Dirr Arabidopsisthe Rosetta stone of flowering time? The differential expression of these genes was confirmed by real time quantitative RT-PCR Table 2and their upregulation in leaves was also observed data not shown. Molecular genetic studies using Arabidopsis have revealed that four major floral-promoting pathways — the photoperiod, vernalization, autonomous and gibberellin pathways — work in response to environmental and internal cues Simpson et al. A fime strain carrying an swi6- mutation Allshire et arabidopsiz. Arabidopsis, the Rosetta stone of flowering time?

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Gordon G. Simpson, et al. Science , ; DOI: If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.

Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www. Copyright by the American Association for the Advancement of Science; all rights reserved. Will the model developed for Arabidopsis unlock the complexities of flowering time control in all plants, as the Rosetta stone did for Egyptian hieroglyphics?

Multiple environmental and endogenous inputs regulate when plants? The molecular genetic dissection of? This framework provides the basis to understand the evolution of different reproductive strategies and how? T he major developmental transition in flowering plants is the switch from vegetative to reproductive development.

The correct timing of this transition is essential to maximize reproductive success given the requirement for synchronous flowering in out-crossing species and the dependence on favorable conditions for optimal seed set 1. Distinct reproductive strategies have evolved in different plant species. Many plants respond to environmental cues to control flowering time, particularly those that indicate seasonal change.

Arabidopsis flowering, for instance, is accelerated by conditions that reliably indicate the passage of winter and the onset of spring and summer, such as a long period of cold temperature, elevated ambient growth temperatures, and increasing day length. In contrast, flowering in rice is promoted by short days. Flowering is also promoted in response to stresses such as overcrowding perceived as changed light quality input , nutrient deficiency, heat, and drought.

In addition, endogenous signals regulate the floral transition. Many plants pass through a juvenile phase, in which flowering cannot occur, to ensure that sufficient reserves accumulate to sustain floral development and seed set. The multiple inputs converge to regulate the developmental fate of the shoot apical meristem. The capacity to adapt morphological development in response to environmental cues highlights a fundamental difference between plants and animals.

In plants, most development occurs postembryonically through the continuous production of stem cells at the shoot and root apical meristems. Email: caroline. However, the molecular basis of other responses, such as the promotion of flowering by age and ambient temperature, is still poorly understood.

In addition, the genetic approach has identified floral pathways for which physiological inputs are not yet known. Vernalization requirement and response. A requirement for vernalization is a reproductive strategy adopted by many species and bred into Fig. A Two Arabidopsis plants, sown at several crops to ensure the same time. The plant on the left is already? B An electron tatively and flower in micrograph of an extremely early?

C The consequence of the favorable condiloss of? The lfy mutant on the left has tions of spring. In Araleaf-like structures in place of the? FRI endogenous cues and to understand how they codes a novel protein with two predicted coiledare integrated to effect the up-regulation of coil domains that functions to promote the acthe floral meristem identity genes. FLC encodes a MADS box transcription established through molecular genetic analyfactor that is a repressor of the floral transition, sis in Arabidopsis.

We then describe how and there is a quantitative relation between FLC changes in the quantitative interactions of the mRNA levels and the timing of flowering 4, multiple inputs alters the predominance of the 5. By promoting the accumulation of FLC different pathways that can account for phemRNA, FRI represses the floral transition to notypic plasticity in response to environmensuch a degree that it overrides the influence of tal change and the evolution of distinct reprootherwise favorable conditions 7.

Vernalizaductive variants adapted to local microclition results in a quantitative reduction in FLC mates. Images of Arabidopsis plants flowering at different times and defective in floral meristem identity function are shown in Fig.

The challenge in flowering time research has been to define the pathways mediating www. This observation provides a molecular been reviewed The circadian oscillator and time-keeping mechanisms to promote explanation for the quantitative nature of the controls many aspects of plant biology in flowering. FLC mRNA levels readdition to flowering time, and several AraAlthough photoreceptors contribute light main low after plants have been returned to bidopsis mutants lhy, cca1, gi, elf3, toc1, ztl, input signals to the circadian system, this is warm temperatures, explaining the mitotic stafkf1 that affect day length— dependent flownot their only role in the photoperiodic conbility of vernalization; however, FLC mRNA ering also disrupt other circadian-regulated trol of flowering.

A single photoreceptor levels are reset after meiosis. Vernalization can processes [see 17, 18 ]. Vernalization is factor with two B-box type zinc fingers That is, there is a clear tempodays, whereas ectopic overexpression of CO late in long days, but are strongly delayed ral separation between the cold treatment and promotes early flowering independently of in night break experiments where nightflowering, suggesting that vernalization has an day length The level of CO expression time dark is interrupted by a light period epigenetic basis.

Far-red nm and blue The mechanism of vernalization has tion and lhy gain-of-function mutants Red light nm inhibits this process vrn mutants. One flowering through PHYB, D, and such mutant, vrn2, has been charE function across a range of speacterized in detail The initial cies 27—30 independently of cold-dependent reduction in FLC clock function and not involving mRNA levels is not affected by transcriptional regulation of CO the vrn2 mutation. However, vrn2 Therefore, VRN2 is reto activate downstream targets.

Multiple input pathways regulating the transition to? CRY2 mediates blue light inhibithe establishment of FLC repres- The inputs promote or repress activation of genes termed? VRN2 encodes a nuclear pathway integrators. Depending on the wavelength of light, the light sibly through a direct interaction quality pathway can repress or promote. The precise molecular protein with homology to Poly- events involved are uncertain in most cases, so the pathways have between the proteins In turn the?

The components of the different input that reach the plant is unlikely to and Su z 12 of Drosophila. This pathways have been omitted to simplify the diagram. The represen- provide seasonal information, but it suggests that VRN2 functions to tation that the input pathways act independently of each other is is important for monitoring local likely to be an oversimpli? An important manision of FLC through an effect on festation of light quality effects is chromatin organization in a manner analoBoth of these mutants flower late and affect the shade avoidance response.

Light reflected gous to Polycomb-group protein complexes other circadian processes. It is not yet in early flowering elf3 mutants 18 , which red light by chlorophyll.

Far-red enriched known which molecules are involved in the also have perturbed clock function but flower light thus serves as a signal of neighborly reduction of FLC mRNA levels during the early. Their output effects on CO expression competition and results in accelerated flowcold, but genes such as HOS1 12 and can, therefore, explain the opposite effects on ering and life cycle completion in a crowded EARL1 13 are candidates for regulatory flowering time caused by these mutations.

Consistent with this, the expression of FT, an The autonomous pathway. Mutants of the Photoperiod and light quality control. At immediate target of CO 20 , correlates with autonomous pathway fca, fy, fpa, ld, fld, and higher latitudes, changing day length is a changes in CO expression in these different fve are late-flowering in long days and short fundamental feature of seasonal progression.

In Arabidopsis, light is peak CO abundance occurs in darkness FCA and FPA both whilst the duration of the day and night is the mechanism by which CO activity was encode RNA binding proteins, so a key quesmeasured by the circadian clock oscillator effective only in long days; indeed, CO protion being addressed is whether FLC is post The molecular basis of signal input to tein appears to be particularly unstable FCA and FPA transcripts are themselves alternatively processed 36, 38 , and in the case of FCA this limits the accumulation of active protein and delays flowering The convergence of the autonomous, vernalization, and FRI repression pathways on FLC raises the question of how pathway predominance is established.

Active FRI alleles are epistatic have the predominant effect to FCA, but this epistasis can be reversed through bypassing the posttranscriptional regulation of FCA expression This quantitative, antagonistic relation of FRI with the autonomous and vernalization pathways may have evolved to enable fine-tuning of FLC levels.

FLC is a potent block of the up-regulation of genes that in turn activate the floral meristem-identity genes. Efficient release of this block is a prerequisite for the promotive photoperiod and gibberellin GA signal transduction pathways to have an effect.

An unresolved issue is whether there is an endogenous input signal to the autonomous pathway. The autonomous pathway may monitor developmental age; plants must pass through a juvenile phase and reach the adult vegetative phase before they will flower.

However, as yet there is no positive evidence for this hypothesis. Therefore, FLC does not appear to be a central repressor that prevents flowering in an agedependent manner. The autonomous pathway may not be dynamically regulated but instead may function constitutively to maintain low levels of FLC expression throughout development.

Some features of this pathway may also be environmentally regulated, as recent findings indicate that the control of flowering by ambient temperature may involve an FLCindependent function of the autonomous pathway The gibberellin pathway. All the currently known phytohormones GAs, auxin, cytokition of GA3 and Arabidopsis mutations such as spindly, which cause constitutively active GA signaling In contrast, mutants that block GA signaling gai or GA biosynthesis ga delay flowering, particularly in short days The GA pathway is genetically distinct from the PHYB repression 31 , vernalization 45, 46 , autonomous 45 , and photoperiod 9, 47 pathways.

As with the autonomous pathway, the input signal that regulates GA activity in the control of flowering is unknown. Plants overexpressing LFY or FT flower early, illustrating that forced ectopic overexpression of either is sufficient to accelerate flowering time 49, 50, However, both lines pass through a vegetative phase 49, 50, In contrast, plants ectopically overexpressing both LFY and FT flower so early that the seedling has time to develop only cotyledons and occasionally one or two subtending leaves bracts 49, 50 Fig.

Because the cotyledons are formed during embryogenesis, these plants flower without passing through a large vegetative phase. This phenotype reveals the combined importance of these integrators and emphasizes the fact that their functions are not redundant. A missing piece in our understanding of how the multiple floral pathways are integrated involves the input of a relatively large number of floral repressors that were identified from early flowering mutants. The first of these mutants to be described were the emf mutants.


Arabidopsis, the Rosetta stone of flowering time?







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