The E3 Ubiquitin Ligase HOS1 Regulates Arabidopsis Flowering by Mediating CONSTANS Degradation Under Cold Stress

Abstract

Background: Intermittent cold stress delays flowering. This results from interaction between the cold and photoperiodic pathways. Results: CO protein is degraded through an HOS1-mediated ubiquitination mechanism during brief cold treatments. Conclusion: CO acts as a molecular hub that integrates photoperiod and cold signals into the flowering genetic pathways. Significance: The CO-HOS1 module is crucial for fine-tuning of photoperiodic flowering under short term temperature fluctuations.

Red Light-Mediated Degradation of CONSTANS by the E3 Ubiquitin Ligase HOS1 Regulates Photoperiodic Flowering in Arabidopsis

“…The early flowering phenotype of hos1 mutants is due to the precocious upregulation of FT expression levels and is completely suppressed by mutations in the CO gene in the Landsberg erecta (Ler) background (Lazaro et al, 2012). Moreover, we showed that HOS1 physically interacts with CO in Arabidopsis and that the CO protein abnormally accumulates in the hos1 mutant during the early hours of the daylight period (Jung et al, 2012;Lazaro et al, 2012). Altogether, these observations suggest that a complex regulatory mechanism ensures that CO accumulates only at the end of a LD, allowing a correct seasonal response.…”

“…Previous results showed that HOS1 and CO proteins physically interact in planta (Jung et al, 2012;Lazaro et al, 2012). To explore the possibility of a direct interaction between phyB and HOS1 or CO, we expressed the C-terminal part of HOS1 (cHOS1), the C-terminal part of phyB (cPHYB), and full-length CO and COP1 in yeast cells to perform two-hybrid experiments (Yu et al, 2008).…”

“…To explore the possibility of a direct interaction between phyB and HOS1 or CO, we expressed the C-terminal part of HOS1 (cHOS1), the C-terminal part of phyB (cPHYB), and full-length CO and COP1 in yeast cells to perform two-hybrid experiments (Yu et al, 2008). Based on the evidence that the C-terminal domain of phyB is directly involved in interactions with signaling partners (Quail et al, 1995;Ni et al, 1998) and that only cHOS1 interacts with CO in yeast (Jung et al, 2012), we used these fragments instead of the full-length proteins in the yeast two-hybrid assays. As shown in Figure 6A, we observed that when we coexpressed cHOS1 together with either CO or cPHYB, yeast cells could grow in selective medium, revealing a protein interaction between them, while the coexpression of cHOS1 and COP1 did not allow yeast cells to grow, meaning that no direct interaction takes place between these two E3 ubiquitin ligases.…”

“…A recent report has shown that intermittent cold treatments trigger the degradation of CO through a HOS1-mediated ubiquitination process (Jung et al, 2012). Interestingly, cold-induced regulation of CO abundance occurs independently of phyB because the effects of cold on CO degradation are maintained in the phyB-9 background (Jung et al, 2012(Jung et al, , 2014.…”

“…A recent report has shown that intermittent cold treatments trigger the degradation of CO through a HOS1-mediated ubiquitination process (Jung et al, 2012). Interestingly, cold-induced regulation of CO abundance occurs independently of phyB because the effects of cold on CO degradation are maintained in the phyB-9 background (Jung et al, 2012(Jung et al, , 2014. Here, we present evidence indicating that the degradation of CO under RL is prevented in the hos1 mutant at normal growth temperatures; therefore, we propose that HOS1 is involved in the RL-mediated degradation of CO. Complementation studies reveal that phyB plays a key role in the vascular tissue to regulate flowering time, similarly to CO and HOS1 Lazaro et al, 2012).…”