FT Expression Research Articles

Is the role of the short-day solely to switch off the CONSTANS in Arabidopsis?

Plants in the genus Arabidopsis are facultative LD plants that flower much earlier under LD conditions than SD regimens, with the photoperiod (or LD) pathway contributing to floral acceleration. LHY and CCA1 genes, among other factors, have central roles in the circadian clock of Arabidopsis, which plays a key role in measuring day length. GI gene mediates the circadian clock and floral activator genes, CO and FT, to control photoperiodic flowering. GI is required to set the peak phase of CO expression at the end of the light period under LD conditions, so that the CO protein is stabilized and activated by light to increase FT expression. However, recent studies have demonstrated that the role of SDs is not solely to switch off CO activity. For example, GI interacts with SPY, a negative regulator of the GA signal. The flowering times of gi mutants were still significantly later under SD conditions than LD regimes, which suggests that GI has a potential role in accelerating the start of flowering, even under SDs. Over-expression of either FT or TSF genes caused early flowering, and the acceleration of flowering was enhanced under SDs, suggesting that SDs have an additional role to that in the LHY/CCA1-GI-CO-FT pathway. In this short review, we discuss the hidden roles of SDs in controlling flowering based on recent studies of the molecular genetics of flowering time in Arabidopsis.

CONSTANS acts in the phloem to regulate a systemic signal that induces photoperiodic flowering of Arabidopsis.

Flower development at the shoot apex is initiated in response to environmental cues. Day length is one of the most important of these and is perceived in the leaves. A systemic signal, called the floral stimulus or florigen, is then transmitted from the leaves through the phloem and induces floral development at the shoot apex. Genetic analysis in Arabidopsis identified a pathway of genes required for the initiation of flowering in response to day length. The nuclear zinc-finger protein CONSTANS (CO) plays a central role in this pathway, and in response to long days activates the transcription of FT, which encodes a RAF-kinase-inhibitor-like protein. We show using grafting approaches that CO acts non-cell autonomously to trigger flowering. Although CO is expressed widely, its misexpression from phloem-specific promoters, but not from meristem-specific promoters, is sufficient to induce early flowering and complement the co mutation. The mechanism by which CO triggers flowering from the phloem involves the cell-autonomous activation of FT expression. Genetic approaches indicate that CO activates flowering through both FT-dependent and FT-independent processes, whereas FT acts both in the phloem and the meristem to trigger flowering. We propose that, partly through the activation of FT, CO regulates the synthesis or transport of a systemic flowering signal, thereby positioning this signal within the established hierarchy of regulatory proteins that controls flowering.

EARLY BOLTING IN SHORT DAYS Is Related to Chromatin Remodeling Factors and Regulates Flowering in Arabidopsis by Repressing FT

The timing of flowering initiation depends on the balanced expression of a complex network of genes that are regulated by both endogenous and environmental factors. We showed previously that mutations at the EARLY BOLTING IN SHORT DAYS (EBS) locus of Arabidopsis result in an acceleration of flowering, especially in noninductive photoperiods (short days), and other phenotypic anomalies. We have identified the EBS gene and demonstrate that it encodes a nuclear protein that contains a bromoadjacent homology domain and a plant homeodomain Zn finger. Both types of motif are thought to mediate protein-protein interactions and occur in transcriptional regulators involved in chromatin remodeling, suggesting that EBS is part of a transcriptional repressor complex that modulates chromatin structure and is required to repress the initiation of flowering in short days. Overexpression of EBS has phenotypic effects similar to those of recessive ebs mutations, suggesting that both might disrupt the formation of protein complexes that contain EBS. Analysis of the expression of flowering-time genes in ebs mutants and in EBS-overexpressing plants indicates that EBS participates in the regulation of flowering time by specifically repressing the expression of FT, a key gene in the integration of floral promotion pathways in Arabidopsis.

Arabidopsis TERMINAL FLOWER2 (TFL2) negatively regulates FT expression

Arabidopsis TERMINAL FLOWER2 (TFL2) negatively regulates FT expression

Effects of sugar on vegetative development and floral transition in Arabidopsis.

Although sugar has been suggested to promote floral transition in many plant species, growth on high concentrations (5% [w/v]) of sucrose (Suc) significantly delayed flowering time, causing an increase in the number of leaves at the time of flowering in Arabidopsis. The effect of high concentrations of Suc seemed to be metabolic rather than osmotic. The delay of floral transition was due to extension of the late vegetative phase, which resulted in a delayed activation of LFY expression. In addition, growth on low concentrations (1% [w/v]) of Suc slightly inhibited flowering in wild-type plants. This delay resulted from effects on the early vegetative phase. This inhibition was more pronounced in tfl1, an early flowering mutant, than in the wild type. Although 1% (w/v) Suc was reported to promote floral transition of late-flowering mutants such as co, fca, and gi, floral transition in these mutants was delayed by a further increase in Suc concentration. These results suggest that sugar may affect floral transition by activating or inhibiting genes that act to control floral transition, depending on the concentration of sugars, the genetic background of the plants, and when the sugar is introduced. Growth on 1% (w/v) Suc did not restore the reduced expression levels of FT and SOC1/AGL20 in co or fca mutants. Rather, expression of FT and SOC1/AGL20 was repressed by 1% (w/v) Suc in wild-type background. The possible effects of sugar on gene expression to promote floral transition are discussed.

Expression of intestinal brush-border membrane hydrolases and ferritin after segmental ischemia-reperfusion in rats.

Jejunal expression of three brush-border membrane (BBM) enzymes, intestinal alkaline phosphatase (IAP), lactose-phlorizin hydrolase (LPH), and sucrase-isomaltase (SI), and a cytosolic protein, ferritin (Ft), was investigated after transient segmental ischemia-reperfusion (I/R). I/R reduced mucosal IAP, LPH, and SI mRNAs to 36%, 11%, and 38% of normal jejunal levels after 3 h of reperfusion and to 22%, 8%, and 51% of normal jejunal levels after 6 h of reperfusion, respectively. Intriguingly, in the internal control jejunum IAP and LPH mRNAs also decreased significantly. LPH and SI mRNA rapidly recovered to levels significantly higher than those of normal jejunum at 12 h, whereas IAP mRNA levels did not recover until 48 h. Enzyme activity paralleled changes in mRNA levels in the ischemic reperfused jejunum. Electrophoretic mobility shift assays showed that I/R significantly increased SI footprinting 1 (SIF1) binding activity. The mobility of one of the DNA-protein complexes was further retarded in the presence of anti-Cdx-2 antibody, suggesting that either Cdx-2 or a related protein was interacting with the SIF1 sequences. Similar to BBM enzymes, cytosolic Ft mRNA and protein were significantly decreased at 3 and 6 h after I/R. By 12 h, Ft mRNA, but not Ft protein, had increased to higher than normal levels. We conclude that a rapid recovery of BBM mRNAs and enzymes occurs in regenerating mucosa after upper villus damage. The increase of SIF1 binding protein activity after I/R may enhance SI, and perhaps LPH, gene transcription. The expression of Ft is regulated at both pretranslational and translational levels.

GENERALIZING THE TWIST-AND-FLIP PARADIGM

In this paper we generalize the horseshoe twist theorem of Brown and Chua [1991] and derive a wide class of ODEs, with and without dissipation terms, for which the Poincare map can be expressed in closed form as FTFT where T is a generalized twist. We show how to approximate the Poincaré maps of nonlinear ODEs with continuous periodic forcing by Poincare maps which have a closed-form expression of the form FT 1 T 2 … T n where the T i are twists. We extend the twist-and-flip map to three dimensions with and without damping. Further, we demonstrate how to use the square-wave analysis to argue for the existence of a twist-and-flip paradigm for the Poincare map of the van der Pol equation with square-wave forcing. We apply this analysis to the cavitation bubble oscillator that appears in Parlitz et al. [1991] and prove a variation of the horseshoe twist theorem for the twist-and-shift map, which models the cavitation bubble oscillator. We present illustrations of the diversity of the dynamics that can be found in the generalized twist-and-flip map, and we use a pair of twist maps to provide a specific and very simple illustration of the Smale horseshoe. Finally, we use the twist-and-shift map of the cavitation bubble oscillators to demonstrate that the addition of sufficient linear damping to a dynamical system having PBS (Poincaré–Birkhoff–Smale) chaos may cause the chaos to become detectable in computer simulations.