In the WT, in short days as well as in long days, LFY expression progressively increases until it reaches a trigger level and flowering is induced (Blazquez et al., 2002). GA treatment has no major effect on LFY expression in the WT, as it increases irrespective of GA treatments (Blazquez et al., 1998).

The LFY promoter contains a putative GA-Responsive Element (GARE) that presents similarities with consensus binding sites for MYB transcription factors on animals. The deletion of this motif has no effect on LFY activity in long days, but makes LFY inactive in short days.

This suggests a mechanism involving a MYB transcription factor (such as MYB33) in the GA-dependent regulation of LFY activity (Blazquez and Weigel, 2000).

See: The GA transduction pathway regulates LFY through AtMYB33

GA1 is a positive regulator of LFY expression:

• LFY expression is lower in ga1-3 mutants: In long day conditions, the initial level of LFY expression is lower, and the increase of LFY expression is delayed compared to the WT (Wilson and Somerville, 1992; Blazquez  et al., 1998). In 9h days (which can be considered as short days), LFY levels progressively increase in the WT, but remain close to the basal level in ga1-3 mutants.
• GA treatment can restore the LFY expression profile in ga1-3 mutants, as well as the WT flowering time (Blazquez  et al., 1998).
• Inhibiting GA biosynthesis by Paclobutrazol mimics the effect of the ga1-3 mutation on LFY expression, and also inhibits flowering (Blazquez  et al., 1998).
• Over-expression of LFY under the control of the 35S promoter partially restores the WT flowering time in ga1-3 mutants, both in LD and SD. However, as the suppression of the ga1-3 phenotype is incomplete, this means that  the GA biosynthesis pathway not only regulates LFY expression, but also the competence to respond to the LFY signal (Blazquez  et al., 1998).

GA treatment up-regulates LFY expression:

In long days, FLY expression is reduced and delayed in the ga1-3 mutants compared to the WT. However, GA treatment significantly restores LFY expression to WT levels, and completely restores the WT flowering time. In short days, GA treatment also increases LFY expression in the WT, but  not in ga1-3 (Blazquez  et al., 1998).

The failure of ga1-3 mutants to flower in short days seems therefore due to the inability to up-regulate LFY expression.