Identification and functional characterization of monofunctional ent-copalyl diphosphate and ent-kaurene synthases in white spruce reveal different patterns for diterpene synthase evolution for primary and secondary metabolism in gymnosperms.

The biosynthesis of the tetracyclic diterpene ent-kaurene is a critical step in the general (primary) metabolism of gibberellin hormones. ent-Kaurene is formed by a two-step cyclization of geranylgeranyl diphosphate via the intermediate ent-copalyl diphosphate. In a lower land plant, the moss Physcomitrella patens, a single bifunctional diterpene synthase (diTPS) catalyzes both steps. In contrast, in angiosperms, the two consecutive cyclizations are catalyzed by two distinct monofunctional enzymes, ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS). The enzyme, or enzymes, responsible for ent-kaurene biosynthesis in gymnosperms has been elusive. However, several bifunctional diTPS of specialized (secondary) metabolism have previously been characterized in gymnosperms, and all known diTPSs for resin acid biosynthesis in conifers are bifunctional. To further understand the evolution of ent-kaurene biosynthesis as well as the evolution of general and specialized diterpenoid metabolisms in gymnosperms, we set out to determine whether conifers use a single bifunctional diTPS or two monofunctional diTPSs in the ent-kaurene pathway. Using a combination of expressed sequence tag, full-length cDNA, genomic DNA, and targeted bacterial artificial chromosome sequencing, we identified two candidate CPS and KS genes from white spruce (Picea glauca) and their orthologs in Sitka spruce (Picea sitchensis). Functional characterization of the recombinant enzymes established that ent-kaurene biosynthesis in white spruce is catalyzed by two monofunctional diTPSs, PgCPS and PgKS. Comparative analysis of gene structures and enzyme functions highlights the molecular evolution of these diTPSs as conserved between gymnosperms and angiosperms. In contrast, diTPSs for specialized metabolism have evolved differently in angiosperms and gymnosperms.

Targeted isolation, sequence assembly and characterization of two white spruce (Picea glauca) BAC clones for terpenoid synthase and cytochrome P450 genes involved in conifer defence reveal insights into a conifer genome.

BACKGROUND: Conifers are a large group of gymnosperm trees which are separated from the angiosperms by more than 300 million years of independent evolution. Conifer genomes are extremely large and contain considerable amounts of repetitive DNA. Currently, conifer sequence resources exist predominantly as expressed sequence tags (ESTs) and full-length (FL)cDNAs. There is no genome sequence available for a conifer or any other gymnosperm. Conifer defence-related genes often group into large families with closely related members. The goals of this study are to assess the feasibility of targeted isolation and sequence assembly of conifer BAC clones containing specific genes from two large gene families, and to characterize large segments of genomic DNA sequence for the first time from a conifer. RESULTS: We used a PCR-based approach to identify BAC clones for two target genes, a terpene synthase (3-carene synthase; 3CAR) and a cytochrome P450 (CYP720B4) from a non-arrayed genomic BAC library of white spruce (Picea glauca). Shotgun genomic fragments isolated from the BAC clones were sequenced to a depth of 15.6- and 16.0-fold coverage, respectively. Assembly and manual curation yielded sequence scaffolds of 172 kbp (3CAR) and 94 kbp (CYP720B4) long. Inspection of the genomic sequences revealed the intron-exon structures, the putative promoter regions and putative cis-regulatory elements of these genes. Sequences related to transposable elements (TEs), high complexity repeats and simple repeats were prevalent and comprised approximately 40% of the sequenced genomic DNA. An in silico simulation of the effect of sequencing depth on the quality of the sequence assembly provides direction for future efforts of conifer genome sequencing. CONCLUSION: We report the first targeted cloning, sequencing, assembly, and annotation of large segments of genomic DNA from a conifer. We demonstrate that genomic BAC clones for individual members of multi-member gene families can be isolated in a gene-specific fashion. The results of the present work provide important new information about the structure and content of conifer genomic DNA that will guide future efforts to sequence and assemble conifer genomes.

Transcriptome and full-length cDNA resources for the mountain pine beetle, Dendroctonus ponderosae Hopkins, a major insect pest of pine forests.

Bark beetles (Coleoptera: Curculionidae: Scolytinae) are major insect pests of many woody plants around the world. The mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins, is a significant historical pest of western North American pine forests. It is currently devastating pine forests in western North America--particularly in British Columbia, Canada--and is beginning to expand its host range eastward into the Canadian boreal forest, which extends to the Atlantic coast of North America. Limited genomic resources are available for this and other bark beetle pests, restricting the use of genomics-based information to help monitor, predict, and manage the spread of these insects. To overcome these limitations, we generated comprehensive transcriptome resources from fourteen full-length enriched cDNA libraries through paired-end Sanger sequencing of 100,000 cDNA clones, and single-end Roche 454 pyrosequencing of three of these cDNA libraries. Hybrid de novo assembly of the 3.4 million sequences resulted in 20,571 isotigs in 14,410 isogroups and 246,848 singletons. In addition, over 2300 non-redundant full-length cDNA clones putatively containing complete open reading frames, including 47 cytochrome P450s, were sequenced fully to high quality. This first large-scale genomics resource for bark beetles provides the relevant sequence information for gene discovery; functional and population genomics; comparative analyses; and for future efforts to annotate the MPB genome. These resources permit the study of this beetle at the molecular level and will inform research in other Dendroctonus spp. and more generally in the Curculionidae and other Coleoptera.