179 1. Introduction The utilization of light-emitting diodes (LEDs) is in- creasing because of their technological improvements and reduction in prices. Because LEDs emit monochromatic light, studying the effects of light quality on plant growth has become important for efficient utilization of LEDs in horticultural production. Plant factories are a promising system of horticultural production where the completely artificial environment may lead to efficient photosynthe- sis, year-round production unaffected by climate, farming without insecticides, and effective utilization of resources such as water (Kozai, 2013). However, plant factories con- sume large amounts of energy for lighting and air condi- tioning, which may result in global warming and high fuel costs and electricity bills. Using LEDs, which consume less electricity and have a long life, may reduce energy consumption and production costs. LEDs emit little ther- mal irradiation and can be used in close proximity with plants, making them suitable for multistage production to increase unit production (Goto, 2012). LEDs are also expected to be useful in lighting culture, where flowering time is controlled by night-time lighting for cut flower production in greenhouses, because incan- descent lamps, which have been commonly used for long- day treatments, are characterized by high energy consump- tion and short life. Chrysanthemum morifolium (hereby referred to as chrysanthemum), the most commonly used cut flower in Japan, is a short-day plant; it is produced year-round by lighting culture, repressing flowering by long-day treatments. Long-day cut flowers can also be produced under short-day conditions during autumn and winter by long-day treatments to promote flowering. The effects of monochromatic light quality on plant growth, morphogenesis, and metabolism need to be inves- tigated for the development of horticultural production us- ing LEDs. Blue, red, and far-red lights are already known to be effective in plant morphogenesis and flowering, and molecular mechanisms underlying the regulation of flowering by each light color have been proposed in stud- ies using the model plant Arabidopsis thaliana, which is a long-day plant. Far-red light or a low red/far-red ratio, which promotes flowering in Arabidopsis, reportedly pro- motes flowering in long-day cut flowers such as Eustoma grandiflorum and Gypsophila paniculata (baby’s breath) (Yamada et al., 2008, 2009; Nishidate et al., 2012). How- ever, studies have not focused on the effects of blue light on flowering; blue light could also be important in timing of flowering. Therefore, this review describes flowering response to blue light and its molecular mechanisms in Arabidopsis and horticultural plants with the aim of ad- vancing research on the utilization of LEDs in horticul- tural production. 2. Flowering response to light quality and its molecu- lar mechanisms Flowering is triggered upon the expression of flower- ing locus T (FT), which is a key flowering integrator, in Flowering response to blue light and its molecular mechanisms in Arabidopsis and horticultural plants T. Shibuya, Y. Kanayama* School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555, Japan. Key words: cut flower, light-emitting diode, lighting culture, plant factory, vegetable. Abstract: Using light-emitting diodes (LEDs) in plant factories and protected horticulture is expected to decrease energy costs and environmental burden. Because LEDs emit monochromatic light, detailed knowledge of plant responses to light quality is essential for efficient and appropriate utilization of LEDs in horticultural production. Timing of flowering is important in cut flower and fruit/vegetable production, and it is often affected by light quality. Red/far-red light is well known to be effective in flowering, and there is abundant knowledge about the effects of red/far-red ratio on flowering of horticultural plants as well as of the model plant Arabidopsis thaliana. However, studies have not focused on the effects of blue light on flowering. Therefore, this review describes the progress in the promotion of flowering by blue light and its molecular mechanisms in Arabidopsis and horticultural plants. Adv. Hort. Sci., 2014 28(4): 179-183 (1) Corresponding author: kanayama@bios.tohoku.ac.jp Received for publication 17 September 2014 Accepted for publication 2 October 2014 Review article 180 the phloem of leaves induced by CONSTANS (CO) under long-day conditions in the photoperiodic flowering path- way proposed for Arabidopsis (An et al., 2004). CO ex- pression is regulated downstream of the circadian clock, and CO mRNA is expressed during the dark period under short-day conditions and during the light period (late af- ternoon) under long-day conditions (Suârez-Lôpez et al., 2001). Because CO protein translated from CO mRNA is degraded through ubiquitination by an E3 ubiquitin li- gase, CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), during the dark period, the expression of FT and consequent induction of flowering are not triggered under short-day conditions (Jang et al. 2008; Liu et al. 2008 b). In Arabidopsis, flowering is promoted under long-day conditions with blue and far-red lights but not with red light (Eskins, 1992). Blue light may induce FT expression by stabilizing CO, as described below. Far-red and blue light signaling through phytochrome A and through CRYPTO- CHROME 1 (CRY1) and CRY2, respectively, stabilize CO protein by an antagonistic function to red light signaling through phytochrome B that induces CO degradation (Mock- ler et al., 1999, 2003; Valverde et al., 2004). As a result, the accumulation of CO protein increases during the light period (late afternoon), and the expression of FT and consequent in- duction of flowering are triggered under long-day conditions. FT protein expressed in leaf phloem moves to the api- cal meristem through sieve tubes (Corbesier et al., 2007) and activates floral identity genes such as APETALA1 and FRUITFUL by interacting with FLOWERING LOCUS D (FD), a bZIP transcription factor (Abe et al., 2005; Wigge et al., 2005). In addition, FT activates SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) to pro- mote flowering (Yoo et al., 2005). 3. Molecular mechanism underlying the promotion of flowering mediated by blue light signaling via FKF1 Flavin-Binding Kelch repeat F-box 1 (FKF1), which has flavin mononucleotide as a chromophore, and GIGANTEA (GI) mediate CO transcription in the photoperiodic flower- ing pathway (Fig. 1) (Nelson et al., 2000; Imaizumi et al., 2003; Martin-Tryon et al., 2007). They are important for the promotion of flowering because in the CO promoter region, FKF1 forms a complex with GI in a blue light-dependent manner, induces the degradation of CYCLING DOF FAC- TOR 1 (CDF1) to repress CO transcription, and promotes CO transcription (Fig. 2) (Imaizumi et al., 2005; Sawa et al., 2007). In detail, the F-BOX domain of FKF1 interacts with Arabidopsis Skp1-like proteins to form the E3 ubiqui- tin ligase complex, and this complex induces CDF1 degra- dation by ubiquitination (Yasuhara et al., 2004; Imaizumi et al., 2005; Sawa et al., 2007). Arabidopsis has multiple CDF family proteins, which redundantly repress CO, and the degradation of another CDF family protein CDF2 is also induced by the interaction of FKF1 with GI in a blue light- dependent manner (Fornara et al., 2009). In addition, FKF1, GI, and CDF1 are proposed to regu- late FT transcription in the same manner as CO transcrip- tion regulation (Sawa and Kay, 2011; Song et al., 2012). Furthermore, FKF1 stabilizes CO by interaction of the LOV domain of FKF1 with CO protein, which is enhanced by blue light (Song et al., 2012). Thus, FKF1 transduces blue light signals to promote flowering in a complex manner. 4. Molecular mechanism underlying the promotion of flowering mediated by blue light signaling via CRY2 CRY is a blue/UVA photoreceptor in plants (Cashmore et al., 1999). In Arabidopsis, the expression and function of CRY2 in vascular bundles regulates flowering (Endo et al., 2007). COP1 controls the accumulation of GI and CO proteins, that is, COP1-mediated degradation of GI and CO proteins during the dark period is repressed by CRY2 during the light period to promote flowering (Fig. 1) (Jang et al., 2008; Liu et al., 2008 b; Yu et al., 2008). COP1- mediated degradation of CO is repressed by blue light- dependent interactions between CRY2, COP1, and SUP- PRESSOR OF PHYA-105 1 (SPA1) (Zuo et al., 2011). CRYPTOCHROME-INTERACTING BASIC-HELIX- LOOP-HELIX 1 (CIB1) has been isolated as an interacting factor of CRY2, and its interaction with CRY2 is promoted by blue light. CIB1 induces FT transcription to promote CRY2-dependent floral initiation (Liu et al., 2008 a). CIB1 is believed to bind to E-box (CANNTG) elements in the promoter region of FT and stimulate FT mRNA expression (Liu et al., 2008 a). CIB family proteins, containing CIB1, form heterodimers, and notably, in comparison with ho- modimers, some of these heterodimers have higher binding Fig. 1 - Proposed model for the induction of FT expression by blue light through two photoreceptors (Jang et al., 2008; Liu et al., 2008 a, b; Sawa and Kay, 2011; Zuo et al., 2011; Song et al., 2012). FKF1 stabilizes CO protein to activate FT expression in a blue light-dependent manner. CRY2 activates CIB1 as a transcription factor to induce FT expression and inhibits COP1, inducing the degradation of CO protein during the light period in a blue light-dependent manner. *See figure 2. 181 affinity to E-box elements. This suggests the importance of the interaction between CIB proteins and E-box elements in the induction of FT expression (Liu et al., 2013 b). Although the interaction of CRY2 with CIB1 plays a role in FT expression in response to blue light, the stabi- lization of CIB1 protein may be controlled by other blue light photoreceptors such as ZEITLUPE (ZTL) and LOV KELCH PROTEIN 2 (LKP2). ZTL and LKP2, belong- ing to the LOV domain of blue light receptors contain- ing FKF1, are required for suppressing the degradation of CIB1 protein by blue light (Liu et al., 2013 a). 5. Flowering response to blue light in long-day horti- cultural plants In some long-day horticultural plants, similar in be- havior to Arabidopsis, the promotion of flowering by blue light under long-day conditions has been reported. Flow- ering is promoted by inducing the expression of FBP28, a petunia homolog of the flowering accelerator SOC1 in Petunia, an important potted and bedding plant (Fukuda et al., 2011). The flowering of a popular cut flower E. gran- diflorum can be promoted under short-day conditions by a night break with blue light as well as with far-red light or a low red/far-red ratio (Yamada et al., 2011). The effect of light quality on flowering has also been investigated in long-day fruit/vegetable production. Ever-bearing straw- berry cultivars (Fragaria x ananassa) are long-day plants in contrast with June-bearing strawberry cultivars, which are short-day plants. Similar to Arabidopsis, the flowering of ever-bearing strawberry cultivars is promoted by long- day treatments with blue as well as far-red light (Nishi- yama and Kanahama, 2009; Yoshida et al., 2012). These reports indicate that the flowering of various long-day horticultural plants is promoted under long-day conditions with blue light. However, flowering is not promoted by long-day treatments with blue light, but it is promoted with far-red light in G. paniculata ‘Bristol Fairy,’ which is a popular cut flower frequently used in flower arrangements (Hori et al., 2011; Nishidate et al., 2012). In a study conducted by Hori et al. (2011), flow- ering in Arabidopsis was reportedly induced with SOC1 expression by long-day treatments with far-red light, whereas it was induced with both FT and SOC1 expres- sion by long-day treatments with blue light. In contrast, although flowering in G. paniculata ‘Bristol Fairy’ was induced with the expression of the SOC1 homolog of G. paniculata (GpSOC1) by long-day treatments with far- red light, flowering was not induced because of the low expression of the FT homolog of G. paniculata (GpFT) and GpSOC1 by long-day treatments with blue light. To the best of our knowledge, there has been no report of blue light receptors FKF1 and CRY in these horticultural plants. Further studies are needed to understand the mo- lecular mechanisms underlying the diversity in flowering response to blue light. 6. Flowering response to blue light in short-day and day-neutral horticultural plants In short-day horticultural plants, flowering is usually repressed by a night break with red light under short-day conditions; there are only a few studies on flowering re- sponse to blue light in short-day horticultural plants. In chrysanthemum, a major short-day cut flower, flowering is repressed by a 4-h night break with blue light or far-red light under short-day (12 h) conditions with blue light (Hi- guchi et al., 2012). However, flowering of chrysanthemum is not repressed by 4-h end-of-day irradiation with blue light under 11-h photoperiods with mixed red and blue lights (Jeong et al., 2014). These studies suggest that in chrysanthemum, blue light affects flowering under some limited conditions, but red light plays a major role in the repression of flowering. Fig. 2 - Proposed model for the induction of CO and FT expression by blue light through FKF1 (Yasuhara et al., 2004; Imaizumi et al., 2005; Sawa et al., 2007; Sawa and Kay, 2011; Song et al., 2012). CDF1 represses CO and FT expression under short-day condi- tions. FKF1 forms a complex with GI and CDF1 on CO and FT promoters in a blue light-dependent manner in the late afternoon under long-day conditions, and then induces the degradation of CDF1 protein by ubiquitination. As a result, the expression of CO and FT are induced because of the absence of CDF1. 182 Solanum lycopersicum (tomato), the most important fruit/vegetable, is well known as a day-neutral plant. To- mato plants flower after vegetative growth regardless of photoperiods, and their flowering is believed to be unaf- fected by photoperiods. However, the node position of the first flower truss under a lower blue/red ratio has been reported to be lower than that under a higher blue/red ra- tio (Nanya et al., 2012); the constitutive expression of the CRY2 homolog of tomato LeCRY2 delays flowering (Gi- liberto et al., 2005), suggesting that blue light represses flowering in tomato plants. It is important to investigate the tomato homologs of genes that play roles in light sig- naling in the photoperiodic flowering pathway because the FT homolog of tomato SFT is a flowering activator, and heterozygosity for loss-of-function alleles of SFT increas- es the yield (Krieger et al., 2010). 7. Concluding remarks Similar to Arabidopsis, blue as well as far-red light-de- pendent promotion of flowering in long-day horticultural plants has been reported. On the other hand, flowering was not promoted by blue light, but by far-red light in a cultivar of another long-day plant, G. paniculata. This diversity in flowering response to blue light is notable, and gaining knowledge about it is important for promoting the utiliza- tion of LEDs in plant factories and lighting culture. The flowering of Arabidopsis is promoted by blue light in a complex manner through FKF1 and CRY. There is a paucity of information on the molecular mechanisms un- derlying blue light signaling in the promotion of flower- ing in horticultural plants, although there are few studies on the relationship between flowering and expression of flowering-related genes under long-day conditions with blue light. Gaining knowledge at the molecular level about blue light signaling in long-day and day-neutral horticul- tural plants would contribute to the introduction of novel traits in molecular breeding. 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