The non-fusion reproductive system of hybrid rice has achieved normal fruiting rates

Since the creation of the hybrid rice fusion-free reproductive system in 2019, the team of Wang Kejian, a researcher at the China Rice Research Institute, has been committed to the research and development from “1” to “many” – optimizing the fusion-free reproductive system and improving the fruiting rate.

On January 6, Molecular Plant published the team’s latest research results online, in which they optimized the hybrid rice-free reproductive system, obtained non-fusion reproductive hybrid rice plants with almost no affected fruiting rate, and achieved a significant improvement in the fruiting rate of the non-fusion reproductive technology system.

Hybrid rice breeding gives full play to the potential of hybrid advantages, achieves high and stable rice yield, and provides an important guarantee for world food security. Hybrid rice offspring seeds undergo heterogeneous phenotypic separation and can no longer be used in agricultural production, requiring annual tedious reproduction of new hybrid F1 generation seeds. Hybrid seeds are highly susceptible to external environmental influences, high production costs, low seed yields and very expensive prices. Therefore, how to achieve hybrid rice retention has always been regarded as the highest goal of hybrid breeding.

In nature, there is an asexual reproduction method that is clonal and reproduced through seeds – non-fusion reproduction. The introduction of this reproductive method into hybrid crops is expected to achieve self-preservation of hybrid crops. In 2019, American scientists and Wang Kejian’s team established artificial fusion-free reproductive systems in conventional rice and hybrid rice, respectively, and obtained cloned seeds of rice for the first time, providing the possibility for hybrid rice to be retained for self-seeding. However, both artificial non-fusion reproductive systems have the problems of low fruiting rate and low proportion of cloned seeds, which limit the application of this technology in crops.

This latest study systematically studies the three homologous genes BBM2, BBM3 and BBM4 of the rice parthenogenesis-inducible gene BBM1, and explores the potential of related genes to induce parthenogenesis. Arabidopsis egg cell-specific promoter pDD45 was used to drive BBM2, BBM3 and BBM4, respectively, and the plants EE-BBM2, EE-BBM3 and EE-BBM4 expressed ectopic in rice egg cells of these three genes were obtained. Then, the offspring of BBMs ectopic expression plants were analyzed with the help of molecular marker technology and flow cytometry, and it was found that BBM4 egg cell ectopic expression plants could induce parthenogenesis, and the haploid induction rate was 3.2%.

The above figure shows the comparison of BBM4 ectopic expression plants with wild-type CY84 phenotypes.

The figure below shows the comparison of diploid offspring and post-haploid representative types of parthenogenetic plants. Photo courtesy of Chinese Academy of Agricultural Sciences

Subsequently, the researchers tested the potential of BBM4 in a non-fusion reproductive system. They successfully obtained Fix2 plants that can undergo fusion-free reproduction in hybrid rice, which not only performed normally in the vegetative growth stage, but also had a setting rate of 80.9~86.1%, which was similar to the normal hybrid rice setting rate of 82.1~86.6%. By cytoploidy assay, plants with cell ploidy diploid and genotype consistent with their parents were obtained in their offspring. The phenotype of these cloned plants was also highly similar to that of wild-type hybrid rice, while maintaining a high setting rate of 80.9~82.0%.

At the same time, although the setting rate of the Fix2 strategy in this study was not affected by the planting conditions, the proportion of cloned seeds was low (1.7%). In the future, by further combining to construct high fruiting rate and high cloning seed induction rate, it is expected to promote the application of hybrid rice’s fusion-free reproductive system.

Comparison of the first generation (top) and second generation (bottom) of Fix2 (bottom) of non-fusion reproductive hybrid rice with the wild-type CY84 phenotype. Photo courtesy of Chinese Academy of Agricultural Sciences

On December 27, 2022, Nature Communications, a team of French and American scientists, published the results of research on efficient hybrid rice without fusion reproduction. By constructing BBM1 and MiMe from previous studies on a vector, a fusion-free germ system with a cloned seed ratio of more than 90% in the hybrid rice BRS-CIRAD 302 variety was achieved. However, the setting rate was still affected to varying degrees under greenhouse planting conditions (the setting rate of non-fusion reproductive rice was 27.0~35.5%, compared with 44.5% for control).

Dr. Wei Xin, assistant researcher Liu Chaolei, Dr. Chen Xi and Lu Hongwei Distinguished Associate Researcher of China Rice Research Institute are the co-first authors of the paper, and Wang Kejian is the corresponding author of the paper. The research was supported by the National Natural Science Foundation of China, the Youth Innovation Special Project of the Chinese Academy of Agricultural Sciences, and the Yazhou Bay Seed Laboratory of Hainan Province. (Source: Li Chen, China Science News)

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