Regulation of flowering time in Arabidopsis

 

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Flowering pathway

Gibberellin pathway

References

 

The Gibberellin transduction pathway

 

GA1 and the GA biosynthesis pathway

 

GA biosynthesis, an overview:

Several reviews focus on GA biosynthesis and catabolism: Hedden and Kamiya, 1997; Lange, 1998; Hedden and Proebsting, 1999; Kamiya and García-Martínez, 1999Yamaguchi and Kamiya, 2000; Hedden and Phillips, 2000; Olszewski et al., 2002; Fleet and Sun, 2005.

The gibberellin biosynthesis pathway is well characterized. The 2 last steps of the biosynthesis produce the bioactive GAs; GA4 and GA1 (Olszewski et al., 2002). Some examples of GA biosynthesis mutants are ga1, ga4, ga5 (Koorneef and van der Veen, 1980, Talon et al., 1990).

 

 

GA1 expression:

In the WT, GA1 expression is very low (Sun et al., 1992). Over-expression of GA1 (under regulation by the 35S promoter) gives no particular phenotype (Sun and Kamiya, 1994).

Transcriptional GUS fusions (Meier et al., 2001) show that GA5 expression is localized,  in young seedlings, in the region between hypocotyl and root, and the rim of the cotyledons. In older plants, it is expressed in the rosette, at the leaf petiole base, leaf rim, hydathodes and abaxial leaf midrib (Meier et al., 2001). The pattern of expression is mostly the same for GA1, but GA1 transcripts are present in all organs of the plant.(Silverstone et al., 1997(2)).

GAs induce flowering and are particularly necessary in short days:

The ga1 mutants were first characterized by Koornneef and van der Veen (1980). The ga1-2, ga1-3 and ga1-4 mutants were obtained by fast-neutrons mutagenesis. The ga1-1, ga1-6, ga1-7, ga1-8 and ga1-9 mutants were obtained by chemical mutagenesis (methane-sulfonate).

The ga1 mutants flower very late in short days, unless exogenous GAs are added. Flowering is also delayed in long days, but to a lesser extent (Koornneef and van der Veen, 1980; Wilson et al., 1992; Sun et al., 1992). In continuous light, ga1-3 mutants flower at the same time as the WT (Wilson et al., 1992), and ga1-2 mutants flower like the WT in very long photoperiods (Jacobsen and Olszewski, 1993).

ga1 is a GA biosynthesis mutant:

In the ga1 mutants, the gibberellin biosynthesis pathway is inactivated. As a result, these mutants are deficient in bioactive GAs. These mutants are late-flowering in long days and extremely late-flowering in short days. However, these mutants are still able to respond to GAs, as the mutant phenotype is suppressed when exogenous GAs are applied. The ga1-3 mutant highlights the importance of the GA pathway in short days (Wilson et al., 1992; Blazquez et al., 1997; Silverstone et al., 1997(2); Moon et al., 2003; Koornneef and van der Veen, 1980).

However, GAs have a role in other development pathways than flowering time regulation, and the ga1 mutants show additional mutant phenotypes such as reduced size, dark green leaves, low fertility, low germination rate and altered flower development (Koornneef and van der Veen, 1980).

The main GA biosynthesis mutant used in flowering time studies is ga1, so we are not going to detail any other mutants of GA biosynthesis here. The ga1 mutant is often used as a genotypic background to study the effects of mutations in the DELLA protein genes such as RGA and GAI (Cheng et al., 2004).

ga1 mutant alleles:

ga1 mutants contain low levels of GAs:

ga1 is a GA-biosynthesis mutant. The level of bioactive GAs was measured by gas chromatography in whole rosettes before flowering. It is as low as 1% of the WT levels in the ga1-3 mutants. However, as small amounts of GAs are actually present in the ga1-3 mutants, it shows that other genes are partially redundant with GA1 (Silverstone et al., 2001; Zeevaart and Talon, 1992). The ga1-2 mutants also have reduced levels of GAs (Jacobsen and Olszewski, 1993).

But ga1 mutants respond normally to GAs:

Exogenous GA treatment on ga1-3 mutants completely restores the WT phenotype, in short days as well as in long days (Koornneef and van der Veen, 1980; Wilson et al., 1992, Moon et al., 2003). It also restores other altered features of the ga1 mutant phenotype, such as the timing of trichome formation (Chien and Sussex, 1996).

The GA biosynthesis pathway is also necessary for the juvenile-adult phase transition:

  • ga1 mutants do not form adult leaves, characterized by the presence of abaxial trichomes. This is a sign that there is no juvenile to adult transition in these mutants, in short days as well as in long days (Chien and Sussex, 1996; Silverstone et al., 1997(2)).
  • Exogenous GA treatment restores the presence of abaxial trichomes on the leaves of ga1 mutants, as well as the flowering time (Silverstone et al., 1997(2)).

Interaction of the GA biosynthesis pathway with other genes and other pathways involved in flowering time regulation: