Large flower size is an important trait that influences the economic value of floricultural plants. Large flower size is conferred by two morphological changes i.e. increased petal number and individual petal size. Increased petal number is induced by enlargement of the floral meristem or homeotic conversion of stamens and carpels into petals. Genes regulating meristematic competence mediate enlargement of the floral meristem, while floral homeotic genes mediate the homeotic conversion of floral organs. Increased individual petal size is induced by increased cell size or cell number. Polyploidization of the nuclear genome increases petal cell size, while increased petal cell number is induced by enhanced expression of the genes involved in auxin signal transduction and cytokinin biosynthesis. In this review, the molecular mechanisms affecting flower size, elucidated mainly using model plants such as Arabidopsis, are summarized. Further, our research on the roles of cytokinin biosynthesis and signal transduction in increased flower size in petunia, induced by the single major gene Grandiflora, is discussed. Based on these results, a possible systematic breeding method for increasing flower size is discussed.

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Flower longevity is one of the most important traits for ornamental plants. Ethylene plays a crucial role in petal senescence in many plant species. Genes involved in ethylene biosynthesis, ethylene perception, and its signal transduction have been identified in several ornamental plants. Genetic modification of ethylene signaling genes has successfully improved flower longevity in several plant species. In contrast, regulatory mechanisms of petal senescence in flowers that show ethylene-independent senescence remain largely unknown. Japanese morning glory (Ipomoea nil) is a proposed model plant for ethylene-independent petal senescence, and an important role of autophagy in petal senescence has been highlighted in this plant. This review focused on our recent findings on ethylene signaling and our attempts to identify factors regulating ethylene-independent petal senescence.

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In Citrus, self-incompatibility (SI) regulation is gametophytic, and this phenomenon is an economically critical problem for some Citrus cultivars without high parthenocarpic ability. Few molecular biological studies of SI in Citrus have been performed, and the molecular mechanism of SI has not been clarified. To investigate the effect of different stages of style development on pollen tube behavior, flower buds of self-incompatible ‘Hyuganatsu’ (Citrus tamurana hort. ex Tanaka) were histologically assayed. When the flower buds were observed 168 hours after pollination, pollen tubes in the self-pollinated flower buds 1 and 3 days before anthesis (DBA) were arrested in the upper part of the styles, while those in flower buds self-pollinated 5 DBA reached the base of the styles. These results revealed that SI in ‘Hyuganatsu’ has not yet occurred in the flower buds 5 DBA, but generated in the flower buds 3 DBA. To search SI-related pistil proteins in Citrus, we profiled a number of protein expressions in ‘Hyuganatsu’ styles of 1, 3, and 5 DBA by two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry. One-hundred thirty-eight protein spots were significantly different in abundance among the three stages, and 17 up-regulated and 26 down-regulated protein spots could be identified. Among the 17 up-regulated proteins, nine up-regulated proteins exhibited the expression pattern of 1 DBA ≥ 3 DBA > 5 DBA, a pattern which reflected the transmission from SC to SI, evaluated by pollen tube growth. BLASTP homology search against the peptide sequences of these proteins was carried out and predicted the proteins related to the maintenance of cell shape, the signaling of pollen tube growth, the flavonol biosynthesis, the responses to various stresses, photosynthesis, and the methionine metabolic process. Among the nine proteins, some may be the SI-related pistil proteins; however, it was not possible to determine the SI-related key protein of the style in Citrus.

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Ceratocystis canker is caused by Ceratocystis fimbriata and is a serious soil disease affecting fig cultures. ‘Masui Dauphine’ is a major fig variety in Japan and is susceptible to this disease. In the present study, the effect of C. fimbriata infection was tested on the growth of 4 fig varieties, ‘Celeste’, ‘Boldido Negra’, ‘Ischia Black’, and ‘Negronne’. Specifically, the potential of these varieties as a disease-resistant rootstock for ‘Masui Dauphine’ was assessed. Fig cuttings of ‘Celeste’ were grown in 350 mL root boxes with sterilized coarse sand. Inoculation with C. fimbriata immediately inhibited root elongation, reduced the root respiration rate from 2 months after inoculation, and caused lesions in the rootlets. The trees of the 4 varieties tested were grown in 3.5-L pots for 5 months. C. fimbriata inoculation caused slight lesions on the underground part of all 4 varieties. Inoculation did not kill any trees and did not reduce root respiration, except with an unreasonable amount of inoculum. No significant reduction in the weights of leaves, shoots or roots was observed except for ‘Celeste’. ‘Masui Dauphine’ trees on the rootstocks of the 4 varieties tested were grown in 22-L containers for 2 years. C. fimbriata inoculation tended to inhibit scion and root growth, and killed a few trees from the 2nd year, except ‘Negronne’. ‘Masui Dauphine’ trees grafted on the rootstocks of the 4 varieties tested, in addition to 2 susceptible varieties, and own-rooted trees, were grown in the field with ceratocystis canker infection. Trees of the 4 varieties tested showed lower mortality and most survived for 5 years. No year-by-year reduction in scion growth was observed for any rootstock variety. Our results demonstrate that the varieties tested are not truly resistant, but are horizontally resistant to ceratocystis canker. We expect these rootstocks to maintain the scion ‘Masui Dauphine’ despite some continuing damage by disease. Of the varieties tested, ‘Negronne’ is the most resistant.

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In Japan, white asparagus spears are grown using two blanching methods to block sunlight: the traditional soil-mound and the film-cover methods. The objective of this study was to investigate the influence of the film-cover and soil-mound methods on components of white asparagus spears related to flavor, especially sweetness and bitterness. We investigated the effects of the two blanching methods on sugar (fructose, glucose, and sucrose) and protodioscin (a major furostanol saponin in white spears) contents in white spears by conducting a spring field survey and an experiment using winter-forcing cultures. No significant differences were observed in sugar content or composition in spears cultivated by the two methods in either the field survey or the experiment. Protodioscin content tended to be higher in spears blanched by the soil-mound method than in spears blanched by the film-cover method in both the field survey and the experiment. These results suggest that differences in the flavor of white spears grown using the two blanching methods are caused mainly by bitterness associated with saponin content, and saponin content may be influenced by soil-borne stresses.

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Shoots near fruits of watermelon (Citrullus lanatus Matsumu. et Nakai) were continuously heated from 5 days after anthesis (DAA) to a minimum of 30°C during night. Temperatures in a plastic greenhouse during 0 a.m. to 6 a.m. fluctuated between 12.7–17.2°C, and the temperature of the surface of the fruit, whose near shoots were heated (designated as a heated fruit) during these hours, was higher by 0.8°C on the average than that of the control, and also the temperature of the flesh of the heated fruit was 1.0–1.5°C higher than that of the control fruit. Pigmentation of the flesh of the heated fruit was accelerated. Cell size of the heated fruit was smaller than that of the control fruit. Higher sucrose phosphate synthase (SPS) activity of the heated fruit resulted in the higher mean sucrose and fructose content of the heated fruit. Therefore, it is reasonable to consider that heating bearing shoots near fruits can accelerate the rate at which cells enlarge and mature, which causes an increase in SPS activity and an increase in the sucrose content in the fruit, especially the outer portions of the fruit.

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Seed production in pat-2 parthenocarpic tomatoes is sufficiently low for the propagation of parthenocarpic tomato cultivars. To establish the technology to promote seed formation in pat-2 parthenocarpic tomato plants, we examined the effects of paclobutrazol, a gibberellin biosynthesis inhibitor, on seed formation. The parthenocarpic F₁ cultivar ‘Renaissance’, that has the homozygote recessive gene pat-2, was treated with paclobutrazol at 0 (control), 0.2, 1, 5, or 25 mg per pot (5.1 L) by irrigation to the culture medium. With increasing application of paclobutrazol, stem diameter increased, while stem length, leaf length, leaf width, and fruit fresh weight decreased. The percentage of fruits with seeds increased with increasing levels of paclobutrazol, and it reached 100% with 1 mg/pot and higher. Seed number per fruit increased from 12 at 0 mg/pot of paclobutrazol to 52 and 74 at 1 and 5 mg/pot, respectively. To confirm the practicality of this technology, the parthenocarpic purebred strain ‘PASK-1’, the seed parent of ‘Renaissance’, was treated with 0 (control), 1, or 5 mg/pot of paclobutrazol. All flowers of ‘PASK-1’ were emasculated before flowering and crossed at flowering with pollen of the parthenocarpic purebred strain ‘PF811K’, the pollen parent of ‘Renaissance’. The percentage of fruits with seeds increased from 59% at 0 mg/pot of paclobutrazol to higher than 95% at 1 and 5 mg/pot. Seed number per fruit increased from 21 at 0 mg/pot of paclobutrazol to 45 and 46 at 1 and 5 mg/pot, respectively. From these results, we concluded that paclobutrazol promotes seed formation in pat-2 parthenocarpic tomato plants.

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Many clubroot resistant (CR) cultivars of Chinese cabbage (Brassica rapa ssp. pekinensis) have been bred so far, but their usage is limited because the capacity for resistance breaks down with time. This degradation is caused by a pathogenic variation in the causal fungus Plasmodiophora brassicae. We attempted to accumulate 3 CR genes, CRa, CRk, and CRc, through marker-assisted selection. Five doubled haploid CR lines with an individual CR locus were used as breeding materials. The CR lines were crossed with each other. A subsequent selection for resistance was performed using sequence characterized amplified region markers in segregating generations. As a result, 4 homozygous lines for 3 resistance genes and the F₁ hybrids between them were developed. CR pyramiding lines were inoculated with 6 field isolates of P. brassicae. The homozygous lines for 3 CR genes, whether selfed or crossed, exhibited exceedingly high resistance against all of the isolates. Morphological characters of F₁ hybrids were comparable to those of a control cultivar, but the degree of heterosis was less than expected, which is probably because of genetic similarity of the parents. The results of this study prove that clubroot resistance can be reinforced through the accumulation of varied resistance genes in B. rapa.

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Lilium brownii var. colchesteri (BRO) has a unique trait in its flower color. The color at anthesis is yellowish cream, and changes to white after one day. To introduce this trait and establish a sexually reproductive cultivation system for cut flower production in Lilium, interspecific crosses of L. × formolongi, L. formosanum, L. longiflorum (as seed parents) and BRO (as a pollen parent) were carried out. Capsules with unviable seeds were found in all crosses. To overcome the incongruity barrier, ovary slices excised from developing ovaries at 7 to 28 days after pollination (DAP) were cultured in Murashige and Skoog medium supplemented with 40 g·L⁻¹ sucrose, 40 g·L⁻¹ D-mannitol and 2.5 g·L⁻¹ gellan gum under 24 h illumination. Hybrid plantlets were obtained from crosses of L. × formolongi × BRO and L. formosanum × BRO, but not from the cross of L. longiflorum × BRO. After acclimatization, 79 (44.1%) of 179 hybrid plants produced flowers within a year of cultivation. The hybrids developed flowers changing from yellowish cream to white during anthesis as does the pollen parent. Twenty-eight of 79 flowered plants sprouted two or more flower stalks, characteristics of L. formosanum. The hybrids exhibited relatively high pollen fertility and produced capsules, but no mature seed was obtained by self-pollination.

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A low red (R): far-red (FR) ratio is known to promote flowering of Arabidopsis and several long-day cut flowers, whereas not much information is available on the effects of single light qualities and their combinations on flowering. Therefore, the effects of light quality on the flowering of Gypsophila paniculata were investigated using light-emitting diodes (LEDs) emitting the following lights: FR, R, and blue (B). Flowering and flower budding were observed under long-day conditions with FR, while no flowering was observed under short-day conditions. Promotion of flowering and flower budding was increased under FR supplemented with R compared to FR alone. Although generally R inhibits flowering in long-day plants, a certain intensity of R, for example, the R:FR ratio between 0.23 and 0.71, may be necessary for effective promotion of flowering. In contrast, B supplementation of FR was not effective at the ratio in this study in inducing flowering and flower budding. The quality of cut flowers produced under long-day conditions with LEDs that promoted flowering was not lower than that under incandescent lamps. These results will provide basic knowledge for the development of LED bulbs as a replacement for incandescent bulbs.

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Application of forchlorfenuron (CPPU) to flower buds induces morphologically different paracorollas, i.e., wide and narrow type, in torenia (Torenia fournieri L.). We investigated the morphological properties and the role of floral homeotic genes in the formation of these two types of paracorolla. The morphology of epidermal cells and distribution pattern of vascular bundles in the wide paracorolla was the same as in the petal; however, in the narrow paracorolla, the morphology of epidermal cells was either petal-like or stamen-like, and the distribution pattern of vascular bundles was stamen-like. In situ hybridization analysis of the floral homeotic genes showed that a class A gene, T. fournieri SQUAMOSA (TfSQUA), and class B genes, TfDEFICIENS (TfDEF) and TfGLOBOSA (TfGLO), were expressed in the broad part of the primordia of the wide paracorolla, as in the petal. Class C genes, TfPLENA1 (TfPLE1) and TfFARINELLI (TfFAR), were only expressed at the margin of the primordia. However, in the primordia of the narrow paracorolla, TfSQUA was only expressed at the margin, but the class B genes and one of the class C genes (TfPLE1) were expressed in a broad section, as in the primordia of the wide paracorolla. This expression pattern in the narrow paracorolla was intermediate between that of the petal and the stamen. In later developmental stages, quantitative real-time PCR analyses showed that, in the wide paracorolla, TfSQUA and class B genes were highly expressed but the expression of class C genes, as in the petal, was low. In the narrow paracorolla, class B genes were also highly expressed as in the petal; however, the expression of both TfSQUA and the class C genes was low, as in the stamen and the petal, respectively. This expression pattern probably reflects the unstable floral organ identity of the narrow paracorolla, and the expression pattern in paracorollas is determined by the site where the paracorolla is formed.

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