MOLECULAR BASES OF SVP REGULATORY FUNCTIONS IN ARABIDOPSIS THALIANA

Flowering time regulation has a strong impact on plant life cycle, since it allows plants to flower and to reproduce under environmental permissive conditions. Several genes are involved in the regulatory pathways that determine the floral transition step, i.e. the switch from the plant vegetative phase to the reproductive phase and the consequent flower formation and fruit set. Among those genes, SHORT VEGETATIVE PHASE (SVP), a MADS box transcription factor, acts as strong repressor of the so called florigen promoting genes, FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1). Moreover, SVP has been also reported to act as a repressor of flower homeotic gene expression, thus ensuring the correct maintenance of floral meristem identity. Due to the relevance of SVP in both such important plant developmental stages, during my Ph.D. research program I tried to elucidate the molecular mechanisms at the basis of SVP activities. That has been done through different and complementary strategies that had the dual aim to identify SVP protein partners and to move the first steps towards the comprehension of the role of chloroplasts and chloroplast-nucleus signaling pathways in SVP functions. Co-immunoprecipitation assays followed by Mass Spectrometry analyses have allowed to draw up a list of Arabidopsis putative robust SVP interactors involved, at different levels, in chromatin organization and histone modification. Interestingly, the detailed characterization of the major Arabidopsis trimethyltransferase enzyme, SET DOMAIN GROUP 2 (SDG2), has revealed the existence of an SVP-SDG2 containing protein complex able to regulate the expression of SVP gene at the vegetative and reproductive meristems, by affecting the H3K4 methylation pattern within the first exon of SVP. Furthermore, our interests on the role of chloroplast-nucleus communication and its possible interactions with the flowering time regulation, have been met through the detailed characterization of two chloroplast-located PENTATRICO-PEPTIDE-REPEAT (PPR) containing proteins, which share three main features: i) they are part of the chloroplast gene expression machinery, ii) they are involved in chloroplast-nucleus communication, iii) they have been reported to be target genes of SVP by ChiP-seq assays. The detailed characterization of the Arabidopsis PPR proteins, GENOME UNCOUPLED 1 (GUN1) and CHLOROPLAST RNA PROCESSING 1 (AtCRP1), has provided the first preliminary insights into how chloroplast-nucleus signaling mechanisms may enable higher plants to more effectively adapt to the ever-changing internal and external conditions and mitigate detrimental effects to fitness.