Acceleration of Flowering during Shade Avoidance in Arabidopsis Alters the Balance betweenFLOWERING LOCUS C-Mediated Repression and Photoperiodic Induction of Flowering  

Abstract

The timing of the floral transition in Arabidopsis (Arabidopsis thaliana) is influenced by a number of environmental signals. Here, we have focused on acceleration of flowering in response to vegetative shade, a condition that is perceived as a decrease in the ratio of red to far-red radiation. We have investigated the contributions of several known flowering-time pathways to this acceleration. The vernalization pathway promotes flowering in response to extended cold via transcriptional repression of the floral inhibitor FLOWERING LOCUS C (FLC); we found that a low red to far-red ratio, unlike cold treatment, lessened the effects of FLC despite continued FLC expression. A low red to far-red ratio required the photoperiod-pathway genes GIGANTEA (GI) and CONSTANS (CO) to fully accelerate flowering in long days and did not promote flowering in short days. Together, these results suggest a model in which far-red enrichment can bypass FLC-mediated late flowering by shifting the balance between FLC-mediated repression and photoperiodic induction of flowering to favor the latter. The extent of this shift was dependent upon environmental parameters, such as the length of far-red exposure. At the molecular level, we found that far-red enrichment generated a phase delay in GI expression and enhanced CO expression and activity at both dawn and dusk. Finally, our analysis of the contribution of PHYTOCHROME AND FLOWERING TIME1 (PFT1) to shade-mediated rapid flowering has led us to suggest a new model for the involvement of PFT1 in light signaling.

The Arabidopsis thaliana Med25 mediator subunit integrates environmental cues to control plant development

“…Our identification of PFT1 as Med25 suggested an indirect effect where PhyB operates via unidentified regulatory transcription factors that in turn interact with Med25 (6). In line with this finding, it was recently reported that Med25/PFT1 has a repressive function in light signaling, which is completely dependent on PhyB, D, and E (13). The consequence of a Med25/PFT1 deletion is therefore not a constitutive or an autonomous effect.…”

“…Reports from other groups combined with our results presented here could be interpreted as if DREB2A and MED25 cooperate to cause down-regulation of a negative regulator of FT, such as, e.g., the FLC gene (24,25). However, based on the model proposed by Wollenberg et al, we rather suggest that Dreb2A acts as a repressor through Med25 to down-regulate the PhyB light signaling pathway (13).…”

PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length

“…Since CO and PRRs contain similar CCT DNA‐binding domains (Matsushika et al , 2000; Gendron et al , 2012), PRRs might also bind along with CO to the FT promoter and contribute to the control of FT transcription, as was recently shown for the interaction between PRRs and PIF transcription factors on their target genes (Soy et al , 2016; Zhu et al , 2016). Morning FT expression is more pronounced when plants are exposed to shade (Wollenberg et al , 2008), suggesting that specific PRRs might also control this process by coupling a distinct flowering signal to CO protein stability or FT induction in the morning.…”

Arabidopsis thaliana life without phytochromes

“…To obtain the quintuple phytochrome mutant, we used phyA-211, a γ-induced deletion (25); phyB-9, a nonsense mutation that eliminates part of the GAF domain needed for phytochrome assembly (4, 41); phyC-2, a T-DNA insertion mutant that does not produce PHYC apoprotein (38); and phyD-201 and phyE-201, two T-DNA insertion mutants described recently (42) that bear insertions in the first exon that also produce deletions of 8 and 26 bp, respectively. All of these mutants are in the Columbia background and are considered null (4,25,38,41,42). The ft-1 allele was previously introgressed in the Col background (43).…”