🌾 Scientists solve hybrid rice's biggest problem: seeds that lose their yield advantage

• Researchers have developed hybrid rice that produces genetically identical seeds generation after generation.
• The system achieves over 99 percent cloning efficiency with normal seed production.
• The technology could eliminate the need for farmers to buy new hybrid seeds every season.

Hybrid rice typically yields 20 to 30 percent more than conventional rice varieties thanks to a phenomenon called heterosis or hybrid vigor. The problem is that this advantage disappears in the next generation when the seeds are replanted. Farmers are therefore forced to buy new hybrid seeds every season at prices that can reach 200 yuan (about 28 US dollars) per kilogram in China and even higher in other countries.

A research team at China National Rice Research Institute has now developed hybrid rice that can reproduce itself almost perfectly through a process called apomixis. This means that seeds develop without fertilization and become genetically identical to the mother plant.

A sperm-specific gene is the key

The researchers analyzed 8,796 publicly available datasets to find genes that are specifically active in sperm cells. They identified a transcription factor that they named HUAXU after a mythological figure from Chinese folklore who according to legend became pregnant without male involvement.

When the researchers activated HUAXU in egg cells of hybrid rice, they were able to induce parthenogenesis, meaning embryo development without fertilization. This resulted in haploid offspring with exactly half the chromosome set.

Over 99 percent cloning efficiency

To create complete synthetic apomixis, the researchers combined the HUAXU system with a technique called MiMe that converts meiosis into mitosis. This allows the egg cells to retain the full chromosome set instead of being halved.

The result was the Fix8 series of hybrid rice lines. When the researchers tested the offspring from five different Fix8 lines, all 464 analyzed plants turned out to be diploid with the same genetic makeup as the mother plant. This means 100 percent cloning efficiency in these trials.

In larger population tests with over a thousand plants, Fix8 line number 3 achieved a cloning efficiency of 99.7 to 99.8 percent. The rare tetraploid plants that occurred could easily be identified and removed due to their distinctive shape.

Normal yield preserved

Previous attempts at synthetic apomixis have often resulted in severely reduced seed production. The Fix8 system solves this problem. Fix8 line number 3 showed seed production of 76.8 to 79.4 percent compared to 77.3 to 80.7 percent for conventional hybrid plants in parallel field trials. The difference was not statistically significant.

The plants also maintained normal height, tiller number, and panicle length across multiple generations. In field trials, the clonal Fix8 plants displayed uniform appearance without the variation normally seen in second-generation hybrid offspring.

Meets commercial requirements

In China, hybrid seeds must meet requirements of at least 97 percent purity to be sold commercially. The Fix8 system's efficiency of over 99 percent exceeds this requirement. The researchers suggest that the technology enables permanent fixation of hybrid vigor, which would eliminate the need to produce new hybrid seeds every season through costly crossing procedures.

The study is published as a preprint and has not yet undergone full scientific peer review. The research was funded by the National Natural Science Foundation of China and national research and development programs.

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