In plants, seasonal changes in day length are perceived in leaves, which initiate long-distance signaling that induces flowering at the shoot apex. The identity of the long-distance signal has yet to be determined. In Arabidopsis , activation of FLOWERING LOCUS T ( FT ) transcription in leaf vascular tissue (phloem) induces flowering. We found that FT messenger RNA is required only transiently in the leaf. In addition, FT fusion proteins expressed specifically in phloem cells move to the apex and move long distances between grafted plants. Finally, we provide evidence that FT does not activate an intermediate messenger in leaves. We conclude that FT protein acts as a long-distance signal that induces Arabidopsis flowering.

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In order to test whether an increased export of carbohydrates by leaves and starch mobilization are critical for floral transition in Arabidopsis thaliana, the Columbia ecotype as well as its starchless mutant pgm and starch-in-excess mutant sex1 were investigated. Induction of flowering was achieved by exposure of plants to either one long day (LD) or one displaced short day (DSD). The following conclusions were drawn: (i) Both the pgm and sex1 mutants have a late-flowering phenotype in days shorter than 16 h. (ii) When inductive treatments cause a large, percentage of induced plants, there is always a large, early and transient increase in carbohydrate export from leaves. By contrast, when an inductive treatment results in only a low percentage of induced plants (pgm plants exposed to one DSD), the export of carbohydrates from leaves is not increased, supporting the idea that phloem carbohydrates have a critical function in floral transition. (iii) Starch mobilization is not required to obtain an increased carbohydrate export when induction is by one LD (extended period of photosynthesis), but is absolutely essential when induction is by one DSD (period of photosynthesis unaffected). (iv) Floral induction apparently increases the capability of the leaf phloem-loading system.

Understanding the complete picture of floral transition is still impaired by the fact that physiological studies mainly concern plant species whose genetics is poorly known, and vice versa. Arabidopsis thaliana has been successfully used to unravel signalling pathways by genetic and molecular approaches, but analyses are still required to determine the physiological signals involved in the control of floral transition. In this work, the putative role of cytokinins was investigated using vegetative plants of Arabidopsis (Columbia) induced to flower synchronously by a single 22 h long day. Cytokinins were analysed in leaf extracts, leaf phloem exudate and in the shoot apical meristem at different times during floral transition. It was found that, in both the leaf tissues and leaf exudate, isopentenyladenine forms of cytokinins increased from 16 h after the start of the long day. At 30 h, the shoot apical meristem of induced plants contained more isopentenyladenine and zeatin than vegetative controls. These cytokinin increases correlate well with the early events of floral transition.

The photoperiodic induction of flowering is a systemic process requiring translocation of a floral stimulus from the leaves to the shoot apical meristem. In response to this stimulus, the apical meristem stops producing leaves to initiate floral development; this switch in morphogenesis involves a change in the identity of the primordia initiated and in phyllotaxis. The physiological study of the floral transition has led to the identification of several putative floral signals such as sucrose, cytokinins, gibberellins, and reduced N-compounds that are translocated in the phloem sap from leaves to the shoot apical meristem. On the other hand, the genetic approach developed more recently in Arabidopsis thaliana allowed the discovery of many genes that control flowering time. These genes function in 'cascades' within four promotive pathways, the 'photoperiodic', 'autonomous', 'vernalization', and 'gibberellin' pathways, which all converge on the 'integrator' genes SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FLOWERING LOCUS T (FT). Recently, several studies have highlighted a role for a product of FT as a component of the floral stimulus or 'florigen'. These recent advances and the proposed mode of action of FT are discussed here.

A system of one-shot induction of flowering in Arabidopsis thaliana, ecotype Columbia, is described. Plants from vernalized seeds are grown for 2 months in 8 h short days at an irradiance of 48 mumol m-2 sec-1 (fluorescent light only). At that age they can be induced to flower by exposure to either a single long day or a single displaced short day. Non-induced plants stay vegetative for at least a further month. Synchrony of induction among the individuals of the population exposed to one long day is of the same order as in the best classical model plants, that is, the fastest individuals are only 6 h ahead of the slowest ones. A further advantage of this system is the large size of plants at the time of induction, allowing easy analysis of changes in leaves, leaf exudate and shoot meristem. The design of such a synchronous system will allow the timings of gene activations and deactivations to be established in the different plant parts, before flowers are initiated.

In many plants the transition from vegetative growth to flowering is controlled by environmental cues. One of these cues is day length or photoperiod, which synchronizes flowering of many species with the changing seasons. Recently, advances have been made in understanding the molecular mechanisms that confer photoperiodic control of flowering and, in particular, how inductive events occurring in the leaf, where photoperiod is perceived, are linked to floral evocation that takes place at the shoot apical meristem. We discuss recent data obtained using molecular genetic approaches on the function of regulatory proteins that control flowering time in Arabidopsis thaliana . These data are compared with the results of physiological analyses of the floral transition, which were performed in a range of species and directed towards identification of the transmitted floral singals.

Background: Arabidopsis thaliana is now the model organism for genetic and molecular plant studies, but growing conditions may still impair the significance and reproducibility of the experimental strategies developed. Besides the use of phytotronic cabinets, controlling plant nutrition may be critical and could be achieved in hydroponics. The availability of such a system would also greatly facilitate studies dealing with root development. However, because of its small size and rosette growth habit, Arabidopsis is hardly grown in standard hydroponic devices and the systems described in the last years are still difficult to transpose at a large scale. Our aim was to design and optimize an up-scalable device that would be adaptable to any experimental conditions.