Regulation of flowering time in Arabidopsis



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Flowering pathway

Gibberellin pathway



The Gibberellin transduction pathway


cross-talk between the GA biosynthesis pathway and the photoperiod pathway


A paper by Blazquez et al. (2002) discusses the possible role of the clock in the Gibberellin (GA) biosynthesis pathway.

The toc1 mutation partially suppresses the ga1-3 phenotype:

In Long Days (LD), it accelerates flowering, and in Short Days (SD), it restores the ability to flower. In both cases, the toc1 ga1-3 mutants flower just a bit later than toc1 mutants. In toc1 ga1-3 and toc1 mutants, LFY expression increases fast (although it does not reach the levels it does in the WT prior to flowering). This means that LFY expression is also regulated by a mechanism independent of GAs (Blazquez et al., 2002).

However, when the same experiment is performed in 21h-cycles, the partial suppression by toc1-1 of the ga1-3 phenotype is not observed any more. As 21h is the duration of the clock endogenous period in the toc1-1 single mutant, this apparent suppression of the ga1-3 phenotype could actually be due only to the effects of TOC1 on the clock.

Also, the expression profiles of flower integrators such as CO, SOC1 and FT, are similar in the WT and toc1-1 mutant in LD and in 21h cycles, but not in SD (Blazquez et al., 2002).

There is some redundancy between the photoperiod and the GA transduction pathways in LD:

The late-flowering mutant phenotype of ga1-3 is enhanced in the double mutant co-2 ga1-3: this mutant does not flower at all in SD or in LD (Reeves and Coupland, 2001; See also: Putterill et al., 1995; Michaels and Amasino, 1999).

Possible response of the GA biosynthesis pathway to the light:

Besides being negatively regulated by the GAs, the GA20ox (GA5) and GA3ox (GA4) transcripts are also regulated by the light (Yamaguchi et al., 1998(2)). However, it is not mentioned if these changes actually affect the flowering time.

These observations could actually be a consequence of the interaction between GI and SPY.

It is also documented that light and photoperiod have an effect on GA biosynthesis and response, for example in germination (Yamaguchi et al., 1998). However, their effect on flowering time is not as well documented (See Kamiya and Garcia-Martinez, 1999).