Research

BACKGROUND:

Many plants flower at a specific time of year to optimize their reproductive success. To predict the timing of seasonal flowering, understanding the governing molecular mechanisms is crucial. Organisms use changes in day length (=photoperiod) and temperature to anticipate upcoming seasonal changes. In my lab, we focus on studying the molecular mechanisms by which plants measure changes in photoperiods. Here I briefly introduce the mechanisms of photoperiodic flowering in Arabidopsis. For further reading regarding this topic, please see the recent reviews [1-4].

Arabidopsis is an ideal system for the study of seasonal flowering mechanisms

Fig1 external coincid. model.al [Converted]

Many organisms use day length (photoperiod) information to prepare for seasonal changes [5-7]. Photoperiodic response was first described in plants in the early twentieth century [6]. Studies in plants were instrumental in enabling researchers to propose and test conceptual models of day-length measurement. Currently, molecular genetic evidence in plants and animals [8-13] most supports the external coincidence model proposed by Collin Pittendriph in 1964 [14]. In this model, light plays two crucial roles. One is entraining the phase of circadian clock oscillation. The other is activating the clock-controlled key regulator that peaks in late afternoon. Photoperiodic responses are only triggered when the presence of daylight (=the external signal) coincides with peak expression of the key regulator (figure above). Thus, the relationship between presence of light and phase of circadian clock components is important for sensing day-length changes. Desynchronization between daily light periods and internal circadian rhythms causes severe reduction of fitness in cyanobacteria, plants, and animals [15-20].

We have been working on the mechanisms of photoperiodic flowering response in Arabidopsis thaliana, which is currently the most suitable organism for studying molecular mechanisms of day-length measurement. It flowers early under long-day (LD) conditions, while flowering is delayed in short days (SD). This drastic difference in developmental patterns enables us to assess the effects of genetic modification on the photoperiodic response more precisely. Based on our findings as well as others, a detailed model for day-length sensing mechanisms in Arabidopsis has been constituted (see details below).

Flowering pathways in the model plant Arabidopsis

Various environmental factors influence the transition from vegetative to reproductive growth (flowering). The major genetic pathways that control the transition can currently be characterized as photoperiodic, vernalization, autonomous, and gibberellin [10,21,22]. The points of the pathways are interrelated. For instance, in Arabidopsis accessions with a functional FLOWERING LOCUS C (FLC) gene (the key regulator of both the vernalization and autonomous pathways [23,24]), photoperiod sensitivity is suppressed unless plants are vernalized. Without vernalization, FLC MADS-box transcription factor directly suppresses the expression of FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) [25-27]. Eventually, the pathways converge to induce the same floral meristem identity genes, such as LEAFY (LFY), APETALA1 (AP1) and other genes [28,29], and lead to flowering. Ambient temperature changes also affect expression of FT [30,31].

A current mechanistic model for day-length measurement in Arabidopsis

The LD photoperiod promotes flowering in Arabidopsis mainly through the function of CONSTANS (CO) and FT proteins [32-36]. CO is a phloem-specific transcription activator of FT [35,37-39] (see the expression pattern of CO below). Since daytime CO protein expression occurs only when days are long, FT is induced in LD, resulting in earlier flowering [35,36,40] (figure left). To accurately measure differences in day length, circadian clock-dependent timing regulation of CO is a crucial mechanism [35,36]. We demonstrated that the timing of formation of the FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) and GIGANTEA (GI) protein complex determines the timing of daytime CO gene expression in LD [41]. GI is a large nuclear protein without known functional domain [42], and FKF1 is a LOV domain containing a blue-light photoreceptor that regulates protein degradation [43,44]. The expression of both FKF1 and GI proteins occurs in the afternoon and is regulated by core clock proteins [42-46]. FKF1-GI complex formation occurs only when the FKF1 LOV domain absorbs blue light [41,44]. The role of the FKF1-GI complex is to remove transcriptional repressors of CO, CYCLING DOF FACTORs (CDFs), in the LD afternoon to facilitate expression of daytime CO [47,48].

Light signals perceived by phytochromes and cryptochromes stabilize CO protein only in LD afternoons when daytime CO expression occurs [40] (figure left). We demonstrated that FKF1 physically binds to CO protein in a blue-light dependent manner and stabilizes CO protein in the afternoon in LD [49]. In the dark, even though CO mRNA is highly accumulated, CO protein is actively degraded at night in both LD and SD by the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPRESSOR OF PHYA-105 1 (SPA1) E3 ubiquitin ligase complex [40,50-54]. Together with transcriptional control, posttranslational regulation ensures that CO protein exists only in LD afternoons when FT is induced. Thus, circadian-clock regulated CO transcription and light-mediated CO protein stability regulation are the core of the day-length sensing mechanisms.

FT protein is a major part of florigen (a long-sought flowering-inducing substance) and it is synthesized in the leaf vasculature [55-59]. FT protein is then translocated to the shoot apical meristem to initiate expression of floral identity genes, which regulate floral development [38,57,60]. To selectively induce the expression of FT in LD, restricting CO protein expression to the LD afternoon is essential.

The CO/FT module is highly conserved in angiosperms[61,62]. Both CO and FT orthologs exist in long-day and short-day plants. The CO/FT module similarly plays an important role in flowering-time regulation of especially long-day plants, including eudicots and monocots (such as Arabidopsis, poplar, barley and wheat)[61-64]. Thus, Arabidopsis is a suitable model for eudicots and monocots for analyzing the molecular mechanism of photoperiodic flowering. In addition, studying the photoperiodic flowering mechanism is important for understanding a major plant reproduction mechanism, one that is directly applicable to improvement in crop yields.


        The picture below shows GUS activity in the CO:GUS plants, indicating CO is expressed in vascular tissues.

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Fundings:



Royalty Research Fund 





National Institute of General Medical Sciences 




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© Takato Imaizumi 2013