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IN THIS ISSUE

Measuring Daylength: Pharbitis Takes a Different Approach

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In many plants species, the initiation of flowering is determined by seasonal changes in daylength. Long-day (LD) plants, such as Arabidopsis, initiate flowering when daylength grows longer, whereas short-day (SD) plants, such as rice, initiate flowering under shorter days. Other cues that affect flowering, such as vernalization (requirement for lengthy cold period) and developmental timing, may be superimposed on daylength responses. The last common ancestor of dicots and monocots existed 150 to 300 million years ago (Sanderson et al., 2004), so Arabidopsis (a dicot) and rice (a monocot) are distantly related, and a comparison of the components involved in the photoperiodic response to flowering in these two species might provide a starting point for understanding the mechanistic basis of these opposing responses. However, photoperiodic responses to flowering have evolved independently several times throughout evolutionary history, underscoring a need to examine a wider range of species to fully understand this phenomenon. For example, the monocot grasses include both LD and SD species, so it is important to ask how LD grasses differ from (or are similar to) the SD grass rice and the LD dicot Arabidopsis.

In this issue of The Plant Cell, Hayama et al. (pages 2988–3000) investigate the photoperiodic response to flowering in the SD dicot Pharbitis (Ipomoea nil; also called Japanese morning glory; see figure ), a member of the Convolvulaceae in the order Solanales. The Solanales are more closely related to Arabidopsis than are the monocot grasses but still are distant relatives to Arabidopsis, which belongs to the Brassicales. The Solanales and Brassicales belong to two distinct major clades within the eudicots, the asterids and rosids, respectively, which are believed to have diverged by the early to mid Cretaceous period (>100 million years ago; Sanderson et al., 2004).

View larger version (63K): [in this window] [in a new window] [as a PowerPoint slide]   Pharbitis Grown under Noninductive LD (Left) and Inductive SD (Right) Conditions.

In Arabidopsis, the transcription factor CONSTANS (CO) activates transcription of FT, and flowering is promoted when CO accumulates to a threshold level in the nucleus (reviewed in Hayama and Coupland, 2004; Kobayashi and Weigel, 2007). CO induces the expression of FT, and the FT protein (possibly also FT mRNA) is transported through the phloem to the shoot apical meristem, where it plays a principal role in the induction of flowering. CO expression in Arabidopsis is regulated by the circadian clock. Under SDs, there is a diurnal peak in expression during the night, whereas under LDs, the expression pattern is broadened and begins to rise in the light. The circadian rhythm of expression, together with degradation of a repressor of CO in the light, stabilization of CO protein in the light, and degradation in the dark, ensure that CO protein only accumulates in the nucleus under LDs and therefore that FT transcription, and subsequent promotion of flowering, occurs under LDs but not SDs (Valverde et al., 2004). Other signaling pathways that affect flowering, such as the autonomous pathway and vernalization, also appear to converge on FT; thus, it represents a major control point for intergrating multiple environmental and developmental signals that influence flowering.

Interestingly, the main differences in the photoperiodic flowering response between Arabidopsis, an LD plant, and rice, an SD plant, appears to be that the rice homolog of CO, called Hd1, acts as a repressor of FT (which in rice is called Hd3a) rather than an activator (Hayama et al., 2003). This suggests that the various mechanisms regulating CO in Arabidopsis and Hd1 in rice (the circadian clock and factors affecting repression of gene expression as well as stabilization and degradation of CO/Hd1 protein) are at least somewhat similar in these two species.

Hayama et al. examined the function and regulation of FT homologs in the SD dicot Pharbitis and compared the photoperiodic mechanisms of Pharbitis with those of Arabidopsis and rice. The authors isolated two homologs of FT in Pharbitis, called Pn FT1 and Pn FT2. (Although the scientific name of Pharbitis is now I. nil, the authors retained the two-letter taxonomic designation Pn, as the species was formerly known as Pharbitis nil and the Pn designation has been widely used in the past). Overexpression of Pn FT1 accelerated flowering in Arabidopsis and Pharbitis, and the accumulation of Pn FT1 and Pn FT2 mRNA occurred only under conditions that promote flowering (SD), suggesting that Pn FT1 functions like At FT and Os Hd3a to promote flowering.

In Arabidopsis, FT expression is strongly dependent on exposure of plants to light and on the circadian rhythm of CO expression, such that FT transcription shows a circadian rhythm under constant light (Harmer et al., 2000; Imaizumi et al., 2005). The prevailing model of the photoperiodic response in Arabidopsis involves light activation of CO protein, which, as noted above, is orchestrated by a number of factors that affect (negatively and positively) CO expression and CO protein stability such that FT expression sufficient to promote flowering only occurs under LDs.

Hayama et al. found that, by contrast, Pn FT1 and FT2 mRNA levels show robust cycling under constant darkness but do not show a circadian rhythm under constant light. The authors concluded that activation of Pn FT in darkness is an important component of the photoperiodic flowering response in Pharbitis, unlike the situation in Arabidopsis. Also, the rhythm determining the timing of Pn FT expression was started at dusk by the light–dark transition, and Pn FT expression was activated only if the night was sufficiently long enough. Furthermore, the pattern of expression of Pn CO, the closest described homolog of At CO in Pharbitis, was not highly correlated with the expression of Pn FT1 and FT2 under darkness, leading the authors to conclude that CO is not a principal regulator of FT expression in Pharbitis. They postulate the existence of another clock-controlled gene, whose phase is set specifically by the light–dark transition, as being a principal factor regulating Pn FT expression.

Identification of such genes will be important to arrive at a complete picture of the regulation of FT activity and the photoperiodic control of flowering in this species. It will also be important to determine the function of CO in Pharbitis. Does it act either to activate or repress FT under any conditions, or does it have another function altogether?

As noted by Hayama and Coupland (2004), there are at least several plant genera, including Nicotiana and Lemna, that include both SD and LD plants, suggesting that a daylength response can diverge rapidly during evolution. The work of Hayama et al. suggests that widely different mechanisms may have evolved to control the photoperiodic response of flowering in plants and that there may be other mechanisms for measuring daylength in addition to those described in Arabidopsis. In addition, the work illustrates the importance of comparative approaches in gaining new insights to this fundamental problem in plant biology.

	Footnotes

 
www.plantcell.org/cgi/doi/10.1105/tpc.107.056713

	REFERENCES

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Related articles in Plant Cell:

A Circadian Rhythm Set by Dusk Determines the Expression of FT Homologs and the Short-Day Photoperiodic Flowering Response in Pharbitis

Ryosuke Hayama, Bhavna Agashe, Elisabeth Luley, Rod King, and George Coupland
Plant Cell 2007 19: 2988-3000. [Abstract] [Full Text]  

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