SIGNALING SALT STRESS TO THE CHROMATIN

Salinity is a major stress for plants and has a severe effect on agricultural yield worldwide. Plants respond to high salt conditions via complex mechanisms, including adjustment of gene expression. Protein kinases play a key role in modulating gene expression by phosphorylation of transcription factors or co-regulatory proteins. Recently, a role emerged for protein kinases of binding to promoters and coding regions in gene expression, but the significance of plant stress- signaling kinases on chromatin function remains elusive. In our recent work we discovered a salt stress-responsive protein kinase which is able to bind to chromatin-associated proteins such as histones and AtDEK3. AtDEK3 is a novel plant chromatin component which we identified as a regulator of chromatin accessibility and salt stress tolerance. In the proposed project we now aim to (I) identify the direct targets of the protein kinase at the chromatin, (II) analyze the interaction between the protein kinase and AtDEK3, (III) study the role of the kinase and AtDEK3 on gene expression in response to salt stress. To address these aims, we will combine molecular, genetic and biochemical techniques with physiological analyses in Arabidopsis thaliana, and we expect to gain novel insights into how salt stress signaling is linked to the chromatin-associated processes.

SIGNALLING SALT STRESS TO THE CHROMATIN In the nucleus of eukaryotes, DNA is tightly packed into chromatin. Despite this complex packing, chromatin is highly dynamic enabling the precise control of diverse biological processes. The balance between restricting and opening of regulatory sequences on the DNA contributes to the complex regulation the expression of gene and needs to be adjusted to the prevailing conditions and is fine-tuned by chromatin remodelling proteins. Despite the eminent and wide-spread importance of the regulation of chromatin accessibility, our knowledge on how signalling inputs are transmitted to the chromatin is limited. Throughout the kingdoms, the Glycogen Synthase Kinase 3 (GSK3) family is known to regulate gene expression by phosphorylation of transcription factors. Our work now revealed a novel function for GSK3 kinases in transcriptional regulation that goes beyond the regulation via transcription factor phosphorylation and involves the modulation of chromatin structure. Unexpectedly, we found that ASK, a member of the GSK3 family from Arabidopsis thaliana, interacts with and phosphorylates the chromatin protein DEK3. DEK3 is an evolutionally conserved chromatin architectural protein regulating different important chromatin-related processes. Remarkably, this specific phosphorylation alters DEK3 protein complex composition, affects nucleosome occupancy and chromatin accessibility that is translated into changes in the expression of DEK3 target genes. We showed that ASK is a new stress-activated protein kinase positively regulating salt stress resistance. Comprehensive biochemical data and genetic analyses revealed that DEK3 phosphorylation by ASK contributes to salt stress tolerance. Based on these findings we propose a model in which GSK3-based signalling is directly linked to the chromatin. In addition to stress responses, plant GSK3 family members have important regulatory functions in development and hormone signalling. Thus, our data, unravelling the molecular interaction between the GSK3-type signalling kinase ASK and the chromatin architectural protein DEK3, will likely have implications well beyond a better understanding of salt stress signalling to the chromatin.