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First published online January 25, 2008
doi: 10.1242/10.1242/dev.008631

Development 135, 767-774 (2008)
Published by The Company of Biologists 2008
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Hd3a and RFT1 are essential for flowering in rice

Reina Komiya, Akiko Ikegami*, Shojiro Tamaki, Shuji Yokoi{dagger} and Ko Shimamoto{ddagger}

Laboratory of Plant Molecular Genetics, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma 630-0101, Japan.


Figure 1
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  		Fig. 1. Expression of RFT1 is similar to Hd3a under SD and LD conditions. (A) Schematic diagram of RFT1 and Hd3a on chromosome 6. The physical distance between RFT1 and Hd3a is 11.5 kb. (B,C) Developmental expression analysis of RFT1 and Hd3a under SD (B) and LD (C) conditions. Expression of both RFT1 and Hd3a increased by 35 days after sowing (DAS), when reproductive transition occurred under SD conditions (B), but not under LD conditions (C). Leaves were collected from wild-type plants (ZT 0; zeitgeber time). (D) Diurnal expression of RFT1 35 DAS under SD conditions. RFT1 and Hd3a show diurnal expression patterns with a peak at ZT 0 (ZT 0=light on). (E,F) Expression analysis of RFT1 and Hd3a in hd1 mutants under SD conditions. Hd1 mutant was induced by Tos17, which was inserted in exon 1 of Hd1. Leaves were collected from wild-type or hd1 plants 35 DAS (ZT 0) under SD conditions.

 

Figure 2
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  		Fig. 2. RFT1 expression is localized in leaf vascular tissues. (A,B) Quantitative RT-PCR analysis of Hd3a and RFT1 expression in rice tissues under SD conditions. RFT1 and Hd3a were expressed in leaf blades (LB), but not in leaf sheaths (LS) or roots (R). Leaf blade, leaf sheaths and roots were collected from wild-type plants grown under SD conditions 35 DAS (ZT 0). (C) RFT1 promoter activity in RFT1::GUS transgenic plants. Transverse leaf section in RFT1::GUS plants 35 DAS under SD conditions (ZT 4). Scale bars: 20 µm.

 

Figure 3
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  		Fig. 3. Functional analysis of RFT1 by RNAi under SD conditions. (A) Hd3a expression in RFT1 RNAi, Hd3a RNAi and double RFT1-Hd3a RNAi plants under SD conditions. (B) RFT1 expression in RNAi plants under SD conditions. Leaves were sampled 35 DAS (ZT 0) under SD conditions. Expression of Hd3a and RFT1 was examined by real-time PCR. Black, wild-type; blue, RFT1 RNAi plants; red, Hd3a RNAi plants; green, double RFT1-Hd3a RNAi plants. (C) Flowering time of RNAi plants under SD conditions. (D) RNAi constructs. Gene-specific regions of Hd3a and RFT1 were used for the Hd3a and RFT1 RNAi constructs. 5'UTR of RFT1 and 3'UTR of Hd3a were used for the double RFT1-Hd3a constructs. (E) Growth of wild-type and RFT1-Hd3a RNAi plants. Ri 1-3, RFT1 RNAi plants; Hi 1-3, Hd3a RNAi plants; Di 1-2, double RFT1-Hd3a RNAi plants. Scale bars: 10 cm.

 

Figure 4
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  		Fig. 4. Expression of RFT1 in Hd3a RNAi plants under SD conditions. (A) Expression of Hd3a in Hd3a RNAi and wild-type plants. Hd3a expression was suppressed at all developmental stages in Hd3a RNAi plants. (B) Expression of RFT1 in Hd3a RNAi and wild-type plants. RFT1 transcription was very low in wild-type plants (black line). However, in Hd3a RNAi plants, RFT1 expression (green, blue and red lines) had increased 30 days before flowering, the stage required for promoting reproductive transition. Leaves were collected from wild-type plants and Hd3a RNAi plants at 35, 50, 70 and 90 DAS (ZT 0) under SD conditions.

 

Figure 5
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  		Fig. 5. Expression of OsMADS14, OsMADS15 and OsMADS50 in Hd3a RNAi and double RFT1-Hd3a RNAi plants. (A-C) Expression of OsMADS genes 35 and 70 DAS in Hd3a RNAi and double RFT1-Hd3a RNAi plants under SD conditions. Leaves were collected from Hd3a RNAi plants at 35 and 70 DAS, or double RFT1-Hd3a RNAi plants at 35, 70 and 120 DAS (ZT 0), under SD conditions. Expression of OsMADS14 (A) and OsMADS15 (B), rice orthologues of Arabidopsis AP1, was suppressed at 35 DAS, but increased at 70 DAS, when RFT1 expression was increased in Hd3a RNAi plants. In double RFT1-Hd3a RNAi plants, expression of OsMADS14 (A) and OsMADS15 (B) was suppressed at 35, 70 and 120 DAS. Expression of OsMADS50 (C), a SOC1 orthologue, was not suppressed in Hd3a RNAi and double RFT1-Hd3a RNAi plants. Hi 1-3, Hd3a RNAi plants; Di 1-2, double RFT1-Hd3a RNAi plants.

 

Figure 6
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  		Fig. 6. Histone modifications at the RFT1 locus in Hd3a RNAi plants. (A) Schematic diagram of RFT1. I through VII represent the regions in which H3K9 acetylation was examined by ChIP. The translation initiation site is +1. Filled boxes represent exons. (B,C) ChIP analysis of H3K9 acetylation levels of RFT1. Relative levels of H3K9 acetylation at various regions of the RFT1 locus were assayed at 35 DAS (B) and 70 DAS (C) in Hd3a RNAi and wild-type plants under SD conditions. Black, wild-type plants; white, Hd3a RNAi plants.

 

Figure 7
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  		Fig. 7. A model for photoperiodic flowering in rice. In wild-type plants, Hd3a promotes transition to the reproductive stage and induces the expression of OsMADS14 and OsMADS15, and flowers about 60 DAS under SD conditions. However, in Hd3a RNAi plants, RFT1 expression starts to increase during later stages, and induces OsMADS14 and OsMADS15 expression under SD conditions. When RFT1 expression is activated, H3K9 acetylation levels are high at the 5'UTR of RFT1. Double RFT1-Hd3a RNAi plants did not flower up to 300 DAS under SD conditions, suggesting that RFT1 and Hd3a are the key regulators of photoperiodic flowering under SD conditions.

 

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© The Company of Biologists Ltd 2008