Analysis on Control Mechanism of Flowering of Rice
Florigen was proposed 75 years ago as an inducer of flowering transported from the leaf to the shoot apex. Florigen is a determinant factor in flowering of plants and it has long been expected to control flowering of plants at our will by taking control of florigen. Although every effort had been made to identify what florigen is, it remained a mystery. From our research on rice protein called Hd3a, it was indicated that after synthesized in the leaf Hd3a moves all the way to the shoot apex and then induces flowering(fig 1). Finally, we discovered that Hd3a protein is moleculer nature of florigen which had been unknown for long time.
Furthermore, we made researches for the purpose of explaining the mechanism of flowering by Hd3a. As a result, we found out that after reaching cells at the shoot apical meristem Hd3a interacts with 14-3-3 protein in cytoplasm first, then moves inside nucleus in the form of Hd3a-14-3-3 complex. Hd3a-14-3-3 complex with bZIP-type transcription factor, complex with bZIP-type, OsFD1, and starts expression of floral meristem identity genes.In conclusion, 14-3-3 protein is considered as an intracellar receptor of florigen Hd3a and a protein complex (Florigen Activation Complex, FAC) composed of Hd3a, 14-3-3 and OsFD1 is supposed to promote flowering.
Most of plants can be categorized as long-day plants whose flowering is promoted when day length is longer than certain level or as short-day plants whose flowering is promoted when length of day is shorter than certain level. Long-day plants and short-day plants show opposite reaction to lengtrh of day. Difference that causes this has been unknown. From our study using rice, we revealed that same as the long-day plant Arabiodopsis thaliana, the short- day plant rice has 3 key gene sets that work on the major pathway controlling flowering. Also, we found that the regulation of florigen gene was reversed between these two species.
1. Elucidation of Flowering Mechanism of Florigen
We evaluate how florigen(Hd3a protein) is transported all the way through the body of plants and how it starts flower bud formation after reaching the shoot apex. So far, we have elucidated that Hd3a and several proteins converged to form a complex to promote flowering. We are to elucidate the mechanism of florigen in inducing flowering making full use of such high technologies as proteomics based on mass spectrometry, bio-imaging and next-generation sequencer.
2. Development of Artificial Florigen
If applied to a plant as it is, it is impossible for florigen to permeate cell membrane and act, since florigen is high molecular weight protein. Therefore, in our study, by adding permeability to Hd3a protein we apply Hd3a to plants to make them flower at our will.
3. Difference between Short-day Plants and Long-day Plants
We analyze how reversal of signaling mechanism of long-day plants and short-day plants is controlled on molecular level.
4. Molecular Mechanism of Diversity of Flowering Time of Rice
Species of rice cultivated around the world show diversity of flowering time. We analyze and evaluate on molecular level how such diversity has been established.
Fig. 4
Chrysanthemum with expression of florigen of rice.
Although chysanthemum usually blooms out in autumn, this chrysanthemum boolms out althrough the year.
(Collaborative study with Dr. Asao of Nara pref.
Agricultural Comprehensive Center.)