Vernalization and Flowering
Last update: Sep 2020
Winter-annual ecotype of Arabidopsis thaliana is an example of a long-day plant that undergoes vernalization, where exposure to low temperatures in winter promotes flowering in spring, as illustrated in Fig.1.
Flowering in Arabidopsis and countless other plants is thought to be controlled by the circadian rhythm, in which a plant's biological activity fluctuates in 24-hour cycles. These fluctuations often persist regardless of the exogenous stimuli, which suggests that the circadian clock is of endogenous origin. Scientists observed that gene expression and protein concentrations fluctuate at regular intervals, giving rise to physiological phenomena such as photosynthesis and flowering. Those circadian rhythm oscillations can be modulated by the amount of light (day length) and temperature. Circadian rhythm is thought to be mediated by groups of transcription factors, which can differ depending on species.
Figure 1. Exposure to cold induces flowering in winter-annual Arabidopsis
Day length is a crucial factor in flowering. Plants can sense the day length and light wavelength using photoreceptors (protein+ light-absorbing pigment), such as phytochrome detecting red/infrared light, or cryptochrome detecting blue light. Photoreceptors are located in the leaves and pass on the light-induced signal to the tip (apex) of the stem. This can induce flowering through the presence of the flowering hormone called florigen.
The transcription of the florigen gene (FT) is controlled by CONSTANS. CONSTANS is one of several transcription factors that promote flowering in Arabidopsis, another one being the FD protein. CONSTANS gene codes for CONSTANS (CO) protein. Transcription of CONSTANS gene and CO protein stability fluctuate depending on day length. CO protein cannot accumulate when days are short due to transcription repression and protein degradation. However, when days are longer, those effects are suppressed via the action of photoreceptors acting in response to late afternoon light, and so CO protein can be accumulated. CO protein then induces the expression of FT. FT protein moves from the leaf to stem tip, where it cooperates with the FD protein (transcription factor) to induce the expression of floral meristem identity gene APETALA1 (AP1). AP1 and several other factors then lead to flower formation. Thanks to this mechanism, which is illustrated in Fig.2, a long-day plant such as Arabidopsis can flower when days get longer.
Figure 2. Accummulation of CONSTANS protein under long-day conditions induces FT, which, together with FD protein, promotes expression of AP1 that induces flowering.
However, long days are not the only condition needed for flowering. Arabidopsis also needs prior exposure to cold in order to flower (vernalization). The critical factor in vernalization is the FLC transcription factor. During autumn, the FLC gene is active, and so FLC represses the FT gene required for flowering. During cold winter, the FLC gene is silenced due to various epigenetic modifications, notably the histone H3 protein (tail) triple methylation (H3K27me3) in the FLC locus, which results in a chromatin status (structure) change that leads to gene repression (Fig.3). Although the FLC gene becomes repressed in winter, Arabidopsis does not flower then, as the FT gene is still silent due to lack of CO protein. Come spring, when days get longer, CO protein accumulates, activating FT that induces flowering (Fig. 4).
Figure 3. Levels of FLC mRNA decrease (gene expression is blocked) due to epigenetic modifications after exposure to cold
Figure 4. FLC is repressed after exposure to cold. As the result, FT can be transcribed after it is induced by CO that accummulates under long-day conditions.
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Figure 1. Taiz, L. and Zeiger, E., 2002. Plant Physiology. 3rd ed. Sunderland, Massachusetts: Sinauer Associates Inc..
Figure 2. McGarry, R. C., & Kragler, F. (2013). Phloem-mobile signals affecting flowers: applications for crop breeding. Trends in plant science, 18(4), 198–206. https://doi.org/10.1016/j.tplants.2013.01.004
Figure 3. Taiz, L. and Zeiger, E., 2002. Plant Physiology. 3rd ed. Sunderland, Massachusetts: Sinauer Associates Inc..
Figure 4. Y. He, T. Chen, X. Zeng. (2020). Genetic and Epigenetic Understanding of the Seasonal Timing of Flowering. Plant Communications. Vol. 1, Issue 1,. https://doi.org/10.1016/j.xplc.2019.100008