Kyeong-Seong Cheon

Crossability of American tree peony ‘High Noon’ as seed parent with Japanese cultivars to breed superior cultivars

The tree peony (Paeonia suffruticosa Andr.) is a valuable ornamental plant. American and French cultivars have desirable flower colors, while, Japanese cultivars have larger flowers with better presentation. We hybridized an American tree peony cultivar ‘High Noon (HN)’ (the seed parent) with 57 different Japanese cultivars (pollen parents) to investigate cross compatibility, with the ultimate goal of obtaining improved hybrids. Of the 1,927 crosses performed, 135 (38 cross combinations) yielded a total of 181 seeds (86 mature) and 22 seedlings. Five of the hybrids have already flowered and exhibited their parent character with large yellow flowers. To investigate the causes of cross incompatibility, we examined pollen tube growth in a cross between ‘HN’ and a Japanese cultivar. The result indicated that there are three most important causes of incompatibility, namely, abnormal pollen tube growth, the failure of fertilized ovules, and poor seed germination. Although cross compatibility in each combination was low, crosses between ‘HN’ and Japanese cultivars might be successful with the right paternal plants. In this study, 13 Japanese cultivars had higher cross compatibility with ‘HN’ than the others. Our results will enhance tree peony breeding and guide the selection of parents for hybridization.

Inheritance of the narrow leaf mutation in traditional Japanese evergreen azaleas

Morphological mutations have been admired and preserved in the classical cultivars of the Japanese evergreen azalea since the Edo era (1603–1867). One such mutation is the presence of narrow leaves and petals. In this study, we investigated the characteristics and inheritance patterns of the narrow leaf-type mutational form of Rhododendron macrosepalum ‘Seigaiha’, R. kaempferi ‘Kin-kujyaku’, and R. kiusianum Yanagi-ba mutant. The leaves of all cultivars showed weak development in the transverse plane and thickened margins. Subsequently, based on this qualitative character, the inheritance of the narrow leaf mutation was investigated. F1 seedlings of cross combinations between normal and narrow leaf-type plants had normal leaf-type. All F1 progenies of cross combinations between narrow leaf-type plants had narrow leaves like their parents. In addition, seedlings obtained from backcross and sib-cross tests using narrow leaf mutants segregated into normal and narrow leaf-types at a 1:1 and 3:1 ratio, respectively. These results indicate that the narrow leaf trait of three different azalea cultivars, ‘Seigaiha’, ‘Kin-kujyaku’, and R. kiusianum Yanagi-ba mutant, is probably a recessive mutation caused by a single, common gene.

Long-lasting Corolla Cultivars in Japanese Azaleas: A Mutant AP3/DEF Homolog Identified in Traditional Azalea Cultivars from More Than 300 Years Ago

Floral shape in higher plants typically requires genetic regulation through MADS transcription factors. In Japan, hundreds of azalea cultivars including flower shape mutations have been selected from the diversity of endogenous species and natural hybrids since the early 17th century, the Edo era (1603–1867). The long-lasting trait, known as “Misome-shō” in Japanese, has been identified in several species and cultivar groups of evergreen azaleas (Rhododendron L.) from three hundred years ago in Japan. However, the natural mutation conferring the long-lasting trait in azalea remains unknown. Here, we showed MADS-box gene mutations in long-lasting flowers, R. kaempferi ‘Nikkō-misome,’ R. macrosepalum ‘Kochō-zoroi,’ R. indicum ‘Chōjyu-hō,’ and R. × hannoense ‘Amagi-beni-chōjyu.’ All of the long-lasting flowers exhibited small-sized corollas with stomata during long blooming. In the long-lasting flowers, transcript of the APETALA3 (AP3)/DEFICIENS (DEF) homolog was reduced, and an LTR-retrotransposon was independently inserted into exons 1, 2, and 7 or an unknown sequence in exon 1 in gDNA of each cultivar. This insertion apparently abolished the normal mRNA sequence of the AP3/DEF homolog in long-lasting flowers. Also, long-lasting flowers were shown from F2 hybrids that had homozygous ap3/def alleles. Therefore, we concluded that the loss of function of the AP3/DEF homolog through a transposable element insertion may confer a stable long-lasting mutation in evergreen azaleas.