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

Kyeong-Seong Cheon,Keisuke Tasaki,Nobuo Kobayashi,Akira Nakatsuka

doi:10.3389/fpls.2017.02239

Abstract

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.

Highlights

Species of the genus Rhododendron (Ericaceae), subgenus Tsutsusi, section Tsutsusi are important genetic resources for evergreen azalea cultivars used as ornamental shrubs or potted azaleas in many regions worldwide, including Asia, Europe, and America
The flowering periods, which facilitate corolla maintenance after flowering, were longer in long-lasting flowers than in normal flowers (Figure 2J). Based on these observations of floral morphologies, we confirmed that longlasting flowers have small-sized corollas with stomata during long blooming period and that the corollas of long-lasting flowers show the conversion of normal corollas to sepaloid corollas, whereas the stamens remain unaffected
As case of herbaceous peony (Paeonia lactiflora), spontaneous corolla mutant in natural condition was closely associated with selective expression alterations of duplicated AP3 and PI genes, because down-regulation of duplicated B-class genes might result in reduction of AP3-PI heterodimer in sepaloid corolla (Gong et al, 2017)

Summary

Introduction

Species of the genus Rhododendron (Ericaceae), subgenus Tsutsusi, section Tsutsusi are important genetic resources for evergreen azalea cultivars used as ornamental shrubs or potted azaleas in many regions worldwide, including Asia, Europe, and America. A monograph on azaleas, “Kinshu-makura” (Figure 1A), edited in 1692, described more than 300 azalea cultivars, and several of the cultivars described in this monograph still exist and have unique flower and leaf characteristics (Ito, 1984). This monograph provided the first description of ‘Misome-guruma’ (Figure 1B), a cultivar possessing small light-red flowers, long stamens, and a long blooming season, with the flowers turning greenish in June (Ito, 1984). Long-lasting flower was shown as recessive trait to normal flower and was controlled by single gene (Gobara et al, 2017)

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