The F-box protein FKF1 inhibits dimerization of COP1 in the control of photoperiodic flowering

Ganesh M Nawkar,Takato Imaizumi,Yasuhito Sakuraba,Su-Hyun Han,Joon-Yung Cha,Min-Young Kang,C Robertson Mcclung,Nam-Chon Paek,Sang Yeol Lee,Byoung-Doo Lee,Mi Ri Kim,Woe-Yeon Kim

doi:10.1038/s41467-017-02476-2

Abstract

In Arabidopsis thaliana, CONSTANS (CO) plays an essential role in the regulation of photoperiodic flowering under long-day conditions. CO protein is stable only in the afternoon of long days, when it induces the expression of FLOWERING LOCUS T (FT), which promotes flowering. The blue-light photoreceptor FLAVIN-BINDING, KELCH REPEAT, F-BOX1 (FKF1) interacts with CO and stabilizes it by an unknown mechanism. Here, we provide genetic and biochemical evidence that FKF1 inhibits CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1)-dependent CO degradation. Light-activated FKF1 has no apparent effect on COP1 stability but can interact with and negatively regulate COP1. We show that FKF1 can inhibit COP1 homo-dimerization. Mutation of the coiled-coil domain in COP1, which prevents dimer formation, impairs COP1 function in coordinating flowering time. Based on these results, we propose a model whereby the light- and day length-dependent interaction between FKF1 and COP1 controls CO stability to regulate flowering time.

Highlights

We found that the FKF1-overexpressing plants, such as 35S::Myc-FKF1 #3 and 35S::FKF1 #18, showed an early-flowering phenotype compared with wild type (WT, Col-0), and 35S::MycFKF1 #3/cop[1,2,3,4] plants flowered earlier than WT but similar to cop[1,2,3,4] in LD
It did not appear that high accumulation of CO negatively affected CO transcription, because native CO mRNA levels in 35S::CO-GFP were similar to those of WT (Fig. 2d). Taking these observations and those of a previous study[12] together, we suggest that FKF1 increases CO stability by reducing CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) function in the late afternoon of LD to induce flowering
In Arabidopsis, CO is a key positive regulator of FLOWERING LOCUS T (FT) transcription in an LD-dependent manner, FT expression is finely controlled by many regulators in other flowering pathways[2,4,6]

Summary

Introduction

FT transcription, controlled by CO, differs remarkably between LD and SD conditions because light signals tightly regulate CO at the posttranslational level Elucidation of these regulatory mechanisms showed that FLAVIN-BINDING, KELCH REPEAT, FBOX1 (FKF1) and CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) control CO stability[10,11,12]. The E3 ubiquitin ligases FKF1 and COP1 play critical roles in controlling photoperiodic flowering by directly regulating CO stability[10,12,15], as FKF1 stabilizes CO in the light and COP1 destabilizes CO in the dark. Other important regulators affect the stability of CO: (i) in the morning, HOS1 and phytochrome B (phyB) decrease CO stability[14,24,25], (ii) in the afternoon of LD, phyA, cryptochrome 2 (cry2), and FKF1 increase CO stability[10,12,26,27], and (iii) in darkness, COP1 mediates degradation of CO15

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Results

Conclusion