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

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 proteasomedependent 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.

Research Progress on Plant RING-Finger Proteins

“…It was also found that HOS1 controls the transcriptional activity of phytochrome interacting factor 4 (PIF4) to participate in phyB-mediated signal light morphogenesis [42]. Another RING-finger protein heading date associated factor 1 (OsHAF1) in rice also participates in the photoperiod response process by the ubiquitination degradation of heading date 1 (HD1) [5].…”

“…Really Interesting New Gene (RING)-finger proteins, as a large family of E3 types, exist widely in eukaryotes. Previous studies have shown that RING-finger proteins are widely involved in the regulation of various physiological and biochemical processes, including plant growth and development, stress resistance, and hormone signaling responses [2][3][4][5][6]. However, compared with known DNA-binding zinc finger domains, the RING-finger domain acts as a protein-protein interaction domain [2,7] and is necessary to catalyze the E3 ligase activity of RING-finger proteins [8].…”

The E3 Ubiquitin Ligase HAF1 Modulates Circadian Accumulation of EARLY FLOWERING3 to Control Heading Date in Rice under Long-Day Conditions

“…To explore the molecular mechanism underlying how HAF1 regulates heading under LDs, we performed yeast two-hybrid assays to examine the possible physical interactions between HAF1 and OsELF3. Due to the autotranscriptional activation activity of HAF1, truncated fragments of HAF1 (amino acids 1-112, 99-619, 288-619, 288-667, and 620-667) with no transcriptional activation activity (Yang et al, 2015) and fulllength HAF1 protein were used to transform yeast cells individually as prey and full-length OsELF3 protein as bait to determine whether HAF1 interacts with OsELF3. The yeast two-hybrid assays showed that constructs containing fragments amino acids 99 to 619 or 288 to 667 of HAF1 and full-length HAF1 protein successfully interacted with OsELF3 ( Figure 1A).…”

“…The yeast two-hybrid assays showed that constructs containing fragments amino acids 99 to 619 or 288 to 667 of HAF1 and full-length HAF1 protein successfully interacted with OsELF3 ( Figure 1A). Considering that the C3HC4 domain is present in the C-terminal region of HAF1 (Yang et al, 2015), its interaction with OsELF3 might occur through the C3HC4 domain of HAF1. We therefore truncated OsELF3 into five fragments (amino acids 1-348, 1-519, 305-519, 305-760, and 503-760) for the interaction assay.…”

“…Although multiple E3 ubiquitin ligases involved in flowering have been characterized in Arabidopsis, only a few have been studied in rice. We previously revealed that HAF1, a C3HC4 RING domain-containing E3 ubiquitin ligase, is essential to precisely modulate the timing of Hd1 accumulation and to ensure an appropriate photoperiodic response under SDs (Yang et al, 2015). Under LDs, the haf1 hd1 double mutant flowers as late as haf1, suggesting that HAF1 may interact with additional components to control heading date under LDs (Yang et al, 2015).…”