Hybrid Rice Production Revolutionized by Apomixis

Apomixis refers to a type of asexual reproduction that allows hybrid rice to produce seeds without sexual reproduction. A recent study led by Professor Li Jiayang at the Institute of Genetics and Developmental Biology (IGDB), part of the Chinese Academy of Sciences, has introduced a new synthetic apomixis system utilizing the rice gene OsWUS. This system enables the rice to achieve normal seed-setting rates while allowing for effective seed production.

The results are published in Plant Communications. The development and application of hybrid rice have significantly boosted grain yields and contributed to food security in China. However, hybrid rice cannot be saved for future planting like traditional rice varieties, primarily due to genetic recombination and trait segregation. This limitation leads to expensive and labor-intensive annual seed production, forcing farmers to buy new seeds each year.

Previous synthetic apomixis efforts often drew from plants that naturally exhibit apomixis. In this study, researchers found that by regulating the rice gene OsWUS, they could induce apomixis, allowing for clonal seed production without disrupting seed-setting rates.

OsWUS, also known as MOC3, is essential for the growth of tiller buds. Earlier investigations by Li's team revealed that mutations in this gene hinder the formation of tiller buds and lead to fewer tillers. Moreover, overexpressing WUS has been shown to promote somatic embryogenesis and shoot regeneration in species such as Arabidopsis.

Working with the Yazhouwan National Laboratory and the China National Rice Research Institute, Li's team designed an ectopic expression vector for OsWUS, controlled by the Arabidopsis egg cell-specific promoter pAtDD45, and introduced it into the hybrid rice variety Chunyou 84 (CY84).

During the transformation process, initial attempts using only OsWUS presented challenges, including poor plant regeneration, dwarfism, and sterility. However, by integrating ectopic expression of OsWUS with the MiMe strategy (using Mitosis instead of Meiosis), the researchers successfully generated lines named Fix3 that displayed normal growth and development.

Confocal microscopy analysis indicated that about 20% of emasculated Fix3 ovules developed embryos independently of fertilization, suggesting that the specific expression of OsWUS in egg cells can partially substitute for fertilization and promote parthenogenesis.

The Fix3 lines demonstrated agronomic characteristics akin to those of wild-type hybrid rice, maintaining standard seed-setting rates and achieving a clonal seed efficiency of around 21.7%. Further studies showed that the offspring from these clonal plants exhibited traits closely resembling those of wild-type hybrid rice, ensuring consistent seed-setting rates.

The Fix3 apomixis system based on OsWUS showed superior clonal seed efficiency compared to the earlier Fix2 system that used OsBBM4, highlighting its potential for practical application. However, in comparison to apomixis systems involving OsBBM1 and dandelion PAR genes, the cloning efficiency of the OsWUS-based system still requires enhancement.

In the future, exploring additional egg cell-specific promoters or integrating more endogenous genes that do not impact seed-setting rates could further boost cloning efficiency while ensuring normal seed yields. Additionally, since there are homologs of OsWUS in several crops, this innovative system might also be applicable to other hybrid crops, expanding the benefits of hybrid vigor.