Assessing the biosynthetic capabilities of secretory glands in Citrus peel.

Siau Sie Voo,Howard D Grimes,B Markus Lange

doi:10.1104/pp.112.194233

Abstract

Epithelial cells (ECs) lining the secretory cavities of Citrus peel have been hypothesized to be responsible for the synthesis of essential oil, but direct evidence for such a role is currently sparse. We used laser-capture microdissection and pressure catapulting to isolate ECs and parenchyma cells (as controls not synthesizing oil) from the peel of young grapefruit (Citrus × paradisi 'Duncan'), isolated RNA, and evaluated transcript patterns based on oligonucleotide microarrays. A Gene Ontology analysis of these data sets indicated an enrichment of genes involved in the biosynthesis of volatile terpenoids and nonvolatile phenylpropanoids in ECs (when compared with parenchyma cells), thus indicating a significant metabolic specialization in this cell type. The gene expression patterns in ECs were consistent with the accumulation of the major essential oil constituents (monoterpenes, prenylated coumarins, and polymethoxylated flavonoids). Morphometric analyses demonstrated that secretory cavities are formed early during fruit development, whereas the expansion of cavities, and thus oil accumulation, correlates with later stages of fruit expansion. Our studies have laid the methodological and experimental groundwork for a vastly improved knowledge of the as yet poorly understood processes controlling essential oil biosynthesis in Citrus peel.

Highlights

Epithelial cells (ECs) lining the secretory cavities of Citrus peel have been hypothesized to be responsible for the synthesis of essential oil, but direct evidence for such a role is currently sparse
Transcripts related to terpenoid synthases, which catalyze the first committed step in the biosynthesis of terpenoid essential oil constituents, were highly expressed in Citrus fruit peel when compared with other tissues and organs (Berger et al, 2007; Dornelas and Mazzafera, 2007; Takita et al, 2007)
Because of the enrichment of this preparation in leucoplasts likely originating from ECs, the authors hypothesized that the ECs were the main location for essential oil biosynthesis

Summary

Introduction

Epithelial cells (ECs) lining the secretory cavities of Citrus peel have been hypothesized to be responsible for the synthesis of essential oil, but direct evidence for such a role is currently sparse. Citrus essential oils are obtained on an industrial scale by cold extraction and generally contain two classes of constituents: a volatile fraction consisting of monoterpenoids and small amounts of sesquiterpenoids (totaling 94%–98% of the oil) and a nonvolatile residue containing fatty acids, sterols, carotenoids, waxes, coumarins, and polymethoxylated flavonoids (2%–6% of the oil; Mondello et al, 2002) These oils are processed into various formulations for industrial. Genome-wide oligonucleotide microarray analyses have been conducted with Citrus peel tissue (Maul et al, 2008; González-Candelas et al, 2010; Matas et al, 2010; Ballester et al, 2011; Hershkovitz et al, 2012) These studies were aimed at evaluating specific developmental processes or stress/ defense responses of Citrus and did not provide direct insights into peel essential oil biosynthesis. The only direct evidence for the site of essential oil biosynthesis comes from the in situ localization of transcripts for putative monoterpene synthases to ECs of rough lemon (Citrus jambhiri; Yamasaki and Akimitsu, 2007)

Methods

Results

Conclusion