Terpenoid synthases in conifers and poplars

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Rapid development of plantation forestry is necessary to meet an increasing demand of fiber and wood for the forest products industry. In addition to delivering high productivity and high quality of cellulose and lignin formation, trees for plantation forestry must resist increasing pressure from insect pests and pathogens. Unlike in agriculture, as practiced in much of the 20th century, forestry cannot rely on massive application of pesticides for crop protection, because forests, including plantation forests, are complex ecosystems providing essential habitats for soil living and above ground organisms. At the current stage of genetic tree improvement, there are unique opportunities to retain or recapture much of the genetic material that determines natural defense and resistance for plantation forestry. Terpenoids and terpenoid synthase (TPS) genes are major components of defense and resistance in conifers and appear to be important for defense biology of poplars as well. Terpenoid defense in forest trees present multigenic traits that are highly variable within species and natural populations. Much of the chemical diversity of terpenoids in trees is determined by variable constitutive and inducible expression of members of large TPS gene families and by multi-product reaction mechanisms of TPS enzymes. Detailed knowledge of the organization and evolution of TPS gene families, TPS gene expression and TPS enzyme biochemistry is critical to maintain natural genetic and chemical diversity of terpenoids in trees selected and improved for plantation forestry. TPS genes can be targeted for development of markers for pest resistance and also as genetic markers for formation of terpenoid extractives that affect mills and toxicity of efflux water in the pulp and paper industries. In addition, the biochemical machinery of resin terpenoid formation and resin secretion in conifers can conceivably be harnessed for biotechnological production of high value compounds in renewable plantation forest resources. Conifers and poplars also emit massive amounts of reactive hemiterpenes and monoterpenes into the atmosphere. Another system of plantation forestry, Eucalyptus (not discussed in this chapter due to lack of published studies of TPS genes in this system) also emits monoterpenes in large quantitities and is protected against many herbivores by terpenoids.

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... In monoterpene synthases, the N-terminal plastid-targeting sequence is generally found in the region upstream of the conserved RR-motif; it contains high numbers of serine and threonine residues with low numbers of acidic residues . Functions of individual terpene synthases must be experimentally tested as gene sequences are highly similar, and therefore gene function cannot be predicted from sequence information alone Bohlmann et al. 2004;). ...
... We use variation within species which are polymorphic for concentrations or presence of monoterpenes to provide an insight into their ecological ramifications and larger-scale consequences, against the background of intra-specific variation in other traits. This approach can also be used to inform strategies of selection or transgenic manipulation for commercial purposes or to provide additional tools to reveal ecological function (Bohlmann et al., 2004;Aharoni et al., 2005;Schuman & Baldwin, Chapter 15;Dicke et al., Chapter 16). ...
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Plant secondary metabolites (PSMs) such as terpenes and phenolic compounds are known to have numerous ecological roles, notably in defence against herbivores, pathogens and abiotic stresses and in interactions with competitors and mutualists. This book reviews recent developments in the field to provide a synthesis of the function, ecology and evolution of PSMs, revealing our increased awareness of their integrative role in connecting natural systems. It emphasises the multiple roles of secondary metabolites in mediating the interactions between organisms and their environment at a range of scales of ecological organisation, demonstrating how genes encoding for PSM biosynthetic enzymes can have effects from the cellular scale within individual plants all the way to global environmental processes. A range of recent methodological advances, including molecular, transgenic and metabolomic techniques, are illustrated and promising directions for future studies are identified, making this a valuable reference for researchers and graduate students in the field.
In the last five years, much progress has been made in the characterization of conifer defenses using an integrated biochemical, molecular, and genomics approach. With a focus on the fascinating processes of traumatic resin duct development in direct defense and on induced volatile emissions in possible tritrophic defense, we have developed species of spruce, Sitka spruce, and Norway spruce, as two of the best characterized systems of conifer defense research at the molecular and biochemical levels. The economic importance of spruce for the forest industry and the enormous economic, environmental, and social impacts of recent, unprecedented outbreaks of conifer insect pests has mandated substantial new investment into conifer genomics research with an emphasis on the genomics of defense and resistance of conifers against insect pests.
Diterpene resin acids are prominent defense compounds against insect pests and pathogens in conifers. Biochemical and molecular analyses in grand fir (Abies grandis), Norway spruce (Picea abies), and loblolly pine (Pinus taeda) have identified two classes of genes and enzymes that generate much of the structural diversity of terpenoid defense compounds: The terpenoid synthases (TPS) and cytochrome P450 monooxgenases (P450). Using a single substrate, geranylgeranyl diphosphate, families of single-product and multi-product diterpene synthases generate an array of cyclic diterpene olefins. These diterpenes are converted to diterpene resin acids by activity of one or more P450 enzymes. A few conifer diterpene synthases have previously been cloned and characterized in grand fir and in Norway spruce. We have also previously shown that the loblolly pine P450 abietadienol/abietadienal oxidase (PtAO) catalyzes multiple oxidations of several diterpene alcohols and aldehydes. Conifer diterpene synthases are thought to function in plastids while P450s can also be localized to plastids or to the endoplasmic reticulum (ER). Here, we show that a loblolly pine cDNA (PtTPS-LAS) encodes a typical multi-product conifer diterpene synthase that forms levopimaradiene, abietadiene, palustradiene, and neoabietadiene similar to the grand fir abietadiene synthase and Norway spruce levopimaradiene/abietadiene synthase. Subcellular targeting of PtTPS-LAS and PtAO to plastids and ER, respectively, was shown with green fluorescent fusion protein expression in tobacco cells. These data suggest that enzymes for conifer diterpene resin acid biosynthesis are localized to at least two different subcellular compartments, plastids and ER, requiring efficient transport of intermediates and secretion of diterpene resin acids into the extracelluar space.
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