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Review
, 39 (10), 715-721

The Effect of Fluctuations in Photoperiod and Ambient Temperature on the Timing of Flowering: Time to Move on Natural Environmental Conditions

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Review

The Effect of Fluctuations in Photoperiod and Ambient Temperature on the Timing of Flowering: Time to Move on Natural Environmental Conditions

Young Hun Song. Mol Cells.

Abstract

Plants have become physiologically adapted to a seasonally shifting environment by evolving many sensory mechanisms. Seasonal flowering is a good example of adaptation to local environmental demands and is crucial for maximizing reproductive fitness. Photoperiod and temperature are major environmental stimuli that control flowering through expression of a floral inducer, FLOWERING LOCUS T (FT) protein. Recent discoveries made using the model plant Arabidopsis thaliana have shown that the functions of photoreceptors are essential for the timing of FT gene induction, via modulation of the transcriptional activator CONSTANS (CO) at transcriptional and posttranslational levels in response to seasonal variations. The activation of FT transcription by the fine-tuned CO protein enables plants to switch from vegetative growth to flowering under inductive environmental conditions. The present review briefly summarizes our current understanding of the molecular mechanisms by which the information of environmental stimuli is sensed and transduced to trigger FT induction in leaves.

Keywords: CONSTANS; FLOWERING LOCUS T; Photoperiod; flowering; temperature fluctuation.

Figures

Fig. 1
Fig. 1
Photoperiodic flowering regulation by the induction of FT expression under long-day conditions. In Arabidopsis, a high abundance of CDF proteins accumulates and represses the expression CO and FT genes simultaneously by binding to promoter regions of these genes in the morning. FKF1 protein, the expression of which coincides with that of GI in the afternoon, absorbs blue light and forms a protein complex with GI. The protein complex stimulates the degradation of CDF proteins on CO and FT promoters. Removal of CDF repression enables FBH transcription factors to gain access to the CO promoter. FBH proteins activate CO transcription throughout the rest of day. CO protein is post-translationally modulated by light quality. The protein is degraded by the COP1–SPAs complex in the dark, but is sequestered from the complex by the inhibitory function of blue light-activated CRY2. In the morning, CO is stabilized by the far-red light photoreceptor PHYA and immediately destabilized by the blue light photoreceptor ZTL and the formation of a protein complex between HOS1 and red light-absorbed PHYB. In the afternoon, CO accumulates as a consequence of the activities of FKF1, GI, and PHYA. Together with GI, blue light activation of FKF1 increases CO stability. Thus, GI indirectly regulates CO stability via the inhibition of ZTL function. In turn, FT expression is induced by CO and CIBs. CO binds to the FT promoter directly as well as indirectly with other transcription factors. In addition, CIB proteins interact with blue light-activated CRY2 and directly bind to the FT promoter. These CO and CIBs activate FT transcription at dusk, leading to the promotion of reproductive transition in long days.
Fig. 2
Fig. 2
The effect of temperature changes on FT induction. FLC, a strong floral regulator, associates with the first intron of the FT gene and represses the expression of the gene. Vernalization removes FLC repression of the FT locus. In short days, low temperature (≈16°C) increases the occupancy of H2A.Z nucleosomes on the FT locus, resulting in the inhibition of PIF4 binding to the FT promoter under this condition. However, high temperature (≈27°C) stimulates the de-association of H2A.Z from the FT locus and induces CO accumulation in the afternoon. These cause the formation of a CO-PIF4 complex that binds to the FT promoter and activates its transcription. SVP represses FT expression by binding to the FT promoter in response to low temperature in long days. Temperature fluctuations increase SVP activity at dusk and CO stability, which are also regulated by photoperiod, at night in long days, leading to changes in the gene expression profiles of FT in a day length-dependent manner.

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