The Gibberellin Pathway

Gibberellins are a class of plant growth regulators that affect many aspects of plant development (Ross et al., 1999). Gibberellins promote the flowering of many long-day plants, including Arabidopsis (Lang, 1965; Zeevaart, 1983; Putterill et al., 1995; figure 1.1). Under short days, the application of exogenous gibberellic acid (GA) leads to a dramatic promotion of flowering in Arabidopsis (Langridge, 1957).

The GIBBERELLIN RESPONSIVE 1 (GA1) gene encodes the ent-copalyl diphosphate synthase enzyme, which catalyses an early step in GA biosynthesis (Koornneef and van der Veen, 1980; Sun and Kamiya, 1994) . The ga1-3 mutant has severely reduced levels of GA, and was reported to never flower when grown under short days (Wilson et al., 1992; Reeves and Coupland, 2001) . Flowering in this mutant is slightly later than wild-type under long days, suggesting that there is redundancy between the photoperiod and gibberellin pathways, and that the non-flowering of ga1-3 mutants under short days is due to the inactivity of both pathways (Koornneef and van der Veen, 1980; Koornneef M et al., 1985; Wilson et al., 1992; Reeves and Coupland, 2001) . The double mutant co-2 ga1-3, which combines mutations that impair the photoperiod and gibberellin pathways, often does not flower at all (Putterill et al., 1995) . In contrast to mutants with reduced GA, spindly (spy) mutants exhibit a constitutive GA response and are early flowering (Jacobsen and Olszewski, 1993) .

Gibberellins role in vernalisation

Although GA has been implicated in vernalisation (Dennis et al., 1998) , ga1-3 mutants are still responsive to vernalisation (Chandler et al., 2000; Reeves and Coupland, 2001). This suggests that gibberellins do not play a central role in the vernalisation response.

DELLA proteins

GA acts, in part through destabilisation of a family of related proteins known as DELLA proteins. There are 5 known DELLA proteins in Arabidopsis (Fleck and Harberd, 2002) . Three of these are involved in GA response: GIBBERELLIC ACID INSENSTIVE (GAI), REPRESSOR OF GA1-3 (RGA) and RGA-LIKE1 (RGL1) (Koornneef, 1985; Wilson et al, 1992; Fleck and Harberd, 2002). Double mutant analysis demonstrated that the lateflowering caused by reduced GA biosynthesis in ga1-3 mutants was partly a result of active repression of GA responses by GAI and RGA (King et al., 2001) .

Gibberellins act upon genes that play key roles in the integration of Arabidopsis flowering pathways. Expression of SOC1 is increased under short days on addition of exogenous GA (Moon et al., 2003) . SOC1 expression is also increased by the photoperiod, autonomous and vernalisation pathways (Hepworth et al., 2002; Moon et al., 2003) . Additionally GA is involved in increasing the expression of the meristem identity gene LFY (Blazquez et al., 1998; Nilsson et al., 1998) . The role of LFY and SOC1 in the integration of flowering time pathways is discussed later.

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