Dedifferentiation signifies the capacity of somatic cells to acquire stem cell-like properties. This process can be induced during normal development and as a response to various stimuli, such as pathogen infection and wounding. Dedifferentiation also characterizes the transition of differentiated leaf cells into protoplasts (plant cells devoid of cell walls), a transition accompanied by widespread chromatin decondensation. Transcriptome profiling of dedifferentiating protoplast cells revealed striking similarities with senescing cells; both display a large increase in the expression of genes of specific transcription factor (TF) families, including ANAC, WRKY, bZIP, and C2H2. Further analysis showed that leaves induced to senesce by exposure to dark display characteristic features of dedifferentiating cells, including chromatin decondensation, disruption of the nucleolus, and condensation of rRNA genes. Considering that premature senescence can be induced by various stress conditions both in plant and animal cells, our results suggest that the response of plant and also animal cells to certain stresses converges on cellular dedifferentiation whereby cells first acquire stem cell-like state prior to acquisition of a new cell fate (e.g., reentry into the cell cycle or death).

Background: Previous data suggested that senescing cells or cells exposed to acute stress may acquire stem cell properties characterized by open chromatin conformation and by promiscuous expression of transcription factor genes. To further explore the link between stress response and dedifferentiation, we generated transgenic plants in which a reporter AtMBD6-GFP is controlled by a meristem-specific promoter derived from the ANAC2 gene together with the analysis of chromatin conformation. Results: We found that ANAC2 promoter is essentially active in the shoot and the root apical meristems including leaf primordia. ANAC2 was activated in mature leaves following exposure to various stress conditions including protoplasting and dark. This activity was associated with decondensation of pericentric but not of centromeric chromatin. Using epigenetic mutants, ddm1 and kyp/suvh4, we found that compaction at centromeric chromatin persists despite a significant reduction in DNA and histone methylation. Conclusions: Our results suggest that extreme environmental signals trigger plant somatic cells to acquire stem cell properties before assuming a new cell fate. Results also pointed to distinct mechanisms involved in controlling chromatin compaction at chromocenter and that compaction of centromeric chromatin may not be dependent on epigenetic means driven by DDM1 and KYP/SUVH4 chromatin modifier proteins. Developmental Dynamics 242:1121-1133, 2013. V C 2013 Wiley Periodicals, Inc.Key words: stress response; dedifferentiation; chromatin reorganization; DNA methylation; histone modifications; Arabidopsis Key Findings:Acute stress induces, in leaf cells, the activity of a meristem-specific promoter derived from the ANAC2 transcription factor-encoding gene. Stressed leaf cells acquire a dedifferentiated state characterized by open chromatin conformation. Stress-induced chromatin decondensation is selective for peri-centromeric but not centromeric region.Maintenance of hetrochromatin at centromeric region is not dependent on DNA and histone methylation driven by DDM1 and KYP/SUVH4 proteins.