Biochemical Signalling to the Shoot Apical Meristem

Phytohormones influence many diverse developmental processes ranging from seed germination to root, shoot and flower formation. Mutational analysis using Arabidopsis has been instrumental in determining the individual components of specific hormone signal transduction pathways. While no hormone transduction pathway is completely understood, the genes identified to date suggest that simple molecular rules can be established to explain how plant hormone signals are transduced (McCourt, 1999; Hay et al., 2004).

Gibberellins

Floral initiation and floral-organ development are both regulated by the phyto-hormone GA in some but not all species. GA is diffusible and accelerates flowering in wild type particularly in SDs and thus would be a potential candidate for a flowering signal. However, because it is synthesized throughout the plant, it is unlikely to serve as the long-sought after universal and primary longdistance flowering signal (Colasanti and Sundaresan, 2000). Moreover, the fact that GA mutants can still flower in response to photoperiod implies that the daylength pathway must use signals additional to GA. Nevertheless the role for GA as a major signal in the floral process is indisputable. GA control of flowering has been studied in many species, most importantly in Arabidopsis. This is crucial for understanding and connecting the hormone signalling pathways to the well-established floral genetic development routes. Additionally, research in the grasses highlights the importance of the GA signal.

Gibberellins promote flowering of Arabidopsis by activating the LFY promoter

Arabidopsis mutants defective in GA biosynthesis or signalling have demonstrated that endogenous GAs are involved in the promotion of flowering, although the requirement for GAs in SDs is more critical (Jacobsen and Ols-zewski, 1993). It has previously been shown that LFY promoter activity is reduced in mutants defective in GA biosynthesis and that the failure of ga1-3 mutants to flower in SDs can be overcome by constitutive expression of LFY. Conversely, constitutive GA signalling in SPINDLY (SPY) mutants causes an increase in LFY promoter activity. GA thus affects flowering through a pathway that controls LFY transcription (Fig. 3.2A) (Blazquez et al., 1998). The

Biochemical Signaling Flowering

Critical photoperiod

GA5+

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