General Introduction to the study of flowering time

The transition from vegetative to reproductive development represents one of the major phase changes during the life cycle of a plant. This transition is initiated by both endogenous (e.g. age and developmental phase) and environmental signals. The most important environmental signals that affect flowering time are those associated with the changing seasons; temperature and photoperiod, although other external stimuli such as light quality and nutrition can also play a role in particular locations. The complex interactions of endogenous and environmental stimuli act to maximise the reproductive success of a plant, by ensuring that flowering occurs only under conditions and at the time, that are favourable for fertilisation and seed formation (Coupland, 1995b) .

Investigation of flowering time

both enhances our knowledge of plant biology and potentially provides commercial benefits in increasing the yield of crop plants. In many crop species optimal yield is dependent upon the time of flowering, and the manipulation of this trait can provide commercial advantages. For example, loss of the flowering time associated photoperiod response in maize and winter wheat varieties are beneficial traits as they increase the range in which plants are able to grow (Paulsen, 1987; Paterson, 1995; Streck et al., 2003) .

During the floral transition, major changes occur at the shoot apical meristem (SAM). The SAM is a group of stem cells formed during embryogenesis. All aerial tissues of a plant, such as stems, leaves and flowers are derived from the SAM (Weigel and Jurgens, 2002) . The floral transition can be considered as a reprogramming of the meristem so that it forms flowers and reproductive organs rather than vegetative organs such as leaves (Howell, 1998).

The model plant Arabidopsis thaliana and its use in flowering-time studies

The use of molecular genetics in the model plant Arabidopsisthaliana has proved to be a powerful approach for studying the floral transition. Arabidopsis thaliana, also known as Mouse-ear or Thale cress, is a member of the Brassicaceae family, and was chosen as a model organism for many reasons.

The major advantages of Arabidopsis for molecular-genetic experiments are its small size, diploid nature, short generation time, ability to produce large numbers of seeds and a small genome (Schmidt, 1998) . The subsequent determination of the full genome sequence facilitated gene isolation (AGI, 2000) , and enabled reverse genetics approaches aimed at isolating mutations in every gene (Alonso et al., 2003).

Flowering time of Arabidopsis is influenced by many of the environmental stimuli that control flowering in other species, making it an ideal species in which to study the genetic control of these processes. Many accessions of Arabidopsis also show different flowering responses to environmental conditions. A combination of quantitative genetics and the isolation of induced mutations that delay or accelerate flowering has identified at least 80 genes involved in flowering-time control in Arabidopsis (Araki, 2001) .

Many of these genes have homologues in other plant species, and in some of these related roles in flowering time control have been demonstrated. The analysis of flowering time control in Arabidopsis therefore provides a starting point for understanding the regulation of flowering in other species (Mouradov et al., 2002; Hayama and Coupland, 2003).

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