image: In hybrid rice breeding, sexual reproduction causes heterosis decay due to meiotic recombination. The Fix4 synthetic apomixis system, established entirely via genome editing technology (targeting the OsOSD1, OsPAIR1, OsREC8 and OsPLDα2 genes), can stably generate clonal seeds in hybrid rice and maintain normal seed-setting rates.
Credit: ©Science China Press
The development of synthetic apomixis enables the fixation of heterosis, which is a breakthrough that promises to transform conventional hybrid breeding strategies and trigger a new wave of green revolution in agricultural production. At present, the engineered synthetic apomixis system, which is entirely based on genome editing, exhibits reduced fertility, thereby limiting its practical applications. Recently, the research team led by Kejian Wang at the China National Rice Research Institute combined MiMe-related genes with the OsPLDα2 gene through genome editing technology, creating a new apomixis system termed Fix4 (Fixation of hybrids 4). This system not only produces stable and heritable clonal seeds but also shows a normal seed-setting rate, providing theoretical support and innovative solutions for accelerating the application of apomixis technology in hybrid rice production.
Heterosis, where cross-pollinated plants exhibit higher yield and enhanced resistance compared to their self-pollinated counterparts, has long been exploited in the hybrid seed industry, representing one of the landmark innovations in modern agriculture. However, unlike seeds from inbred lines, seeds harvested from hybrid plants cannot be used for subsequent generations of production due to genetic and phenotypic segregation. Consequently, hybrid seeds must be produced annually, which is a labor-intensive, costly, and environmentally demanding process, making hybrid seeds more expensive than inbred seeds. Apomixis is a form of asexual reproduction that produces seeds with a genetic makeup identical to that of the maternal parent. Although natural apomixis has evolved in over 400 plant species, this valuable trait is notably absent in major crops. Therefore, the introgression of apomixis into crops is considered the holy grail of agriculture and has attracted intense interest from plant biologists and the seed industry.
In recent years, synthetic apomixis systems have been engineered in rice by combining modifications of meiosis and fertilization. These systems can be categorized into two strategies based on the used technologies. The first combines MiMe (Mitosis instead of Meiosis) with ectopic expression of parthenogenesis-related genes (e.g., OsBBM1, OsBBM4, ToPAR, OsWUS) in egg cells, while the second integrates MiMe with mutation of haploid induction genes (e.g., OsMTL). "By simultaneously editing MiMe-associated genes and OsMTL via CRISPR/Cas9, we achieved the foundational breakthrough in hybrid rice synthetic apomixis—from concept to reality," said Dr. Kejian Wang, corresponding author of the study and a principal investigator at the China National Rice Research Institute (CNRRI). "The MiMe-Osmtl system is entirely dependent on genome editing technology. However, this system exhibits a very low seed setting rate, primarily due to the negative effects caused by the mutation of the OsMTL gene.”
The OsPLDα2 gene, which encodes a phospholipase specifically located in mature rice pollen, exhibits haploid induction capability after mutation and notably displays no reduction in fertility. "This discovery has ignited our curiosity: could harnessing this gene pave the way for the hybrid rice apomixis system that maintains full seed-setting rates?” said Fengyue Hu, first author of the study and a doctoral candidate at the CNRRI. They further employed the CRISPR/Cas9 technology to simultaneously mutate the MiMe-related genes (OsOSD1, OsPAIR1, OsREC8) and OsPLDα2 to establish the Fixation of Hybrids 4 (Fix4) system in hybrid rice. As expected, the newly established synthetic apomixis system Fix4 induced heritable clonal seeds as well as maintained normal seed-setting rates comparable to those of wild-type plants.
“We have been attempting to identify new haploid induction genes to replace the OsMTL gene for years. The target gene should not only possess haploid induction ability but also exhibit no negative effects,” highlighted Dr. Chaolei Liu, co-first author of the study and an associate researcher at the CNRRI. "We have identified the OsPLDα2 gene, which not only demonstrates haploid induction capacity in rice but also exhibits functional conservation across a wide range of crops, including maize and cabbage, highlighting its broad applicability in both monocotyledonous and dicotyledonous plants.”
"For this new system, future integration with transient transformation technologies could enable the development of transgene-free synthetic apomixis system. Furthermore, we are exploring strategies to enhance clonal seed efficiency and engineer non-transgenic apomixis systems with both high fertility and high induction rates," envisioned Dr. Kejian Wang.