Chinese Scientists Identify Gene to Mechanize Production of Hybrid Rice Seeds

Scientists in China have found a gene that could help fully mechanize the production of hybrid rice seeds which can bypass current time and labor-intensive manual methods that limit breeding efforts. Researchers found that by creating rice lines with both smaller and larger grain sizes, it became a simple matter of mechanically separating the hybrid seeds from the rest using a sieve.

In early June, the team from the Chinese Academy of Science, Hainan Seed Industry Laboratory, and China National Rice Research Institute, wrote a paper published in the peer-reviewed journal Nature Plants, stating, “Hybrid rice has achieved high grain yield and greatly contributes to food security, but the manual-labor-intensive hybrid seed production process limits fully mechanized hybrid rice breeding.”

“For next-generation hybrid seed production, the use of small-grain male sterile lines to mechanically separate small hybrid seeds from the mixed harvest is promising,” they added.

The challenge was searching for a gene that could enable a rice line to have a small grain size without reducing the seed number and yield. However, the research team discovered that it was possible with the “ideal grain-size gene” GSE3. Field tests showed that rice lines with the gene did not negatively impact hybrid seed numbers. In fact, it led to a 21 to 38% increase.

Pioneered by China, hybrid rice is rice bred from two genetically different parents, resulting in significantly higher rice yields. The initial high-yield commercial strains were created in China, with the country now being the world’s largest producer and consumer of hybrid rice.

“Rice yield has increased by 20 to 30% due to the use of hybrid rice in the past few decades, thereby contributing greatly to food security,” the team stated. Hybrid rice is the first generation offspring of a rice plant that is fertilized by the pollen of a genetically different rice plant and achieved by pollinating a sterile male line with a self-pollinating restorer line.

The team wrote in their paper, “The restorer line needs to be grown near the male sterile line in separate rows to provide enough pollen for hybridization. To avoid seed contamination, manual labor is used to remove the restorer line before harvesting hybrid seeds,” and added that around 150,000 tonnes of seed are wasted in China each year.

Many methods to plant a mix of the sterile and restorer lines and harvest them together before mechanically separating the seeds have been proposed but, according to the researchers, the methods have “inherent shortcomings.” The team said that there’s been a proposal to make the male sterile line smaller, which then would create smaller hybrid seeds, and making the restorer line bigger would actually allow for mechanical separation using a “simple sifter.”

The team of researchers crossed the Tianyouhuazhan (TYHZ) “super-hybrid rice” variety with others, including the small-grain Xiaoligeng variety. They discovered that crossing TYHZ with Xiaoligeng created a smaller male sterile line, which they then later identified was due to the GSE3 gene.

Called Xiaoqiao A (XQA), the team’s new male sterile line was bred with a large grain-size restorer line that they previously created and named Da huazhan (DHZ). The first-generation hybrid seeds produced from this cross were smaller, allowing them to be mechanically separated from larger seeds produced by the self-pollination of the restorer line.

The researchers found a sieve aperture width of under 2.08mm (0.08 inch), the purity of hybrid seeds sifted by their grain thickness was around 96% which meets the standards for commercial production. According to the paper, the traditional separation methods have a purity of 96 to 98%.

The team found that while the hybrid seed yield was lower, the seed number per plot was higher using the XQA and DHZ lines, which is vital as this “is a determinant of commercial hybrid seed production.”

“These field trials thus reveal that the ideal male sterile line (XQA) and the restorer line (DHZ) allow for fully mechanized hybrid rice breeding,” the team wrote. However, creating rice lines with the GSE3 gene still presents a challenge as conventional breeding approaches are also time and labor-intensive. To get around this, the research team developed a “one-step method to generate the loss-of-function mutants of GSE3” using the well-known gene-editing tool CRISPR-Cas9.

Aside from rice, the researchers said that GSE3 could also potentially be used in other crops to improve male sterile lines for mechanized hybrid seed production.