Formation of the Unusual Semivolatile Diterpene Rhizathalene by the Arabidopsis Class I Terpene Synthase TPS08 in the Root Stele Is Involved in Defense against Belowground Herbivory

Abstract

Secondary metabolites are major constituents of plant defense against herbivore attack. Relatively little is known about the cell type-specific formation and antiherbivore activities of secondary compounds in roots despite the substantial impact of root herbivory on plant performance and fitness. Here, we describe the constitutive formation of semivolatile diterpenes called rhizathalenes by the class I terpene synthase (TPS) 08 in roots of Arabidopsis thaliana. The primary enzymatic product of TPS08, rhizathalene A, which is produced from the substrate all-trans geranylgeranyl diphosphate, represents a so far unidentified class of tricyclic diterpene carbon skeletons with an unusual tricyclic spiro-hydrindane structure. Protein targeting and administration of stable isotope precursors indicate that rhizathalenes are biosynthesized in root leucoplasts. TPS08 expression is largely localized to the root stele, suggesting a centric and gradual release of its diterpene products into the peripheral root cell layers. We demonstrate that roots of Arabidopsis tps08 mutant plants, grown aeroponically and in potting substrate, are more susceptible to herbivory by the opportunistic root herbivore fungus gnat (Bradysia spp) and suffer substantial removal of peripheral tissue at larval feeding sites. Our work provides evidence for the in vivo role of semivolatile diterpene metabolites as local antifeedants in belowground direct defense against root-feeding insects.

In Planta Variation of Volatile Biosynthesis: An Alternative Biosynthetic Route to the Formation of the Pathogen-Induced Volatile Homoterpene DMNT via Triterpene Degradation in Arabidopsis Roots

“…Additionally, a pericycle-specific expression at the root differentiation zone could be important both under constitutive and induced conditions as a barrier to avoid vasculature invasion by root pathogens. A similar pericycle-specific expression profile was found for rhizathalene synthase in Arabidopsis roots (Vaughan et al, 2013). The expanded expression in the elongation zone, an area that is preferred by microbial pathogen infection, positively correlates with potential defensive functions of arabidiol-derived compounds.…”

“…In addition, an analysis of T-DNA insertion lines of several root-expressed terpene synthase genes (Vaughan et al, 2013) did not lead to the identification of a gene involved in DMNT formation. However, treatment of Arabidopsis hairy roots with lovastatin, an inhibitor of the isopentenyl diphosphate producing mevalonate pathway in the cytosol, severely reduced jasmonate-induced emission of DMNT, while this was not the case when fosmidomycin, an inhibitor of the plastidial methylerythritol phosphate pathway, was applied (Supplemental Figure 7).…”

Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize

“…Notably, ZmAN2 is located proximal to a quantitative trait locus mapped to bin 1.03 that is associated with root growth (Rahman et al, 2011), and ZmKSL4 colocates in bin 1.08 with quantitative trait loci associated with both drought tolerance (Tuberosa et al, 2002) and abscisic acid biosynthesis, which can further mediate drought-induced phytoalexin biosynthesis in maize roots (Vaughan et al, 2015). Our findings here contribute to a growing body of knowledge demonstrating roles for root diterpenoids, including momilactone phytoalexins in rice (Toyomasu et al, 2008), the antiherbivory activity of rhizathalene in Arabidopsis (Arabidopsis thaliana; Vaughan et al, 2013), and drought tolerance mediated by isorosmanol in rosemary (Rosmarinus officinalis; Munné-Bosch and Alegre, 2000).…”

Biosynthesis of the microtubule-destabilizing diterpene pseudolaric acid B from golden larch involves an unusual diterpene synthase

“…2C). Additional quantum chemical calculations (23,24) of 1 H and 13 C NMR chemical shifts for several pseudolaratriene isomers and diastereomers of each verified the structure shown in Fig. 2C (Dataset S1) and were consistent with all available NMR data (chemical shifts, coupling constants, and NOE experiments).…”