Clonal reproduction maintain uniformity and preserve the superior traits of hybrid plants across generations. “Hybrid tomato seeds are more expensive than gold by weight”, says Charles Underwood, Max Planck Institute for Plant Breeding Research.
Yazhong Wang, Charles Underwood, and Raphael Mercier (from left). Photo credit: Frank Vinken
When different varieties of one plant species are crossed with each other, their hybrid offspring are often more robust and grow more quickly than their parents. However, in the next generation this effect disappears again. New methods make it possible to preserve the advantageous qualities of these kinds of hybrid plants for the long term and to deliberately design plants with four sets of chromosomes rather than two. The techniques should make it easier to breed particularly high-yielding and resistant crops that could feed a growing global population even in times of climate crisis.
Over the last five years, Charles Underwood, Group Leader at the Max Planck Institute for Plant Breeding Research (Cologne, Germany) and Professor at the Radboud University (Nijmegen, the Netherlands), and his team have focused on the complexities of plant reproduction. They use tomato as a model system to study seed development and develop clonal reproduction technologies.
"The reproduction of plants is a fascinating field," Charles explains. "We study how plants make pollen and seeds, using tomato as a genetic model. While tomato is an important crop, it also provides an excellent system to explore reproduction at a genetic level."
Recently, his team achieved the creation of clonal sex cells in tomato. This innovation enables plants to accurately pass on all genetic information from both parents, moving away from the normal situation, where offspring inherit only half the genetic material from each parent.
The science behind clonal reproduction
The research makes use of genome editing as a tool. "We developed a system where both the egg and sperm are clonal, carrying 100% of the parent's genetic information," Charles Underwood states. "This was achieved by editing three specific genes involved in meiosis, the cell division process responsible for forming sex cells."
A clonal egg from one parent was fused with a clonal sperm from another parent. This allowed the offspring to carry double the genetic information of a standard tomato plant, resulting in plants with 48 chromosomes instead of the typical 24. The factors affected by this doubling of the genetic information may include important agronomic factors, including yield and robustness. "The cultivated potato – a close relative of tomato – has naturally doubled to 48 chromosomes. This is thought to contribute to the vigor and robustness of potato."
The team's work in tomato represents the first engineered system – in any plant or animal species - where a clonal egg from one parent is fertilized by a clonal sperm coming from another parent. There is a lot of potential to apply this to other crop types as well, according to Underwood.
Collaboration plays a vital role in advancing this research. Underwood acknowledges the contributions of colleagues, including efforts to develop similar systems in rice. "While our work in tomato is unique, it complements broader efforts in the scientific community to explore clonal reproduction across crops."
Applications in plant breeding and the industry
Hybrid tomato seeds are notoriously expensive, often costing around one euro per seed. Charles Underwood elaborates, "Hybrid seeds are currently produced through manual processes—pollinating flowers by hand and extracting seeds from those fruits. It's labor-intensive, making hybrid tomato seeds more expensive than gold by weight."
The ability to propagate hybrid plants through clonal seeds could drastically reduce these costs. "If a hybrid plant could produce seeds clonally, the production costs would plummet. This would not only make hybrid seeds more accessible but could also increase genetic diversity in seed catalogs," he adds.
Beyond cost reduction, clonal seed production could address long-standing challenges in horticulture. Underwood explains, "Traditional breeding methods result in genetic segregation, leading to variability in plant performance. This is why farmers do not save seeds from hybrid plants. Clonal reproduction would maintain uniformity and preserve the superior traits of hybrid plants across generations."
This technology can also increase the diversity of commercially available hybrids. "Currently, most seed companies offer limited varieties of hybrid tomatoes due to the high cost of developing and producing them. With clonal propagation, it becomes feasible to expand these offerings significantly," Underwood suggests.
Formation of gametes in plants: In meiotic cells, pairs of homologous chromosomes first form and exchange DNA segments. These pairs then align along a single plane. The previously duplicated genetic material is separated in an orthogonal direction, resulting in four cells, each with a unique set of chromosomes. In the MiMe process, meiosis is bypassed, and the exchange of DNA between chromosomes is omitted. Instead, two cells are formed, each with a double set of chromosomes identical to that of the original meiotic cell. © Charles Underwood
Building on natural engineered systems
"In nature, some plants like dandelions reproduce clonally, maintaining genetic and phenotypic stability across generations," Underwood says. "There's also evidence from engineered systems in crops like rice, showing that clonal reproduction can enhance uniformity and agronomic performance."
"Our next steps include refining the technology to produce fully clonal seeds in tomato," says Underwood. "Currently, we've succeeded in creating clonal eggs, while converting these into seeds that can self-propagate is an ongoing focus."
"We aim to make this research understandable, bridging the gap between advanced science and its implications for growers and the horticultural industry," he says.
As clonal seed production moves closer to practical application, its impact can be massive. From cost savings to enhanced diversity and uniformity, the benefits are still to be fully understood. "Ultimately, our goal is to make hybrid seeds readily available for farmers in all nations, and facilitate sustainable, high-performance, agriculture around the world," Underwood concludes.
There’s a barrier to the application of this technique, however – namely the strict EU regulations concerning genetically modified crops. These restrict techniques like MiMe that are based on genome editing, i.e. the targeted alteration or deactivation of genes. “The EU should follow the example of the USA and Great Britain and make it easier to cultivate genome-edited plants. Ultimately, we need to make future food production more efficient so that we can feed a growing global population in times of more frequent extreme climatic events. Here, hybrids that are made more high-yielding and more robust with genetic scissors can make a contribution,” says Mercier.
Other researchers are therefore also calling for modernized genetic technology legislation in the EU that takes into account new techniques and findings, since the existing legislation is now more than 20 years old. A legislative proposal by the European Commission that would facilitate the approval of genome-edited plants was approved by the European Parliament at the beginning of the year. Now the EU member states have to agree on a final version of the text for the legislation.
Some complementary data
Crossing two varieties can produce hybrid daughter plants with particularly advantageous traits, but these traits are then lost in the following generation. With the MiMe technique, hybrid plants without meiosis can form clonal sex cells. Clonal egg cells can be used to develop clonal plants without fertilization. The genetic material of these new plants is identical to that of the mother plant, preserving the high performance of hybrid plants for the long term.
The MiMe technique can also be applied in polyploid genome design. This offers a route to increase the genetic diversity within a single plant by, for example, engineering plants with four sets of chromosomes rather than two.
Underwood's research is supported by the Max Planck Society for the Advancement of Science (a non-governmental and non-profit association), the European Research Council (an agency established by the European Union to fund excellent frontier research), and the Radboud University (a public research university).
More information on this topic can be found here:
Wang, Y., Fuentes, R.R., van Rengs,W.M.J., Effgen, S., Zaidan, M.W.A.M., Franzen, R., Susanto, T., Fernandes, J.B., Mercier, R. and Underwood, C.J.: "Harnessing clonal gametes in hybrid crops to engineer polyploid genomes." Nature Genetics 56, 1075–1079, (2024). https://www.nature.com/articles/s41588-024-01750-6
For more information:
Max Planck Institute for Plant Breeding Research
Charles Underwood
Sources: hortidaily.com, mpipz.mpg.de
