Clonal seed formation without fertilization — Yokohama City University discovers novel rice strain

A research group led by Assistant Professor Kaoru Tonosaki and Professor Tetsu Kinoshita of the Kihara Institute for Biological Research at Yokohama City University has announced the discovery of a rice strain that produces clonal seeds without fertilization. By modifying two genes encoding the proteins of the Polycomb repressive complex which are involved in histone modification (one of the regulatory mechanisms of gene expression in rice), the researchers found that embryos, endosperm-like tissues, or both could be induced simultaneously without fertilization. These results are expected to promote the apomixis induction technology in rice. (Apomixis means reproduction without fertilization, made possible by means of asexual seed formation.) The results were presented at the 144th Meeting of the Japanese Society of Breeding held at Kobe University from September 16 to 18.

There are 40 known genera and 400 known species of plants. Many plant species require fertilization for seed formation, whereas some plant species form clonal seeds of maternal origin. This mode of reproduction not requiring fertilization is known as apomixis.

Current seed production is based on hybrid vigor, which allows the easy inheritance of superior traits. Only the F1 progeny (hybrid generation 1), obtained from the cross between the paternal and maternal strains, is available for sale. As recombination occurs in the next generation, similar traits cannot be obtained in the F1 progeny from crossing.

Since the introduction of apomixis into F1 would considerably reduce the labor involved in seed production and breeding, researchers have long been studying asexual seed formation. However, the underlying mechanism has yet to be clarified and its introduction into breeding has not been realized. While studying the molecular mechanisms of seed development, the research group had shown that the Polycomb repressive complexes are crucial for seed development. In 2000, they had observed the enlargement of unfertilized ovaries due to the loss of function of a factor in the Polycomb repressive complex. The Polycomb repressive complex comprises several protein complexes that have been reported to repress the function of target genes through histone modifications.

In this study, the research group conducted an experiment that disrupted the function of two proteins that made up the Polycomb repressive complex. Then, the researchers used hot-water treatment to kill the pollen and continued observing the ovaries. Consequently, enlarged ovaries appeared in approximately half of the flowers. Embryos or endosperm-like tissue formed in the enlarged ovaries, Formation of both of these was also observed in a low percentage, about 10% of them. Such variations are common in the suppression of epigenetic gene expression by means of histone modifications. The ploidy (i.e., the number of chromosome sets) of these tissues was examined and found to be of maternal origin. However, even with both, there was no germination potential.

Tonosaki commented, "In the future, we hope to unravel the mechanism by which embryos and endosperm-like tissues are induced. By discovering important genes involved in this process, we hope to contribute to future rice breeding by developing the apomixis induction technology in rice."

Journal Information
Publication: The Plant Cell
Title: Mutation of the imprinted gene OsEMF2a induces autonomous endosperm development and delayed cellularization in rice
DOI: 10.1093/plcell/koaa006