Circadian clock components in Arabidopsis III. LHY/CCA1/GI in regulating the floral integrator genes LFY/SOC1/FT to control flowering time and shoot architecture

LHY and CCA1 are clock components and TFL1 encodes a shoot-identity gene in Arabidopsis. When combined with tfl1, lhy cca1 results in precocious and ectopic expression of a floral integrator gene, LFY. We have shown that lhy cca1 and GI-ox cause early flowering and increase the expression of two floral integrator genes, FT and SOC1. FT and SOC1 are required for the early flowering of GI-ox under SD. Here we demonstrate that tfl1 dramatically reduced the generation time of GI-ox as well as lhy cca1 plants. tfl1 enhanced the precocious expression of the meristem-identity gene AP1 in GI-ox in a similar way to that in lhy cca1. However, tfl1 did not affect the mRNA levels of FT and SOC1 in lhy cca1 and GI-ox, suggesting that the additive phenotypes in lhy cca1 tfl1 (and GI-ox tfl1) are attributable to the concurrent up- regulation of three genes, FT/SOC1 and LFY. The terminal flower phenotype of tfl1 was enhanced by lhy cca1 and GI-ox under SD, suggesting that a proper balance between FT and TFL1 with antagonistic roles is important for the photoperiodic control of architecture in Arabidopsis. Our results indicate that GI mediates between the circadian clock and three floral integrator genes, FT, SOC1 and LFY, to control the photoperiodic flowering.