Fig. 4.

GI regulates growth by acting predominantly in the evening. (A) Waveforms of GI::LUC expression in the NILs in LDs of 16 h. (B) Pearson coefficient (R) of correlations between GI::LUC expression at different times of the day with hypocotyl length and PIF4 mRNA levels at ZT 20 h in the NILs. R indicates the strength of the correlations, with 1 and −1 indicating perfect positive and negative correlations, respectively. (C) A model for the regulation of GI expression by natural alleles during the day, and how this affects PIF4 expression and growth. Light signaling mediated by PHYB is repressed by the circadian clock in the morning of a long day. Clock repression is released later during the day, so that light activates GI expression until it reaches its peak in the evening. GI then contributes to the repression of PIF4 early in the night, so that growth is less efficiently promoted. Weak PHYB alleles cause less GI accumulation (blue line), more PIF4 transcription, and more growth. GI could hypothetically regulate PIF4 transcription by interacting with the EC, as represented by the dashed lines. Blue and red lines represent the GI waveform when influenced by weak or strong PHYB alleles, respectively. Numbers in italics indicate representative GI peak times.