VAL1/HSI2 promotes Arabidopsis flowering by repressing the organ boundary genes BLADE ON PETIOLE 1 (BOP1) and BOP2
The transition to reproductive development is a critical step in the plant lifecycle that relies on the integration of intrinsic and environmental signals. Several different pathways have been described for the control of flowering time that function downstream of the perception of environmental cues such as day length (photoperiodic pathway) and seasonal temperature (vernalization and ambient temperature pathways). In addition, the phytohormone gibberellin (GA) induces the floral transition under non-inductive photoperiod. In the model plant Arabidopsis thaliana, the transcriptional repressor VAL1/HSI2 triggers the stable repression of the floral repressor FLOWERING LOCUS C (FLC) during vernalization. However, the involvement of VAL1 in other flowering pathways has remained unclear. In this work, we combined a genetic and transcriptomic approach to investigate the requirement of VAL1 for the activation of flowering at different day length conditions. We found that VAL1, and not its sister protein VAL2, is required to induce the floral transition both under long and short days. Specifically in short days, the delayed flowering time of val1 mutant plants can be fully bypassed by the exogenous application of GA. We have been able to demonstrate that VAL1 induction of flowering occurs via the direct epigenetic repression of the organ boundary genes BLADE ON PETIOLE 1 (BOP1) and BOP2. Our work thus expands the repertoire of VAL target genes and further demonstrates the pleiotropic role of VAL factors in the regulation of Arabidopsis development. Overall design: RNA-seq from 6 weeks old shoot apic meristem in Arabidopsis thaliana plants growth at short day.