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Stress-induced activation of heterochromatic transcription.

Tittel-Elmer M, Bucher E, Broger L, Mathieu O, Paszkowski J, Vaillant I - PLoS Genet. (2010)

Bottom Line: We have found that heterochromatin-associated silencing in Arabidopsis plants subjected to a particular temperature regime is released in a genome-wide manner.This occurs without alteration of repressive epigenetic modifications and does not involve common epigenetic mechanisms.Thus, our results reveal new regulatory aspects of transcriptional repression in constitutive heterochromatin and open up possibilities to identify the molecular mechanisms involved.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Biology, University of Geneva, Geneva, Switzerland.

ABSTRACT
Constitutive heterochromatin comprising the centromeric and telomeric parts of chromosomes includes DNA marked by high levels of methylation associated with histones modified by repressive marks. These epigenetic modifications silence transcription and ensure stable inheritance of this inert state. Although environmental cues can alter epigenetic marks and lead to modulation of the transcription of genes located in euchromatic parts of the chromosomes, there is no evidence that external stimuli can globally destabilize silencing of constitutive heterochromatin. We have found that heterochromatin-associated silencing in Arabidopsis plants subjected to a particular temperature regime is released in a genome-wide manner. This occurs without alteration of repressive epigenetic modifications and does not involve common epigenetic mechanisms. Such induced release of silencing is mostly transient, and rapid restoration of the silent state occurs without the involvement of factors known to be required for silencing initiation. Thus, our results reveal new regulatory aspects of transcriptional repression in constitutive heterochromatin and open up possibilities to identify the molecular mechanisms involved.

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Related in: MedlinePlus

A temperature shift can release transcriptional silencing of a transgenic locus.Representative images of histochemical staining for GUS activity (left) performed on seedlings grown under the conditions defined on the right. Plants grown for 3 days at 21°C were transferred to 4°C for 3–9 weeks (a–c) and then shifted to either 21°C (d–f) or 37°C (g–i) for 1 day. Seedlings at 3, 7, and 9 days post-sowing were transferred at 4°C for 1 week and shifted to 37°C for 1 day (j–l), or directly shifted to 37°C for 1 day omitting the cold treatment (m–o).
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pgen-1001175-g001: A temperature shift can release transcriptional silencing of a transgenic locus.Representative images of histochemical staining for GUS activity (left) performed on seedlings grown under the conditions defined on the right. Plants grown for 3 days at 21°C were transferred to 4°C for 3–9 weeks (a–c) and then shifted to either 21°C (d–f) or 37°C (g–i) for 1 day. Seedlings at 3, 7, and 9 days post-sowing were transferred at 4°C for 1 week and shifted to 37°C for 1 day (j–l), or directly shifted to 37°C for 1 day omitting the cold treatment (m–o).

Mentions: Three-day-old seedlings were exposed to a long cold period (4°C) known to alter DNA methylation [49] and also to influence silencing mediated by polycomb-group proteins, which is best illustrated by the vernalization process [50]–[52]. Cold-exposed and control seedlings were subsequently subjected to histochemical GUS staining (Figure 1A–1C). Three or 6 weeks of cold treatment did not destabilize GUS silencing (Figure 1A and 1B); however, seedlings transferred to 4°C for 9 weeks showed weak TGS release manifested by occasional patches of GUS staining in a proportion of seedlings (Figure 1C). When seedlings were returned to 21°C for 24 h following the cold treatment, GUS staining was also detected in seedlings placed in the cold for only 6 weeks, and this shift led to increased GUS staining intensity in plants grown at 4°C for 9 weeks (Figure 1D–1F). Therefore, we concluded that, in addition to cold treatment, a temperature shift may also contribute to the release of TGS. To test this, we extended the range of the temperature shifts from 21°C to 37°C (Figure 1G–1I). While no GUS expression was observed in plants kept in the cold for only 3 weeks and then placed at 21°C for 24 h, a temperature shift to 37°C instead of 21°C resulted in very clear GUS activity (Figure 1G). This activity remained at a similar level when longer cold periods were applied, suggesting that the length of the cold period preceding the temperature shift to 37°C was not a limiting factor for the release of TGS (Figure 1H and 1I). To further examine this, we shortened the cold period to 1 week or even omitted it prior to the temperature shift to 37°C. For these experiments, we used seedlings at three stages (3, 7, and 9 days after sowing) in order to assess also whether silencing release can be effective over a broader span of early plant development. One week of cold treatment followed by a shift to 37°C for 24 h was sufficient to release silencing of GUS locus at all three developmental stages of the seedlings (Figure 1J–1L). Omission of the cold period prior to the shift to 37°C resulted not only in less uniform and less pronounced TGS release (Figure 1M–1O) but also caused plant lethality (not shown). Therefore, the cold period before the shift to high temperature increased both plant viability and the amplitude of TGS suppression. Shortening the period at 37°C to 15 h permitted most of this treatment (12 h) to be performed during the light phase of the applied photoperiod and promoted plant survival. The shortening of the time at 37°C had no influence on the degree of silencing release (Figure 1K and Figure 2D and data not shown).


Stress-induced activation of heterochromatic transcription.

Tittel-Elmer M, Bucher E, Broger L, Mathieu O, Paszkowski J, Vaillant I - PLoS Genet. (2010)

A temperature shift can release transcriptional silencing of a transgenic locus.Representative images of histochemical staining for GUS activity (left) performed on seedlings grown under the conditions defined on the right. Plants grown for 3 days at 21°C were transferred to 4°C for 3–9 weeks (a–c) and then shifted to either 21°C (d–f) or 37°C (g–i) for 1 day. Seedlings at 3, 7, and 9 days post-sowing were transferred at 4°C for 1 week and shifted to 37°C for 1 day (j–l), or directly shifted to 37°C for 1 day omitting the cold treatment (m–o).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2965753&req=5

pgen-1001175-g001: A temperature shift can release transcriptional silencing of a transgenic locus.Representative images of histochemical staining for GUS activity (left) performed on seedlings grown under the conditions defined on the right. Plants grown for 3 days at 21°C were transferred to 4°C for 3–9 weeks (a–c) and then shifted to either 21°C (d–f) or 37°C (g–i) for 1 day. Seedlings at 3, 7, and 9 days post-sowing were transferred at 4°C for 1 week and shifted to 37°C for 1 day (j–l), or directly shifted to 37°C for 1 day omitting the cold treatment (m–o).
Mentions: Three-day-old seedlings were exposed to a long cold period (4°C) known to alter DNA methylation [49] and also to influence silencing mediated by polycomb-group proteins, which is best illustrated by the vernalization process [50]–[52]. Cold-exposed and control seedlings were subsequently subjected to histochemical GUS staining (Figure 1A–1C). Three or 6 weeks of cold treatment did not destabilize GUS silencing (Figure 1A and 1B); however, seedlings transferred to 4°C for 9 weeks showed weak TGS release manifested by occasional patches of GUS staining in a proportion of seedlings (Figure 1C). When seedlings were returned to 21°C for 24 h following the cold treatment, GUS staining was also detected in seedlings placed in the cold for only 6 weeks, and this shift led to increased GUS staining intensity in plants grown at 4°C for 9 weeks (Figure 1D–1F). Therefore, we concluded that, in addition to cold treatment, a temperature shift may also contribute to the release of TGS. To test this, we extended the range of the temperature shifts from 21°C to 37°C (Figure 1G–1I). While no GUS expression was observed in plants kept in the cold for only 3 weeks and then placed at 21°C for 24 h, a temperature shift to 37°C instead of 21°C resulted in very clear GUS activity (Figure 1G). This activity remained at a similar level when longer cold periods were applied, suggesting that the length of the cold period preceding the temperature shift to 37°C was not a limiting factor for the release of TGS (Figure 1H and 1I). To further examine this, we shortened the cold period to 1 week or even omitted it prior to the temperature shift to 37°C. For these experiments, we used seedlings at three stages (3, 7, and 9 days after sowing) in order to assess also whether silencing release can be effective over a broader span of early plant development. One week of cold treatment followed by a shift to 37°C for 24 h was sufficient to release silencing of GUS locus at all three developmental stages of the seedlings (Figure 1J–1L). Omission of the cold period prior to the shift to 37°C resulted not only in less uniform and less pronounced TGS release (Figure 1M–1O) but also caused plant lethality (not shown). Therefore, the cold period before the shift to high temperature increased both plant viability and the amplitude of TGS suppression. Shortening the period at 37°C to 15 h permitted most of this treatment (12 h) to be performed during the light phase of the applied photoperiod and promoted plant survival. The shortening of the time at 37°C had no influence on the degree of silencing release (Figure 1K and Figure 2D and data not shown).

Bottom Line: We have found that heterochromatin-associated silencing in Arabidopsis plants subjected to a particular temperature regime is released in a genome-wide manner.This occurs without alteration of repressive epigenetic modifications and does not involve common epigenetic mechanisms.Thus, our results reveal new regulatory aspects of transcriptional repression in constitutive heterochromatin and open up possibilities to identify the molecular mechanisms involved.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Biology, University of Geneva, Geneva, Switzerland.

ABSTRACT
Constitutive heterochromatin comprising the centromeric and telomeric parts of chromosomes includes DNA marked by high levels of methylation associated with histones modified by repressive marks. These epigenetic modifications silence transcription and ensure stable inheritance of this inert state. Although environmental cues can alter epigenetic marks and lead to modulation of the transcription of genes located in euchromatic parts of the chromosomes, there is no evidence that external stimuli can globally destabilize silencing of constitutive heterochromatin. We have found that heterochromatin-associated silencing in Arabidopsis plants subjected to a particular temperature regime is released in a genome-wide manner. This occurs without alteration of repressive epigenetic modifications and does not involve common epigenetic mechanisms. Such induced release of silencing is mostly transient, and rapid restoration of the silent state occurs without the involvement of factors known to be required for silencing initiation. Thus, our results reveal new regulatory aspects of transcriptional repression in constitutive heterochromatin and open up possibilities to identify the molecular mechanisms involved.

Show MeSH
Related in: MedlinePlus