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Petal color change and expansion growth of Ipomoea tricolor cv. Heavenly Blue. (A) Whole ower growth. The right photos are half-cut buds. Bar: 2.0 cm. (B) Transverse section of petals. Bar: 50 m m. (C) Structure of heavenly blue anthocyanin (HBA) (D) Cryo-SEM appearance of a fully-opened petal at 0 h. Bar: 100 m m. (E) Cell volume increase of adaxial colored epidermis during the last 24 h. (n 1⁄4 9, mean e SD). ***: signicant dierences ( P < 0.001) between À 24 h; § : signicant dierence ( P < 0.05) between À 12 h; § § § : signicant dierence ( P < 0.001) between À 12 h. 

Petal color change and expansion growth of Ipomoea tricolor cv. Heavenly Blue. (A) Whole ower growth. The right photos are half-cut buds. Bar: 2.0 cm. (B) Transverse section of petals. Bar: 50 m m. (C) Structure of heavenly blue anthocyanin (HBA) (D) Cryo-SEM appearance of a fully-opened petal at 0 h. Bar: 100 m m. (E) Cell volume increase of adaxial colored epidermis during the last 24 h. (n 1⁄4 9, mean e SD). ***: signicant dierences ( P < 0.001) between À 24 h; § : signicant dierence ( P < 0.05) between À 12 h; § § § : signicant dierence ( P < 0.001) between À 12 h. 

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Petal color change in morning glory Ipomoea tricolor cv. Heavenly Blue, from red to blue, during the flower-opening period is due to an unusual increase in vacuolar pH (pHv) from 6.6 to 7.7 in colored epidermal cells. We clarified that this pHv increase is involved in tonoplast-localized Na+/H+ exchanger (NHX). However, the mechanism of pHv increas...

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... by X-ray crystallographic analysis of blue pigments from Commelina commu- nis 9)-11) and Centaurea cyanus. 10),12 ) The petals of blue morning glory, Ipomoea tri- color cv. Heavenly Blue is the rst example that proved the ower color change by pH change. The petals exhibit sky-blue when full-opened, but in the bud stage the petal color is red (Fig. 1A), although the responsible pigment is the same heavenly blue anthocyanin, HBA (Fig. 1C), 13) during all the ower- ing stages. 14) The petal color change was in fact found to be due to an unusual increase of pHv from 6.6 to 7.7 in colored cells. 14) Followed by this obser- vation, we also found that this pHv increase was linked with ...
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... and Centaurea cyanus. 10),12 ) The petals of blue morning glory, Ipomoea tri- color cv. Heavenly Blue is the rst example that proved the ower color change by pH change. The petals exhibit sky-blue when full-opened, but in the bud stage the petal color is red (Fig. 1A), although the responsible pigment is the same heavenly blue anthocyanin, HBA (Fig. 1C), 13) during all the ower- ing stages. 14) The petal color change was in fact found to be due to an unusual increase of pHv from 6.6 to 7.7 in colored cells. 14) Followed by this obser- vation, we also found that this pHv increase was linked with up-regulation of the tonoplast-localized Na þ /H þ exchanger (ItNHX1). 15) In plants, the ...
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... in petal color, pHv and cell vol- ume during ower-opening period. The time- course of morning glory petal-color change during the ower-opening period is shown in Fig. 1A. At À 24 h, petal color is red. Blue color development starts around À 6 h, and reaches completion after À 3 h. Both adaxial and abaxial epidermal vacuoles are colored (Fig. 1B) due to the presence of a single anthocyanin, HBA, 13) in solution (Fig. 1C). Adaxial cells have a typical conical shape, while abaxial cells are at and it was ...
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... in petal color, pHv and cell vol- ume during ower-opening period. The time- course of morning glory petal-color change during the ower-opening period is shown in Fig. 1A. At À 24 h, petal color is red. Blue color development starts around À 6 h, and reaches completion after À 3 h. Both adaxial and abaxial epidermal vacuoles are colored (Fig. 1B) due to the presence of a single anthocyanin, HBA, 13) in solution (Fig. 1C). Adaxial cells have a typical conical shape, while abaxial cells are at and it was clearly shown that cell expansion growth occurred during blooming (Fig. 1B, ...
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... The time- course of morning glory petal-color change during the ower-opening period is shown in Fig. 1A. At À 24 h, petal color is red. Blue color development starts around À 6 h, and reaches completion after À 3 h. Both adaxial and abaxial epidermal vacuoles are colored (Fig. 1B) due to the presence of a single anthocyanin, HBA, 13) in solution (Fig. 1C). Adaxial cells have a typical conical shape, while abaxial cells are at and it was clearly shown that cell expansion growth occurred during blooming (Fig. 1B, ...
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... starts around À 6 h, and reaches completion after À 3 h. Both adaxial and abaxial epidermal vacuoles are colored (Fig. 1B) due to the presence of a single anthocyanin, HBA, 13) in solution (Fig. 1C). Adaxial cells have a typical conical shape, while abaxial cells are at and it was clearly shown that cell expansion growth occurred during blooming (Fig. 1B, ...
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... calculated cell volume on the adaxial side using photographs of transverse sections. In normal growing petals, the average cell volume at À 24 h was 3.7 e 0.9 pl, and the volume increased gradually in two separate phases, reaching 11.0 e 3.2 pl at 0 h (Fig. 1E). In the rst stage (À 24 h to À 6 h), a slow increase in cell volume was observed without cell color change, whereas during the second stage (À 6 h to 0 h) the cell volume increase was more rapid and coupled with bluing. Because the cell color change from red to blue is solely dependent upon the change in pHv from acidic to alkaline ...
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... For ower-opening, deviation growth with cell expansion growth occurs, and accu- mulation of osmoticum into vacuoles is essential. In several owers organic solutes plays the role. 31)-33) In petals of I. tricolor cv. Heavenly Blue, the colored epidermal cells grew to more than three times their original size during the last 24 h of ower-opening (Fig. 1B, 1E). This expansive cell growth may be caused by two dierent mechanisms. Cell enlarge- ment during the rst stage (À 24 h to À 12 h) with- out a change in cell color may be coordinated with the uptake of neutral organic compounds, since the K þ concentration was shown to decrease during this period. However, in the second stage (À 6 h to 0 ...
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... normal cells the following phenomena oc- curred synchronously with the expression of ItNHX1: cell-color bluing (Fig. 1B), pHv increase, 14),15) K þ ac- cumulation ( Fig. 2A), and cell expansion growth ( Fig. 1E) Mechanism of vacuolar alkalization of col- ored cells by ItNHX. In the tonoplast, a number of channels, transporters and pumps work to regu- late ion homeostasis. 6),7),39) Therefore, pHv is deter- mined by the ion balance in a dynamic ...
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... normal cells the following phenomena oc- curred synchronously with the expression of ItNHX1: cell-color bluing (Fig. 1B), pHv increase, 14),15) K þ ac- cumulation ( Fig. 2A), and cell expansion growth ( Fig. 1E) Mechanism of vacuolar alkalization of col- ored cells by ItNHX. In the tonoplast, a number of channels, transporters and pumps work to regu- late ion homeostasis. 6),7),39) Therefore, pHv is deter- mined by the ion balance in a dynamic equilibrium condition. We calculated whether such a high pHv level (7.7 at 0 h) could be maintained ...

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The Japanese morning glory (Ipomoea nil) and petunia (Petunia hybrida), locally called “Asagao” and “Tsukubane-asagao”, respectively, are popular garden plants. They have been utilized as model plants for studying the genetic basis of floricultural traits, especially anthocyanin pigmentation in flower petals. In their long history of genetic studies, many mutations affecting flower pigmentation have been characterized, and both structural and regulatory genes for the anthocyanin biosynthesis pathway have been identified. In this review, we will summarize recent advances in the understanding of flower pigmentation in the two species with respect to flower hue and color patterning. Regarding flower hue, we will describe a novel enhancer of flavonoid production that controls the intensity of flower pigmentation, new aspects related to a flavonoid glucosyltransferase that has been known for a long time, and the regulatory mechanisms of vacuolar pH being a key determinant of red and blue coloration. On color patterning, we describe particular flower patterns regulated by epigenetic and RNA-silencing mechanisms. As high-quality whole genome sequences of the Japanese morning glory and petunia wild parents (P. axillaris and P. inflata, respectively) were published in 2016, further study on flower pigmentation will be accelerated.