More Efficient Hybrid Rice Breeding Using Female Sterility Technique

Research conducted by Hong Kong Baptist University (HKBU) involving the use of a pioneering female infertility technique has led to a breakthrough in the production of hybrid rice seeds. Compared to the “three-line” male-sterility technique commonly used in hybrid rice seed production, the new approach improves the efficiency of hybrid rice production by eliminating rice seeds that have been produced due to the self-pollination of the “restorative lineage”.

This new technique enables fully automatic harvesting of hybrid seeds by machines, which can significantly reduce harvesting costs. The research results were recently published in Cell search.

Male infertility technique has high harvesting costs

Self-pollinated plants are known to maintain their genomic homozygosity and therefore their offspring may have the same characteristics over generations.

Heterosis, which refers to the increase in growth rate due to heterozygosity of the genome resulting from the hybridization of distant relatives, is difficult to exploit with autogamous plants. In the wild, rice is usually raised by self-pollination.

However, in recent decades, scientists – following the pioneering work of Professor Yuan Longping, the “father of hybrid rice” – have developed hybrid rice breeding techniques by exploiting male sterile genes, and these techniques can produce hybrid seeds with the normally self-pollinating rice plants in large quantities. China and other countries around the world have widely used the male sterility technique to produce hybrid rice seeds, which has resulted in a substantial increase in rice yields.

The male-sterility technique first selects cultivars, ie plant varieties, of the “male-sterile line” of rice as pollen receptors. Rice cultivars from the “restorer line” with normal fertility act as pollen donors and are grown close to the “male sterile line” to facilitate pollen transfer for hybridization. However, self-pollinating seeds can also be produced by the “restoration line” and must be removed manually to avoid mixing them with the hybrid seeds before mechanical harvesting, resulting in high harvesting costs.

In theory, using sterile female rice as a “restorative line” is ideal because it cannot produce self-pollinating seeds. However, this approach has not been taken because female sterile rice germplasm remains extremely rare in nature and female sterile plants have difficulty self-reproducing.

TFS1 mutation presents female sterility

After nearly a decade of ongoing study, a research team led by Professor Zhang Jianhua, Full Professor of HKBU’s Department of Biology, successfully identified a mutation in the “spontaneous heat-sensitive female infertility 1” gene ( TFS1) in elite rice. cultivar during rice paddy production. This genetic mutation exhibits female sterility under normal or high temperature conditions (i.e. above 25°C), and fertility is partially resumed under low temperature conditions (i.e. above 25°C). i.e. 23°C). It does not present any defect in its vegetative growth.

The team observed that rice carrying the TFS1 gene mutation can produce healthy pollen with normal male fertility. Rice with normal fertility can produce normal seeds after receiving pollen from rice with the TFS1 gene mutation. Further investigations revealed that under normal or high temperature conditions, after the pollen landed on the rice stigma with the TFS1 gene mutation, the pollen tubes that developed from the pollen cannot enter into the embryo sac. Embryos therefore do not develop and seeds cannot be produced. But under low temperature conditions, the ability to fertilize and develop embryos is partially recovered.

Following genetic analysis using gene cloning and molecular techniques, the team discovered that the female infertility mutation is created by a point mutation in the gene region of Argonaute7 (AGO7), a member of the Argonaute protein complex (AGO) which is responsible for the production of many small interfering RNAs, namely tasiR-ARFs. Downregulation of these tasiR-ARFs regulates pollen tube entry into the embryo sac, but it failed under regular or high temperature conditions in rice with the TFS1 mutation, and therefore double fertilization cannot be obtained.

No need to remove “restoration lines” before harvest

To assess the potential for using TFS1 as a genetic tool for hybrid rice production, the team conducted field trials in Hong Kong and Hunan Province in mainland China. The TFS1 gene mutation was introduced into three rice cultivars by introgression and genome editing to create temperature-sensitive female-sterile germplasm. They acted as “restorative lines” for pollen donation. Three other male-sterile rice cultivars were used as “male-sterile lines”.

The team planted the “restorer lines” separately alongside the “sterile male lines” as in traditional hybrid breeding, or mixed them randomly on the farm when planting. In both planting arrangements, more than 30% of panicles of “male sterile lines” in Hong Kong and 40% in Hunan Province produced hybrid seed. The proportion of seed sets is similar to yields from hybrid production using existing “restoration lines”, but hybrid rice seed can be harvested without the removal of “restoration lines”.

Great market potential with reduced harvesting costs

Professor Zhang said, “Nowadays, hybrid rice seed production is still a labor-intensive process in agriculture. Female sterility, if it can be introduced into a “restoration line” as a pure pollen donor, has great potential to reduce costs, as the male and female parents of hybrid rice can be grown and harvested together by machines without worrying about the purity of the seeds.

“Our research results provide a suitable trait for fully mechanized hybrid rice breeding, and our genetic tool has shown great promise for commercial applications. lines.”