The phytohormone abscisic acid (ABA) mediates drought responses in plants and, in particular, triggers stomatal closure. Snf1-related kinase 2 (SnRK2) proteins from several plant species have been implicated in ABA-signaling pathways. In Arabidopsis (Arabidopsis thaliana) guard cells, OPEN STOMATA 1 (OST1)/SRK2E/SnRK2-6 is a critical positive regulator of ABA signal transduction. A better understanding of the mechanisms responsible for SnRK2 protein kinase activation is thus a major goal toward understanding ABA signal transduction. Here, we report successful purification of OST1 produced in Escherichia coli: The protein is active and autophosphorylates. Using mass spectrometry, we identified five target residues of autophosphorylation in recombinant OST1. Sequence analysis delineates two conserved boxes located in the carboxy-terminal moiety of OST1 after the catalytic domain: the SnRK2-specific box (glutamine-303 to proline-318) and the ABA-specific box (leucine-333 to methionine-362). Site-directed mutagenesis and serial deletions reveal that serine (Ser)-175 in the activation loop and the SnRK2-specific box are critical for the activity of recombinant OST1 kinase. Targeted expression of variants of OST1 kinase in guard cells uncovered additional features that are critical for OST1 function in ABA signaling, although not required for OST1 kinase activity: Ser-7, Ser-18, and Ser-29 and the ABA-specific box. Ser-7, Ser-18, Ser-29, and Ser-43 represent putative targets for regulatory phosphorylation and the ABA-specific box may be a target for the binding of signaling partners in guard cells.

SignificanceDNA damage triggers a highly conserved response that coordinates processes necessary to maintain genome integrity, including cell cycle arrest, DNA repair, and cell death. Despite the identification of primary transcription factors (TFs) that control these processes, knowledge regarding the downstream genes and regulatory networks controlled by these TFs remains poorly understood. Using Arabidopsis, we generated the first model of the DNA damage response transcriptional network, revealing 11 coexpressed gene groups with distinct biological functions and cis-regulatory features. Our characterization of this model demonstrates that SOG1 and three MYB3R TFs are, respectively, the major activator and repressors within this network, coordinating the rapid induction of DNA repair genes and TF cascades as well as the subsequent repression of cell cycle genes.

The spatial organization of chromatin can be subject to extensive remodeling in plant somatic cells in response to developmental and environmental signals. However, the mechanisms controlling these dynamic changes and their functional impact on nuclear activity are poorly understood. Here, we determined that light perception triggers a switch between two different nuclear architectural schemes during Arabidopsis postembryonic development. Whereas progressive nucleus expansion and heterochromatin rearrangements in cotyledon cells are achieved similarly under light and dark conditions during germination, the later steps that lead to mature nuclear phenotypes are intimately associated with the photomorphogenic transition in an organ-specific manner. The light signaling integrators DE-ETIOLATED 1 and CONSTITUTIVE PHOTOMORPHOGENIC 1 maintain heterochromatin in a decondensed state in etiolated cotyledons. In contrast, under light conditions cryptochrome-mediated photoperception releases nuclear expansion and heterochromatin compaction within conspicuous chromocenters. For all tested loci, chromatin condensation during photomorphogenesis does not detectably rely on DNA methylation-based processes. Notwithstanding, the efficiency of transcriptional gene silencing may be impacted during the transition, as based on the reactivation of transposable element-driven reporter genes. Finally, we report that global engagement of RNA polymerase II in transcription is highly increased under light conditions, suggesting that cotyledon photomorphogenesis involves a transition from globally quiescent to more active transcriptional states. Given these findings, we propose that light-triggered changes in nuclear architecture underlie interplays between heterochromatin reorganization and transcriptional reprogramming associated with the establishment of photosynthesis. plant development | photomorphogenesis | light signaling | nuclear organization | heterochromatin C hromatin allows dense packaging of chromosomal DNA into a small and constrained nuclear space. It also serves as a structural framework for regulatory processes that control the functional status of genome domains with variable sizes, notably by dynamically influencing the subnuclear partitioning of generich and repeat-rich domains within euchromatic and heterochromatic regions, respectively (reviewed in refs. 1-3). These chromatin-based processes impact plasticity in the regulation of nuclear programs during both vegetative and reproductive phases of the plant life cycle (recently reviewed in refs. 4-8). In many cases, such events combine local chromatin variations with prominent changes in nuclear architecture and higher-order chromatin organization.Spatial chromatin organization is well exemplified by the extreme cases of transcriptionally silent chromocenters that contain the majority of ribosomal DNA repeats, such as the (peri)centromeric domains of mice and several plant species that include transposable elements (TE) and non-TE repeated sequences (9,