As featured in Arable Farming Magazine
Improving efficiency of hybrid wheat breeding
by Arable Farming Nov/Dec 2020 issue
Work to control wheat fertility and help breeders create better varieties is nearing completion. Andrew Blake reports.
The genes which control pollen production in plants are surprisingly similar across a range of species.
This is a surprise finding from a research project* begun in October 2016 based on data from a Crop Improvement Network project funded by the Biotechnology and Biological Sciences Research Council (BBSRC).
The work is led by Prof Zoe Wilson, of the University of Nottingham.
She says: “Our initial BBSRC grant enabled us to interact with AHDB and breeding companies and to work with them to develop a targeted programme to further investigate pollen development and the control of male fertility.
“We’re trying to identify ways to control male fertility in wheat in a reversible manner and to understand how the development of pollen – the ‘male side’ of plant reproduction – is controlled.” Understanding that and being able to control male fertility should make the outcrossing used in hybrid breeding easier and more reliable, adds Prof Wilson.
Hybrid crops can boost yields without needing more inputs which may damage the environment.
The challenge is to avoid the natural tendency of many crops, such as wheat, to self-fertilise before outcrossing.
“Hybrid seed production also relies on having effective males to pollinate the female lines,” she says.
“So traits for optimal pollen production, viability and release are also of importance and are being targeted as part of the research programme.
“We’re using several approaches to identify and characterise mutants in the genes that control pollen development in wheat and barley.” Chemicals, gene editing and genetic manipulation are all being used.
Knowledge from other ‘model’ plant systems, including the weed arabidopsis and rice, was first used to identify the genes likely to be involved.
“We targeted mutants in these genes using chemical mutagenesis and then crossed these lines to stop the function of all gene copies.
“Bread wheat is made of three separate parental genomes [i.e.
six copies of each gene], so these mutants needed to be crossed together to generate mutations in all copies of the genes involved in pollen development – a very time consuming exercise.” As well as making these crosses, the mutants have also been crossed into different commercial backgrounds to enable future use by breeders.
Using gene editing and genetic manipulation allows mutants controlling the levels of expression of key genes in both wheat and barley to be generated much faster, says Prof Wilson.
“It means we can test the systems earlier and understand what these genes are doing to make pollen and what can go wrong with this process.
“This is important as fertility is critical to crop yields and especially so under volatile environmental conditions and climate change.” Barley as well as wheat is being used to help further the work because its reproductive system is similar to that in wheat; but barley, a diploid, has only two genome copies and so is genetically much simpler than hexaploid bread wheat with its copies from three genetic backgrounds.
“We’re characterising all these mutants to determine how fertility is impacted and to help understand which genes are altered and how they work together to facilitate the formation of functional pollen.
“We’re also looking at how the changes occur under different environmental conditions as these can be mechanisms to reverse fertility for breeding purposes.”
Microscopy is being used to characterise pollen development in the mutants alongside molecular techniques to determine which genes are switched on or off as a result of the mutations.
The research is also examining the impact of the changes in relation to the chemical structure of the pollen grains, she adds.
A key message from the project so far is that the genetic networks driving pollen development are much the same across diverse plant species and the critical ones for wheat and barley have been identified.
The discovery should help make it easier to generate more reliable male sterile lines for use in hybrid breeding.
“We’ve generated lines with mutations in both copies of all three of the different wheat genomes.
When they’re combined they impact on fertility.
“They’re currently being analysed to understand the changes involved and assess their potential as breeding tools for hybrid breeding,” says Prof Wilson.
Seed cost remains a challenge
The concept of hybrid wheat has been around for many decades and countless programmes have been initiated and shut down, says Jacob Lage, head of hybrid wheat and pre-breeding at KWS UK.
“Many companies are now refocusing on hybrid wheat, believing the tools and genetic knowledge developed over the past 10-15 years puts us in a position to make hybrid wheat a success.
“It’s already possible to produce hybrid wheat, using a chemical to sterilise the female plants or employing a cytoplasmatic male sterility system, but neither methods work as smoothly and efficiently as we’d like.
“This BBSRC and industry supported project looks at some of the essential elements needed to develop a more efficient system.
Although the research is not expected to deliver a final hybrid system, it will help us understand pollen development much better.” Hybridisation, however, is not the only bottleneck to address, says Dr Lage.
“If we’re to successfully commercialise hybrid wheat, we must do so at a competitive seed price.
“We’re confident wheat growers will benefit from increased production and stability, but the extra seed cost must not exceed the expected gain in the field.
“One challenge for hybrid seed production is that wheat is predominantly self-pollinated with limited pollen being spread to other plants.
“Prof Wilson’s project should help develop male lines which shed large amounts which can be carried by wind to nearby sterile female plants.
“System development and better pollen production are topics of interest to most breeding companies, so it was an easy decision to join forces with several of our competitors to support the work.
“We all actively contribute to the project and it has been great to see how it has progressed,” he adds.