BRM Complex in Arabidopsis Adopts ncBAF-like Composition and Requires BRD Subunits for Assembly and Stability.

ATP-dependent SWI/SNF chromatin remodelling complexes are conserved multi-subunit assemblies that control genome activity. Functions of SWI/SNF complexes in plant development and growth have been well established, but the architecture of particular assemblies is unclear. In this study, we elucidate the organization of Arabidopsis SWI/SNF complexes formed around a BRM catalytic subunit, and define the requirement of bromodomain-containing proteins BRD1/2/13 for the formation and stability of the entire complex. Using affinity purification followed by mass spectrometry, we identify a set of BRM-associated subunits and demonstrate that the BRM complexes strongly resemble mammalian non-canonical BAF complexes. Furthermore, we identify BDH1 and 2 proteins as components of the BRM complex and, using mutant analyses, show that BDH1/2 are important for vegetative and generative development, as well as hormonal responses. We further show that BRD1/2/13 represent unique subunits of the BRM complexes, and their depletion severely affects the integrity of the complex, resulting in the formation of residual assemblies. Finally, analyses of BRM complexes after proteasome inhibition revealed the existence of a module consisting of the ATPase, ARP, and BDH proteins, assembled with other subunits in a BRD-dependent manner. Together, our results suggest modular organization of plant SWI/SNF complexes and provide a biochemical explanation for mutant phenotypes.

Norway spruce deploys tissue-specific responses during acclimation to cold.

Climate change in the conifer-dominated boreal forest is expected to lead to warmer but more dynamic winter air temperatures, reducing the depth and duration of snow cover and lowering winter soil temperatures. To gain insight into the mechanisms that have enabled conifers to dominate extreme cold environments, we performed genome-wide RNA-Seq analysis from needles and roots of non-dormant two-year Norway spruce (Picea abies (L.) H. Karst), and contrasted these response to herbaceous model Arabidopsis We show that the main transcriptional response of Norway spruce needles exposed to cold was delayed relative to Arabidopsis, and this delay was associated with slower development of freezing tolerance. Despite this difference in timing, Norway spruce principally utilizes early response transcription factors (TFs) belonging to the same gene families as Arabidopsis, indicating broad evolutionary conservation of cold response networks. In keeping with their different metabolic and developmental states, needles and root of Norway spruce showed contrasting results. Regulatory network analysis identified both conserved TFs with known roles in cold acclimation (e.g. homologs of ICE1, AKS3, and of the NAC and AP2/ERF superfamilies), but also a root-specific bHLH101 homolog, providing functional insights into cold stress response strategies in Norway spruce.

Bromodomain-containing subunits BRD1, BRD2, and BRD13 are required for proper functioning of SWI/SNF complexes in Arabidopsis.

SWI/SNF chromatin remodelers are evolutionarily conserved multiprotein complexes that use the energy of ATP hydrolysis to change chromatin structure. A characteristic feature of SWI/SNF remodelers is the occurrence in both the catalytic ATPase subunit and some auxiliary subunits, of bromodomains, the protein motifs capable of binding acetylated histones. Here, we report that the Arabidopsis bromodomain-containing proteins BRD1, BRD2, and BRD13 are likely true SWI/SNF subunits that interact with the core SWI/SNF components SWI3C and SWP73B. Loss of function of each single BRD protein caused early flowering but had a negligible effect on other developmental pathways. By contrast, a brd triple mutation (brdx3) led to more pronounced developmental abnormalities, indicating functional redundancy among the BRD proteins. The brdx3 phenotypes, including hypersensitivity to abscisic acid and the gibberellin biosynthesis inhibitor paclobutrazol, resembled those of swi/snf mutants. Furthermore, the BRM protein level and occupancy at the direct target loci SCL3, ABI5, and SVP were reduced in the brdx3 mutant background. Finally, a brdx3 brm-3 quadruple mutant, in which SWI/SNF complexes were devoid of all constituent bromodomains, phenocopied a loss-of-function mutation in BRM. Taken together, our results demonstrate the relevance of BRDs as SWI/SNF subunits and suggest their cooperation with the bromodomain of BRM ATPase.

Circadian and Plastid Signaling Pathways Are Integrated to Ensure Correct Expression of the CBF and COR Genes during Photoperiodic Growth.

The circadian clock synchronizes a wide range of biological processes with the day/night cycle, and correct circadian regulation is essential for photosynthetic activity and plant growth. We describe here a mechanism where a plastid signal converges with the circadian clock to fine-tune the regulation of nuclear gene expression in Arabidopsis (Arabidopsis thaliana). Diurnal oscillations of tetrapyrrole levels in the chloroplasts contribute to the regulation of the nucleus-encoded transcription factors C-REPEAT BINDING FACTORS (CBFs). The plastid signal triggered by tetrapyrrole accumulation inhibits the activity of cytosolic HEAT SHOCK PROTEIN90 and, as a consequence, the maturation and stability of the clock component ZEITLUPE (ZTL). ZTL negatively regulates the transcription factor LONG HYPOCOTYL5 (HY5) and PSEUDO-RESPONSE REGULATOR5 (PRR5). Thus, low levels of ZTL result in a HY5- and PRR5-mediated repression of CBF3 and PRR5-mediated repression of CBF1 and CBF2 expression. The plastid signal thereby contributes to the rhythm of CBF expression and the downstream COLD RESPONSIVE expression during day/night cycles. These findings provide insight into how plastid signals converge with, and impact upon, the activity of well-defined clock components involved in circadian regulation.