Phytochrome B regulates Heading date 1 (Hd1)-mediated expression of rice florigen Hd3a and critical day length in rice
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Many plants require circadian clock and light information for the photoperiodic control of flowering. In Arabidopsis, a long-day plant (LDP), flowering is triggered by the circadian clock-controlled expression of CONSTANS (CO) and light stabilization of the CO protein to induce FT (FLOWERING LOCUS T). In rice, a short-day plant (SDP), the CO ortholog Heading date 1 (Hd1) regulates FT ortholog Hd3a, but regulation of Hd3a by Hd1 differs from that in Arabidopsis. Here, we report that phytochrome B (phyB)-mediated suppression of Hd3a is a primary cause of long-day suppression of flowering in rice, based on the three complementary discoveries. First, overexpression of Hd1 causes a delay in flowering under SD conditions and this effect requires phyB, suggesting that light modulates Hd1 control of Hd3a transcription. Second, a single extension of day length decreases Hd3a expression proportionately with the length of daylight. Third, Hd1 protein levels in Hd1-overexpressing plants are not altered in the presence of light. These results also suggest that phyB-mediated suppression of Hd3a expression is a component of the molecular mechanism for critical day length in rice.
KeywordsFlowering Florigen Oryza sativa Photoperiod Phytochrome B
We thank Drs. H. Tsuji and H. L. Wong for their critical reading of and suggestion for this manuscript, Dr. N. Inagaki for providing phyB mutants, and H. Shimizu, T. Watabe, J. Naritomi and M. Nobuhara for their technical assistance and production of transgenic plants. We are grateful for the gift of the GATEWAY-based transforming vectors from Dr. T. Nakagawa. This research was supported by Grants-in-Aid for Scientific Research on Priority Areas (Grant 10182102 to K.S.) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, and by the Rice Genome Programs (Grant IP-1006 to T.S. and M.T.). This work was also supported in part by Global COE Program in NAIST, MEXT, Japan. R.I. was supported by fellowships from the Japanese Society for the Promotion of Science during this study.
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