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A pair of floral regulators sets critical day length for Hd3a florigen expression in rice


The critical day length triggering photoperiodic flowering is set as an acute, accurate threshold in many short-day plants, including rice1,2. Here, we show that, unlike the Arabidopsis florigen gene FT3, the rice florigen gene Hd3a (Heading date 3a) is toggled by only a 30-min day-length reduction. Hd3a expression is induced by Ehd1 (Early heading date 1) expression when blue light coincides with the morning phase set by OsGIGANTEA(OsGI)-dependent circadian clocks. Ehd1 expression is repressed by both night breaks under short-day conditions and morning light signals under long-day conditions. Ghd7 (Grain number, plant height and heading date 7) was acutely induced when phytochrome signals coincided with a photosensitive phase set differently by distinct photoperiods and this induction repressed Ehd1 the next morning. Thus, two distinct gating mechanisms—of the floral promoter Ehd1 and the floral repressor Ghd7—could enable manipulation of slight differences in day length to control Hd3a transcription with a critical day-length threshold.

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Figure 1: Levels of Hd3a, Ehd1 and Ghd7 mRNA under various day-length conditions.
Figure 2: Analysis of Ehd1 expression.
Figure 3: Analysis of the effect of night breaks.
Figure 4: Analysis of gated expression of Ghd7.
Figure 5: Sustainability of Ghd7 activity in repressing Ehd1 expression the next morning.

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We thank N. Tanabe and K. Toyoshima for their assistance in plasmid construction and plant transformation. This work was supported by grants from the Ministry of Agriculture, Forestry and Fisheries of Japan (Genomics for Agricultural Innovation, GPN0001) to T.I. and from the Ministry of Education, Culture, Sports, Science and Technology Grant-in-Aid For Young Scientists (B) (20770040) to H.I.

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H.I. performed the majority of the experiments. Y.N. and M.Y. provided the NIL(Ghd7) plants. T.I. came up with the study design and organized this work. Both H.I. and T.I. wrote the manuscript.

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Correspondence to Takeshi Izawa.

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The authors declare no competing financial interests.

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Itoh, H., Nonoue, Y., Yano, M. et al. A pair of floral regulators sets critical day length for Hd3a florigen expression in rice. Nat Genet 42, 635–638 (2010).

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