Terpene Specialized Metabolism in Arabidopsis thaliana

Abstract

Terpenes constitute the largest class of plant secondary (or specialized) metabolites, which are compounds of ecological function in plant defense or the attraction of beneficial organisms. Using biochemical and genetic approaches, nearly all Arabidopsis thaliana (Arabidopsis) enzymes of the core biosynthetic pathways producing the 5-carbon building blocks of terpenes have been characterized and closer insight has been gained into the transcriptional and posttranscriptional/translational mechanisms regulating these pathways. The biochemical function of most prenyltransferases, the downstream enzymes that condense the C(5)-precursors into central 10-, 15-, and 20-carbon prenyldiphosphate intermediates, has been described, although the function of several isoforms of C(20)-prenyltranferases is not well understood. Prenyl diphosphates are converted to a variety of C(10)-, C(15)-, and C(20)-terpene products by enzymes of the terpene synthase (TPS) family. Genomic organization of the 32 Arabidopsis TPS genes indicates a species-specific divergence of terpene synthases with tissue- and cell-type specific expression profiles that may have emerged under selection pressures by different organisms. Pseudogenization, differential expression, and subcellular segregation of TPS genes and enzymes contribute to the natural variation of terpene biosynthesis among Arabidopsis accessions (ecotypes) and species. Arabidopsis will remain an important model to investigate the metabolic organization and molecular regulatory networks of terpene specialized metabolism in relation to the biological activities of terpenes.

Figure 1.

Terpene biosynthesis pathways and their subcellular organization in Arabidopsis. The dashed line indicates a possible partial or complete location of MVA pathway enzymes in peroxisomes as predicted by Sapir-Mir et al. (2008). Dashed arrows indicate more than one enzymatic step. Enzymes are in red (cytosol/peroxisome), orange (plastid), or yellow (mitochondrion). Abbreviations for enzymes and metabolites are as described in the text.

Figure 2.

Reaction mechanisms and enzyme products of selected Arabidopsis monoterpene synthases (a), sesquiterpene synthases (b), and diterpene synthases (c). TPS enzymes are marked by red boxes.

Figure 3.

Protein domain structure of bi-functional class I/II TPSs and Arabidopsis class I and class II TPSs according to the structural evolutionary model by Cao et al. (2010). DXDD and DDXXD motifs are conserved in functionally active class II (β) and class I (α) domains, respectively.

Figure 4.

Clades of the Arabidopsis TPS family (a) and chromosomal Arabidopsis TPS gene clusters (b). (a) Arabidopsis TPS amino acid sequences were aligned by the MUSCLE algorithm and a tree was built with the GENEIOUS program (Biomatters Ltd.). Bootstrap values larger than 50% are shown. Colors indicate enzymes of TPS subfamilies a (green), b (purple), c (dark blue), e/f (light blue), and g (orange). Functionally characterized TPS proteins are shaded, (b) TPS pseudogenes (with AGI number) or gene fragments (non-annotated) are marked with grey arrows. Numbers indicate distances in kb between genes.

Figure 5.

Tissue- and cell-type specific Arabidopsis TPS gene expression patters according to RT-PCR and promoter-GUS reporter gene analyses. Spatial expression is shown for flowers (a), insect-damaged leaves (b), and roots (c). Arrows in (c) mark gene expression in specific root cell types. Numbers indicate different TPS genes. Only those genes are shown, for which enzymatic products have been detected in vivo.

Figure 6.

Inter- and intra-specific variation of terpene volatile emission and TPS gene expression and function in Arabidopsis thaliana and Arabidopsis lyrata.