FLOWERING LOCUS T (FT) is a conserved promoter of flowering that acts downstream of various regulatory pathways, including one that mediates photoperiodic induction through CONSTANS (CO), and is expressed in the vasculature of cotyledons and leaves. A bZIP transcription factor, FD, preferentially expressed in the shoot apex is required for FT to promote flowering. FD and FT are interdependent partners through protein interaction and act at the shoot apex to promote floral transition and to initiate floral development through transcriptional activation of a floral meristem identity gene, APETALA1 (AP1). FT may represent a long-distance signal in flowering.

Flowering in Arabidopsis is promoted via several interacting pathways. A photoperiod-dependent pathway relays signals from photoreceptors to a transcription factor gene, CONSTANS (CO), which activates downstream meristem identity genes such as LEAFY (LFY). FT, together with LFY, promotes flowering and is positively regulated by CO. Loss of FT causes delay in flowering, whereas overexpression of FT results in precocious flowering independent of CO or photoperiod. FT acts in part downstream of CO and mediates signals for flowering in an antagonistic manner with its homologous gene, TERMINAL FLOWER1 (TFL1).

Genetic studies, using floral homeotic mutants, have led to the ABC model of flower development. This model proposes that the combinatorial action of three sets of genes, the A, B and C function genes, specify the four floral organs (sepals, petals, stamens and carpels) in the concentric floral whorls. However, attempts to convert vegetative organs into floral organs by altering the expression of ABC genes have been unsuccessful. Here we show that the class B proteins of Arabidopsis, PISTILLATA (PI) and APETALA3 (AP3), interact with APETALA1 (AP1, a class A protein) and SEPALLATA3 (SEP3, previously AGL9), and with AGAMOUS (AG, a class C protein) through SEP3. We also show that vegetative leaves of triply transgenic plants, 35S::PI;35S::AP3;35S::AP1 or 35S::PI;35S::AP3;35S::SEP3, are transformed into petaloid organs and that those of 35S::PI;35S::AP3;35S::SEP3;35S::AG are transformed into staminoid organs. Our findings indicate that the formation of ternary and quaternary complexes of ABC proteins may be the molecular basis of the ABC model, and that the flower-specific expression of SEP3 restricts the action of the ABC genes to the flower.

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Distinctive from that of the animal system, the basic plan of the plant body is the continuous formation of a structural unit, composed of a stem with a meristem at the top and lateral organs continuously forming at the meristem. Therefore, mechanisms controlling the formation, maintenance, and development of a meristem will be a key to understanding the body plan of higher plants. Genetic analyses of filamentous flower (fil) mutants have indicated that FIL is required for the maintenance and growth of inflorescence and floral meristems, and of floral organs of Arabidopsis thaliana. FIL encodes a protein carrying a zinc finger and a HMG box-like domain, which is known to work as a transcription regulator. As expected, the FIL protein was shown to have a nuclear location. In situ hybridization clearly demonstrated that FIL is expressed only at the abaxial side of primordia of leaves and floral organs. Transgenic plants, ectopically expressing FIL, formed filament-like leaves with randomly arranged cells at the leaf margin. Our results indicate that cells at the abaxial side of the lateral organs are responsible for the normal development of the organs as well as for maintaining the activity of meristems.[Key Words: Arabidopsis; FIL; zinc finger; HMG; nuclear protein; abaxial-adaxial development]Received January 29, 1999; revised version accepted March 2, 1999.Differing from the situation in animal systems, maintenance of the meristem activity throughout life is a key for the patterning of the plant body. The shoot apical meristem and the meristem of the main root are formed in embryonic development and kept active after germination. During vegetative growth, leaves are continuously formed in a strictly controlled fashion from the shoot apical meristem. After the plant shifts to reproductive growth, the meristem converts to an inflorescence meristem, which in turn forms floral meristems. The floral meristem generates floral organs at predetermined positions. Genetic and molecular studies are under way to help understand the genetic regulatory system supporting meristem activity: control of cell division, formation of lateral organ primordia, and maintenance of the meristem structure. Recent studies using Arabidopsis, snapdragon, and maize have started to unveil the genetic basis of these molecular mechanisms (Okada and Shimura 1994).The leaf meristems are formed in a helical manner at the top of the inflorescence. When the leaf primordia begin to form, the petiole and flattened leaf blade are developed and the leaves show epinasty. Mature leaves show abaxial-adaxial polarity. The adaxial surface bears glossy, dark green epidermal cells, and produces many trichomes. On the contrary, the abaxial surface shows matte, gray-green epidermal cells and does not produce any trichomes. As the plant grows, the adaxial surface of the newly made leaves undergoes a gradual reduction in production of trichomes, and the abaxial surface starts to produce them. An Antirrhinum phantastica (phan) mutant produces filamentous leaves th...

The flowering time of plants is tightly regulated by both promotive and repressive factors. Molecular genetic studies using Arabidopsis have identified several epigenetic repressors that regulate flowering time. TERMINAL FLOWER2 ( TFL2 ), which encodes a homolog of HETEROCHROMATIN PROTEIN1, represses FLOWERING LOCUS T ( FT ) expression, which is induced by the activator CONSTANS (CO) in response to the long-day signal. Here, we show that TFL2 , CO , and FT are expressed together in leaf vascular tissues and that TFL2 represses FT expression continuously throughout development. Mutations in TFL2 derepress FT expression within the vascular tissues of leaves, resulting in daylength-independent early flowering. TFL2 can reduce FT expression even when CO is overexpressed. However, FT expression reaches a level sufficient for floral induction even in the presence of TFL2 , suggesting that TFL2 does not maintain FT in a silent state or inhibit it completely; rather, it counteracts the effect of CO on FT activation.

In Arabidopsis, several genetic pathways controlling the floral transition (flowering) are integrated at the transcriptional regulation of FT, LFY and SOC1. TSF is the closest homolog of FT in Arabidopsis. TSF expression was induced rapidly upon activation of CONSTANS (CO). The mRNA levels of TSF and FT showed similar patterns of diurnal oscillation and response to photoperiods: an evening peak, higher levels in long day (LD) than in short day (SD) conditions, and immediate up-regulation upon day-length extension. These observations suggest that TSF is a direct regulatory target of CO. tsf mutation delayed flowering in SD conditions and enhanced the phenotype of ft in both LD and SD conditions. TSF and FT also shared similar modes of regulation by FLC, an integrator of autonomous and vernalization pathways, and other factors such as EBS and PHYB. Consistently, TSF overexpression caused a precocious flowering phenotype independent of photoperiods or CO, or FLC. These observations suggest that TSF is a new member of the floral pathway integrators and promotes flowering largely redundantly with FT but makes a distinct contribution in SD conditions. TSF and FT seem to act independently of each other and of LFY, and partially upstream of SOC1. Interestingly, the expression patterns of TSF and FT in seedlings did not overlap, although both were expressed in the phloem tissues. Our work revealed additional complexity and spatial aspects of the regulatory network at the pathway integration level. We propose that the phloem is the site where multiple regulatory pathways are integrated at the transcriptional regulation of FT and TSF.

Vernalization is the process by which sensing a prolonged exposure to winter cold leads to competence to flower in the spring. In winter annual Arabidopsis thaliana accessions, flowering is suppressed in the fall by expression of the potent floral repressor FLOWERING LOCUS C (FLC). Vernalization promotes flowering via epigenetic repression of FLC. Repression is accompanied by a series of histone modifications of FLC chromatin that include dimethylation of histone H3 at Lys9 (H3K9) and Lys27 (H3K27). Here, we report that A. thaliana LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) is necessary to maintain the epigenetically repressed state of FLC upon return to warm conditions typical of spring. LHP1 is enriched at FLC chromatin after prolonged exposure to cold, and LHP1 activity is needed to maintain the increased levels of H3K9 dimethylation at FLC chromatin that are characteristic of the vernalized state.