Regulation of flowering time in Arabidopsis

 

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Flowering pathway

Gibberellin pathway

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The Gibberellin transduction pathway

 

Auto-regulation of GA biosynthesis by negative feedback

 

GAs down-regulate GA4 and GA5 expression:

A negative feedback loop regulates the GA biosynthesis pathway. This regulation affects the  transcription of GA20ox (GA5) and 3b-hydroxylase (GA4), on the 2 last steps in the biosynthesis of the bioactive GAs (Hedden and Kamiya, 1997; Cowling et al., 1998).

The levels of bioactive GAs the plant contains can be estimated by the level of GA4 expression. In the WT, GA4 expression is usually low. GA4 is up-regulated in case of GA deficiency, and down-regulated upon GA treatment. This system of feedback regulates the GA biosynthesis pathway (Dill et al., 2001).

 Bioactive GAs (such as GA4 , but not GA9) can down-regulate the expression of GA20ox and GA3ox (Chiang  et al., 1995; Phillips  et al., 1995; Xu  et al., 1995; Yamaguchi  et al., 1998(1); Cowling et al., 1998).

Plants carrying GA5-LUC fusions show that GA5 is negatively regulated by the GAs: In presence of the GA3 gibberellin, GA5 expression decreases by 50%. On the contrary, when treated with the GA biosynthesis inhibitor ancymidol, GA5 expression level doubles (Meier et al., 2001, see also Phillips et al., 1995; Xu  et al., 1995).

ga1 mutants over-express GA4 and GA5:

GA4 expression is higher in the ga1-3 background (in ga1-3  as well as in the rga-24 ga1-3 mutants) than in the WT. This shows the part of GA1 in this negative feedback regulation (Silverstone et al., 1998; Cowling et al., 1998; Silverstone et al., 2001; Dill and Sun, 2001).

The ga1-3 mutation does not alter the feedback regulation of GA biosynthesis:

In different mutants containing different combinations of ga1-3, rga-24 and gai-t6 mutations, GA4 levels are very low in case of GA treatment, and similar to the WT levels. This means that the feedback regulation of GA synthesis is not altered by the ga1-3 mutation (Cowling et al., 1998; Silverstone et al., 2001).

Although the GA4 expression is higher in the ga1-3 mutant than in the WT, it is not detectable in the leaky ga1-6 mutant. This means that moderate levels of GAs are still able to reduce GA4 expression in the ga1-6 mutant (Silverstone et al., 1998).

However, in both ga1-3 and ga1-6 mutants, GA treatment decreases GA4 expression, like it does in the WT. This means that the feedback regulation of GA biosynthesis is still functional in the absence of GA1 (Silverstone et al., 1998; Silverstone et al., 2001) and can be induced by the GAs themselves. However, as GA4 expression is higher in ga1-3 than in the WT, the negative feedback is weaker in the ga1-3 mutants than in the WT.

De-repressing the GA response pathway disrupts the negative feedback regulation:

When the GA response pathway is de-repressed (such as in rga gai-t6 mutants), both the GA levels and GA20ox and GA3ox transcript levels decrease (Dill and Sun, 2001; Silverstone et al., 2001).

On the contrary, when the repressors of the GA transduction pathway show gain-of-function mutations (such as gai-1), both GA levels and GA20ox and GA3ox expression levels are increased (Talon et al., 1990; Xu et al., 1995; ; Cowling et al., 1998; Peng et al., 1999 (1 and 2)).

Other genes interact with the GA biosynthesis pathway: