The plant breeders’ dilemma: sex or no sex?

The ability to switch between sexual and asexual modes of reproduction in plants is being exploited to help some of the world’s poorest crop farmers.

Farmer inspecting sorghum ears in a field.

Farmer inspecting sorghum ears in a field.

Farmer inspecting sorghum ears in a field.

By exploiting quirks in plant reproduction, an international team of researchers funded by the Bill & Melinda Gates Foundation and headed by Professor Anna Koltunow of The University of Queensland are seeking to improve the bottom line for farmers.

The endgame is to increase yields and hardiness while reducing on-farm costs thereby improving agricultural productivity, profitability and food security.

First in line to benefit are poor smallholder farmers in sub-Saharan Africa who rely on exceptionally drought tolerant and hardy crop species to survive. Of prime importance are sorghum and cowpea, domesticated in Africa, and important subsistence food crops.

Because hybrids – the offspring of in-bred, genetically divergent parents – can be exceptionally tough and highly productive, they are the initial focus of this work.

Professor Koltunow explains that significantly higher yield gains are possible by cultivating hybrids, with both sorghum and maize routinely cultivated commercially in this way.

The downside for farmers is that this yield-boosting hybrid vigour – technically known as ‘heterosis’ – is expressed only in the first generation of progeny.

“Hybrids are valuable to agriculture but the benefit is offset by the cost to the farmer of purchasing hybrid seed every year,”

- Professor Koltunow

“For smallholders to benefit from hybrids, the new breeding method we are developing will allow farmers to retain and re-sow hybrid seed for multiple generations.”

The right expertise

Professor Koltunow has a formidable record of accomplishment in plant reproduction research and translating that research into gains in agricultural sectors.

Her research focus is on the molecular and genetic mechanisms that regulate development of plant reproductive cell types and, therefore, seed and fruit formation.

Accomplishments include developing seedless fruit, a sought-after commodity in the horticultural sector.

She was selected by the Bill & Melinda Gates Foundation to head an international collaborative research venture between six research organisations and a multinational seed company to develop technologies so smallholder African farmers could economically save seed from cowpea and sorghum hybrids.

Professor Koltunow led the first five-year phase of this project, called Capturing Heterosis, while at CSIRO.

This project finished in July 2019 and the second phase, also funded by the Bill & Melinda Gates Foundation – Hy-Gain – commenced in March 2020, at the University of Queensland.

Members of the Hy-Gain research team at UQ’s Crop Research Unit at Gatton.

Members of the Hy-Gain research team at UQ’s Crop Research Unit at Gatton. Pictured L to R:  Professor David Jordan, Dr Emma Mace and Professor Ian Godwin.

Members of the Hy-Gain research team at UQ’s Crop Research Unit at Gatton. Pictured L to R:  Professor David Jordan, Dr Emma Mace and Professor Ian Godwin.

Professor Koltunow, who in 2019 took up a Professorial Research Fellow role at QAAFI will again lead the project.

The Hy-Gain team includes QAAFI’s world-leading sorghum researchers, Professor David Jordan, Dr Emma Mace and Professor Ian Godwin, who have expertise in breeding, genetics and transformation.

Associate Prof Brett Ferguson from The University of Queensland’s School of Agriculture and Food Sciences (SAFS) is contributing leading expertise in legume biotechnology.

These researchers are linked to five prestigious international research institutes and a multinational seed company, which collectively form Hy-Gain – a public-private research team to promote food security in the developing world.

The five-year project includes in-kind funding from several of the participating institutions.

“Together, the Hy-Gain project team is developing new ways to breed sorghum and cowpea varieties that make it possible to achieve large gains in yields, while increasing resilience to diseases and environmental stress,”

- Professor Koltunow

The key to preserving intergenerational hybrid vigour lies with one of the quirkiest aspects of plant sexual biology: ‘apomixis’, which is also known as asexual seed formation.

Asexuality

Cowpea flower

Cowpea flower

Cowpea flower

There are many non-agronomic plants that can reproduce asexually, meaning they bypass the need for meiosis during gamete formation and fertilisation by male sex cells to form a seed.

Sorghum and cowpea are not among them. “If a hybrid seed is equipped with genetic switches to make female sex cells by mitosis and also produce embryos without fertilisation, the hybrid would be self-seeding, and able to make clones of itself via seed,” Professor Koltunow says.

“We are developing prototypes of such plants and testing if this will lock in those valuable hybrid characteristics when the hybrid seed was planted, flowered and produced more seed."

- Professor Koltunow

In developing countries, this would allow hybrid seed to be retained and re-sown on-farm for multiple generations, with the farmer able to realise yield gains, and pocket the income otherwise required to purchase hybrid seed each year.

“In developed countries, the technology will reduce seed costs and increase the rate of genetic improvement.”

Professor Koltunow says the strategy was looking very promising for sorghum but there was much work to do. Cowpea, like most legumes, is not normally cultivated as a hybrid.

A suite of tools is required to enable the genes to switch on in the right place at the right time.

The Hy-Gain research collaboration is expanding knowledge of cowpea genomics (analyses of cowpea genetic materials), transcriptomics (to analyse changes in gene expression) and using state-of-the-art transformation techniques (to specifically alter cowpea genes) to support the project goals.

Messing with sex

Sorghum redheads in a field.

Sorghum red heads in a field.

Sorghum red heads in a field.

One of the most striking aspects of the advances made in the prior research phase under Professor Koltunow’s leadership is the relatively small number of changes required to switch to an asexual mode of seed formation.

The changes prevent the female sex cell from undergoing a special kind of cell division (meiosis) that would normally halve the number of chromosomes passed onto the offspring and also diversify the genetic content of those chromosomes.

By keeping a full complement of (unshuffled) chromosomes, the female sex cell is competent to form the embryo of the seed, without fertilisation, through the action of another gene.

“The ultimate goal of our work, in the long term, is to deliver African-adapted sorghum and cowpea varieties with improved yield and resilience traits,” she says.

“In order to do that, we need to ensure that we develop a technology that can be readily used in breeding.

We need flexibility to switch between sexual and asexual seed formation.

This would enable us to rapidly develop new hybrid varieties, targeted to specific agro-ecological regions, and support improvement of non-hybrid varieties with resilience and quality traits to get them to the grower faster, avoiding varietal stagnation.

“We are already factoring these considerations into the next phase of the project, if Hy-Gain is as successful as we hope”.

If developed as a practical breeding tool, this technology has numerous other applications, including within Australia’s sorghum breeding program, and also in adapting the grains industry to climate impacts that are confronting the nation with hotter and drier growing conditions.

Cowpea  is a kind of legume plant with young pods and this plant is relatively resistant to dryness and is usually planted in the yard as a family food reserve.
Sorghum grains in wooden bowl in hands of Sri Lankan woman
Sorghum red seeds

Contact

Hy-Gain Project Manager
Email: 
hygain@uq.edu.au