SUMMARYDEAD-box RNA helicases are involved in many aspects of RNA metabolism and in diverse biological processes in plants. Arabidopsis thaliana mutants of two DEAD-box RNA helicases, STRESS RESPONSE SUPPRESSOR1 (STRS1) and STRS2 were previously shown to exhibit tolerance to abiotic stresses and upregulated stress-responsive gene expression. Here, we show that Arabidopsis STRS-overexpressing lines displayed a less tolerant phenotype and reduced expression of stress-induced genes confirming the STRSs as attenuators of Arabidopsis stress responses. GFP-STRS fusion proteins exhibited localization to the nucleolus, nucleoplasm and chromocenters and exhibited relocalization in response to abscisic acid (ABA) treatment and various stresses. This relocalization was reversed when stress treatments were removed. The STRS proteins displayed mis-localization in specific gene-silencing mutants and exhibited RNA-dependent ATPase and RNA-unwinding activities. In particular, STRS2 showed mis-localization in three out of four mutants of the RNA-directed DNA methylation (RdDM) pathway while STRS1 was mis-localized in the hd2c mutant that is defective in histone deacetylase activity. Furthermore, heterochromatic RdDM target loci displayed reduced DNA methylation and increased expression in the strs mutants. Taken together, our findings suggest that the STRS proteins are involved in epigenetic silencing of gene expression to bring about suppression of the Arabidopsis stress response.

Stem cells are commonly defined by their developmental capabilities, namely, self-renewal and multitype differentiation, yet the biology of stem cells and their inherent features both in plants and animals are only beginning to be elucidated. In this review article we highlight the stem cell state in plants with reference to animals and the plastic nature of plant somatic cells often referred to as totipotency as well as the essence of cellular dedifferentiation. Based on recent published data, we illustrate the picture of stem cells with emphasis on their open chromatin conformation. We discuss the process of dedifferentiation and highlight its transient nature, its distinction from re-entry into the cell cycle and its activation following exposure to stress. We also discuss the potential hazard that can be brought about by stress-induced dedifferentiation and its major impact on the genome, which can undergo stochastic, abnormal reorganization leading to genetic variation by means of DNA transposition and/or DNA recombination.

Zygophyllum dumosum Boiss. is a perennial Saharo-Arabian phytogeographical element and a dominant shrub on the rocky limestone southeast-facing slopes of the Negev desert. The plant is highly active during the winter, and semideciduous during the dry summer, i.e., it sheds its leaflets, while leaving the thick, fleshy petiole green and rather active during the dry season. Being resistant to extreme perennial drought, Z. dumosum appears to provide an intriguing model plant for studying epigenetic mechanisms associated with drought tolerance in natural habitats. The transition from the wet to the dry season was accompanied by a significant decrease in nuclear size and with posttranslational modifications of histone H3 N-terminal tail. Dimethylation of H3 at lysine 4 (H3K4)--a modification associated with active gene expression--was found to be high during the wet season but gradually diminished on progression to the dry season. Unexpectedly, H3K9 di- and trimethylation as well as H3K27 di- and trimethylation could not be detected in Z. dumosum; H3K9 monomethylation appears to be prominent in Z. dumosum during the wet but not during the dry season. Contrary to Z. dumosum, H3K9 dimethylation was detected in other desert plants, including Artemisia sieberi, Anabasis articulata and Haloxylon scoparium. Taken together, our results demonstrate dynamic genome organization and unique pattern of histone H3 methylation displayed by Z. dumosum, which could have an adaptive value in variable environments of the Negev desert.

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.

Fruit tree production is challenged by climate change, which is characterized by heat waves, warmer winters, increased storms, and recurrent droughts. The technology of top netting may provide a partial solution, as it alleviates climatic effects by microclimate manipulation. The tree physiological performance is improved under the nets, with an increased productivity and quality. The application of photoselective nets, which also alter the light spectrum, may result in additional horticultural improvements. We present the results of a 5-year experimental study on Valencia oranges, examining three nets: red, pearl, and transparent. Each net was tested at three fertigation conditions: a field standard (100%, I100) and two reduced fertigation regimes, which were 80% (I80) and 60% (I60) of the standard. The average multi-annual yield under the red and pearl nets with I100 and I80 and transparent net with I100 was significantly higher than that of the control trees. While the multi-annual yield increase under the red net I80 was due to the increase in the fruit number, in other treatments, the effect was mostly due to induction in the individual fruit weight. The data presented here show that an increased productivity of orange trees grown under photoselective nets, particularly the red net, with its specific spectral properties, was achieved with a considerable water-saving effect.