Abstract
BackgroundIn conifers, terpene synthases (TPSs) of the gymnosperm-specific TPS-d subfamily form a diverse array of mono-, sesqui-, and diterpenoid compounds, which are components of the oleoresin secretions and volatile emissions. These compounds contribute to defence against herbivores and pathogens and perhaps also protect against abiotic stress.ResultsThe availability of extensive transcriptome resources in the form of expressed sequence tags (ESTs) and full-length cDNAs in several spruce (Picea) species allowed us to estimate that a conifer genome contains at least 69 unique and transcriptionally active TPS genes. This number is comparable to the number of TPSs found in any of the sequenced and well-annotated angiosperm genomes. We functionally characterized a total of 21 spruce TPSs: 12 from Sitka spruce (P. sitchensis), 5 from white spruce (P. glauca), and 4 from hybrid white spruce (P. glauca × P. engelmannii), which included 15 monoterpene synthases, 4 sesquiterpene synthases, and 2 diterpene synthases.ConclusionsThe functional diversity of these characterized TPSs parallels the diversity of terpenoids found in the oleoresin and volatile emissions of Sitka spruce and provides a context for understanding this chemical diversity at the molecular and mechanistic levels. The comparative characterization of Sitka spruce and Norway spruce diterpene synthases revealed the natural occurrence of TPS sequence variants between closely related spruce species, confirming a previous prediction from site-directed mutagenesis and modelling.
Highlights
In conifers, terpene synthases (TPSs) of the gymnosperm-specific TPS-d subfamily form a diverse array of mono, sesqui, and diterpenoid compounds, which are components of the oleoresin secretions and volatile emissions
The rate of gene discovery was dependent on the depth of expressed sequence tags (ESTs) sequencing (Table 1), the substantially deeper EST sequence coverage in white spruce (242,931 ESTs) did not result in a proportional increase of TPS discovery relative to Sitka spruce (174,384 ESTs) and hybrid white spruce (26,350 ESTs), suggesting that the majority of expressed TPSs in the tissues sequenced were captured at the depth of sequencing probed in white spruce and Sitka spruce
The estimate of at least 69 TPSs in white spruce is comparable to the number of putatively active TPS genes found in the sequenced genomes of angiosperms and is perhaps a good approximation of the total number of transcriptionally active TPS genes in a conifer species
Summary
Terpene synthases (TPSs) of the gymnosperm-specific TPS-d subfamily form a diverse array of mono-, sesqui-, and diterpenoid compounds, which are components of the oleoresin secretions and volatile emissions These compounds contribute to defence against herbivores and pathogens and perhaps protect against abiotic stress. The genes encoding putatively active mono-, sesqui-, and di-TPSs number at least 32 in the Arabidopsis (Arabidopsis thaliana) genome [6], at least in the rice (Oryza sativa) genome [7], at least in the poplar (Populus trichocarpa) genome [8], and at least 69 in the genome of a highly inbred grapevine (Vitis vinifera) Pinot Noir variety [9,10] All of these angiosperm genomes contain clusters of duplicated TPS genes. Targeted BAC sequencing of a few conifer TPSs from white spruce (Picea glauca) did not reveal any genomic clustering of multiple TPS genes in this conifer genome [11,12]
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