AGL24 acts in concert with SOC1 and FUL during Arabidopsis floral transition

Arabidopsis plants flower in response to long days (LDs). Exposure of leaves to inductive day lengths activates expression of FLOWERING LOCUS T (FT) protein which moves to the shoot apical meristem (SAM) to induce developmental reprogramming. SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) are induced by FT at the apex. We previously screened the SAM for mRNAs of genes required to promote the floral transition in response to photoperiod, and conducted detailed expression and functional analyses on several putative candidates. Here, we show that expression of AGAMOUS-LIKE 24 (AGL24) is detected at the SAM under SD conditions and increases upon exposure to LDs. Mutations in AGL24 further delay flowering of a soc1 ful double mutant, suggesting that flowering is controlled by AGL24 partly independently of SOC1 and FUL.     

Developmental physiology of floral initiation in Nicotiana tabacum L.

The central process in the making of a multicellular organismis the fating of cells and tissues for their terminal phenotypes.The formation of a flower from a shoot apical meristem completesa sequence of fating processes initiated in embryogenesis. Thefating of a vegetative meristem of Nicotiana tabacum L. to initiatea flower involves at least two signals and two developmentalstates. A signal from the roots maintains vegetative growth,or prevents flowering, in the young seedling. As the plant grows,the vegetative meristem gains greater competence to respondto the floral stimulus from the leaves until it is evoked, byfloral stimulus, into a florally determined state. The florallydetermined state is then expressed. These developmental processesnot only establish the time of floral initiation, but also regulateplant size as measured by the number of nodes produced. Key words: Plant size, floral stimulus, competence, floral determination, induction     

Regulatory networks that function to specify flower meristems require the function of homeobox genes PENNYWISE and POUND-FOOLISH in Arabidopsis

Flowering is a major developmental phase change that transforms the fate of the shoot apical meristem (SAM) from a leaf-bearing vegetative meristem to that of a flower-producing inflorescence meristem. In Arabidopsis, floral meristems are specified on the periphery of the inflorescence meristem by the combined activities of the FLOWERING LOCUS T (FT)–FD complex and the flower meristem identity gene, LEAFY ( LFY ). Two redundant functioning homeobox genes, PENNYWISE ( PNY ) and POUND-FOOLISH ( PNF ), which are expressed in the vegetative and inflorescence SAM, regulate patterning events during reproductive development, including floral specification. To determine the role of PNY and PNF in the floral specification network, we characterized the genetic relationship of these homeobox genes with LFY and FT . Results from this study demonstrate that LFY functions downstream of PNY and PNF. Ectopic expression of LFY promotes flower formation in pny pnf plants, while the flower specification activity of ectopic FT is severely attenuated. Genetic analysis shows that when mutations in pny and pnf genes are combined with lfy , a synergistic phenotype is displayed that significantly reduces floral specification and alters inflorescence patterning events. In conclusion, results from this study support a model in which PNY and PNF promote LFY expression during reproductive development. At the same time, the flower formation activity of FT is dependent upon the function of PNY and PNF.     

EMF genes interact with late-flowering genes in regulating floral initiation genes during shoot development in Arabidopsis thaliana

To investigate the mechanisms regulating the initiation of floral development in Arabidopsis, a construct containing beta-glucuronidase (GUS) gene driven by APETALA1 promoter (AP1::GUS) was introduced into emf fwa and emf ft double mutants. GUS activity was strongly detected on shoot meristem of emf1-1 single mutants harboring AP1::GUS construct just 5 d after germination. By contrast, GUS activity was undetectable on emf1-1 fwa-1, emf1-1 ft-1, emf2-1 fwa-1, emf2-3 fwa-1 and emf2-3 ft-1 double mutants harboring AP1::GUS construct 10 d after germination. GUS activity was only weakly detected on the apical meristem of 20-day-old emf1-1 fwa-1 and emf2-1 fwa-1 seedlings. During this time, only sessile leaves were produced. Further analysis indicated that AP1 was strongly expressed in 10-day-old emf1-1 and emf2-1 single mutants. Its expression was significantly reduced in all emf1-1 or emf2-1 late-flowering double mutants tested. Similar to AP1, the expression of LEAFY (LFY) was also high in emf1-1 and emf2-1 single mutants and reduced in emf1-1 or emf2-1 late-flowering double mutants. Our results indicate that the precocious expression of AP1 and LFY is dependent not only on the low EMF and FWA activities but also on the expression of most of the late-flowering genes such as FT, FCA, FE, CO and GI. These data also reveal that most late-flowering genes may function downstream of EMF or in pathways distinct from EMF to activate genes specified floral meristem identity during shoot maturation in Arabidopsis.     

Leaf-produced floral signals

Florigen is the hypothetical leaf-produced signal that induces floral initiation at the shoot apex. The nature of florigen has remained elusive for more than 70 years. But recent progress toward understanding the regulatory network for flowering in Arabidopsis has led to the suggestion that FLOWERING LOCUS T (FT) or its product is the mobile flower-inducing signal that moves from an induced leaf through the phloem to the shoot apex. In the past year, physical and chemical evidence has shown that it is FT protein, and not FT mRNA, that moves from induced leaves to the apical meristem. These results have established that FT is the main, if not the only, component of the universal florigen.     

Regulation of Flowering Time by MicroRNAs

The shoot apical meristem (SAM) continuously produces lateral organs in plants.Based on the identity of the lateral organs,the life cycle of a plant can be divided into two phases:vegetative and reproductive.The SAM produces leaves during the vegetative phase,whereas it gives rise to flowers in the reproductive phase (reviewed in Poethig,2003).The floral transition,namely the switch from vegetative to reproductive growth,is controlled by diverse endogenous and exogenous cues such as age,hormones,photoperiod,and temperature (reviewed in B(a)urle and Dean,2006;Srikanth and Schmid,2011;Andres and Coupland,2012). The model annual Arabidopsis thaliana has been extensively used for the dissection of the molecular mechanism underlying the floral transition during the last two decades.The molecular and genetic analyses have revealed five flowering time pathways,including age,autonomous,gibberellins (GAs),photoperiod and vernalization (reviewed in Amasino and Michaels,2010).Growing lines of evidence indicate that there are extensive crosstalks,feedback or feed-forward loops between the components within these pathways,and that these multiple floral inductive cues are integrated into a set of floral promoting MADS-box genes including APETALA 1 (AP1),SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1),FRUITFULL (FUL) and LEAFY (LFY) (Amasino and Michaels,2010;Lee and Lee,2010;Srikanth and Schmid,2011).     

Dissection of floral induction pathways using global expression analysis

Flowering of the reference plant Arabidopsis thaliana is controlled by several signaling pathways, which converge on a small set of genes that function as pathway integrators. We have analyzed the genomic response to one type of floral inductive signal, photoperiod, to dissect the function of several genes transducing this stimulus, including CONSTANS, thought to be the major output of the photoperiod pathway. Comparing the effects of CONSTANS with those of FLOWERING LOCUS T, which integrates inputs from CONSTANS and other floral inductive pathways, we find that expression profiles of shoot apices from plants with mutations in either gene are very similar. In contrast, a mutation in LEAFY, which also acts downstream of CONSTANS, has much more limited effects. Another pathway integrator, SUPPRESSOR OF OVEREXPRESSION OF CO 1, is responsive to acute induction by photoperiod even in the presence of the floral repressor encoded by FLOWERING LOCUS C. We have discovered a large group of potential floral repressors that are down-regulated upon photoperiodic induction. These include two AP2 domain-encoding genes that can repress flowering. The two paralogous genes, SCHLAFMUTZE and SCHNARCHZAPFEN, share a signature with partial complementarity to the miR172 microRNA, whose precursor we show to be induced upon flowering. These and related findings on SPL genes suggest that microRNAs play an important role in the regulation of flowering.     

The gentian orthologs of the FT/TFL1 gene family control floral initiation in Gentiana

Gentians are herbaceous perennials blooming in summer through autumn. Although they are popular ornamental flowers in Japan, the regulation of their timing of flowering has not been studied. We identified and characterized gentian orthologs of the Arabidopsis FT/TFL1 gene family to elucidate the mechanisms of flowering initiation. We isolated three gentian orthologs of FT and TFL1, denoted GtFT1, GtFT2 and GtTFL1. Since up-regulation of GtFT1 and GtFT2 as well as down-regulation of GtTFL1 promoted floral initiation in gentian plantlets, these genes affected floral initiation in a similar way to Arabidopsis FT and TFL1. The expression levels of GtFT1 and GtFT2 in leaves of late-flowering gentian increased prior to floral initiation, whereas GtTFL1 was highly expressed in shoot apical meristem at the vegetative stage and decreased drastically just before flowering initiation. Comparison of gene expression patterns showed that GtFT1 expression increased earlier in early-flowering than in late-flowering gentian, whereas the timing of the increase in GtFT2 expression was similar in early- and late-flowering plants. The GtTFL1 expression in early-flowering gentian was extremely low throughout the vegetative and reproductive stages. These results indicated that either the up-regulation of GtFT1 or the down-regulation of GtTFL1 may determine flowering time. Furthermore, we found that early-flowering but not late-flowering gentians have a 320 bp insertion in the promoter region of GtTFL1. Thus, the negligible expression of GtTFL1 in early-flowering lines may be due to this insertion, resulting in a shortened vegetative stage.     

Specification of reproductive meristems requires the combined function of SHOOT MERISTEMLESS and floral integrators FLOWERING LOCUS T and FD during Arabidopsis inflorescence development

In Arabidopsis floral meristems are specified on the periphery of the inflorescence meristem by the combined activities of the FLOWERING LOCUS T (FT)-FD complex and the flower meristem identity gene LEAFY. The floral specification activity of FT is dependent upon two related BELL1-like homeobox (BLH) genes PENNYWISE (PNY) and POUND-FOOLISH (PNF) which are required for floral evocation. PNY and PNF interact with a subset of KNOTTED1-LIKE homeobox proteins including SHOOT MERISTEMLESS (STM). Genetic analyses show that these BLH proteins function with STM to specify flowers and internodes during inflorescence development. In this study, experimental evidence demonstrates that the specification of flower and coflorescence meristems requires the combined activities of FT-FD and STM. FT and FD also regulate meristem maintenance during inflorescence development. In plants with reduced STM function, ectopic FT and FD promote the formation of axillary meristems during inflorescence development. Lastly, gene expression studies indicate that STM functions with FT-FD and AGAMOUS-LIKE 24 (AGL24)-SUPPRESSOR OF OVEREXPRESSION OF CONTANS1 (SOC1) complexes to up-regulate flower meristem identity genes during inflorescence development.     

Normal and Abnormal Development in the Arabidopsis Vegetative Shoot Apex

Vegetative development in the Arabidopsis shoot apex follows both sequential and repetitive steps. Early in development, the young vegetative meristem is flat and has a rectangular shape with bilateral symmetry. The first pair of leaf primordia is radially symmetrical and is initiated on opposite sides of the meristem. As development proceeds, the meristem changes first to a bilaterally symmetrical trapezoid and then to a radially symmetrical dome. Vegetative development from the domed meristem continues as leaves are initiated in a repetitive manner. Abnormal development of the vegetative shoot apex is described for a number of mutants. The mutants we describe fall into at least three classes: (1) lesions in the shoot apex that do not show an apparent alteration in the shoot apical meristem, (2) lesions in the apical meristem that also (directly or indirectly) alter leaf primordia, and (3) lesions in the apical meristem that alter meristem size and leaf number but not leaf morphology. These mutations provide tools both to genetically analyze vegetative development of the shoot apex and to learn how vegetative development influences floral development.