeding programme. We will assess how this increase in grain weight is manifested in different genetic backgrounds and how it translates into yield under commercial growing conditions.

This project will generate knowledge not only on the newly identified gene, but also on additional genes that are involved in the processes affecting floral organ size. This has been a hitherto unexplored avenue and as such holds potential to increase yields in ways not previously used by breeders.

Beneficiaries of this work will be UK wheat breeding companies, which will be able to use perfect genetic markers to select for increased grain weight within UK commercial varieties. This is novel variation which is currently not within the UK gene pool, but which we have introduced through several years of work. This project will also generate additional induced variation in this gene (through chemical mutagenesis and gene editing) which, subject to EU legislation, will be amenable for use in UK farms. Breeders will also be able to combine genes affecting grain weight as described above using simple genetic markers, thereby simplifying the selection of these genes.

Farmers will benefit from this impact as they will be able to grow wheat varieties with increased yield and with defined genetic loci controlling this trait. This is important as until now, any knowledge of how specific varieties behave across environments in terms of yield are only relatable to that specific variety. As we learn which genes are responsible for yield in each variety, we will be able to extrapolate yield data across varieties and environments, potentially helping us to better understand yield stability across space and time. This will have important consequences for farmer practices, as we should be able to predict yield more consistently and with science-based indicators.

UK consumers will benefit from this impact as wheat constitutes a main staple in British diets. On average, each person consumes roughly 60 kg of wheat flour per year and 99.8% of UK households purchase bread at least once yearly. This is equivalent to nearly 11 million loaves of bread consumed each day in the UK alone. Being able to breed and grow local varieties with improved yield will help keep food prices low and at a more stable price over the year. This is especially relevant for lower income households, which assign a larger percentage of their food budget to basic groceries (such as bread and milk) than higher income households.