| Plants compensate for their immobility by an enhanced ability to detect different types of external cues, which allows them to coordinate their growth and development with changing environmental conditions. The switch from vegetative growth to reproductive growth (flowering) is a critical developmental transition for angiosperms, and as such, is regulated by a complex network of genes. To identify new regulators of flowering time in the model plant Arabidopsis thaliana, we performed a genetic screen to identify early-flowering mutants. Our study has revealed two genes that can affect flowering time. The first gene, AtTPR, encodes a protein that is part of the nuclear pore complex (NPC). The NPC is responsible for controlling the cellular traffic of macromolecules between the nucleus and the cytoplasm. Our results show that mRNAs are retained in the nucleus in the attpr mutant, which precludes them from being translated into proteins. As a result, the early-flowering phenotype of the mutant can be explained by the nuclear retention of mRNAs coding for proteins that repress flowering. The second gene that we identified in the screen is ATXR6, which encodes a chromatin-modifying protein. Chromatin is primarily composed of histones and DNA, which are both chemically modified by specific proteins. These modifications serve as signals to regulate the compaction of the chromatin and the transcriptional status of different loci in the genome. The ATXR6 protein and its close homolog ATXR5 are responsible for the deposition of a single methyl group at lysine 27 of histone 3 (H3K27me1). We discovered that this specific modification promotes the condensation of the chromatin and gene silencing. In addition, our work unveiled an additional role for H3K27me1 in controlling DNA replication. Taken together, our work on AtTPR, ATXR5 and ATXR6 shows that flowering time is a useful readout for the identification of genes required for the global regulation of gene expression in Arabidopsis. |
