Unmixed Blessing?
Researchers are Closing in on the long-sought goal of creating high-yielding hybrid crops that can be Propagated Indefinitely
18 MAY 2023 BYERIK STOKSTAD
Science, Vol 380, Issue 6646.
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.
Get more great content like this delivered right to you!
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.
ADVERTISEMENT
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.
Some methods of hybrid rice production employ helicopters to blow pollen from one rice strain to another.RICK SÁ
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 Arabidopsis, it 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. .... '
