The isoprene-responsive phosphoproteome provides new insights into the putative signalling pathways and novel roles of isoprene.

Many plants, especially trees, emit isoprene in a highly light- and temperature-dependent manner. The advantages for plants that emit, if any, have been difficult to determine. Direct effects on membranes have been disproven. New insights have been obtained by RNA sequencing, proteomic and metabolomic studies. We determined the responses of the phosphoproteome to exposure of Arabidopsis leaves to isoprene in the gas phase for either 1 or 5 h. Isoprene effects that were not apparent from RNA sequencing and other methods but were apparent in the phosphoproteome include effects on chloroplast movement proteins and membrane remodelling proteins. Several receptor kinases were found to have altered phosphorylation levels. To test whether potential isoprene receptors could be identified, we used molecular dynamics simulations to test for proteins that might have strong binding to isoprene and, therefore might act as receptors. Although many Arabidopsis proteins were found to have slightly higher binding affinities than a reference set of Homo sapiens proteins, no specific receptor kinase was found to have a very high binding affinity. The changes in chloroplast movement, photosynthesis capacity and so forth, found in this work, are consistent with isoprene responses being especially useful in the upper canopy of trees.

Hydroxymethylbutenyl diphosphate accumulation reveals MEP pathway regulation for high CO2-induced suppression of isoprene emission.

Isoprene is emitted by some plants and is the most abundant biogenic hydrocarbon entering the atmosphere. Multiple studies have elucidated protective roles of isoprene against several environmental stresses, including high temperature, excessive ozone, and herbivory attack. However, isoprene emission adversely affects atmospheric chemistry by contributing to ozone production and aerosol formation. Thus, understanding the regulation of isoprene emission in response to varying environmental conditions, for example, elevated CO2, is critical to comprehend how plants will respond to climate change. Isoprene emission decreases with increasing CO2 concentration; however, the underlying mechanism of this response is currently unknown. We demonstrated that high-CO2-mediated suppression of isoprene emission is independent of photosynthesis and light intensity, but it is reduced with increasing temperature. Furthermore, we measured methylerythritol 4-phosphate (MEP) pathway metabolites in poplar leaves harvested at ambient and high CO2 to identify why isoprene emission is reduced under high CO2. We found that hydroxymethylbutenyl diphosphate (HMBDP) was increased and dimethylallyl diphosphate (DMADP) decreased at high CO2. This implies that high CO2 impeded the conversion of HMBDP to DMADP, possibly through the inhibition of HMBDP reductase activity, resulting in reduced isoprene emission. We further demonstrated that although this phenomenon appears similar to abscisic acid (ABA)-dependent stomatal regulation, it is unrelated as ABA treatment did not alter the effect of elevated CO2 on the suppression of isoprene emission. Thus, this study provides a comprehensive understanding of the regulation of the MEP pathway and isoprene emission in the face of increasing CO2.

Relationship between seasonal variation in isoprene emission and plant hormone profiles in the tropical plant Ficus septica.

In Ficus septica, the short-term control of isoprene production and, therefore, isoprene emission has been linked to the hormone balance between auxin (IAA) and jasmonic acid (JA). However, the relationship between long-term changes in isoprene emission and that of plant hormones remains unknown. This study tracked isoprene emissions from F. septica leaves, plant hormone concentrations and signalling gene expression, MEP pathway metabolite concentrations, and related enzyme gene expression for 1 year in the field to better understand the role of plant hormones and their long-term control. Seasonality of isoprenes was mainly driven by temperature- and light-dependent variations in substrate availability through the MEP route, as well as transcriptional and post-transcriptional control of isoprene synthase (IspS). Isoprene emissions are seasonally correlated with plant hormone levels. This was especially evident in the cytokinin profiles, which decreased in summer and increased in winter. Only 4-hydroxy-3-methylbut-2-butenyl-4-diphosphate (HMBDP) exhibited a positive connection with cytokinins among the MEP metabolites examined, suggesting that HMBDP and its biosynthetic enzyme, HMBDP synthase (HDS), play a role in channelling of MEP pathway metabolites to cytokinin production. Thus, it is probable that cytokinins have potential feed-forward regulation of isoprene production. Under long-term natural conditions, the hormonal balance of IAA/JA-Ile was not associated with IspS transcripts or isoprene emissions. This study builds on prior work by revealing differences between short- and long-term hormonal modulation of isoprene emissions in the tropical tree F. septica.

Automatization of metabolite extraction for high-throughput metabolomics: case study on transgenic isoprene-emitting birch.

Metabolomics studies are becoming increasingly common for understanding how plant metabolism responds to changes in environmental conditions, genetic manipulations and treatments. Despite the recent advances in metabolomics workflow, the sample preparation process still limits the high-throughput analysis in large-scale studies. Here, we present a highly flexible robotic system that integrates liquid handling, sonication, centrifugation, solvent evaporation and sample transfer processed in 96-well plates to automatize the metabolite extraction from leaf samples. We transferred an established manual extraction protocol performed to a robotic system, and with this, we show the optimization steps required to improve reproducibility and obtain comparable results in terms of extraction efficiency and accuracy. We then tested the robotic system to analyze the metabolomes of wild-type and four transgenic silver birch (Betula pendula Roth) lines under unstressed conditions. Birch trees were engineered to overexpress the poplar (Populus × canescens) isoprene synthase and to emit various amounts of isoprene. By fitting the different isoprene emission capacities of the transgenic trees with their leaf metabolomes, we observed an isoprene-dependent upregulation of some flavonoids and other secondary metabolites as well as carbohydrates, amino acid and lipid metabolites. By contrast, the disaccharide sucrose was found to be strongly negatively correlated to isoprene emission. The presented study illustrates the power of integrating robotics to increase the sample throughput, reduce human errors and labor time, and to ensure a fully controlled, monitored and standardized sample preparation procedure. Due to its modular and flexible structure, the robotic system can be easily adapted to other extraction protocols for the analysis of various tissues or plant species to achieve high-throughput metabolomics in plant research.

Peering down the sink: A review of isoprene metabolism by bacteria.

Isoprene (2-methyl-1,3-butadiene) is emitted to the atmosphere each year in sufficient quantities to rival methane (>500 Tg C yr-1 ), primarily due to emission by trees and other plants. Chemical reactions of isoprene with other atmospheric compounds, such as hydroxyl radicals and inorganic nitrogen species (NOx ), have implications for global warming and local air quality, respectively. For many years, it has been estimated that soil-dwelling bacteria consume a significant amount of isoprene (~20 Tg C yr-1 ), but the mechanisms underlying the biological sink for isoprene have been poorly understood. Studies have indicated or confirmed the ability of diverse bacterial genera to degrade isoprene, whether by the canonical iso-type isoprene degradation pathway or through other less well-characterized mechanisms. Here, we review current knowledge of isoprene metabolism and highlight key areas for further research. In particular, examples of isoprene-degraders that do not utilize the isoprene monooxygenase have been identified in recent years. This has fascinating implications both for the mechanism of isoprene uptake by bacteria, and also for the ecology of isoprene-degraders in the environments.

Origin, evolution, and future of isoprene and nitric oxide interactions within leaves.

Photolytic generation of nitric oxide (NO), isoprene, and reactive oxygen species (ROS) pre-dated life on Earth (~4 billion years ago). However, isoprene-ROS-NO interactions became relevant to climate chemistry ~50 million years ago, after aquatic and terrestrial ecosystems became dominated by isoprene-emitting diatoms and angiosperms. Today, NO and NO2 (together referred to as NOx) are dangerous biogenic gaseous atmospheric pollutants. In plants, NO, with its multiple sources and sinks, acts as a secondary messenger that regulates development at low doses and induces cell death at high doses. Likewise, biogenic isoprene is a putative antioxidant and hormone 'enabler' that hastens plant (and leaf) growth and reproduction, and improves plant tolerance to transient abiotic stresses. Using examples from controlled-chamber simulation and field studies of isoprene oxidation, we discuss the likely nature and extent of isoprene oxidation within leaves. We argue that isoprene-NO interactions vary greatly among plant species, driven by differences in isoprene emission rate and nitrate assimilation capacity (i.e. NO sink strength), ROS availability, and the within-leaf ratio between free-NO and isoprene. In a warmer and CO2-fertilized future climate, antagonism between isoprene and NO within leaves will probably occur in a NO-rich (relative to present) environment, yielding a greater proportion of isoprene oxidation products, and inducing major changes in NO-mediated growth and stress responses.

Molecular characteristics of isoprene synthase and its control effects on isoprene emissions from tropical trees.

The isoprene emission rate from plants is simulated by a function of light intensity and leaf temperature, and the G-93 formula is the most extensively applied algorithm for this purpose. Isoprene is biosynthesized by the enzyme isoprene synthase (IspS), and instantly emitted from the leaf. Enzyme kinetics of IspS and substrate availability are important factors involved in the short-term leaf-level control of isoprene emissions. It is thus assumed that the parameters of G-93 may correlate with the kinetics of IspSs, however, at present there is no data available on the relationship between these two parameters. In this investigation, six IspS genes from tropical trees were cloned, their properties characterized, and the relationship between the enzyme kinetics of IspSs and the parameters of G-93 examined. There was a negative correlation between the enzyme kinetics of IspS Km and parameter CT1 of G93, which is used to define the temperature dependency of isoprene emissions. However, performance constant of IspS (kcat/Km) only showed slight positive correlation with CT1.suggesting that the enzyme kinetics of IspS has limited significance in controlling the temperature response of isoprene emissions. The molecular structure of IspS was further elucidated using a molecular dynamics simulation with a focus on the active site in the 6 α-helices bundle. The simulation of the enzyme-substrate complex of IspS from B. variegata predicted a new metal binding domain in helix F (E383) and catalytic motif FXRDRLXE in the A-C loop that could involve the deprotonation of dimethylallyl diphosphate (DMADP) to form a carbocation. Notably, after the binding of a metal ion and DMADP, the active-site closure mechanism was found to involve conformational alterations in the helix H-α1 and transition from a loose to tight enclosure of the 6 α-helices bundles to tune the active pocket size. The characteristics identified for the IspSs from tropical trees could help to explain regional isoprene emissions in tropical areas.

Isoprene measurements to assess plant hydrocarbon emissions and the methylerythritol pathway.

Isoprene is the most abundant non-methane hydrocarbon emitted to the atmosphere and a target of biotechnology as a source of biofuels or chemical feedstock. Measurements of the amount of isoprene or the rate of production of isoprene are important for atmospheric chemistry, evaluating biotechnology processes, and can provide information on the capacity and regulation of the methyl erythritol 4-phosphate pathway found in plants and bacteria. In this chapter we discuss techniques, and their strengths and weaknesses, of methods in common use for measuring isoprene. There are many sources of isoprene for measurements including emissions from leaves and head space analysis of reactions involving recombinant enzymes or bacterial/fungal cultures. Similarly, there are a variety of detection methods including several mass spectrometer methods that are useful for examining rates of labeling of isoprene when carbon isotopes are used.

Investigations of specialised metabolites of endophyte Diaporthe destruens hosted in Illigera orbiculata C. Y. Wu.

A chemical investigation of the endophytic fungus Diaporthe destruens from the Hernandiaceae plant Illigera orbiculata C. Y. Wu collected from southern Yunnan Province, China, led to the isolation of six undescribed compounds, including two azaphilone analogs, which are a pair of epimers (13R-hydroxy-chermesinone A and 13S-hydroxy-chermesinone A); a pyrrole derivative (1-(4-(methoxymethyl)-1H-pyrrol-3-yl)ethan-1-one); an isoindolone derivative (4-hydroxy-6-methoxyisoindolin-1-one); a benzylbenzene derivative (destruensine A) and a conjectural fragment of polyketide ((2R,4R)-2-(methoxymethyl)pentane-1,4-diol) along with nine known compounds. Their structures were elucidated by spectroscopic methods and HRESIMS, and the absolute configurations were further confirmed by electronic circular dichroism (ECD) and chemical derivatization. The antimicrobial activities, anti-acetylcholinesterase activities, antiproliferation, and NO production inhibitory effects of compounds 1-15 were evaluated.

Elevated ozone inhibits isoprene emission of a diploid and a triploid genotype of Populus tomentosa by different mechanisms.

Ozone (O3) pollution affects plant growth and isoprene (ISO) emission. However, the response mechanism of isoprene emission rate (ISOrate) to elevated O3 (EO3) remains poorly understood. ISOrate was investigated in two genotypes (diploid and triploid) of Chinese white poplar (Populus tomentosa Carr.) exposed to EO3 in an open top chamber system. The triploid genotype had higher photosynthetic rate (A) and stomatal conductance (gs) than the diploid one. EO3 significantly decreased A, gs, and ISOrate of middle and lower leaves in both genotypes. In the diploid genotype, the reduction of ISOrate was caused by a systematic decrease related to ISO synthesis capacity, as indicated by decreased contents of the isoprene precursor dimethylallyl diphosphate and decreased isoprene synthase protein and activity. On the other hand, the negative effect of O3 on ISOrate of the triploid genotype did not result from inhibited ISO synthesis capacity, but from increased ISO oxidative loss within the leaf. Our findings will be useful for breeding poplar genotypes with high yield and lower ISOrate, depending on local atmospheric volatile organic compound/NOx ratio, to cope with both the rising O3 concentrations and increasing biomass demand. They can also inform the incorporation of O3 effects into process-based models of isoprene emission.

Isoprene Emission Influences the Proteomic Profile of Arabidopsis Plants under Well-Watered and Drought-Stress Conditions.

Isoprene is a small lipophilic molecule synthesized in plastids and abundantly released into the atmosphere. Isoprene-emitting plants are better protected against abiotic stresses, but the mechanism of action of isoprene is still under debate. In this study, we compared the physiological responses and proteomic profiles of Arabidopsis which express the isoprene synthase (ISPS) gene and emit isoprene with those of non-emitting plants under both drought-stress (DS) and well-watered (WW) conditions. We aimed to investigate whether isoprene-emitting plants displayed a different proteomic profile that is consistent with the metabolic changes already reported. Only ISPS DS plants were able to maintain the same photosynthesis and fresh weight of WW plants. LC-MS/MS-based proteomic analysis revealed changes in protein abundance that were dependent on the capacity for emitting isoprene in addition to those caused by the DS. The majority of the proteins changed in response to the interaction between DS and isoprene emission. These include proteins that are associated with the activation of secondary metabolisms leading to ABA, trehalose, and proline accumulations. Overall, our proteomic data suggest that isoprene exerts its protective mechanism at different levels: under drought stress, isoprene affects the abundance of chloroplast proteins, confirming a strong direct or indirect antioxidant action and also modulates signaling and hormone pathways, especially those controlling ABA synthesis. Unexpectedly, isoprene also alters membrane trafficking.

Synthetic metabolic pathways for conversion of CO2 into secreted short-to medium-chain hydrocarbons using cyanobacteria.

The objective of this study was to implement direct sunlight-driven conversion of CO2 into a naturally excreted ready-to-use fuel. We engineered four different synthetic metabolic modules for biosynthesis of short-to medium-chain length hydrocarbons in the model cyanobacterium Synechocystis sp. PCC 6803. In module 1, the combination of a truncated clostridial n-butanol pathway with over-expression of the native cyanobacterial aldehyde deformylating oxygenase resulted in small quantities of propane when cultured under closed conditions. Direct conversion of CO2 into propane was only observed in strains with CRISPRi-mediated repression of three native putative aldehyde reductases. In module 2, three different pathways towards pentane were evaluated based on the polyunsaturated fatty acid linoleic acid as an intermediate. Through combinatorial evaluation of reaction ingredients, it was concluded that linoleic acid undergoes a spontaneous non-enzymatic reaction to yield pentane and hexanal. When Synechocystis was added to the reaction, hexanal was converted into 1-hexanol, but there was no further stimulation of pentane biosynthesis even in the Synechocystis strains expressing GmLOX1. For modules 3 and 4, several different acyl-ACP thioesterases were evaluated in combination with two different decarboxylases. Small quantities of 1-heptene and 1-nonene were observed in strains expressing the desaturase-like enzyme UndB from Pseudomonas mendocina in combination with C8-C10 preferring thioesterases ('CaFatB3.5 and 'ChoFatB2.2). When UndB instead was combined with a C12-specific 'UcFatB1 thioesterase, this resulted in a ten-fold increase of alkene biosynthesis. When UndB was replaced with the light-dependent FAP decarboxylase, both undecane and tridecane accumulated, albeit with a 10-fold drop in productivity. Preliminary optimization of the RBS, promoter and gene order in some of the synthetic operons resulted in improved 1-alkene productivity, reaching a titer of 230 mg/L after 10 d with 15% carbon partitioning. In conclusion, the direct bioconversion of CO2 into secreted and ready-to-use hydrocarbon fuel was implemented with several different metabolic systems. Optimal productivity was observed with UndB and a C12 chain-length specific thioesterase, although further optimization of the entire biosynthetic system is still possible.

Isoprene enhances leaf cytokinin metabolism and induces early senescence.

Isoprene, a major biogenic volatile hydrocarbon of climate-relevance, indisputably mitigates abiotic stresses in emitting plants. However functional relevance of constitutive isoprene emission in unstressed plants remains contested. Isoprene and cytokinins (CKs) are synthesized from a common substrate and pathway in chloroplasts. It was postulated that isoprene emission may affect CK-metabolism. Using transgenic isoprene-emitting (IE) Arabidopsis and isoprene nonemitting (NE) RNA-interference grey poplars (paired with respective NE and IE genotypes), the life of individual IE and NE leaves from emergence to abscission was followed under stress-free conditions. We monitored plant growth rate, aboveground developmental phenotype, modelled leaf photosynthetic energy status, quantified the abundance of leaf CKs, analysed Arabidopsis and poplar leaf transcriptomes by RNA-sequencing in presence and absence of isoprene during leaf senescence. Isoprene emission by unstressed leaves enhanced the abundance of CKs (isopentenyl adenine and its precursor) by > 200%, significantly upregulated genes coding for CK-synthesis, CK-signalling and CK-degradation, hastened plant development, increased chloroplast metabolic rate, altered photosynthetic energy status, induced early leaf senescence in both Arabidopsis and poplar. IE leaves senesced sooner even in decapitated poplars where source-sink relationships and hormone homeostasis were perturbed. Constitutive isoprene emission significantly accelerates CK-led leaf and organismal development and induces early senescence independent of growth constraints. Isoprene emission provides an early-riser evolutionary advantage and shortens lifecycle duration to assist rapid diversification in unstressed emitters.

Metabolic Engineering of Escherichia coli for De Novo Production of 1,5-Pentanediol from Glucose.

1,5-Pentanediol (1,5-PDO) is an important C5 building block for the synthesis of different value-added polyurethanes and polyesters. However, no natural metabolic pathway exists for the biosynthesis of 1,5-PDO. Herein we designed and constructed a promising nonnatural pathway for de novo production of 1,5-PDO from cheap carbohydrates. This biosynthesis route expands natural lysine pathways and employs two artificial metabolic modules to sequentially convert lysine into 5-hydroxyvalerate (5-HV) and 1,5-PDO via 5-hydroxyvaleryl-CoA. Theoretically, the 5-hydroxyvaleryl-CoA-based pathway is more energy-efficient than a recently published carboxylic acid reductase-based pathway for 1,5-PDO production. By combining strategies of systematic enzyme screening, pathway balancing, and transporter engineering, we successfully constructed a minimally engineered Escherichia coli strain capable of producing 3.19 g/L of 5-HV and 0.35 g/L of 1,5-PDO in a medium containing 20 g/L of glucose and 5 g/L lysine. Introducing the synthetic modules into a lysine producer and enhancing NADPH supply enabled the strain to accumulate 1.04 g/L of 5-HV and 0.12 g/L of 1,5-PDO using glucose as the main carbon source. This work lays the basis for the development of a biological route for 1,5-PDO production from renewable bioresources.

Bacterial synthesis of C3-C5 diols via extending amino acid catabolism.

Amino acids are naturally occurring and structurally diverse metabolites in biological system, whose potentials for chemical expansion, however, have not been fully explored. Here, we devise a metabolic platform capable of producing industrially important C3-C5 diols from amino acids. The presented platform combines the natural catabolism of charged amino acids with a catalytically efficient and thermodynamically favorable diol formation pathway, created by expanding the substrate scope of the carboxylic acid reductase toward noncognate ω-hydroxylic acids. Using the established platform as gateways, seven different diol-convertible amino acids are converted to diols including 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol. Particularly, we afford to optimize the production of 1,4-butanediol and demonstrate the de novo production of 1,5-pentanediol from glucose, with titers reaching 1.41 and 0.97 g l-1, respectively. Our work presents a metabolic platform that enriches the pathway repertoire for nonnatural diols with feedstock flexibility to both sugar and protein hydrolysates.

The olfactory coreceptor IR8a governs larval feces-mediated competition avoidance in a hawkmoth.

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta (Sphingidae), oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars can face food competition, as well as an increased exposure to predators and induced plant defenses. Here, we show that feces from conspecific caterpillars are sufficient to deter a female M. sexta from ovipositing on a plant and that this deterrence is based on the feces-emitted carboxylic acids 3-methylpentanoic acid and hexanoic acid. Using a combination of genome editing (CRISPR-Cas9), electrophysiological recordings, calcium imaging, and behavioral analyses, we demonstrate that ionotropic receptor 8a (IR8a) is essential for acid-mediated feces avoidance in ovipositing hawkmoths.

Isovaleronitrile co-induced with its precursor, l-leucine, by herbivory in the common evening primrose stimulates foraging behavior of the predatory blue shield bug.

Herbivore-induced plant volatiles play important roles in plant-insect and plant-plant interactions. The common evening primrose, Oenothera biennis, is often infested by the flea beetle, Altica oleracea, on which the predatory blue shield bug, Zicrona caerulea, is usually found. This observation suggests that the predatory bug can discriminate infested plants from intact ones to locate its prey. In this study, l-leucine-derived nitrogen-containing compounds [isovaleronitrile (3-methylbutanenitrile), (E/Z)-isovaleraldoxime and 3-methyl-1-nitrobutane] and some terpenes were identified as a characteristic volatile blend from herbivore-infested O. biennis leaves by gas chromatography/mass spectrometry, chemical synthesis, and incorporation assays using deuterium-labeled l-leucine. Volatile emission was also elicited by exogenous methyl jasmonate (MeJA), but not by mechanical damage. l-Leucine accumulated temporarily in O. biennis leaves after MeJA treatment prior to isovaleronitrile emission. Behavioral assays revealed that Z. caerulea showed a strong preference for herbivore-infested leaves, their volatiles, and isovaleronitrile in laboratory conditions.

Microbial cycling of isoprene, the most abundantly produced biological volatile organic compound on Earth.

Isoprene (2-methyl-1,3-butadiene), the most abundantly produced biogenic volatile organic compound (BVOC) on Earth, is highly reactive and can have diverse and often detrimental atmospheric effects, which impact on climate and health. Most isoprene is produced by terrestrial plants, but (micro)algal production is important in aquatic environments, and the relative bacterial contribution remains unknown. Soils are a sink for isoprene, and bacteria that can use isoprene as a carbon and energy source have been cultivated and also identified using cultivation-independent methods from soils, leaves and coastal/marine environments. Bacteria belonging to the Actinobacteria are most frequently isolated and identified, and Proteobacteria have also been shown to degrade isoprene. In the freshwater-sediment isolate, Rhodococcus strain AD45, initial oxidation of isoprene to 1,2-epoxy-isoprene is catalyzed by a multicomponent isoprene monooxygenase encoded by the genes isoABCDEF. The resultant epoxide is converted to a glutathione conjugate by a glutathione S-transferase encoded by isoI, and further degraded by enzymes encoded by isoGHJ. Genome sequence analysis of actinobacterial isolates belonging to the genera Rhodococcus, Mycobacterium and Gordonia has revealed that isoABCDEF and isoGHIJ are linked in an operon, either on a plasmid or the chromosome. In Rhodococcus strain AD45 both isoprene and epoxy-isoprene induce a high level of transcription of 22 contiguous genes, including isoABCDEF and isoGHIJ. Sequence analysis of the isoA gene, encoding the large subunit of the oxygenase component of isoprene monooxygenase, from isolates has facilitated the development of PCR primers that are proving valuable in investigating the ecology of uncultivated isoprene-degrading bacteria.

Biotechnological potential for degradation of isoprene: a review.

Isoprene, the ubiquitous, highly emitted non-methane volatile hydrocarbon, affects atmospheric chemistry and human health, and this makes its removal from the contaminated environment imperative. Physicochemical degradation of isoprene is inefficient and generates secondary pollutants. Therefore, biodegradation can be considered as the safer approach for its efficient abatement. This review summarizes efforts in this regard that led to tracking the diverse groups of isoprene degrading bacteria such as Methanotrophs, Xanthobacter, Nocardia, Alcaligenes, Rhodococcus, Actinobacteria, Alphaproteobacteria, Bacteriodetes, Pseudomonas, and Alcanivorax. Biodegradation of isoprene by such bacteria in batch and continuous modes has been elaborated. The products, pathways and the key enzymes associated with isoprene biodegradation have also been presented.

Evidence That Isoprene Emission Is Not Limited by Cytosolic Metabolites. Exogenous Malate Does Not Invert the Reverse Sensitivity of Isoprene Emission to High [CO2].

Isoprene is synthesized via the chloroplastic 2-C-methyl-d-erythritol 4-phosphate/1-deoxy-d-xylulose 5-phosphate pathway (MEP/DOXP), and its synthesis is directly related to photosynthesis, except under high CO2 concentration, when the rate of photosynthesis increases but isoprene emission decreases. Suppression of MEP/DOXP pathway activity by high CO2 has been explained either by limited supply of the cytosolic substrate precursor, phosphoenolpyruvate (PEP), into chloroplast as the result of enhanced activity of cytosolic PEP carboxylase or by limited supply of energetic and reductive equivalents. We tested the PEP-limitation hypotheses by feeding leaves with the PEP carboxylase competitive inhibitors malate and diethyl oxalacetate (DOA) in the strong isoprene emitter hybrid aspen (Populus tremula × Populus tremuloides). Malate feeding resulted in the inhibition of net assimilation, photosynthetic electron transport, and isoprene emission rates, but DOA feeding did not affect any of these processes except at very high application concentrations. Both malate and DOA did not alter the sensitivity of isoprene emission to high CO2 concentration. Malate inhibition of isoprene emission was associated with enhanced chloroplastic reductive status that suppressed light reactions of photosynthesis, ultimately leading to reduced isoprene substrate dimethylallyl diphosphate pool size. Additional experiments with altered oxygen concentrations in conditions of feedback-limited and non-feedback-limited photosynthesis further indicated that changes in isoprene emission rate in control and malate-inhibited leaves were associated with changes in the share of ATP and reductive equivalent supply for isoprene synthesis. The results of this study collectively indicate that malate importantly controls the chloroplast reductive status and, thereby, affects isoprene emission, but they do not support the hypothesis that cytosolic metabolite availability alters the response of isoprene emission to changes in atmospheric composition.