Investigation of a potential function of COR27 and COR28 in phytochrome-mediated chromatin remodelling

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Light is one of the most important environmental factors controlling almost all aspect of plant growth and development. Plants possess a plethora of proteins involved in light perception and light signal transduction to sense and adapt to the light conditions in their environment. In addition, many factors involved in light signalling also have a function in crosstalk between different signalling pathways to integrate internal and external stimuli. Light signalling in plants begins with the perception of light quality and quantity by photoreceptors. Among them, phytochromes (phys) are mainly responsible for red- and far- red light-dependent responses. With COLD REGULATED GENE 27 (COR27), and its homologue COR28, we have identified two proteins that interact directly with phyA and phyB and play a role in light-dependent hypocotyl growth, induction of flowering, regulation of circadian rhythms, and freezing tolerance. Thus, they are likely involved in the integration of light and temperature signals. The cor27-2 cor28-2 double mutant is hypersensitive to red light in a phyB-dependent manner and flowers late under long-day conditions, while seedlings overexpressing COR27 or COR28 are hyposensitive to red light. Furthermore, we have shown that COR27 and COR28 are targets of the CONSTITUTIVELY PHOTOMORPHOGENIC 1/SUPPRESSOR OF phyA-105 (COP1/SPA) E3 ubiquitin ligase complex and stabilised by light, probably through photoreceptor-dependent inactivation of COP1/SPA. Recently, COR27 and COR28 were shown to bind to ELONGATED HYPOCOTYL 5 (HY5), a central transcription factor involved in light signalling, thereby repressing the transcriptional activity of HY5. However, RNAseq data and phenotypic analysis revealed that the effect of COR27 and COR28 on seedling development is largely independent of HY5. Therefore, the main pathways of COR27 and COR28 action remain elusive. In the proposed project, we plan to investigate the relevance of the direct association of COR27 and COR28 with phyA and phyB, and examine potential COR27/COR28 interaction partners that we recently identified by AP-MS and that point to a role of COR27 and COR28 in chromatin remodelling. Consequently, we aim to analyse whether COR27 and COR28 affect chromatin density and histone modifications, and we want to identify direct target genes of COR27 and COR28 using a ChIP-seq approach. Overall, the aim of the proposed project is to understand how phytochromes, COR27 and COR28, and factors involved in regulation of chromatin density and histone modifications act together to control light-regulated gene expression.