Recent advances in flower color variation and patterning of Japanese morning glory and petunia

Abstract

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.

Keywords: Ipomoea; RNA silencing; anthocyanin; epigenetics; floral pigmentation pattern; petunia; vacuolar pH.

Fig. 1

Simplified flavonoid biosynthesis pathway. The enzymes in the pathway are: CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanidin synthase; 3GT, UDP-glucose:flavonoid 3-O-glucosyltransferase; FNS, flavone synthase; FLS, flavonol synthase. The arrowheads indicate modification steps of anthocyanins, which are mediated by glycosyltransferases, acyltransferases, and methyltransferases. Hypothesis 1 of EFP function: EFP enhances CHS activity. Hypothesis 2 of EFP function: EFP interacts with biosynthesis enzymes and forms a metabolon complex.

Fig. 2

Flower phenotypes of flower hue mutants and the genomic structure of the mutated loci. The gray and white boxes indicate the coding and the untranslated regions, respectively. Flower phenotype of efp-1 mutant (A) and the structure of the EFP gene (B). The efp-1 mutant shows pale-colored flowers with normal pigmented spots and sectors. The arrowheads indicate insertion sites of Tpn13, Tpn14, and a 17-bp sequence of the efp-1, efp-2, and efp-3 mutations, respectively. Flower phenotypes of dk-1 (C) and dk-2 (D) mutants and the structure of the 3GT gene (E). The dk-1 and dk-2 mutants display pale and dull-colored flowers. The flower color in the dk-2 mutant is slightly darker than that in the dk-1 mutant. The arrowheads show a 4-bp sequence and Tpn10 insertion sites of the dk-1 and dk-2 mutations, respectively.

Fig. 3

Flower color patterns of the Japanese morning glory and petunia. A–E: Flower phenotypes of the Q531 line of the dk2-mutant of the Japanese morning glory. (A) Ruled plants confer non-clonal spots and sectors, hakeme-shibori (brush marks variegation). Apparently clonal spots and sectors are also occasionally observed in ruled plants. (B) Plain plants display pale pigmentation petals. (C) Self-colored plants display fully pigmented flowers. These plants carry an identical dk-2 mutation, and Tpn10 seems to be able to transpose only in plain plants. (D) Somatic reversions caused by Tpn10 excisions are occasionally observed in plain plants. (E) Germinal revertant from a plain plant. (F–I) Naturally occurring bicolor RNAi mutants of the petunia. Flower phenotypes of Star (F), Picotee (G), and the bicolor cultivar ‘Night Sky’ (Ball Seed Co.) (I). (H) Genome structure of CHS siRNA-producing locus of the bicolor petunia. Two copies of CHS genes are tandemly located.

Fig. 4

H⁺ transporters regulate vacuolar pH, affecting the flower color of petunias (A–E) and I. nil (F–H). (A) In wild-type petunias, PH3 activates PH1 and PH5 expression, and the heteromeric complex of two P-ATPases, PH1 and PH5, mediate hyperacidification. (B) In the ph3 mutant, PH1 and PH5 are not expressed, resulting in an increase of vacuolar pH. This mutant line accumulates cyanidin derivatives that exhibit dull gray flowers. (C) PH1 is necessary for the H⁺ pump activity of PH5, and PH5 alone cannot rescue the ph3 phenotype. (D) The ph3 phenotype is rescued by the co-expression of PH1 and PH5. (E) The rescued phenotype in (D) is canceled by the expression of 35S:NHX1. NHX exchanges cations and H⁺, resulting in an increase in vacuolar pH. (F) Flower buds of the wild-type I. nil show lower vacuolar pH and red petals. (G) During flower opening, flower color changes from red to blue. In the same stage, PURPLE/InNHX1 (purple circle) is accumulated and mediates vacuolar alkalization. (H) The pr mutant shows partial vacuolar alkalization, and red flower buds change into purple flowers. The pH values of petal homogenates are presented, and those estimated from spectra are shown in parentheses. Scale bars represent 1 cm.