Mechanism of floral scent production in Osmanthus fragrans and the production and regulation of its key floral constituents, \u03b2-ionone and linalool

Yuanji Han,Yong Li,Fude Shang,Qian Zhu,Xiaodan Wang,Ke Li,Meifang Dong,Hongyun Wang

doi:10.1038/s41438-019-0189-4

Abstract

Sweet osmanthus (Osmanthus fragrans Lour.) is among the top ten most well-known flowers in China and is recognized as both an aromatic plant and ornamental flower. Here, manual sectioning, scanning electron microscopy, and transmission electron microscopy of sweet osmanthus petals revealed that large amounts of lipids are present inside the petal cells and on the cell surfaces. However, no secretory structures were observed. Instead, the petal cells protrude slightly outward, and the surfaces of the cells are adorned with highly regular brush-shaped hairs. The surfaces of the ‘Yingui’ petals possessed mostly curled and more numerous hairs, whereas the ‘Dangui’ petals possessed fewer brush-shaped and more sparsely arranged hairs. In addition, many granular substances were attached to the brush-shaped hairs, and the granules were denser on the hairs of the ‘Yingui’ petals compared to the hairs on the ‘Dangui’ petals. Furthermore, 35 aromatic components in the ‘Yingui’ petals and 30 aromatic components in the ‘Dangui’ petals were detected via GC-MS. The main aromatic component of the ‘Yingui’ petals was β-ionone, whereas that of the ‘Dangui’ petals was linalool and its oxides. Transcriptome sequencing and qRT-PCR indicated that the high β-ionone content in the ‘Yingui’ petals was due to the overexpression of CCD1 and CCD4 and that the high linalool content in the ‘Dangui’ petals was due to the overexpression of MECS, HDR, IDI1, and LIS1, which function upstream of the linalool synthetic pathway. In particular, the expression levels of CCD4 and LIS1 were upregulated by 5.5- and 5.1-fold in the ‘Yingui’ and ‘Dangui’ petals, respectively. One transcription factor (ERF61) was cloned and named, and the expression pattern of ERF61 in sweet osmanthus petals was found to be generally consistent with that of CCD4. Tobacco transformation experiments, yeast one-hybrid experiments, and electrophoretic mobility shift assays indicated that ERF61 binds to the CCD4 promoter and stimulates CCD4 expression, thereby regulating the synthesis of β-ionone in sweet osmanthus petals.

Highlights

Floral fragrance, color, shape, surface structure, and nectar glands can affect the interactions of higher plants with pollinators, and floral fragrance, in particular, canHan et al Horticulture Research (2019)6:106 floral scents have been reported to originate from unique “aromatic glands”[5,6,7,8,9,10].In plants with aromatic glands, different parts of the flower can differentiate into flap, cilia, or hair-shaped glands that function in the production and emission of aromatic substances[9,10]
After Sudan Black staining, the petal cells appeared brown-black, and brown-black granules could be observed inside the cells (Fig. 1), which indicated that the petal cells contained large amounts of lipids
The present study used Sudan Black to stain sweet osmanthus petals. Both the epidermal and palisade cells of the petals were stained brown-black, and brown-black granules were observed within the cells. This indicated that the sweet osmanthus petal cells contained large amounts of lipids, which could be related to the floral scent

Summary

Introduction

In plants with aromatic glands, different parts of the flower can differentiate into flap-, cilia-, or hair-shaped glands that function in the production and emission of aromatic substances[9,10]. After volatile oils are produced in the secretory cells of aromatic glands, the oils are usually immediately released and disperse within a short period of time[5]. The mechanisms underlying floral scent production likely vary among plants and remain poorly understood. The fragrance of the flower contains as many as 30 potentially exploitable chemical substances[11,12,13]. The flowering period, floral color, and other characteristics are used to classify sweet osmanthus cultivars into four groups: Yingui, Jingui, Dangui, and Sijigui[14,15]

Methods

Results

Conclusion