Regulation of Oleoresinosis in Grand Fir (Abies grandis)1

Abstract

Grand fir (Abies grandis Lindl.) has been developed as a model system for the study of wound-induced oleoresinosis in conifers as a response to insect attack. Oleoresin is a roughly equal mixture of turpentine (85% monoterpenes [C 10 ] and 15% sesquiterpenes [C 15 ]) and rosin (diterpene [C 20 ] resin acids) that acts to seal wounds and is toxic to both invading insects and their pathogenic fungal symbionts. The dynamic regulation of wound-induced oleoresin formation was studied over 29 d at the enzyme level by in vitro assay of the three classes of synthases directly responsible for the formation of monoterpenes, sesquiterpenes, and diterpenes from the corresponding C 10 , C 15 , and C 20 prenyl diphosphate precursors, and at the gene level by RNA-blot hybridization using terpene synthase class-directed DNA probes. In overall appearance, the shapes of the time-course curves for all classes of synthase activities are similar, suggesting coordinate formation of all of the terpenoid types. However, closer inspection indicates that the monoterpene synthases arise earlier, as shown by an abbreviated time course over 6 to 48 h. RNA-blot analyses indicated that the genes for all three classes of enzymes are transcriptionally activated in response to wounding, with the monoterpene synthases up-regulated first (transcripts detectable 2 h after wounding), in agreement with the results of cell-free assays of monoterpene synthase activity, followed by the coordinately regulated sesquiterpene synthases and diterpene synthases (transcription beginning on d 3-4). The differential timing in the production of oleoresin components of this defense response is consistent with the immediate formation of monoterpenes to act as insect toxins and their later generation at solvent levels for the mobilization of resin acids responsible for wound sealing.

Terpenoid-based defenses in conifers: cDNA cloning, characterization, and functional expression of wound-inducible ( E )-α-bisabolene synthase from grand fir ( Abies grandis )

“…Following wounding, the accumulation of ag1 mRNA increased transiently to a maximum at 11-14 d, demonstrating that bisabolene synthase mRNA induction is responsible for this presumed sesquiterpene-based defense response in grand fir. The increase of ag1 mRNA accumulation is relatively slow compared with the rapid transcriptional activation of monoterpene synthases, for which maximum steady state mRNA levels are reached 1-2 d after wounding (13,15). This differential gene activation suggests that both early and late terpenoid-based defense responses are triggered by wounding, possibly involving independent signaling pathways.…”

“…Therefore, grand fir has been developed as a model system for the study of the regulation of defensive terpene biosynthesis in conifers (12)(13)(14)(15). The induced production of oleoresin monoterpenes and diterpene resin acids has been well defined (11,(16)(17)(18).…”

“…The induced production of oleoresin monoterpenes and diterpene resin acids has been well defined (11,(16)(17)(18). Recent cDNA cloning of abietadiene synthase (19), the major wound-inducible diterpene synthase of grand fir, and of three wound-inducible monoterpene synthases (13), led to the structural analysis of the corresponding catalysts and to the molecular dissection of induced monoterpene and diterpene biosynthesis (14,15).…”

Plant terpenoid synthases: Molecular biology and phylogenetic analysis

“…Grand fir has been developed as a model system to study conifer defense responses to bark beetles and their associated fungal pathogens, a process that involves induced production of monoterpenes, sesquiterpenes, and diterpenes (57,87). Monoterpenes are toxic to invaders and furnish the volatile solvent for the diterpene resin acids, which upon monoterpene evaporation harden to a mechanical barrier that seals the wound site (88).…”

Role of Arginine-304 in the Diphosphate-Triggered Active Site Closure Mechanism of Trichodiene Synthase,

“…In comparison, plants produce C 10 , C 15 , and C 20 cyclic terpenoids for defense against fungi and insects. For example, Abies grandis (Grand fir) secretes multiple cyclic terpenoids in response to stem wounds and insect attack (7). Interestingly, cyclic terpenoids contribute to the desirable taste and aroma of plants such as Salvia officinalis (sage) (8), leading to their use as culinary flavorings.…”