When different varieties of a plant species are crossbred, their hybrid offspring often exhibit greater vigor and faster growth than the parent plants. However, this effect typically fades in subsequent generations.
New methods now allow scientists to retain these beneficial traits in hybrid plants over the long term and to create plants with four sets of chromosomes instead of the usual two, according to a press release from the Max Planck Institute for Plant Breeding Research. These techniques could significantly simplify the breeding of high-yield, resilient crops capable of sustaining a growing global population, even in the face of climate challenges.
Hybrid crop varieties are faster-growing and more resilient to abiotic and biotic stresses than their inbred counterparts, with hybrid maize producing yields 30% higher. However, the heterosis effect is temporary, as yield gains and uniformity are lost by the second generation due to meiotic cell division, which recombines genetic material. If hybrids could be propagated asexually or cloned through seeds, they could maintain their full genetic makeup and beneficial traits in future generations, significantly reducing production costs and expanding the availability of hybrid varieties.
Raphaël Mercier, head of the Department of Chromosome Biology at the Max Planck Institute for Plant Breeding Research, and Research Group Leader Charles Underwood are developing a method to produce clonal hybrid seeds. Their work is showcased in greenhouses with various plants, including thale cress, barley, potatoes, and tomatoes. Two key conditions must be met: the mother plant’s genetic material must be preserved in the female gamete by preventing meiotic cell division, allowing for a clonal egg cell; and the new plant must develop from this clonal egg cell without fertilization to avoid excess chromosomes. Mercier states: “We need to overcome both meiosis and fertilization to produce genetically identical seeds,” a method that could extend the hybrid state indefinitely.
Mercier began his research in 2009 at the INRA Jean-Pierre Bourgin Institute in France, focusing on the genes involved in meiotic cell division and the development of egg and pollen cells. He identified three genes that differentiate meiosis from mitosis, and by deactivating them, he successfully reverted meiosis to mitosis, resulting in clonal germ cells with genetic material identical to the mother plant.
In 2016, Mercier and Emmanuel Guiderdoni applied this process, known as MiMe (mitosis instead of meiosis), to rice, marking the first time it was used on a crop. The same three genes regulate meiosis in both thale cress and rice, allowing for genetically identical egg cells.
In 2019, Mercier and Venkatesan Sundaresan addressed clonal seed reproduction by activating the BBM1 gene in egg cells, initiating embryonic development without fertilization. This method, though currently yielding 30% fewer seeds than traditional methods, shows promise, and Mercier is hopeful for improvements. They applied the MiMe technique to supermarket hybrid tomatoes, where Charles Underwood’s team created clonal sex cells by fertilizing a clonal egg with clonal sperm from different plants. This “polyploid genome design” produced plants with four chromosome sets, resulting in “super-hybrids” similar to those in crops like wheat, rapeseed, banana, and potato.
Disease-Resistant Potato Varieties
In an LED-lit greenhouse, a scientist showcases a tomato plant with a quadruple set of chromosomes, the first instance of clonal sex cells from different parents fusing in any plant or animal. Next to it is a sturdier plant with smaller fruits, a hybrid of a MiMe tomato and the wild relative Solanum pennellii, which is known for its heat, drought, and salinity resistance.
The potato, closely related to tomatoes, is another candidate for the MiMe technique. Underwood emphasizes the urgent need for disease-resistant potato varieties to adapt to changing climates, as many existing varieties, like ‘Russet Burbank,’ are over a century old. Potato blight poses a threat to both plants and tubers, and incorporating genetic material from wild potato species could enhance disease resistance while preserving desirable traits, potentially reducing pesticide use. Since potatoes are harvested for tubers rather than seeds, the lower seed production of MiMe hybrids is less critical for yield compared to crops like rice.
Potato cultivation faces challenges from diseases like potato blight, which can cause severe yield losses and historically contributed to the Irish famine. The MiMe technique aims to use genetic material from wild potato species to breed domestic varieties that resist blight while maintaining typical characteristics, potentially reducing pesticide use. Raphaël Mercier emphasizes the promise of MiMe hybrid potatoes, noting that since they are harvested for tubers rather than seeds, reduced seed production does not adversely affect yield as it does in crops like rice.
The application of the MiMe technique faces a barrier due to strict EU regulations on genetically modified crops, which restrict genome editing methods. Raphaël Mercier urges the EU to adopt more flexible policies like those in the USA and Great Britain to facilitate the cultivation of genome-edited plants, essential for efficient food production amid increasing climate challenges.
Other researchers are advocating for updated EU genetic technology legislation, as the current regulations are over 20 years old. A recent proposal by the European Commission to ease approval for genome-edited plants has been approved by the European Parliament, pending agreement from EU Member States.
Ultimately, the decision on the adoption of these plants in Europe lies with politicians and, importantly, consumer acceptance. Interestingly, the MiMe technique resembles natural reproductive methods seen in plants like dandelions and certain blackberries, which reproduce without female meiosis or fertilization.
