Browsing by Author "Huh, Jung-Hyun"
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- Biochemical, Molecular and Functional Analysis of Volatile Terpene Formation in Arabidopsis RootsHuh, Jung-Hyun (Virginia Tech, 2011-07-20)Plants produce secondary (or specialized) metabolites to respond to a variety of environmental changes and threats. Especially, volatile compounds released by plants facilitate short and long distance interaction with both beneficial and harmful organisms. Comparatively little is known about the organization and role of specialized metabolism in root tissues. In this study, we have investigated the root-specific formation and function of volatile terpenes in the model plant Arabidopsis. As one objective, we have characterized the two root-specific terpene synthases, TPS22 and TPS25. Both enzymes catalyze the formation of several volatile sesquiterpenes with (E)-β-farnesene as the major product. TPS22 and TPS25 are expressed in the root in distinct different cell type-specific patterns and both genes are induced by jasmonic acid. Unexpectedly, both TPS proteins are localized to mitochondria, demonstrating a subcellular localization of terpene specialized metabolism in compartments other than the cytosol and plastids. (E)-β-Farnesene is produced at low concentrations suggesting posttranslational modifications of the TPS proteins and/or limited substrate availability in mitochondria. We hypothesize that the mitochondrial localization of TPS22 and TPS25 reflects evolutionary plasticity in subcellular compartmentation of TPS proteins with emerging or declining activity. Since (E)-β-farnesene inhibits Arabidopsis root growth in vitro, mitochondrial targeting of both proteins may fine tune (E)-β-farnesene concentrations to prevent possible autotoxic or inhibitory effects of this terpene in vivo. We further investigated the role of volatile terpenes in Arabidopsis roots in interaction with the soil-borne oomycete, Pythium irregulare. Infection of roots with P. irregulare causes emission of the C11-homoterpene (or better called C4-norterpene) 4,8-dimethylnona-1,3,7-triene (DMNT), which is a common volatile induced by biotic stress in aerial parts of plants but was not previously known to be produced in plant roots. We demonstrate that DMNT is synthesized by a novel, root-specific pathway via oxidative degradation of the C30-triterpene, arabidiol. DMNT exhibits inhibitory effects on P. irregulare mycelium growth and oospore germination in vitro. Moreover, arabidiol and DMNT biosynthetic mutants were found to be more susceptible to P. irregulare infection and showed higher rates of Pythium colonization in comparison to wild type plants. Together, our studies demonstrate differences and plasticity in the metabolic organization and function of terpenes in roots in comparison to aboveground plant tissues.
- Identification of a Dolabellane Type Diterpene Synthase and other Root-Expressed Diterpene Synthases in ArabidopsisWang, Qiang; Jia, Meirong; Huh, Jung-Hyun; Muchlinski, Andrew; Peters, Reuben J.; Tholl, Dorothea (Frontiers, 2016-11-25)Arabidopsis thaliana maintains a complex metabolism for the production of secondary or specialized metabolites. Such metabolites include volatile and semivolatile terpenes, which have been associated with direct and indirect defensive activities in flowers and leaves. In comparison, the structural diversity and function of terpenes in Arabidopsis roots has remained largely unexplored despite a substantial number of root-expressed genes in the Arabidopsis terpene synthase (TPS) gene family. We show that five root-expressed TPSs of an expanded subfamily-a type clade in the Arabidopsis TPS family function as class I diterpene synthases that predominantly convert geranylgeranyl diphosphate (GGPP) to different semi-volatile diterpene products, which are in part detectable at low levels in the ecotypes Columbia (Col) and Cape Verde Island (Cvi). The enzyme TPS20 produces a macrocyclic dolabellane diterpene alcohol and a dolabellane-related diterpene olefin named dolathaliatriene with a so far unknown C6- C11 bicyclic scaffold besides several minor olefin products. The TPS20 compounds occur in all tissues of Cvi but are absent in the Col ecotype because of deletion and substitution mutations in the Col TPS20 sequence. The primary TPS20 diterpene products retard the growth of the root rot pathogen Pythium irregulare but only at concentrations exceeding those in planta. Together, our results demonstrate that divergence and pseudogenization in the Arabidopsis TPS gene family allow for structural plasticity in diterpene profiles of above- and belowground tissues.