LEAFY activity is post‐transcriptionally regulated by BLADE ON PETIOLE 2 and CULLIN 3 in Arabidopsis

Summary The Arabidopsis LEAFY ( LFY ) transcription factor is a key regulator of floral meristem emergence and identity. LFY interacts genetically and physically with UNUSUAL FLORAL ORGANS , a substrate adaptor of CULLIN 1– RING ubiquitin ligase complexes ( CRL 1). The functionally redundant genes BLADE ON PETIOLE 1 ( BOP 1 ) and ‐ 2 ( BOP 2 ) are potential candidates to regulate LFY activity and have recently been shown to be substrate adaptors of CULLIN 3 ( CUL 3)– RING ubiquitin ligases ( CRL 3). We tested the hypothesis that LFY activity is controlled by BOP s and CUL 3 s in plants and that LFY is a substrate for ubiquitination by BOP ‐containing CRL 3 complexes. When constitutively expressed, LFY activity is fully dependent on BOP 2 as well as on CUL 3A and B to regulate target genes such as APETALA 1 and to induce ectopic flower formation. We also show that LFY and BOP 2 proteins interact physically and that LFY ‐dependent ubiquitinated species are produced in vitro in a reconstituted cell‐free CRL 3 system in the presence of LFY , BOP 2 and CUL 3. This new post‐translational regulation of LFY activity by CRL 3 complexes makes it a unique transcription factor subjected to a positive dual regulation by both CRL 1 and CRL 3 complexes and suggests a novel mechanism for promoting flower development.