The RING-Finger Ubiquitin Ligase HAF1 Mediates Heading date 1 Degradation during Photoperiodic Flowering in Rice.

Ying Yang,Yizhou He,Huijun Zhang,Debao Fu,Tao Liu,Changyin Wu,Xianghua Li,Chunmei Zhu

doi:10.1105/tpc.15.00320

Abstract

The photoperiodic response is one of the most important factors determining heading date in rice (Oryza sativa). Although rhythmic expression patterns of flowering time genes have been reported to fine-tune the photoperiodic response, posttranslational regulation of key flowering regulators has seldom been elucidated in rice. Heading date 1 (Hd1) encodes a zinc finger transcription factor that plays a crucial role in the photoperiodic response, which determines rice regional adaptability. However, little is known about the molecular mechanisms of Hd1 accumulation during the photoperiod response. Here, we identify a C3HC4 RING domain-containing E3 ubiquitin ligase, Heading date Associated Factor 1 (HAF1), which physically interacts with Hd1. HAF1 mediates ubiquitination and targets Hd1 for degradation via the 26S proteasome-dependent pathway. The haf1 mutant exhibits a later flowering heading date under both short-day and long-day conditions. In addition, the haf1 hd1 double mutant headed as late as hd1 plants under short-day conditions but exhibited a heading date similar to haf1 under long-day conditions, thus indicating that HAF1 may determine heading date mainly through Hd1 under short-day conditions. Moreover, high levels of Hd1 accumulate in haf1. Our results suggest that HAF1 is essential to precise modulation of the timing of Hd1 accumulation during the photoperiod response in rice.

Highlights

Rice (Oryza sativa) is one of the most important cereal crops in the world
The expression of two florigen genes, RFT1 in LD and Hd3a in SD (Tamaki et al, 2007; Komiya et al, 2008), was significantly reduced in haf1 (Figures 4I and 4J). These results suggested that Heading date Associated Factor 1 (HAF1) functions upstream of Ehd1, Heading date 1 (Hd1), RFT1, and Hd3a in the LD and SD photoperiodic flowering pathway
Ubiquitin-mediated degradation of proteins in the photoperiodic flowering pathway has been well characterized in Arabidopsis, little is known about the molecular mechanism responsible for circadian accumulation of flowering regulators in rice

Summary

Introduction

Rice (Oryza sativa) is one of the most important cereal crops in the world. Heading date in rice is a critical determinant of regional adaptability and grain production. The molecular basis for regulation of flowering has been extensively studied using Arabidopsis thaliana and rice, which represent long-day (LD) and SD plants (Song et al, 2015), respectively. A previous study showed that overexpression of rice GI suppressed Hd3a expression, resulting in late flowering under both SD and LD conditions (Hayama et al, 2003). This result indicated that rice GI functions oppositely to GI in Arabidopsis and suppresses flowering in rice (Fowler et al., 1999; Hayama et al, 2003). Further investigation showed that increased expression of Hd1 coincides with overexpression of GI, suggesting that GI-promoted Hd1 homolog expression is conserved between rice and Arabidopsis (Hayama et al, 2003)

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