Enzymatic synthesis of methylbutenol from dimethylallyl diphosphate in needles of Pinus sabiniana

Metabolomics and transcriptomics of pheromone biosynthesis in an aggressive forest pest Ips typographus

Eurasian spruce bark beetle, Ips typographus, is a destructive pest in spruce forests. The ability of I. typographus to colonise host trees depends on its massive aggregation behaviour mediated by aggregation pheromones, consisting of 2-methyl-3-buten-2-ol and cis-verbenol. Other biologically active compounds such as ipsdienol and verbenone have also been detected in the beetle. Biosynthesis of 2-methyl-3-buten-2-ol and ipsdienol de novo from mevalonate and that of cis-verbenol from α-pinene sequestrated from the host have been reported in preliminary studies. However, knowledge on the molecular mechanisms underlying pheromone biosynthesis in this pest is currently limited. In this study, we performed metabolomic and differential gene expression (DGE) analysis for the pheromone-producing life stages of I. typographus. The highest amounts of 2-methyl-3-buten-2-ol (238 ng/gut) and cis-verbenol (23 ng/gut) were found in the fed male gut (colonisation stage) and the immature male gut (early stage), respectively. We also determined the amount of verbenyl oleate (the possible storage form of cis-verbenol), a monoterpenyl fatty acid ester, to be approximately 1604 ng/mg in the immature stage in the beetle body. DGE analysis revealed possible candidate genes involved in the biosynthesis of the quantified pheromones and related compounds. A novel hemiterpene-synthesising candidate isoprenyl-di-phosphate synthase Ityp09271 gene proposed for 2-methyl-3-buten-2-ol synthesis was found to be highly expressed only in the fed male beetle gut. Putative cytochrome P450 genes involved in cis/trans-verbenol synthesis and an esterase gene Ityp11977, which could regulate verbenyl oleate synthesis, were identified in the immature male gut. Our findings from the molecular analysis of pheromone-producing gene families are the first such results reported for I. typographus. With further characterisation of the identified genes, we can develop novel strategies to disrupt the aggregation behaviour of I. typographus and thereby prevent vegetation loss.

Bacterial degradation of tert-amyl alcohol proceeds via hemiterpene 2-methyl-3-buten-2-ol by employing the tertiary alcohol desaturase function of the Rieske nonheme mononuclear iron oxygenase MdpJ

Tertiary alcohols, such as tert-butyl alcohol (TBA) and tert-amyl alcohol (TAA) and higher homologues, are only slowly degraded microbially. The conversion of TBA seems to proceed via hydroxylation to 2-methylpropan-1,2-diol, which is further oxidized to 2-hydroxyisobutyric acid. By analogy, a branched pathway is expected for the degradation of TAA, as this molecule possesses several potential hydroxylation sites. In Aquincola tertiaricarbonis L108 and Methylibium petroleiphilum PM1, a likely candidate catalyst for hydroxylations is the putative tertiary alcohol monooxygenase MdpJ. However, by comparing metabolite accumulations in wild-type strains of L108 and PM1 and in two mdpJ knockout mutants of strain L108, we could clearly show that MdpJ is not hydroxylating TAA to diols but functions as a desaturase, resulting in the formation of the hemiterpene 2-methyl-3-buten-2-ol. The latter is further processed via the hemiterpenes prenol, prenal, and 3-methylcrotonic acid. Likewise, 3-methyl-3-pentanol is degraded via 3-methyl-1-penten-3-ol. Wild-type strain L108 and mdpJ knockout mutants formed isoamylene and isoprene from TAA and 2-methyl-3-buten-2-ol, respectively. It is likely that this dehydratase activity is catalyzed by a not-yet-characterized enzyme postulated for the isomerization of 2-methyl-3-buten-2-ol and prenol. The vitamin requirements of strain L108 growing on TAA and the occurrence of 3-methylcrotonic acid as a metabolite indicate that TAA and hemiterpene degradation are linked with the catabolic route of the amino acid leucine, including an involvement of the biotin-dependent 3-methylcrotonyl coenzyme A (3-methylcrotonyl-CoA) carboxylase LiuBD. Evolutionary aspects of favored desaturase versus hydroxylation pathways for TAA conversion and the possible role of MdpJ in the degradation of higher tertiary alcohols are discussed.

Biochemical characterization and homology modeling of methylbutenol synthase and implications for understanding hemiterpene synthase evolution in plants

2-Methyl-3-buten-2-ol (MBO) is a five-carbon alcohol produced and emitted in large quantities by many species of pine native to western North America. MBO is structurally and biosynthetically related to isoprene and can have an important impact on regional atmospheric chemistry. The gene for MBO synthase was identified from Pinus sabiniana, and the protein encoded was functionally characterized. MBO synthase is a bifunctional enzyme that produces both MBO and isoprene in a ratio of ~90:1. Divalent cations are required for activity, whereas monovalent cations are not. MBO production is enhanced by K(+), whereas isoprene production is inhibited by K(+) such that, at physiologically relevant [K(+)], little or no isoprene emission should be detected from MBO-emitting trees. The K(m) of MBO synthase for dimethylallyl diphosphate (20 mm) is comparable with that observed for angiosperm isoprene synthases and 3 orders of magnitude higher than that observed for monoterpene and sesquiterpene synthases. Phylogenetic analysis showed that MBO synthase falls into the TPS-d1 group (gymnosperm monoterpene synthases) and is most closely related to linalool synthase from Picea abies. Structural modeling showed that up to three phenylalanine residues restrict the size of the active site and may be responsible for making this a hemiterpene synthase rather than a monoterpene synthase. One of these residues is homologous to a Phe residue found in the active site of isoprene synthases. The remaining two Phe residues do not have homologs in isoprene synthases but occupy the same space as a second Phe residue that closes off the isoprene synthase active site.

Abiotic stresses and induced BVOCs

Plants produce a wide spectrum of biogenic volatile organic compounds (BVOCs) in various tissues above and below ground to communicate with other plants and organisms. However, BVOCs also have various functions in biotic and abiotic stresses. For example abiotic stresses enhance BVOCs emission rates and patterns, altering the communication with other organisms and the photochemical cycles. Recent new insights on biosynthesis and eco-physiological control of constitutive or induced BVOCs have led to formulation of hypotheses on their functions which are presented in this review. Specifically, oxidative and thermal stresses are relieved in the presence of volatile terpenes. Terpenes, C6 compounds, and methyl salicylate are thought to promote direct and indirect defence by modulating the signalling that biochemically activate defence pathways.

Thermal history regulates methylbutenol basal emission rate in Pinus ponderosa

Methylbutenol (MBO) is a 5-carbon alcohol that is emitted by many pines in western North America, which may have important impacts on the tropospheric chemistry of this region. In this study, we document seasonal changes in basal MBO emission rates and test several models predicting these changes based on thermal history. These models represent extensions of the ISO G93 model that add a correction factor C(basal), allowing MBO basal emission rates to change as a function of thermal history. These models also allow the calculation of a new emission parameter E(standard30), which represents the inherent capacity of a plant to produce MBO, independent of current or past environmental conditions. Most single-component models exhibited large departures in early and late season, and predicted day-to-day changes in basal emission rate with temporal offsets of up to 3 d relative to measured basal emission rates. Adding a second variable describing thermal history at a longer time scale improved early and late season model performance while retaining the day-to-day performance of the parent single-component model. Out of the models tested, the T(amb),T(max7) model exhibited the best combination of day-to-day and seasonal predictions of basal MBO emission rates.

Differential accumulation of dimethylallyl diphosphate in leaves and needles of isoprene- and methylbutenol-emitting and nonemitting species

The biosynthesis and emission of volatile plant terpenoids, such as isoprene and methylbutenol (MBO), depend on the chloroplastic production of dimethylallyl diphosphate (DMAPP). To date, it has been difficult to study the relationship of cellular DMAPP levels to emission of these volatiles because of the lack of a sensitive assay for DMAPP in plant tissues. Using a recent DMAPP assay developed in our laboratories, we report that species with the highest potential for isoprene and MBO production also exhibit elevated light-dependent DMAPP production, ranging from 110% to 1,063%. Even species that do not produce significant amounts of volatile terpenoids, however, exhibit some potential for light-dependent production of DMAPP. We used a nonaqueous fractionation technique to determine the intracellular distribution of DMAPP in isoprene-emitting cottonwood (Populus deltoides) leaves; approximately 65% to 70% of the DMAPP recovered at midday occurred in the chloroplasts, indicating that most of the light-dependent production of DMAPP was chloroplastic in origin. The midday concentration of chloroplastic DMAPP in cottonwood leaves is estimated to be 0.13 to 3.0 mM, which is consistent with the relatively high K(m)s that have been reported for isoprene synthases (0.5-8 mM). The results provide support for the hypothesis that the light dependence of isoprene and MBO emissions is in part due to controls over DMAPP production.

Diurnal variation of dimethylallyl diphosphate concentrations in oak (Quercus robur) leaves

In the present work a rapid and sensitive non-radioactive assay for the determination of cellular dimethylallyl diphosphate (DMADP) was developed and used for the analysis of the diurnal variation of DMADP levels in oak leaves. The method is based on the acid-catalysed hydrolysis of DMADP to isoprene, which subsequently is determined by gas chromatography. Diurnal variation of cellular DMADP levels in oak leaves of young saplings was measured on 6 days in 1998 and 1999, showing a 2 to 3-fold light-dependent increase from approximately 15 pmol mg-1 DW in the night to 31-75 pmol mg-1 DW around noon. The leaf DMADP contents showed a significant positive correlation with net assimilation and isoprene emission rates, indicating that the availability of cellular DMADP might be an important regulatory factor of leaf isoprene emission.

Nonradioactive assay for cellular dimethylallyl diphosphate

A sensitive, nonradioactive method was developed to measure cellular levels of dimethylallyl diphosphate (DMAPP), a central intermediate of isoprenoid metabolism in nature. The assay is based on the hydrolysis of DMAPP in acid to the volatile hydrocarbon isoprene (2-methyl-1,3-butadiene), with subsequent analysis of isoprene by headspace gas chromatography with reduction gas detection. In the assay, cell samples are directly acidified with 4 M H(2)SO(4) in sealed reaction vials. Therefore, there is no need to extract metabolites, purify them, and keep them stable prior to analysis, and degradative enzymatic activities are destroyed. DMAPP levels of 23 +/- 4 nmol (g fresh weight)(-1) [ca. 85 nmol (g dry weight)(-1)] and 80 +/- 14 nmol (g fresh weight)(-1) [ca. 296 nmol (g dry weight)(-1)] were measured in dark- and light-adapted leaves of Populus deltoides (Eastern cottonwood), respectively. Evidence is presented to show that DMAPP is the major leaf metabolite giving rise to isoprene following acid hydrolysis. DMAPP levels in Bacillus subtilis and Saccharomyces cerevisiae were determined to be 40.8 +/- 16.7 pmol (OD(600))(-1) [ca. 638 pmol (mg dry weight)(-1)] and 6.3 +/- 3.7 pmol (OD(600))(-1) [ca. 139 pmol (mg dry weight)(-1)], respectively. The method should be suitable for any cell or tissue type and isolated cellular organelles.