If implemented in agriculture, the propagation process of a commercial hybrid rice strain through cloning explored in the 2022 study could lower the price of hybrid rice seeds
What is plant breeding and hybrid plants? – The Filial 1 (F1) hybrid
Plant breeding is an activity whose goal is to improve the genetic patterns of plant varieties. Humans have been engaging in this activity since the beginning of agriculture thousands of years ago. Since then, plant breeders have utilised observation to optimise the beneficial genetic characteristics of plants to create desired results. Biologist Gregor Johann Mendel’s work on plant hybridization has contributed to the emergence of Modern Scientific Plant Breeding.
The crossing of two pure lines of plants results in a Filial 1 (F1) hybrid. This offspring ends up being more vigorous than the two parent plants. That’s called hybrid vigour and is a manifestation of heterozygosity. F1 hybrids in seed crops have been utilised in agriculture because of their qualities, but they have a downside, as they need to be renewed at each crop season.
CRISPR/Cas9 – About gene editing
Gene editing is a field of research that seeks to modify the genes of living organisms. It allows scientists to make precise changes in genetic codes and can be applied to medicine, agriculture, and other industries. DNA can be corrected, added, or deleted via genome editing in various cell types and organisms, including plants. In crop breeding, plant breeders can use gene editing to induce functional mutations in plants.
CRISPR/Cas9 is a system utilised in genome editing. The scientists behind the CRISPR/Cas9 genetic scissors research, Emmanuelle Charpentier and Jennifer A. Doudna, were awarded the 2020 Nobel Prize in Chemistry. In the field of plant breeding, professionals can use gene editing to alter plants’ characteristics in a quicker and more precise manner than when using traditional plant breeding techniques.
Apomixis versus Synthetic Apomixis
Apomixis is a mode of reproduction that has been observed in a few hundred plant species. Its peculiarity is that the seeds it produces are identical to the mother plant. In angiosperms, Apomixis is the asexual process that generates a seed from the maternal tissues of the ovule without going through the meiosis and fertilisation processes, which results in the formation of a clonal embryo.
Synthetic Apomixis is the Apomixis induced in sexual crops. By preserving favourable genotypes through generations, synthetic Apomixis offers several avenues for use in plant breeding, and it can allow plant breeders to maintain hybrid vigour in economically significant plant species, circumnavigating the shuffling of genetic material that characterises sexual reproduction. Using synthetic Apomixis in hybrid production could lower the costs of hybrid seed production and expedite the seed production cycle.
F1 hybrids: the yearly costs to produce a new batch of seeds
«When we have F1 hybrids, like most plants and animals, reproduce sexually, the genetic material gets shuffled through the generations. So if you let an F1 hybrid reproduce spontaneously, it will reproduce sexually. Then the second generation will not be an F1 hybrid anymore but would have genes mixed in, which would cause a loss of the hybrid vigour».
This means that for hybrids today, like hybrids of tomato, maize, or any hybrid we have in the fields or the gardens, you have to consider the yearly costs to produce a new batch of seeds. That’s why the seeds of tomatoes are so expensive, and the farmers have to buy the seeds every year. Explained Prof. Raphael Mercier, Ph.D., Director and Scientific Member of the Max Planck Institute for Plant Breeding Research in Cologne, Germany.
Raphael Mercier – director of the Max Planck Institute for Plant Breeding
«If we could implement Apomixis in those F1 hybrids, because the Apomixis leads to clonal seeds and progeny, the progeny of F1 hybrids would be equivalent to the F1 hybrids, and then you can propagate them forever. This means you can make hybrids cheaper and affordable for more people, and you could get hybrid vigour in more plants than we can today».
Another potential benefit of Apomixis is that it would be easier to develop more varieties of hybrids. It’s difficult today to have these hybrids as you need to control the two parents and the crosses. But if we could propagate any new F1 hybrid by just harvesting the seeds, it would probably trigger more diversity.
Mitosis instead of Meiosis (MiMe) mutation
The study A male-expressed rice embryogenic trigger redirected for asexual propagation through seeds was published in the journal Nature in 2019. This paper describes the work of a team of researchers at the University of California, Davis, led by Professor Venkatesan Sundaresan and Assistant Professor Imtiyaz Khanday, who triggered Apomixis in rice plants with between ten to thirty percent of the resulting seeds being clones.
The study demonstrates the viability of asexual reproduction in crops, which may allow for the clonal maintenance of hybrids. Two elements have been utilised to trigger asexual propagation in rice in this research: the BABY BOOM (BBM) gene and the Mitosis instead of Meiosis (MiMe) mutation.
BABY BOOM, the key trigger for embryogenesis
BABY BOOM is a transcription factor, a gene that controls the other genes, like a master regulator. This gene is responsible for starting the biogenesis in the seeds. In particular, this gene triggers embryogenesis only after fertilisation. As it’s not active before fertilisation.
«The absence of expression of the gene in the female gamete prevents embryogenesis from occurring before fertilisation. In the paper from 2019, our colleagues in California showed that BABY BOOM is a key trigger for embryogenesis and that if we express that gene in female gametes, this makes an embryo without that fertilisation, which proves the function of that gene. We just need to express that gene in female gametes, and then you have an embryo going». Said Prof. Raphael Mercier.
The MiMe mutation: a combination of three mutations
«The MiMe mutation is the other half of the story. Meiosis is a special cell division that produces gametes in both plants and animals. During Meiosis, there is a shuffling of the chromosomes. The MiMe mutation is a combination of three mutations in three genes involved in Meiosis. These mutations are the actors of this shuffling of chromosomes. So when the three genes are off, this process of shuffling the genetic information by Meiosis does not occur, and then the gametes contain all the genetic information of the mother plant. If we combine these two, we have a seed containing an embryo identical to the mother plant».
While the 2019 paper proved that by combining MiMe and BABY BOOM, it is possible to produce clonal seeds of rice (Oryza sativa), the 2022 study High-frequency synthetic apomixis in hybrid rice expanded on the previous study’s research.
Combining MiMe mutations and the parthenogenesis inducer BABY BOOM
For this study, an international team of researchers from the University of Montpellier, the Max Planck Institute for Plant Breeding Research, the Plant Genetic Resources Research Institute, and the University of California Davis combined MiMe mutations and the parthenogenesis inducer BABY BOOM in a single step to trigger synthetic Apomixis in a variety of F1 hybrid of rice, the commercial F1 hybrid BRS-CIRAD 302 rice.
«Engineering crops able to perform synthetic apomixis is difficult, because to be able to do that, and to do that efficiently, we need to understand in detail the process of reproduction. That’s because apomixis is derived from sexual production. In evolution, some plants have done that by modification of sexual reproduction. So we need to understand how it works to be able to replicate what happened in evolution in those apomictic plants that produce the crops we use today».
A rice variety commonly used and hybrid rice
They realised that doing that in a single step efficiency improved, switching from between ten to thirty percent of the plants obtained through this process being clonal to more than ninety -five percent of these seeds being clonal and keeping the phenotype of the F1 hybrid over three generations, bringing this process closer to one that could be employed in agriculture.
«In the 2022 study, the principle was the same as the previous one, but how we implemented it in detail differed. In addition, in the first study, we used a rice variety commonly used in laboratories, while in the 2022 study, we used a commercial hybrid rice, so closer to real agriculture». Explained Professor Raphael Mercier, one of the authors of both the 2019 and 2022 studies.
Clonal hybrid rice plants – what’s next for the clonal crops?
If implemented in agriculture, the propagation process of a commercial hybrid rice strain through cloning explored in the 2022 study could lower the price of hybrid rice seed, enabling low-income farmers to access disease-resistant and high-yielding hybrid rice variants. As of now, the result of this method, the clonal hybrid rice plants, have yet to be exposed to the conditions of a field.
«Those plants need to be tested in the field because, as we showed in the paper, the plants issued through Apomixis are like the F1 hybrids. But that happened in a greenhouse, and the greenhouse in the research lab is like a five-star hotel where the plants are well treated. There the water is controlled, as well as everything else. The next step will be to put them in the field in normal conditions and to see if they perform as well as the F1 hybrid». Concluded Professor Mercier.
Prof. Raphael Mercier,
Ph.D., has been the Director and Scientific Member of the Max Planck Institute for Plant Breeding Research since 2019.
