Regulation of flowering time in Arabidopsis

 

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Flowering pathway

Gibberellin pathway

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The Gibberellin transduction pathway

 

Cross-talk between the Gibberellin response pathway and the autonomous pathway

 

FPA is a positive regulator of flowering that is part of the autonomous pathway. The fpa1-3 and fpa1-4 mutants are late-flowering (Reeves and Coupland, 2001). However, FPA is also positively regulated by GAs (Meier et al., 2001).

Plants carrying the LUC-FPA promoter fusion show increased LUC activity in the fpa1-3 and fpa1-4 backgrounds, and also have an apparent GA-related mutant phenotype. This indicates a possible crosstalk between the autonomous and GA pathways (Meier et al., 2001).

Seeds containing a GA5-LUC fusion were irradiated, then screened for plants with altered GA5 expression, as indicated by a higher or lower expression of LUC. Several mutants were isolated. Among them, two mutants showing a very high expression of GA5 were affected on the FPA gene (Meier et al., 2001).

GA signaling seems repressed in  fpa1-3 and fpa1-4:

Contrary to the WT, fpa1-3 and fpa1-4 do not respond to GA treatment. Besides, the  fpa1-4 germination is less affected than the WT by the GA biosynthesis inhibitor ancymidol. The fpa mutants seem therefore to be partially insensitive to GAs (Meier et al., 2001).

What is the effect observed on the GA biosynthesis pathway?: In the WT, GA treatment down-regulates GA5 expression in a negative feedback loop (Phillips et al., 1995; Xu et al., 1995). In the fpa1-3 mutant, GA5 expression is higher than in the WT, but its feedback regulation does not seem affected.

In the fpa1-3 mutant, the expression of GAI is lower than in the WT, which is consistent with the increase of GA5 expression. Meier et al. (2001) proposed that, in the fpa1-3 and fpa1-4 mutants, the high expression of GA5 might be indirect, and happen through FLC (Meier et al., 2001).

flf mutants may repress GA signaling through FLC:

FLC is a negative regulator of the GA activity in the apex (Michaels and Amasino, 1999; Sheldon et al., 1999). The flf mutants (they over-express FLC ) are insensitive to exogenous GAs (Sheldon et al., 1999). Moreover, the fpa1 mutant has increased FLC transcript levels, which is all consistent with the findings that fpa1-3 and fpa1-4 have low GA sensitivity. So, it may be possible that FLC levels (protein or gene expression) repress the GA signaling pathway (Meier et al., 2001).