The popularization of hybrid rice has increased the yield of rice per unit area in the mainland by about 20%, which has greatly ensured the food security of the mainland. The traditional hybrid rice seed production process mainly includes interval transplanting of the paternal parent (restorer line) and female parent (sterile line), manual pollination, manual removal of the paternal parent in advance, and harvesting of hybrid seeds. Therefore, the traditional hybrid rice seed production process is highly dependent on manual labor, and the cost of seed production is high. In addition, about 150 million kg of male seed is lost annually due to the early removal of the paternal parent during seed production. With the rising labor cost, the cost of hybrid rice seed production is also increasing, which also increases the cost of farmers' seed use and reduces the economic benefits of hybrid rice planting. Therefore, it is urgent to develop a mechanized seed production method suitable for hybrid rice to realize the industrial upgrading of hybrid rice seed production.
Mixed planting of sterile lines and restorer lines, taking advantage of the difference in seed size between sterile lines and restorer lines, and mechanical sorting of hybrid seeds and restorer line seeds through a sieve is a very promising and simple method. There are only a few attempts to breed small-grain sterile lines by traditional breeding methods, and combine them with large-grain restorer lines for mechanical sorting of rice hybrid seeds, but they have not been applied to hybrid rice seed production on a large scale. It is also difficult for traditional breeding methods to achieve rapid improvement of existing varieties to adapt to the mechanized seed production of hybrid rice. The difficulty and core of mechanized hybrid seed production using grain size regulation genes is to find the ideal grain size regulating genes and cultivate ideal small-grain sterile lines. Loss-of-function mutations in ideal grain size genes should significantly reduce grain size (especially grain thickness); The recessive mutation in this gene results in small grains, which affect the grain size through the maternal parent. It does not affect important agronomic traits such as plant height, spike type, flowering time, etc. At the same time, the small-grain sterile line could not have a negative impact on the number of F1 hybrids and the yield of hybrid rice. So far, although it has been found that the loss of function of many genes leads to smaller rice grains, it also negatively affects other agronomic traits. Therefore, it is urgent to find the ideal grain size regulating gene and cultivate small-grain sterile lines suitable for mechanized seed production.
In order to find the ideal grain size regulating genes to rapidly breed/improve new hybrid rice varieties suitable for mechanized seed production, the research group of Li Yunhai from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences, the team of Zhu Xudong and Wang Yuexing from the China Rice Research Institute, the team of Wang Dekai from Zhejiang Sci-Tech University and the team of Luo Yuehua from Hainan University worked together to improve the parents of Tianyou Huazhan to meet the needs of mechanized seed production. Tianyou Huazhan is an indica three-line high-yield and high-quality hybrid indica rice variety, which has been promoted and planted for more than 30 million mu, and its sterile line, maintenance line and restorer line are Tianfeng A, Tianfeng B and Huazhan respectively. The ideal small-grain maintainer line Xiaoxiao B (XQB) and the corresponding ideal small-grain sterile line Xiaoxiao A (XQA) were cultivated by crossing the small-grain rice variety XLG with Tianfeng B (the maintainer line of the three-line hybrid rice Tianyouhuazhan). At the same time, the large-grain rice variety Kuangsijiadi was crossed with Huazhan to breed a new large-grain restorer line Dahuazhan (DHZ), and the field experiments showed that the small A and large-grain restorer lines (Dahuazhan, DHZ) could realize the mechanized seed production of hybrid rice, and the number of hybrid seeds per unit area of the hybrid rice was increased by about 20%, and the yield of hybrid rice was not affected.
The researchers further cloned GSE3, the ideal micrograin gene in Petite A and Petite B. At the same time, the researchers conducted a large-scale mutagenesis screening for decades to find the ideal small-grain mutant to breed the ideal small-grain sterile line/maintainer line, and finally identified the mutant M238 with significantly smaller grains and increased grain number per panicle. Studies have shown that m238 is a new allele of GSE3. The loss of GSE3 function led to a significant decrease in grain size, a significant increase in tillering and grain number per panicle, while there was no significant difference in other agronomic traits. GSE3 encodes a GCN5-related N-acetyltransferase that affects histone acetylation levels. GSE3 can be recruited by the transcription factor GS2 to the promoter of co-regulated grain size genes, and the grain size can be regulated by affecting the histone acetylation status of the promoter region.
By gene editing the GSE3 genes of Tianyou Huazhan's sterile line Tianfeng A (TFA) and maintainer line Tianfeng B (TFB), the corresponding ideal small-grain sterile lines TFAgse3-cri3 and TFBgse3-cri3 were created. Field experiments showed that the combination of TFAgse3-cri3 and large-grain restorer line (large-grain Huazhan, DHZ) could also realize the mechanized seed production of hybrid rice, and the number of hybrid seeds per unit area increased by 21.2%, and did not affect the yield of hybrid rice. Super hybrid rice Yliangyou 900 (YLY900) is an indica two-line hybrid rice variety with a yield of more than 15 tons per hectare, and its sterile line and restorer line Y58S and R900 are respectively. The researchers created the ideal small-grain sterile line Y58Sgse3-cri4 by gene-editing the sterile line Y58S of Liangyou 900, and the combination of Y58Sgse3-cri4 and the restorer line R900 could also realize the mechanized seed production of hybrid rice, and the number of hybrid seeds per unit area increased by 38.3%, which greatly improved the seed production efficiency and did not affect the yield of hybrid rice.
Therefore, this study found an ideal grain size regulator gene GSE3 and revealed the molecular mechanism of GSE3 regulating grain size. This paper proposes a strategy to rapidly improve the excellent hybrid rice parents in the current production and realize the mechanized seed production of hybrid rice: when the grain thickness difference between the restorer line and the male sterile line is relatively large, the mechanized seed production of hybrid rice can be realized by gene editing of the GSE3 gene in the male sterile line. When the difference in grain thickness between the restorer line and the male sterile line is relatively small, the mechanized seed production of hybrid rice can be realized by editing the GSE3 gene in the male sterile line and the GS2 gene or other macrograin genes in the restorer line. In addition, this research provides a convenient solution for mechanized seed production of other important crops.
The study was published online in the journal Nature Plants on June 3, 2024 (DOI: 10.1038/s41477-024-01720-0) with the title of "Modulation of histone acetylation enables fully mechanized hybrid rice breeding". Wang Yuexing, a researcher at the China Rice Research Institute, is the first author of the paper, and Li Yunhai, a researcher from the Institute of Genetics and Development, and Zhu Xudong, a researcher from the China Rice Research Institute, are the co-corresponding authors. Professor Wang Dekai of Zhejiang Sci-Tech University and Professor Luo Yuehua of Hainan University participated in the research. The research was supported by the National Key R&D Program of China, the Pilot Project of the Chinese Academy of Sciences, the Hainan Provincial Seed Industry Laboratory and the Hainan Provincial Key R&D Program.
Figure: Gene editing GSE3 rapidly improved hybrid rice to realize mechanized seed production of hybrid rice
a. Comparison of traditional seed production methods and new generation mechanized seed production methods. In the process of traditional seed production, restorer lines and sterile lines are generally planted at row ratio intervals of 1:6 to 1:10, and the restorer lines in the field need to be manually removed after pollination. The new generation of mechanized seed production method is to plant sterile lines and restorer lines together, harvest hybrid seeds and restorer lines of seeds, and mechanically separate hybrid seeds with sieves according to the difference in seed size of hybrid seeds and restorer lines. b, GSE3-GS2 module regulates grain size, GSE3 positively regulates grain size, GSE3 can be recruited by the transcription factor GS2 to the promoter of the co-regulated grain size gene, and the grain size can be regulated by affecting the histone acetylation state of the promoter region. c. GSE3 gene in gene-edited sterile lines or GS2 gene in gene-edited restorer lines or other large-grain genes can quickly realize mechanized seed production of hybrid rice.
Source: Authorized by Li Yunhai Laboratory