Complex Signaling Networks Underlying Blue-Light-Mediated Floral Transition in Plants

Abstract

Blue light (BL) is important in regulating floral transition. In a controlled environment production system, BL can be manipulated easily and precisely in aspects like peak wavelength, intensity, duration, and co-action with other wavelengths. However, the results of previous studies about BL-mediated floral transition are inconsistent, which implies that an in-depth critical examination of the relevant physiological mechanisms is necessary. This review consolidates the recent findings on the role of BL in mediating floral transition not only in model plants, such as Arabidopsis thaliana, but also in crops, especially horticultural crops. The photoreceptors, floral integrator proteins, signal pathways, and key network components involved in BL-mediated floral transition are critically reviewed. This review provides possible explanations for the contrasting results of previous studies on BL-mediated flowering; it provides valuable information to explain and develop BL manipulation strategies for mediating flowering, especially in horticultural plants. The review also identifies the knowledge gaps and outlines future directions for research in related fields.

Keywords: flower induction; future directions; photoreceptors; physiological mechanisms; signal pathways.

Figure 1

An illustration of the functions of photoreceptors and photosynthetic pigments involved in blue-light-mediated flowering in terms of their sensed signals, role in flowering, and response to blue light. CRY1/2 = cryptochrome1/2; PHYA/B/C/D/E = phytochrome A/B/C/D/E; ZTL = ZEITLUPE; FKF1 = FLAVIN-BINDING, KELCHREPEAT, F-BOX; LKP2 = LOV KELCH PROTEIN2; PHOT1/2 = phototropin1/2; HAL3 = Halotolerance protein; and Chl = Chlorophyll. If both flowering promotion and repression are shown together, this indicates that the role of this receptor depends on environmental conditions or plant genotypes. If both the positive and negative effects of blue light are shown together, this indicates that the blue light effect may vary with light intensity or background light. * indicates that there is only indirect evidence.

Figure 2

An illustration of the characterization of floral integrator proteins in terms of their role in flowering and their response to blue light. FT = FLOWERING LOCUS T; SOC1 = SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1; AGL24 = AGAMOUS-LIKE 24; SPL = SQUAMOSA PROMOTER BINDING-LIKE; FUL = FRUITFULL; LFY = LEAFY; TFL1 = TERMINAL FLOWER 1; FLC = FLOWERING LOCUS C; SVP = SHORT VEGETATIVE PHASE; TEM1 = TEM-PRANILLO1; and FLM = FLOWERING LOCUS M. If both positive and negative effects of blue light are shown together, this indicates the blue light effect may vary with plant genotype and the application of blue light.

Figure 3

A simplified diagram of the photoperiod pathway in blue-light-mediated floral transition in Arabidopsis thaliana. FT = FLOWERING LOCUS T, a florigen; CO = CONSTANS, a central transcription factor in photoperiod pathway. The involved main blue light photoreceptors include CRY1/2 = cryptochrome 1/2; PHYA/B = phytochrome A/B; ZTL = ZEITLUPE, FKF1 = FLAVIN-BINDING, KELCHREPEAT, F-BOX1; and LKP2 = LOV KELCH PROTEIN2. The involved key circadian clock components include GI = GIGANTEA, an important clock output member; ELF3 = EARLY FLOWERING 3, an important clock input member; PRRs = PSUEDO RESPONSE REGULATORs, core clock components; and CCA1/LHY = CIRCADIAN CLOCK ASSOCIATED 1/LATE ELONGATED HYPOCOTYL, two core clock components. Other involved key transcription factors include CDF1 = CYCLING DOF FACTOR1; CIBs = CRYPTOCHROME-INTERACTING basic helix–loop–helixes; and COP1/SPA = CONSTITUTIVE PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYA-105. Each gray hexagon with two transcription factors inside indicates a protein complex. Under long day conditions, GI and FKF1 form a complex, leading to the degradation of CDF and thus an increase in CO transcript abundance; however, under short day conditions, the expression of GI precedes that of FKF1, which disrupts the formation of the GI–FKF1 complex and thus reduces the abundance of CO [151]. The dashed lines indicate that the action can be affected by many factors or the involved detailed action mechanism is still unclear. To be clear, only the key pathway components are presented in the diagram, and information about other pathway components can be found in the manuscript’s text. Also, the regulations at the transcriptional and translational levels are not distinguished in the figure for simplification, and the relevant information can be found in the text.

Figure 4

A simplified diagram of the light quality pathway (or shade pathway) in blue-light-mediated floral transition in Arabidopsis thaliana. The involved floral integrator proteins include FT = FLOWERING LOCUS T, a florigen; SPL = SQUAMOSA PROMOTER BINDING-LIKE; FUL/LFY/AP1 = FRUITFULL/LEAFY/APETALA1; and FLC = FLOWERING LOCUS C. The involved main blue light photoreceptors include CRY1 = cryptochrome 1; and PHY A/B = phytochrome A/B. The involved key transcription factors include PIF4/5/7 = PHYTOCHROME INTERACTING FACTOR4/5/7, an important group in the shade pathway; CO = CONSTANS, a central component in the photoperiod pathway; ELF4 = EARLY FLOWERING 4, an important clock component; PFT1 = PHYTOCHROME AND FLOWERING TIME 1; FHY3/FAR1 = HYPOCOTYL3/FAR-RED IMPAIRED RESPONSE1; FOF2 = F-box of Flowering 2; and VOZ2 = VASCULAR PLANT ONE-ZINC FINGER 2. To be clear, only the key pathway components are presented in the diagram, and information about other pathway components can be found in the manuscript’s text. Also, the regulations at the transcriptional and translational levels are not distinguished in the figure for simplification, and the relevant information can be found in the text.

Figure 5

A simplified diagram of the light quantity pathway (or photosynthesis pathway) in blue-light-mediated floral transition. FT = FLOWERING LOCUS T, a florigen; SPL/3/4/5 = SQUAMOSA PROMOTER BINDING-LIKE 3/4/5, a group of positive floral integrator proteins. PHOTs = phototropins, blue light photoreceptors. CO = CONSTANS, a central component in the photoperiod pathway; T6P = trehalose-6-phosphate, a carbohydrate from sucrose; TPS1 = T6P synthase1; GBSS = GRANULE BOUND STARCH SYNTHASE. To be clear, only the key pathway components are presented in the diagram, and information about other pathway components can be found in the manuscript’s text. Also, the regulations at the transcriptional and translational levels are not distinguished in the figure for simplification, and the relevant information can be found in the text.

Figure 6

An illustration of the functions of key pathway components in blue-light-mediated flowering in terms of their involved flowering pathways, roles in flowering, and response to blue light. CO = CONSTANS; GI = GI-GANTEA; PRRs = PSUEDO RESPONSE REGULATORS; ELF3/4 = EARLY FLOWERING3/4; LUX = LUX ARRHYTHMO; COP1/SPA = CONSTITUTIVE PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYTOCHROME A; CIB = CRYPTOCHROME-INTERACTING basic helix–loop–helix; CDF = CYCLING DOF FACTOR; PIFs = PHYTOCHROME INTERACTING FACTORS; HY5 = ELONGATED HYPOCOTYL 5; HRB1 = Hypersensitive to red and blue protein; TOE1/2/3 = TARGET OF EAT1/2/3; and BBXs = B-box containing proteins. If both flowering promotion and repression are shown together, this indicates that the role of this pathway component depends on specific pathway or member. If both positive and negative effects of blue light are shown together, this indicates that the blue light effect may vary with light intensity or component member. For BBXs, although some members show positive responses to blue light, other members are unknown.

Figure 7

A simplified diagram of the main components of the circadian clock in Arabidopsis thaliana. The figure is adapted and modified from the literature [170]. CCA1/LHY = CIRCADIAN CLOCK ASSOCIATED 1/LATE ELONGATED HYPOCOTYL; PRR9/7/5 = PSUEDO RESPONSE REGULATOR9/7/5; TOC1 = Timing of CAB2 Expression 1; LUX = LUX ARRHYTHMO; ELF3/4 = EARLY FLOWERING 3/4; GI = GIGANTEA. Among these clock components, ELF3 and GI are important clock input and output members, respectively. In the diagram, the yellow components are expressed in the morning, purple components are expressed midday, and blue components are expressed in the evening.