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.

Monitoring the distribution and subsequent effects of nanoparticle contaminants in aquatic ecosystems will be pivotal to developing regulations that minimize their environmental footprint. The present study focused on the link between nanoparticle characteristics and Daphnia magna body burden using gold nanoparticles (AuNPs) with different size, shape, and surface charge configurations as model particles. Uptake followed first-order kinetics across the entire concentration range for all particles except the cationic rods, which demonstrated 2 distinct uptake patterns. Elimination followed the 2-compartment model for all particle configurations. Multiple regression analysis identified size and surface charge as controlling influences over AuNP uptake and elimination, whereas shape was regarded as inconsequential to both processes. Examination of the lumen-microvilli interface produced no evidence to indicate assimilation of the AuNPs used in the present study. Instead, these nanoparticles were restricted to the gut lumen and the carapace, where ingestion efficiency and adsorption were the primary determinants of total body burden. Models developed from the present data predict that D. magna will amass a higher body burden of larger cationic AuNPs at high concentration exposures and larger anionic AuNPs at low concentration exposures. A survey of the nanoparticle literature revealed that these trends were consistent with observations for certain nanomaterial exposures but could not be applied indiscriminately to all nanoparticle types and species.