A novel repressor of floral transition, MEE3, an abiotic stress regulated protein, functions as an activator of FLC by binding to its promoter in Arabidopsis

• We show for the first time that MEE3 may act as a repressor of floral transition.
• MEE3 functions by activating FLC transcription via binding to its promoter in Arabidopsis.
• FT is downstream of and down-regulated by MEE3.

The floral transition is a key step for the sexual reproduction in the life cycle of flowering plants. FLOWERING LOCUS T (FT), is known as a floral promoter and integrator while FLOWERING LOCUS C (FLC) is a floral repressor inhibiting several floral promoters including FT. Arabidopsis MATERNAL EFFECT EMBRYO ARREST 3 (MEE3), predicted to be a member of I-box-binding-like proteins in MYB-related family, was initially briefly reported to control embryo development but without detailed description. Whether and how MEE3 is involved in floral transition remains totally unknown. In this work, we show for the first time that MEE3 may act as a novel repressor of floral transition by functioning as an activator of FLC via binding to its promoter in Arabidopsis. Overexpression of MEE3 caused a delay in flowering time under both long-day and short-day conditions. RNA-seq and RT-qPCR assays revealed that MEE3 upregulated FLC and downregulated FT and some other floral transition and floral meristem identity genes. Genetic analyses showed that MEE3 repressed floral transition and was likely upstream of FLC. A further combination of yeast one-hybrid, EMSA and ChIP-qPCR assays demonstrated that MEE3 is bound to the promoter of FLC.