Plants, as sessile organisms, need to sense and adapt to heterogeneous environments and have developed sophisticated responses by changing their cellular physiology, gene regulation, and genome stability. Recent work demonstrated heritable stress effects on the control of genome stability in plants—a phenomenon that was suggested to be of epigenetic nature. Here, we show that temperature and UV-B stress cause immediate and heritable changes in the epigenetic control of a silent reporter gene in Arabidopsis. This stress-mediated release of gene silencing correlated with pronounced alterations in histone occupancy and in histone H3 acetylation but did not involve adjustments in DNA methylation. We observed transmission of stress effects on reporter gene silencing to non-stressed progeny, but this effect was restricted to areas consisting of a small number of cells and limited to a few non-stressed progeny generations. Furthermore, stress-induced release of gene silencing was antagonized and reset during seed aging. The transient nature of this phenomenon highlights the ability of plants to restrict stress-induced relaxation of epigenetic control mechanisms, which likely contributes to safeguarding genome integrity.

SUMMARYThis work shows that a successful heat response depends on the integrity of epigenetic pathways and provides evidence that heat-dependent gene expression is influenced by closely located transposon sequences and read-through transcription.

SummaryWe report the characterization of rice OsHKT1 (Oryza sativa ssp. indica) homologous to the wheat K + / Na + -symporter HKT1. Expression of OsHKT1 in the yeast strain CY162 defective in K + -uptake restored growth at mM and mM concentrations of K + and mediated hypersensitivity to Na + . When expressed in Xenopus oocytes, rice OsHKT1 showed uptake characteristics of a Na + -transporter but mediated transport of other alkali cations as well. OsHKT1 expression was analysed in salt-tolerant rice Pokkali and salt-sensitive IR29 in response to external cation concentrations. OsHKT1 is expressed in roots and leaves. Exposure to Na + , Rb + , Li + , and Cs + reduced OsHKT1 transcript amounts in both varieties and, in some cases, incompletely spliced transcripts were observed. By in situ hybridizations the expression of OsHKT1 was localized to the root epidermis and the vascular tissue inside the endodermis. In leaves, OsHKT1 showed strongest signals in cells surrounding the vasculature. The repression of OsHKT1 in the two rice varieties during salt stress was different in various cell types with main differences in the root vascular tissue. The data suggest control over HKT expression as a factor that may distinguish salt stress-sensitive and stress-tolerant lines. Differences in transcript expression in space and time in different lines of the same species appear to be a component of ion homeostasis correlated with salt sensitivity and tolerance.

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Background: Plants respond to extracellularly perceived abiotic stresses such as low temperature, drought, and salinity by activation of complex intracellular signaling cascades that regulate acclimatory biochemical and physiological changes. Protein kinases are major signal transduction factors that have a central role in mediating acclimation to environmental changes in eukaryotic organisms. In this study, we characterized the function of the sucrose nonfermenting 1-related protein kinase2 (SnRK2) SAPK4 in the salt stress response of rice.

This study characterizes the expression and functional significance of the member of the matrix metalloproteinase (MMP) family At2-MMP from Arabidopsis. By transcript analysis, expression of At2-MMP was found in leaves and roots of juvenile Arabidopsis and leaves, roots, and inflorescences of mature flowering plants showing strong increase of transcript abundance with aging. Cell specificity of expression of At2-MMP was studied by in situ hybridizations in leaves and flowers of Arabidopsis. In leaves, the gene was expressed in the phloem, in developing xylem elements, epidermal cells, and neighboring mesophyll cell layers. In flowers, signals were localized in pistils, ovules, and receptacles. In an Arabidopsis mutant (at2-mmp-1) carrying a tDNA insertion in At2-MMP, neither germination nor development of plants was modified in comparison to the wild type in the juvenile rosette stage. Starting with the onset of shoots, growth of roots, leaves, and shoots was inhibited compared with the wild type, and the plants were characterized by late flowering. Besides the flowering, at2-mmp-1 plants showed fast degradation of chlorophyll in leaves and early senescence. These results demonstrate the involvement of At2-MMP in plant growth, morphogenesis, and development with particular relevance for flowering and senescence.The family of zinc-dependent endopeptidases that has been particularly characterized in vertebrates is divided into four subfamilies based on structural and functional characteristics: matrix metalloproteinases (matrixins), adamalysins, serralysins, and astacins. All members of these zinc metalloproteinases have a similar structure with the conserved consensus sequence HEXXHXXGXXH in their catalytic site and a conserved methionine residue that forms the "Met-turn" structure (1). Members of the subfamily of matrix metalloproteinases are gelatinases, collagenases, and stromelysins (2). Matrix metalloproteinases are synthesized as prepro-enzymes with a signaling peptide targeting the enzyme to the extracellular space. The pre-domain is followed by a pro-peptide with a conserved PRCG(V/N)PD motif that contains the so-called "cysteine switch." This Cys residue ligates the catalytic zinc thus maintaining the latency of the inactive pro-enzymes (3). In vitro, pro-matrix metalloproteinases can be activated proteolytically by proteinases as well as by mercurial compounds and reactive oxygen, for example, whereas in vivo activation by proteinases is most likely (3). The function of matrix metalloproteinases is the degradation and remodeling of the extracellular matrix (2). The enzymes play a role in development, embryogenesis, and organ morphogenesis but also in wound healing in vertebrates (4). They also participate in pathological processes such as cancer and arthritis (5). In vertebrates, the enzyme activity of matrix metalloproteinases is transcriptionally regulated as well by proteolytic activation of the mature enzyme from the inactive pro-enzyme and by co-secretion with endogenous inhibitors (4).In addition ...

* Genomic predictions were filtered by retaining only hits to the InterPro database and cleaned from the cephalopod contamination with BLAST searches against the NCBI nr database. * * Transcriptome assemblies were filtered with BLAST searches against the RefSeq database as detailed in Materials and Methods, Section "Assembly and Filtering of Dicyemid Sequences."

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