Control of Flowering Time Yaron Y Levy and Caroline Dean†

49 Control of ¯owering time Yaron Y Levy∗ and Caroline Dean² The multiple promotive and repressive pathways controlling Figure 1 ¯owering have been further de®ned by analysis of genetic interactions and the activation of ¯oral meristem identity Vegetative meristem genes. Cloning of additional genes in these pathways has uncovered some of the molecular processes that control the timing of the transition to reproductive development. FCA LD FPA FVE VRN Autonomous Vernalization Addresses promotion Department of Molecular Genetics, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK; ∗ ? e-mail: [email protected] + + ²e-mail: [email protected] Light perception ? PHY ELF3 + – Current Opinion in Plant Biology 1998, 1:49±54 http://biomednet.com/elecref/1369526600100049 Repression LFY AP1 Current Biology Ltd ISSN 1369-5266 AG TFL1 EMF1 Abbreviations FRI GA gibberellin Photoperiod FLC PHY phytochrome CO TFL1 FWA CLF FT Introduction To reproduce under favourable conditions, plants moder- ate their intrinsic developmental timing with cues from Inflorescence/floral the environment, particularly day length, light quality, and meristem temperature. Physiological studies have led to a general Current Opinion in Plant Biology `multifactorial model' [1] which attempts to account for the diverse ¯owering responses observed in a variety of Physiological pathways and genes controlling ¯owering in Arabidopsis. Physiological studies have identi®ed different pathways species. In this model, growth regulators and assimilates that either promote (+) or repress ( ± ) the transition of the apical act as ¯oral promoters and inhibitors which are required in meristem from vegetative to in¯orescence/¯oral development. appropriate concentrations and at particular times before Only genes speci®cally mentioned in the text are included in ¯owering is triggered. The genetics of ¯owering time this ®gure. The circadian clock (left side of ®gure) is implicated •• in the measurement of photoperiod via the perception of light. support this model [2 ,3,4] (Figure 1). Multiple genetic Arabidopsis strains in which ¯owering is promoted by vernalization pathways have been identi®ed, some of which promote also show strong acceleration of ¯owering by far-red-enriched ¯owering and some of which repress it. Some genes act light, so vernalization and perception of light quality appear to be independently of growth conditions, while others mediate closely related processes. The inputs from the different pathways are somehow integrated (symbolised by a question mark) and eventually responses to environmental cues. In this review, we focus lead to activation of in¯orescence/¯oral meristem identity genes. A on recent advances in our understanding of the control of major future goal is to analyse the interactions of the different genes ¯owering time, drawing mainly from work with Arabidopsis to de®ne these physiological processes in terms of genetic pathways. as a model system. Just do it: autonomous promotion the levels of certain FCA transcripts resulted in earlier The autonomous promotion pathway (Figure 1) is consid- ¯owering. This result suggests that FCA is part of a ered to promote the transition from vegetative growth to post-transcriptional regulatory cascade in which alternative ¯owering, independently of environmental cues. Cloning RNA splicing is an important point of control. and analysis of the FCA gene, a component of this pathway, demonstrated a role for post-transcriptional regulation in The rather general expression of FCA throughout the this pathway [5••]. The FCA protein is an RNA-binding plant was similar to that observed for another gene protein with a high degree of similarity, within the of the autonomous promotion pathway, LD [6], and RNA-recognition motifs, to Drosophila proteins SX-1 and is consistent with genes in this pathway functioning ELAV. These proteins function in ¯y development to throughout development [7]. Furner et al. [8] used X-rays alternatively splice transcripts in the sex-determination to generate plants with sectors of fca tissue in an otherwise and neuronal differentiation pathways respectively. The wild-type background. Analysis of fca sectors in the two FCA transcript itself is alternatively spliced and increasing inner layers (L2 and L3), which were marked by loss 50 Growth and development of a gene near FCA involved in pigmentation, showed GA signaling [19]. GAs are clearly involved in multiple that FCA is not required in the entire apical meristem in processes related to ¯owering, and the interaction of GAs order to produce a phenotypically normal plant [8]. These and phytochrome-mediated signaling pathways is com- results imply that FCA or downstream signals produced plex. Analysis of mutants de®cient in both phytochrome in the L1 (epidermal) layer or in adjacent wild-type and GA responses has shown that a fully functional GA L2/L3 tissue can diffuse within the plant and rescue system is necessary for the full expression of at least the phenotype of the fca sectors. Translocated signals one manifestation of phytochrome de®ciency, an elongated which promote ¯owering have been demonstrated in the hypocotyl [20]. Increased responsiveness of phyB mutants elegant genetic analysis of ¯owering time in pea [3]. The to exogenous GAs [21] and the interesting phenomenon of phenotype of the pea mutant gigas, which is de®cient in a ¯oral meristem reversion [22] also suggest an interaction graft-transmissible ¯oral stimulus and is more responsive between phytochrome and GA signaling. to vernalization, has led to speculation about whether GIGAS is a pea orthologue of FCA [9••]. Vernalization: promotion by cold temperature A long cold temperature treatment (i.e., a winter season) Seeing the light: photoperiodic induction induces or accelerates ¯owering in many species. This Many plants monitor day length (®gure 1) as a cue for phenomenon, known as vernalization, has a number of ¯owering at the correct time of year. The promotion unusual features that suggest an epigenetic mechanism as of ¯owering by photoperiod has received considerable its basis [23]. Burn et al. [24] proposed that vernalization attention over the years with clear evidence that promotive causes general DNA demethylation which allows expres- and repressive signals, made in the leaves, are translocated sion of kaurenoic acid hydroxylase, an important enzyme in the phloem exudate to the apex [1]. While there in GA biosynthesis. This hypothesis was tested by trans- have been considerable efforts to identify transcripts and forming Arabidopsis plants with a construct expressing an substances induced in leaves that might form the basis antisense transcript of a methyltransferase gene (MET1), of this ¯oral signal, speci®c candidates remain elusive resulting in plants with substantially reduced levels of [10±12]. The recent development of a method to induce cytosine methylation [25]. Many developmental abnormal- synchronous ¯owering in Arabidopsis in response to one ities were seen in these plants, but consistent with the long day photoperiod [13] will facilitate the important above hypothesis, the antisense methyltransferase plants integration of genetics and physiology studies when ¯owered earlier than the wild-type [26]. Two other studies similar experiments are done with Arabidopsis mutants. An have also addressed the role of methylation in ¯owering. exciting recent development has been the demonstration Ronemus et al. [27], using a similar MET1-antisense that there is a connection between the endogenous construct, and Kakutani et al. [28], working with the circadian clock and the control of ¯owering time, probably ddm1 mutant, which has decreased DNA methylation but via the photoperiodic promotion pathway (Figure 1). unaltered methyltransferase activity, noted late ¯owering Mutations at the ELF3 locus result in an elongated as a frequently appearing phenotype in their plant hypocotyl (especially under blue light) and early ¯owering lines. These data imply that methylation has a role in which is insensitive to photoperiod [14]. elf3 shows no establishing or maintaining different developmental states circadian rhythm in continuous light, thus showing a of the meristem; Ronemus et al. [27] speculate that there is connection between the circadian clock and the control a gradient of increasing methylation during development, of ¯owering time [15•] and suggesting a role for ELF3 in acting to change meristem competency and determinacy. linking light perception to circadian rhythms. It would be interesting, therefore, to establish whether this gradual increase in methylation during development There has also been a focus on regulatory events occurring is related to the changes in ¯oral repressor concentration at the shoot apex as it switches from vegetative to proposed by the `controller of phase switch' hypothesis reproductive development. Experiments with cultured [29]. The level of repressor activity is proposed to decrease apices of Lolium demonstrated that photoperiodic induc- over time due to an internal (developmental) program, tion resulted from two signals acting at the apex [16]. as well as being modulated by external (environmental) The ®rst signal, of an unknown nature, switched the signals. Switches in phase (e.g., from in¯orescesce to developmental fate of the shoot meristem cells from ¯oral development) are proposed to occur when repressor commitment to produce leaves to commitment to produce activity drops below the critical level for maintaining the ¯owers,

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