Bruno-like proteins modulate flowering time via 3′ UTR-dependent decay of SOC1 mRNA

The Bruno RNA-binding protein (RBP) has been shown to initially repress the translation of oskar mRNA during Drosophila oogenesis and later to be involved in a broad range of RNA regulation. Here, we show that homologous constitutive overexpression of each of two Arabidopsis thaliana Bruno-like genes, AtBRN1 and AtBRN2, delayed the flowering time, while the atbrn1 atbrn2-3 double mutant flowered early and exhibited increased expression of APETALA1 (AP1) and LEAFY (LFY) transcripts. Crossing of 35S::AtBRNs with SOC1 101-D plants demonstrated that 35S::AtBRNs suppress an early-flowering phenotype of SOC1 101-D in which the coding sequence (CDS) with the 3 UTR of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) gene is overexpressed. However, this early-flowering phenotype by SOC1 overexpression was maintained in the plants coexpressing 35S::AtBRNs and 35S::SOC1 without the 3 UTR (3 UTR). Using yeast three-hybrid, electrophoretic mobility shift, RNA immunoprecipitation, and protoplast transient assays, we found that AtBRNs bind to the 3 UTR of SOC1 RNA and participate in mRNA decay, which was mediated by the distal region of the SOC1 3 UTR. Overall, AtBRNs repress SOC1 activity in a 3 UTR-dependent manner, thereby controlling the flowering time in Arabidopsis.