Photoperiodic flowering: time measurement mechanisms in leaves

Abstract

Many plants use information about changing day length (photoperiod) to align their flowering time with seasonal changes to increase reproductive success. A mechanism for photoperiodic time measurement is present in leaves, and the day-length-specific induction of the FLOWERING LOCUS T (FT) gene, which encodes florigen, is a major final output of the pathway. Here, we summarize the current understanding of the molecular mechanisms by which photoperiodic information is perceived in order to trigger FT expression in Arabidopsis as well as in the primary cereals wheat, barley, and rice. In these plants, the differences in photoperiod are measured by interactions between circadian-clock-regulated components, such as CONSTANS (CO), and light signaling. The interactions happen under certain day-length conditions, as previously predicted by the external coincidence model. In these plants, the coincidence mechanisms are governed by multilayered regulation with numerous conserved as well as unique regulatory components, highlighting the breadth of photoperiodic regulation across plant species.

Keywords: CONSTANS; FLOWERING LOCUS T; external coincidence model; florigen; photoperiodism; seasonal flowering.

Figure 1

Models of induction of the photoperiodic response. (a) Bünning’s hypothesis. In this model, organisms possess 12-h-long photophile and skotophile phases delimited by an internal oscillator. When daylight lengthens into the skotophile phase, the photoperiodic response is induced in long-day plants and repressed in short-day plants. (b) The external coincidence model. This model proposes that a photoperiodic response is induced by the activity of a hypothetical enzyme and the presence of its hypothetical substrate. The enzyme is present throughout the day, and light triggers the enzyme to change from the inactive form (Ei) to the active form (Ea). The expression patterns of the substrate are regulated by the circadian clock. Light and temperature change throughout the day and reset the clock each day by adjusting the phases of the clock components. The time when resetting occurs changes throughout the year, causing the phase of the substrate to also change slightly. Therefore, the phases of the maximal amount of the substrate (s-max) are slightly different in long- and short-day conditions. The photoperiodic response is induced only when the amount of substrate is higher than a required threshold and Ea is present at the same time.

Figure 2

Photoperiodic regulation of FT induction in Arabidopsis. The abundance of CCA1 transcript oscillates throughout the day; it is high in the early morning in both long and short days. CCA1 and its homolog LHY bind to promoters of PRR5, FKF1, and GI to repress their expression in the morning. Daily oscillation patterns of PRR5 mRNA expression are antiphasic to those of CCA1. PRR5 protein binds to the CCA1 promoter to form a feedback loop between morning and evening clock components. PRR5 also negatively controls the expression of CDF genes. CDF proteins (CDF1, CDF2, CDF3, and CDF5) act as transcriptional repressors that likely bind to the Dof-binding site (AAAG) in the CO promoter in both long and short days. Daily expression profiles of CDF1 are regulated by the FKF1-GI complex. During long days, the peak expression of FKF1 and GI proteins, which are regulated by the circadian clock, occurs in the afternoon. When FKF1 absorbs blue light, FKF1 interacts with GI. The photo-induced FKF1-GI complex accumulates to high levels in long-day afternoons and degrades CDF proteins on the CO promoter. Once the repression of CO transcription by CDFs is relieved, FBH proteins activate CO gene expression by directly binding to the E-box elements in the CO locus. In contrast to their response to long-day conditions, the expression of FKF1 and GI proteins is out of phase during short days, and FKF1 is expressed mainly in the dark. This causes a low level of FKF1-GI complex formation in the afternoon, and consequently the abundance of CO mRNA remains very low under light. CO protein is the primary activator of FT transcription and shows daily oscillation patterns. The protein accumulates to high levels only in the late afternoon in long days, and its stability is regulated by several factors. PHYB, the COP1-SPA complex, and HOS1 are involved in the degradation of CO. Among these, COP1, SPAs, and HOS1 directly bind to and degrade the protein. PHYB is a red-light photoreceptor, and the function of PHYB is inhibited by PHL through the formation of a protein complex under red light. PHL also interacts with CO. By contrast, the far-red-light photoreceptor PHYA and the blue-light photoreceptors CRY2 and FKF1 stabilize CO. Blue-light-stimulated CRY2 interacts with both COP1 and the SPAs, and the interactions lead to sequestration of CO protein away from the COP1-SPA complex. Through another FKF1-dependent mechanism that aligns with the external coincidence model, FKF1 directly binds to CO in a blue-light-enhanced manner and promotes the stability of the protein in the late afternoon in long days. Many factors regulate FT expression throughout the day during long days. In the morning, CDFs repress FT transcription through direct association with this gene’s promoter. The degradation of CDFs is controlled by the FKF1-GI complex, which also exists on the FT promoter. CO, which is stabilized by FKF1, strongly induces FT expression around dusk in long days by directly binding to the CORE region in the FT promoter as well as by interacting with other FT regulators, namely NF-Y complexes and AS1. NF-Ys bind to the CCAAT boxes located approximately 2 kb and 5.3 kb upstream from the transcription start site (TSS) of the FT gene. These CCAAT-box regions form loops with the CORE region within the TSS, and the timing of loop formation shows diurnal oscillation in long days. This type of FT chromatin dynamic has not been studied in short-day conditions, but it demonstrates how cis-acting sequences away from the TSS influence the activation of FT transcription. In addition to CO, CIB proteins are involved in the activation of FT transcription. CIB proteins, which interact with CRY2 under blue light, directly bind to the E-box located near the TSS on the FT promoter. Hence, the functions of CO and other factors enable FT to be strongly expressed at the end of the day only in long days, which accelerates the time to flowering. Clock marks on each protein symbol indicate that the circadian clock regulates its expression.

Figure 3

Photoperiodic control in the leaves of the long-day cereals wheat, barley, and Brachypodium distachyon. (a) Regulation of FT1 via the vernalization and photoperiodic pathways. The latter pathway may be governed by the coincidence of circadian-clock control of PPD1 and CO as well as by red-light signals mediated through PHYC, which influences the expression of circadian-clock genes in wheat, barley, and Brachypodium. VRN2, a negative regulator of FT1 gene expression, is downregulated by vernalization through VRN1. VRN2 is induced in long days in a PHYC-dependent manner, potentially through PPD1. Whether CO acts in parallel or cooperatively with PPD1 is not known. (b) Diurnal patterns in the gene expression of the key floral-regulator genes CO1 (or CO in Brachypodium), PPD1, and FT1 in strains carrying wild-type or hyperfunctional alleles (solid lines) and strains with reduced or null PHYC activity (dashed red lines). PHYC is nonfunctional in the phyC^AB and phyc-1 lines in wheat and Brachypodium, respectively, whereas HvPhyC-e from an early-flowering barley variety is likely hyperfunctional relative to HvPhyC-l in a late-flowering variety. In the strains carrying nonfunctional phyC alleles (wheat and Brachypodium), the expression of all three floral regulators is altered, whereas the expression of Ppd-H1 is only slightly decreased and that of HvCO1 is not significantly altered in barley. FT1 expression is significantly decreased across the three species in all three of those strains. Wheat CO1 is upregulated, perhaps owing to the release of a negative feedback from FT1 in phyC^AB lines. (c) The changing influence of day length throughout the year as mediated by PHYC. During fall, in winter varieties (i.e., those requiring vernalization), afternoon light causes upregulation of VRN2 gene expression. VRN2 may be downstream of PPD1 and also acts antagonistically to PPD1 to repress FT1 and delay flowering. Cold winter temperatures repress VRN2 expression via VRN1. CO1 and PPD1 genes continue to be transcribed. In spring, day length acts through PHYC, PPD1, and CO1 to activate FT1 expression, which feeds back to further upregulate VRN1 and maintain repression of VRN2. In summer, activation by light further facilitates this process. In wheat, around the time of floral initiation, CO1 begins to decline, perhaps owing to negative feedback from FT1. CO2 begins to be upregulated, perhaps maintaining FT1 expression through the terminal spikelet stage and heading.

Figure 4

Regulation of rice Hd3a and RFT1 expression by photoperiod. (a) The regulatory network controlling expression of Hd3a and RFT1. In rice, the critical day length required for floral induction is determined by two distinct pathways, Hd1-Hd3a and Ghd7-Ehd1-Hd3a/RFT1, which are regulated by the circadian clock and light signaling. The circadian clock regulates diurnal expression of Hd1 through OsGI function. Hd1, which potentially forms a complex with NF-Y, activates Hd3a expression in short days but suppresses it in long days. Red light converts Hd1 activity from activating to repressing Hd3a expression via PHYB. This repressive activity is enhanced by Hd6, which encodes the α subunit of CK2. Expression of Ehd1 and Ghd7 is controlled by the circadian clock and light signaling. Ehd1 activates expression of Hd3a and RFT1 independently of Hd1. OsGI regulates Ehd1 expression by activating OsMADS51 expression, setting a blue-light-dependent gate around dawn. Ghd7 acts as a repressor of Ehd1 expression, and Hd16 promotes repressive activity of Ghd7, potentially through phosphorylation. The Ghd7 transcript is induced by light and increased by lengthening photoperiods. Phytochrome is required for light-dependent-induction of Ghd7. In short-day conditions, low induction of Ghd7 allows induction of Ehd1 to activate Hd3a expression. When day length increases above the critical short-day-length that is required for flowering, Ghd7 is highly induced and is sufficient to suppress Ehd1 and Hd3a expression. Disruption of the circadian clock by decreasing activity of OsELF3-1 and OsPRR37 affects daily expression of floral regulators. OsELF3-1 negatively regulates expression of OsGI, OsPRR37, and Ghd7 in both long- and short-day conditions. OsPRR37 preferentially affects long-day flowering by suppressing Hd3a expression. Upstream regulators of Ehd1 and Ghd7, which contribute to flowering in long-day conditions, are indicated in the blue oval. Long-day-dependent induction of Ehd1 is promoted when OsMADS50 suppresses the negative regulators Ghd7 and OsLFL1. OsLFL1 is also negatively regulated by the OsVIL2-OsEMF2b complex, which is responsible for increasing repressive histone marks (H3K27me3). Lvp1/SDG724 activates OsMADS50 expression by increasing H3K36 methylation. Two plant-homeodomain-containing proteins, OsTrx1/SDG723 and Ehd3, downregulate Ghd7 expression to activate Ehd1 transcription in long-day conditions. Clock marks on each protein symbol indicate that the circadian clock regulates its expression. (b) Diurnal expression of floral regulators. Ghd7 has higher phytochrome-dependent, red-light inducibility around dawn in long-day conditions, shifting to midnight in short-day conditions (orange shaded area). Ehd1 has higher blue-light-dependent inducibility around dawn in both long- and short-day conditions (blue shaded area). In long days, red light induces Ghd7 transcription, leading to suppression of Ehd1 and Hd3a expression. Accumulation of Hd1 transcript in the presence of light suppresses Hd3a expression through PHYB function. In short days, weak expression of Ghd7 allows induction of the Ehd1 gene, leading to activation of Hd3a expression. Under these conditions and through a parallel pathway, Hd1 expression occurs mainly during nighttime and also acts as an activator of Hd3a.