Medically Useful Plant Terpenoids: Biosynthesis, Occurrence, and Mechanism of Action.

Specialized plant terpenoids have found fortuitous uses in medicine due to their evolutionary and biochemical selection for biological activity in animals. However, these highly functionalized natural products are produced through complex biosynthetic pathways for which we have a complete understanding in only a few cases. Here we review some of the most effective and promising plant terpenoids that are currently used in medicine and medical research and provide updates on their biosynthesis, natural occurrence, and mechanism of action in the body. This includes pharmacologically useful plastidic terpenoids such as p-menthane monoterpenoids, cannabinoids, paclitaxel (taxol®), and ingenol mebutate which are derived from the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, as well as cytosolic terpenoids such as thapsigargin and artemisinin produced through the mevalonate (MVA) pathway. We further provide a review of the MEP and MVA precursor pathways which supply the carbon skeletons for the downstream transformations yielding these medically significant natural products.

Measuring the activity of 1-deoxy-D-xylulose 5-phosphate synthase, the first enzyme in the MEP pathway, in plant extracts.

The first enzyme in the methylerythritol phosphate (MEP) pathway is 1-deoxy-D-xylulose 5-phosphate (DXP) synthase (DXS). As such this enzyme is considered to be important in the control of plastidial isoprenoid production. Measuring the activity of DXS in plant extracts is therefore crucial to understanding the regulation of the MEP pathway. Due to the relatively low amounts of DXS, the activity of this enzyme can only be measured using highly sensitive analytical equipment. Here, a method is described to determine the DXS enzyme activity in a crude plant extract, by measuring DXP production directly using high performance liquid chromatography linked to a tandem triple quadrupole mass spectrometry detector (LC-MS/MS).

Pleiotropic regulatory locus 1 (PRL1) integrates the regulation of sugar responses with isoprenoid metabolism in Arabidopsis.

The biosynthesis of isoprenoids in plant cells occurs from precursors produced in the cytosol by the mevalonate (MVA) pathway and in the plastid by the methylerythritol 4-phosphate (MEP) pathway, but little is known about the mechanisms coordinating both pathways. Evidence of the importance of sugar signaling for such coordination in Arabidopsis thaliana is provided here by the characterization of a mutant showing an increased accumulation of MEP-derived isoprenoid products (chlorophylls and carotenoids) without changes in the levels of relevant MEP pathway transcripts, proteins, or enzyme activities. This mutant was found to be a new loss-of-function allele of PRL1 (Pleiotropic Regulatory Locus 1), a gene encoding a conserved WD-protein that functions as a global regulator of sugar, stress, and hormone responses, in part by inhibition of SNF1-related protein kinases (SnRK1). Consistent with the reported role of SnRK1 kinases in the phosphorylation and inactivation of the main regulatory enzyme of the MVA pathway (hydroxymethylglutaryl coenzyme-A reductase), its activity but not transcript or protein levels was reduced in prl1 seedlings. However, the accumulation of MVA-derived end products (sterols) was unaltered in mutant seedlings. Sucrose supplementation to wild-type seedlings phenocopied the prl1 mutation in terms of isoprenoid metabolism, suggesting that the observed isoprenoid phenotypes result from the increased sugar accumulation in the prl1 mutant. In summary, PRL1 appears to coordinate isoprenoid metabolism with sugar, hormone, and stress responses.

Evaluation of Candidate Reference Genes for Real-Time Quantitative PCR of Plant Samples Using Purified cDNA as Template.

Quantitative real-time polymerase chain reaction (qRT-PCR) is a precise method to measure changes in gene transcript level. Accurate quantification requires careful RNA quality assessment, determination of primer efficiency, and selection of an appropriate reference gene. While many experimental procedures for these purposes have been described for mammalian samples, the direct application of these methods to plant samples often introduces unexpected experimental errors due to the complex and variable nature of the ribosomal RNA species present in typical plant extracts. In this paper, we report a simple procedure for the purification and quantification of complementary DNA (cDNA) after reverse transcriptase reactions by microcapillary electrophoresis. The use of purified cDNA allows template concentrations to be more accurately standardized for SYBR Green PCR reactions and increases amplification efficiencies so that these closely resemble those determined by the standard curve method. These advantages facilitate a more precise evaluation of the transcript levels of candidate reference genes under various experimental conditions without bias from differences in reverse transcriptase efficiency, template loading, or the presence of PCR inhibitors following reverse transcription. Using samples from Arabidopsis thaliana and Picea abies (Norway spruce), we demonstrate the value of this approach for selecting reference genes.