The Terpene Synthase Gene Family of Carrot (Daucus carota L.): Identification of QTLs and Candidate Genes Associated with Terpenoid Volatile Compounds.

Jens Keilwagen,Frank Dunemann,Heike Lehnert,Thomas Nothnagel,Thomas Berner,Detlef Ulrich,Holger Budahn

doi:10.3389/fpls.2017.01930

Abstract

Terpenes are an important group of secondary metabolites in carrots influencing taste and flavor, and some of them might also play a role as bioactive substances with an impact on human physiology and health. Understanding the genetic and molecular basis of terpene synthases (TPS) involved in the biosynthesis of volatile terpenoids will provide insights for improving breeding strategies aimed at quality traits and for developing specific carrot chemotypes possibly useful for pharmaceutical applications. Hence, a combination of terpene metabolite profiling, genotyping-by-sequencing (GBS), and genome-wide association study (GWAS) was used in this work to get insights into the genetic control of terpene biosynthesis in carrots and to identify several TPS candidate genes that might be involved in the production of specific monoterpenes. In a panel of 85 carrot cultivars and accessions, metabolite profiling was used to identify 31 terpenoid volatile organic compounds (VOCs) in carrot leaves and roots, and a GBS approach was used to provide dense genome-wide marker coverage (>168,000 SNPs). Based on this data, a total of 30 quantitative trait loci (QTLs) was identified for 15 terpenoid volatiles. Most QTLs were detected for the monoterpene compounds ocimene, sabinene, β-pinene, borneol and bornyl acetate. We identified four genomic regions on three different carrot chromosomes by GWAS which are both associated with high significance (LOD ≥ 5.91) to distinct monoterpenes and to TPS candidate genes, which have been identified by homology-based gene prediction utilizing RNA-seq data. In total, 65 TPS candidate gene models in carrot were identified and assigned to known plant TPS subfamilies with the exception of TPS-d and TPS-h. TPS-b was identified as largest subfamily with 32 TPS candidate genes.

Highlights

Terpenoids, named isoprenoids, are the largest group of plant natural products comprising at least 30,000 different substances and containing a wide assortment of structural types including monoterpenes, sesquiterpenes, diterpenes, and triterpenes (Degenhardt et al, 2009)
The quantitation of the identified terpene volatiles in each carrot accession is presented in Supplementary Table 4. These volatile organic compounds (VOCs) belong to the chemical classes of monoterpenes, sesquiterpenes, and several different terpene-derivatives such as terpene alcohols
20 VOCs could be extracted and identified. The majority of these VOCs are monoterpenes, whereas only four sesquiterpenoids were present in the roots, namely β-caryophyllene, β-farnesene, β-bisabolene and caryophyllene oxide

Summary

Introduction

Terpenoids, named isoprenoids, are the largest group of plant natural products comprising at least 30,000 different substances and containing a wide assortment of structural types including monoterpenes, sesquiterpenes, diterpenes, and triterpenes (Degenhardt et al, 2009). Terpenoids play a considerable physiological role in the primary metabolism as phytohormones (gibberellic acid and abscisic acid), photosynthesis pigments (carotenoids and chlorophylls) and stabilize membrane structural components (sterols). They have important functions for communication and defense of the plants and help to attract pollinators or predators of herbivores (Pichersky and Gershenzon, 2002; Degenhardt et al, 2003). A typical characteristic of monoterpenes and sesquiterpenes is their volatility, and they contribute to the typical flavor and aroma of many plant species (Pichersky and Gershenzon, 2002). In carrots they are important for taste and flavor but are known to influence bitterness (Kramer et al, 2012)

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Conclusion