BBX19 Interacts with CONSTANS to RepressFLOWERING LOCUS TTranscription, Defining a Flowering Time Checkpoint inArabidopsis   

Abstract

The timely transition of vegetative to reproductive development is coordinated through quantitative regulation of floral pathway genes in response to physiological and environmental cues. Here, we show that the circadian-controlled expression of the Arabidopsis thaliana floral transition regulators FLOWERING LOCUS T (FT) and CONSTANS (CO) is antiphasic to that of BBX19, a transcription factor with two B-Box motifs. Diminished expression of BBX19 by RNA interference accelerates flowering, and constitutive expression of BBX19 delays flowering under inductive photoperiods. This delay is not accompanied by the alteration of CO expression levels but rather by a reduction of transcript levels of FT and the FT-regulated genes SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1, LEAFY, and FRUITFUL. Similar to CO, BBX19 is expressed in vasculature. BBX19 and CO colocalize in the nucleus and interact physically in vivo. In transient assays, coinfiltration of 10-fold more CO overcomes the BBX19-mediated repression of FT activation. Substitution of the conserved Cys-25 to Ser in the BBX19 Box1 motif abolishes the BBX19-CO interaction and eliminates the negative regulation of flowering time, while the analogous C76S substitution in the Box2 motif is ineffective. Together, these results implicate BBX19 as a circadian clock output that depletes the active CO pool to accurately monitor daylength and precisely time FT expression.

The Transcriptional Regulator BBX19 Promotes Hypocotyl Growth by Facilitating COP1-Mediated EARLY FLOWERING3 Degradation in Arabidopsis

“…We have also demonstrated that lightgrown ceh1 (for constitutively expressing HPL) mutant plants accumulating high MEcPP levels are severely dwarfed (Xiao et al, 2012). Microarray analyses in conjunction with quantitative PCR identified a number of genes with robustly altered transcript levels in ceh1, among them BBX19, whose expression level is reduced by half (Xiao et al, 2012;Wang et al, 2014). The altered expression level of BBX19 and the severe dwarfism of ceh1 in conjunction with the reported function of BBX19 as a negative regulator of photomorphogenesis prompted us to explore the mode of action of this transcriptional regulator in plant photomorphogenic development.…”

“…We have previously established that mutant Arabidopsis plants (ceh1) accumulating high levels of the stress-specific retrograde signal MEcPP are dwarfed and have reduced BBX19 transcript levels (Xiao et al, 2012;Wang et al, 2014). The reported role of BBX19 in promoting hypocotyl growth (Kumagai et al, 2008) prompted us to examine its function in ceh1 as compared with the Columbia-0 (Col-0) wild-type plants.…”

Arabidopsis B-BOX32 interacts with CONSTANS-LIKE3 to regulate flowering

“…BBX proteins exist in species from algae, mosses, animals, and plants and are functionally diverse (15,16). The founding and best-characterized member of the B-box family is CO. Studies have reported that other members of the BBX family such as COL3/BBX4, BBX6, BBX7 (COL9), BBX19, BBX24/SALT TOLERANCE (STO), microProtein1a (MiP1a)/ BBX31 and BBX32/EMBRYONIC FLOWER-1 INTER-ACTING PROTEIN6 (EIP6) also affect flowering with distinct mechanisms (17)(18)(19)(20)(21)(22).…”

Photoperiodic Regulation of Florigen Function inArabidopsis thaliana

“…which positively regulate the expression of other floral integrator genes such as SOC1, AGL24, FUL, AP1 and LFY at the shoot apex Jung et al, 2011;Yu et al, 2012;Wang et al, 2014). miR172 levels, unlike miR156, are regulated by SPL9 and SPL10, the effect of which modifies TOE1 and TOE2.…”