In early summer, unusual pollinators swoop over rice fields in Texas and Arkansas. Small, nimble helicopters fly low and steady so their rotors blow pollen from one row of plants to another. The flights help RiceTec, a plant breeding company, produce seed for high-yielding, robust varieties of rice grown across the southern United States. It’s an expensive and complicated way to create seed.

But the effort is worthwhile because the seeds sprout into plants with a mysterious robustness and resilience. The phenomenon, called hybrid vigor, comes from crossing two strains of inbred parents. Why hybrids are superior to normal plants is not clear, but one long-standing hypothesis is that favorable versions of genes from one parent dominate poor-performing, recessive genes from the other.

The development of hybrid varieties has boosted the yield of maize, sorghum, and other crops by up to 50% and has resulted in other valuable traits, such as better drought tolerance. But the method is only feasible in some species; there’s no practical way to produce hybrid wheat or soybeans, for example. And when it works, it’s extremely labor intensive.

In rice, seed companies must first develop a strain of plants that can’t self-pollinate. Then come the helicopters, which sweep in pollen from a second strain. The process has to be repeated for each new batch of seed to avoid the reshuffling of genes and loss of favorable traits that happens during ordinary sexual reproduction. “It’s a very imperfect system,” says José Ré, vice president of research at RiceTec.

Plant breeders have long dreamed of an easier, more powerful way to create hybrid seed. In nature, some plant species reproduce clonally: The eggs inside their flowers become embryos without pollination, part of a process called apomixis—“away from mixing” in Greek. If researchers could genetically engineer crops to reproduce through apomixis, the process of creating the first hybrid generation might still be laborious. But then seed companies could much more easily propagate hybrid offspring.

For decades, scientists had limited success. But recent breakthroughs have brought the concept closer to reality. In 2019, an international team reported that it had successfully engineered a line of rice plants that could reproduce clonally—the first instance of synthetic apomixis in a crop. Groups around the world are working to develop apomictic varieties of sorghum, tomatoes, alfalfa, and other crops. There’s a palpable “sense of excitement” in the field, says Mary Gehring, a molecular biologist at the Whitehead Institute and the Massachusetts Institute of Technology who studies development in apomictic plants.

The technology won’t be ready to be commercialized for years. “There’s still an awful lot that we don’t understand about how to make it efficient for agriculture,” says Peggy Ozias-Akins, a geneticist at the University of Georgia. But seed companies are paying attention. Apomictic reproduction would simplify how they produce hybrid seeds, quicken the release of new varieties, and reduce costs. The technology could also benefit smallholder farmers in poorer countries who might not have regular access to commercial hybrid seeds, because they could save seeds produced by the previous year’s crop. “It really would be a big game changer,” says Adam Famoso, a rice breeder at Louisiana State University.

THE DISCOVERY OF VIRGIN birth in plants is widely credited to John Smith, a 19th century botanist who served as the inaugural curator at London’s Royal Botanic Gardens. For a decade, he had watched three holly plants from Australia bear fruit without having ever produced a male flower or anything that resembled pollen, the vehicle for plant sperm. In 1839, Smith reported to the Linnean Society of London that he could grow new plants from the seeds of the hollies. It was an incendiary claim that was met with “incredulity,” according to Thomas Meehan, a botanist writing decades later.

In 1898, however, Swedish botanist Oscar Juel demonstrated, with convincing microscopy, that the egg cells of a plant called alpine catsfoot could develop into embryos in the absence of pollen. Other researchers took a closer look at their own favorite species. As evidence accumulated, more and more botanists began to take the phenomenon seriously. Today, apomictic reproduction has been confirmed in more than 400 plant species but no staple crops.

In the late 1990s, as genetic tools became more readily available, experts were optimistic they could identify the genes behind apomixis and deploy them in crops, creating clones that would bypass the genetic recombination that happens during plant sex, which shuffles away favorable gene combinations. But progress was slow. “People said, ‘OK, we will crack that nut,’” recalls Erik Jongedijk, a plant geneticist at KWS, a major seed company in Europe. And for a long time, “it was never cracked.”

Part of the difficulty stems from the complexity of the reproductive process researchers are trying to modify. During sexual reproduction, gametes—eggs and sperm—are created through meiosis, a process that results in haploid cells, with half the number of chromosomes. To form embryos with a full complement of chromosomes, eggs and sperm need to come together. Many naturally apomictic plants instead create gametes through mitosis, with no change in the chromosome count. The eggs can then turn into an embryo without being fertilized, in a process known as parthenogenesis.

It’s taken decades to identify some of the genes involved and to figure out how to tinker with them. In 2009, a group of scientists led by Raphaël Mercier, a geneticist now at the Max Planck Institute for Plant Breeding Research, showed that if they knocked out three genes involved in meiosis in the model plant Arabidopsisit would make gametes through mitosis, preserving their full set of chromosomes. They named the trio of mutations MiMe, short for “mitosis instead of meiosis.” In 2016, they replicated the feat in rice, showing that the MiMe mutations would create diploid eggs genetically identical to the mother plant.

Meanwhile, other groups were figuring out how to coax egg cells to develop into embryos without being fertilized. In 2002, geneticist Kim Boutilier—now at Wageningen University—made what turned out to be a key discovery when she and her colleagues identified a gene called BABY BOOM in rapeseed. The gene, the team found, triggered the growth of embryos from shoots and leaves when it was turned on in Arabidopsis.

Since then, BABY BOOM–like genes have been found in many plants. In 2015, Ozias-Akins and her colleagues identified the function of one of them in a natural apomict, the grass Pennisetum squamulatum. When they transferred this gene into a closely related grass that reproduces sexually, as well as into rice and maize, it induced parthenogenesis, resulting in haploid embryos that were viable. “Amazingly, it worked,” says Ueli Grossniklaus of the University of Zurich, a developmental geneticist who studies reproduction in Arabidopsis and natural apomicts.

The stage was set for the next step of the engineering process: knocking out the trio of meiosis genes and activating the critical BABY BOOM gene within a single plant.

SCIENTISTS DIDN’T WANT to simply transfer the key embryo activating gene from a grass into rice containing MiMe mutations. Although such a step is technically possible, it comes with a regulatory downside: Crops that are created by transferring a gene between species require lengthy regulatory evaluation before they can go to market. So scientists looked to rice’s own genome instead.

A landmark accomplishment came from someone who didn’t set out to study apomixis. Venkatesan Sundaresan, a development biologist at the University of California (UC), Davis, had been investigating genes expressed in rice as egg and sperm cells fuse and develop into embryos. He and his colleagues noticed that the rice version of BABY BOOM is normally expressed in a plant’s sperm cells, but not the eggs, and that it remains active in the embryo after fertilization. If they could activate BABY BOOM in the egg cells as well, it could make pollination unnecessary, they reasoned.

To test whether that would work, UC Davis team member Imtiyaz Khanday isolated rice’s BABY BOOM and tacked on a promoter that turns the gene on specifically in the egg. The next step was to get the gene back into the plants. Khanday inserted this DNA package into the genome of a plant-infecting bacterium, a standard vehicle for genetically modifying plants. Added to a petri dish of rice cells, the modified bacterium inserted its own DNA—including the added gene—into the crop’s DNA. The cells formed a tumorlike tissue known as callus that could then be coaxed to grow into a seedling.

The resulting plants, which were also modified using CRISPR gene editing to have the MiMe mutations, underwent parthenogenesis and produced clonal seeds. Those seeds germinated and the seedlings were genetically identical to the mother plant, the team reported in 2019 in Nature.

It was a groundbreaking achievement, but the plants weren’t perfect: Only about 30% of their seeds were clonal. (The remainder had been fertilized by pollen produced by the genetically modified plants and were not viable.) So the team continued to tinker with its methods. In a follow-up paper published in Nature Communications in December 2022, the researchers reported that after using a different variety of rice—a commercial hybrid—and adding the MiMe mutations and the BABY BOOM promoter in the same step, they ended up with plants that yielded more than 95% clonal seeds. It’s “absolutely fantastic work,” Jongedijk says.

The group is now looking for funding to test its clonal rice in field trials, says co-author Emmanuel Guiderdoni, a rice geneticist at the French Agricultural Research Centre for International Development. The researchers want to see how their clonal plants fare in harsher conditions than they experienced in the greenhouse.

RESEARCH ON OTHER CROPS is also picking up steam. Anna Koltunow of the University of Queensland is developing apomictic varieties of sorghum and cowpea, important crops for farmers in sub-Saharan Africa. In October 2022, she and her team began field trials in Australia of hybrid sorghum that was genetically modified to be parthenogenic and produce haploid embryos; they plan to add MiMe mutations to strains in the future. At least 10 groups in China are also working on apomictic varieties of cabbage, tomatoes, alfalfa, and other vegetable and forage crops, says Kejian Wang, a geneticist at the China National Rice Research Institute who is developing an apomictic strain of hybrid rice.

One challenge for those researching dicots—the large group of flowering plants that includes beans and vegetable crops—has been that BABY BOOM doesn’t seem to work in them when expressed in egg cells. “We’ve tried really hard, and nothing ever worked,” Grossniklaus says. But now they have another option. In January 2022, a group led by plant geneticist Peter van Dijk of KeyGene, a plant breeding company, reported in Nature Genetics the discovery of PAR, a gene in dandelion—a naturally apomictic dicot—that appears to have a similar function as BABY BOOM. When his team switched on PAR in lettuce, rudimentary embryos formed without fertilization. The search for that gene took more than 15 years. “It’s a beautiful story, but it also demonstrates how much work and effort was needed,” says Tim Sharbel, an evolutionary biologist at the University of Saskatchewan.

Jongedijk expects another 5 to 10 years of research may be needed before synthetic apomixis can be deployed commercially in any crop. For an apomictic hybrid variety to appeal to farmers, 100% of the seeds must be clonal because they won’t want the less vigorous seeds that are produced through normal sexual reproduction. And as with all new crop varieties, scientists will need to conduct extensive field testing to determine how hybrid varieties respond to drought and other stressors.

Further tinkering with genes that control seed development could lead to even more progress. Like most plants, the apomictic rice now under development still needs pollen to fertilize its endosperm—the seed tissue that provides sustenance for the developing embryo. For those strains—as well as for commercial varieties that reproduce sexually—that step is vulnerable to climate change, because pollen can become less viable when it’s exposed to high temperatures. Gehring’s team recently received funding to try to engineer plants that develop endosperm without being fertilized, a feat that some naturally apomictic plants are capable of. If the team is successful, future apomictic crop varieties wouldn’t rely on pollen at all—enabling them to produce bountiful seeds even during heat waves.

As research on apomixis proceeds, some breeders caution that even when perfected, such crops might not succeed in the market. Famoso notes that some countries, especially in Asia, that have resisted genetically modified foods may not want to import apomictic rice. But Jauhar Ali, head of the hybrid rice program at the International Rice Research Institute, is more optimistic. “Gene editing is slowly being accepted and many governments are understanding the importance of this tool for bringing benefits to agriculture,” he says.

“There’s a lot of promise for this technology and hopefully it can make it into a farmer’s field someday,” Famoso says. For now, though, the helicopters will keep flying.