Temperature response of isoprene emission in vivo reflects a combined effect of substrate limitations and isoprene synthase activity: a kinetic analysis

Isoprenoid emissions from Schima superba and Cunninghamia lanceolata: Their responses to elevated temperature by two warming facilities

Isoprenoids (including isoprene (ISO) and monoterpenes (MTs)) are the majority of biogenic volatile organic compounds (BVOCs) which are important carbon-containing secondary metabolites biosynthesized by organisms, especially plant in terrestrial ecosystem. Results of the warming effects on isoprenoid emissions vary within species and warming facilities, and thus conclusions remain controversial. In this study, two typical subtropical tree species seedlings of Schima superba and Cunninghamia lanceolata were cultivated under three conditions, namely no warming (CK) and two warming facilities (with infrared radiators (IR) and heating wires (HW)) in open top chamber (OTC), and the isoprenoid emissions were measured with preconcentor-GC-MS system after warming for one, two and four months. The results showed that the isoprenoid emissions from S. superba and C. lanceolata exhibited uniformity in response to two warming facilities. IR and HW both stimulated isoprenoid emissions in two plants after one month of treatment, with increased ratios of 16.3 % and 72.5 % for S. superba, and 2.47 and 5.96 times for C. lanceolata. However, the emissions were suppressed after four months, with more pronounced effect for HW. The variation in isoprenoid emissions was primarily associated with the levels of Pn, Tr, monoterpene synthase (MTPS) activity. C. lanceolata predominantly released MTs (mainly α-pinene, α-terpene, γ-terpene, and limonene), with 39.7 % to 99.6 % of the total isoprenoid but ISO was only a very minor constituent. For S. superba, MTs constituted 24.7 % to 96.1 % of total isoprenoid. It is noteworthy that HW generated a greater disturbance to physiology activity in plants. Our study provided more comprehensive and more convincing support for integrating temperature-elevation experiments of different ecosystems and assessing response and adaptation of forest carbon cycle to global warming.

Limitations of Plant Stress Tolerance upon Heat and CO2 Exposure in Black Poplar: Assessment of Photosynthetic Traits and Stress Volatile Emissions

Volatile organic compounds (VOCs) emitted by plants may help in understanding the status of a plant's physiology and its coping with mild to severe stress. Future climatic projections reveal that shifts in temperature and CO2 availability will occur, and plants may incur the uncoupling of carbon assimilation and synthesis of key molecules. This study explores the patterns of emissions of key VOCs (isoprene, methanol, acetaldehyde, and acetic acid) emitted by poplar leaves (more than 350) under a combined gradient of temperature (12-42 °C) and air CO2 concentration (400-1500 ppm), along with measurements of photosynthetic rates and stomatal conductance. Isoprene emission exhibited a rise with temperature and CO2 availability, peaking at 39 °C, the temperature at which methanol emission started to peak, illustrating the limit of stress tolerance to severe damage. Isoprene emission was uncoupled from the photosynthesis rate, indicating a shift from the carbon source for isoprene synthesis, while assimilation was decreased. Methanol and acetaldehyde emissions were correlated with stomatal conductance and peaked at 25 °C and 1200 ppm CO2. Acetic acid emissions lacked a clear correlation with stomatal conductance and the emission pattern of its precursor acetaldehyde. This study offers crucial insights into the limitations of photosynthetic carbon and stress tolerance.

Deoxyxylulose 5-Phosphate Synthase Does Not Play a Major Role in Regulating the Methylerythritol 4-Phosphate Pathway in Poplar

The plastidic 2-C-methylerythritol 4-phosphate (MEP) pathway supplies the precursors of a large variety of essential plant isoprenoids, but its regulation is still not well understood. Using metabolic control analysis (MCA), we examined the first enzyme of this pathway, 1-deoxyxylulose 5-phosphate synthase (DXS), in multiple grey poplar (Populus × canescens) lines modified in their DXS activity. Single leaves were dynamically labeled with 13CO2 in an illuminated, climate-controlled gas exchange cuvette coupled to a proton transfer reaction mass spectrometer, and the carbon flux through the MEP pathway was calculated. Carbon was rapidly assimilated into MEP pathway intermediates and labeled both the isoprene released and the IDP+DMADP pool by up to 90%. DXS activity was increased by 25% in lines overexpressing the DXS gene and reduced by 50% in RNA interference lines, while the carbon flux in the MEP pathway was 25-35% greater in overexpressing lines and unchanged in RNA interference lines. Isoprene emission was also not altered in these different genetic backgrounds. By correlating absolute flux to DXS activity under different conditions of light and temperature, the flux control coefficient was found to be low. Among isoprenoid end products, isoprene itself was unchanged in DXS transgenic lines, but the levels of the chlorophylls and most carotenoids measured were 20-30% less in RNA interference lines than in overexpression lines. Our data thus demonstrate that DXS in the isoprene-emitting grey poplar plays only a minor part in controlling flux through the MEP pathway.

A Novel Isoprene Synthase from the Monocot Tree Copernicia prunifera (Arecaceae) Confers Enhanced Drought Tolerance in Transgenic Arabidopsis

The capacity to emit isoprene, among other stresses, protects plants from drought, but the molecular mechanisms underlying this trait are only partly understood. The Arecaceae (palms) constitute a very interesting model system to test the involvement of isoprene in enhancing drought tolerance, as their high isoprene emissions may have contributed to make them hyperdominant in neotropical dry forests, characterized by recurrent and extended periods of drought stress. In this study we isolated and functionally characterized a novel isoprene synthase, the gene responsible for isoprene biosynthesis, from Copernicia prunifera, a palm from seasonally dry tropical forests. When overexpressed in the non-emitter Arabidopsis thaliana, CprISPS conferred significant levels of isoprene emission, together with enhanced tolerance to water limitation throughout plant growth and development, from germination to maturity. CprISPS overexpressors displayed higher germination, cotyledon/leaf greening, water usage efficiency, and survival than WT Arabidopsis under various types of water limitation. This increased drought tolerance was accompanied by a marked transcriptional up-regulation of both ABA-dependent and ABA-independent key drought response genes. Taken together, these results demonstrate the capacity of CprISPS to enhance drought tolerance in Arabidopsis and suggest that isoprene emission could have evolved in Arecaceae as an adaptive mechanism against drought.

Policy-Related Gains in Urban Air Quality May Be Offset by Increased Emissions in a Warming Climate

Air quality policies have made substantial gains by reducing pollutant emissions from the transportation sector. In March 2020, New York City's activities were severely curtailed in response to the COVID-19 pandemic, resulting in 60-90% reductions in human activity. We continuously measured major volatile organic compounds (VOCs) during January-April 2020 and 2021 in Manhattan. Concentrations of many VOCs decreased significantly during the shutdown with variations in daily patterns reflective of human activity perturbations, resulting in a temporary ∼28% reduction in chemical reactivity. However, the limited effect of these dramatic measures was outweighed by larger increases in VOC-related reactivity during the anomalously warm spring 2021. This emphasizes the diminishing returns from transportation-focused policies alone and the risk of increased temperature-dependent emissions undermining policy-related gains in a warming climate.

Age effects of Moso bamboo on leaf isoprene emission characteristics

Isoprene is a highly reactive volatile organic compound that significantly affects atmospheric oxidant capacity, regional air quality, and climate change. Moso bamboo (Phyllostachys edulis), a species widely distributed in tropical and subtropical regions, particularly in China, is a strong isoprene emitter with great potential for carbon sequestration. Carbon sequestration is negatively correlated with culm age; however, the effect of this correlation on isoprene emissions remains unknown. In this study, we investigated the photosynthetic and isoprene emission characteristics of Moso bamboo at different culm ages. The results showed that the age effect on isoprene emission was different from that on photosynthesis; the net photosynthesis rate (Pn) was the highest in young, followed by mature, and then old bamboo, whereas the isoprene emission rate (Iso) was the highest in young, followed by old, and then mature bamboo. Moreover, the percentage of carbon loss as isoprene emission (C-loss) during photosynthesis of old bamboo was 35% higher than that of mature bamboo under standard conditions (leaf temperature: 30°C; light intensity: 1000 µmol m-2 s-1). Therefore, we strongly recommend considering the culm age when establishing an isoprene emission model of Moso bamboo. Additionally, because the Iso and C-loss of old bamboo were higher than those of mature bamboo, we suggest that attention should be paid to the management of bamboo age structure and timely felling of aged bamboo to reduce environmental risk.

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 K_m and parameter C_T1 of G93, which is used to define the temperature dependency of isoprene emissions. However, performance constant of IspS (k_cat/K_m) only showed slight positive correlation with C_T1.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.

Seasonal characteristics of biogenic secondary organic aerosol tracers in a deciduous broadleaf forest in northern Japan

We collected total suspended particulate (TSP) samples from January 2010 to December 2010 at Sapporo deciduous forest to understand the oxidation processes of biogenic volatile organic compounds (BVOCs). The gas chromatography-mass spectrometric technique was applied to determine biogenic secondary organic aerosols (BSOAs) in the TSP samples. We found the predominance of the isoprene SOA (iSOA) tracers (20.6 ng m^-3) followed by α/β-pinene SOA (pSOA) tracers (8.25 ng m^-3) and β-caryophyllene SOA (cSOA) tracer (1.53 ng m^-3) in the forest aerosols. The results showed large isoprene fluxes and relatively high levels of oxidants in the forest atmosphere. The iSOA and pSOA tracers showed a clear seasonal trend with summer and autumn maxima and winter and spring minima. Their seasonal trends were mainly controlled by BVOCs emission from the local broadleaf deciduous forest. Additionally, the regional level of isoprene emissions from the oceanic sources may also be contributed during summertime aerosols. cSOA tracer showed high concentrations in the winter and spring, possibly due to an additional contribution of biomass burning (BB) aerosols from the local or regional BB activities. The biogenic secondary organic carbon (BSOC) was contributed mainly by the oxidation products of isoprene (136 ngC m^-3) followed by β-caryophyllene (63.0 ngC m^-3) and α/β-pinene (35.9 ngC m^-3). The mass concentration ratio (0.92) of pinonic acid + pinic acid and 3-methyl-1,2,3-butanetricarboxylic acid ((PNA + PA)/3-MBTCA) indicates the photochemical transformation of first-generation oxidation products to the higher generation oxidation products. The average ratios of isoprene to α/β-pinene (1.64) and β-caryophyllene (18.6) oxidation products suggested a large difference in the emissions of isoprene and α/β-pinene compared to β-caryophyllene. The cSOA tracers in the forest aerosols are also contributed by BB during the winter and spring. Positive matrix factorization analyses of the BSOA tracers confirmed that organic aerosols of deciduous forests are mostly related to isoprene emissions. This study suggests that isoprene is a more significant precursor for the BSOA than α/β-pinene and β-caryophyllene in a broadleaf deciduous forest.

Stress-Induced Volatile Emissions and Signalling in Inter-Plant Communication

The sessile plant has developed mechanisms to survive the “rough and tumble” of its natural surroundings, aided by its evolved innate immune system. Precise perception and rapid response to stress stimuli confer a fitness edge to the plant against its competitors, guaranteeing greater chances of survival and productivity. Plants can “eavesdrop” on volatile chemical cues from their stressed neighbours and have adapted to use these airborne signals to prepare for impending danger without having to experience the actual stress themselves. The role of volatile organic compounds (VOCs) in plant–plant communication has gained significant attention over the past decade, particularly with regard to the potential of VOCs to prime non-stressed plants for more robust defence responses to future stress challenges. The ecological relevance of such interactions under various environmental stresses has been much debated, and there is a nascent understanding of the mechanisms involved. This review discusses the significance of VOC-mediated inter-plant interactions under both biotic and abiotic stresses and highlights the potential to manipulate outcomes in agricultural systems for sustainable crop protection via enhanced defence. The need to integrate physiological, biochemical, and molecular approaches in understanding the underlying mechanisms and signalling pathways involved in volatile signalling is emphasised.

Biogenic Volatile Organic Compounds and Protein Expressions of Chamaecyparis formosensis and Chamaecyparis obtusa var. formosana Leaves under Different Light Intensities and Temperatures

Both Chamaecyparis formosensis and C. obtusa var. formosana are representative cypresses of high economic value in Taiwan, the southernmost subtropical region where cypresses are found. Both species show differences of their habitats. To find out the effects of environmental factors on the CO₂ assimilation rate and the biogenic volatile organic compound (BVOC) emission of both species, saplings from both species were grown under different light intensity and temperature regimes. The results indicated that the net CO₂ assimilation rates and total BVOC emission rates of both species increased with increasing light intensity. C. formosensis showed a higher magnitude of change, but C. obtusa var. formosana had considerably increased sesquiterpenoid and diterpenoid emission in BVOC under high light intensity. Both species grown under higher temperatures had significantly lower BVOC emission rates. Proteomic analyses revealed that compared to C. formosensis saplings, C. obtusa var. formosana saplings had less differentially expressed proteins in terms of protein species and fold changes in response to the growth conditions. These proteins participated mainly in photosynthesis, carbon metabolism, amino acid and protein processing, signal transduction, and stress mechanisms. These proteins might be the major regulatory factors affecting BVOC emission of these two species under different environments.

Rapid Sampling Protocol of Isoprene Emission Rate of Palm (Arecaceae) Species Using Excised Leaves

The high isoprene emission capacity of palm species can decrease regional air quality and enhance the greenhouse effect when land is converted to palm plantations. Propagation of low-emitting individuals can be a strategy for reducing isoprene emission from palms. However, the identification of low-emitting individuals requires large-scale sampling. Thus, we aimed to develop a rapid method in which the isoprene emission rate of leaf segments is observed. We examined the temperature response and effect of incubation length on the isoprene emission rate of palm leaf and found that leaf temperatures at 25 to 30 °C and an incubation length of 40 min are suitable parameters. To further examine the validity of the method, we applied both the enclosure method and this method to the same sections of leaves. High coefficient of determinations (0.993 and 0.982) between the results of the two methods were detected regardless of seasonal temperature. This result proves that the method is capable of measuring the isoprene emission rate under any growth conditions if the incubation temperature is controlled. By using a water bath tank and a tested light source, we can simply implement a unified environmental control of multiple samples at once, which achieves a higher time efficiency than conventional enclosure measurements.

Improved plant heat shock resistance is introduced differently by heat and insect infestation: the role of volatile emission traits

Heat stress is one of the most important abiotic stresses confronted by plants under global climate change. Plant exposure to abiotic or biotic stress can improve its tolerance to subsequent severe episodes of the same or different stress (stress priming), but so far there is limited comparative information about how pre-exposures to different abiotic and biotic elicitors alter plant resistance to severe heat stress. We exposed the perennial herb Melilotus albus Medik., a species rich in secondary metabolites, to moderate heat stress (35 °C) and greenhouse whitefly (Trialeurodes vaporariorum West.) infestation to comparatively determine whether both pre-treatments could enhance plant tolerance to the subsequent heat shock (45 °C) stress. Plant physiological responses to stress were characterized by photosynthetic traits and volatile organic compound emissions through 72 h recovery. Heat shock treatment reduced net assimilation rate (A) and stomatal conductance in all plants, but heat-primed plants had significantly faster rates of recovery of A than other plants. By the end of the recovery period, A in none of the three heat shock-stressed groups recovered to the control level, but in whitefly-infested plants it reached the pre-heat shock level. In heat-primed plants, the heat shock treatment was associated with a fast rise of monoterpene emissions, and in whitefly-infested plants with benzenoid emissions and an increase in total phenolic content.

Review on plant terpenoid emissions worldwide and in China

Biogenic volatile organic compounds (BVOCs), particularly terpenoids, can significantly drive the formation of ozone (O₃) and secondary organic aerosols (SOA) in the atmosphere, as well as directly or indirectly affect global climate change. Understanding their emission mechanisms and the current progress in emission measurements and estimations are essential for the accurate determination of emission characteristics, as well as for evaluating their roles in atmospheric chemistry and climate change. This review summarizes the mechanisms of terpenoid synthesis and release, biotic and abiotic factors affecting their emissions, development of emission observation techniques, and emission estimations from hundreds of published papers. We provide a review of the main observations and estimations in China, which contributes a significant proportion to the total global BVOC emissions. The review suggests the need for further research on the comprehensive effects of environmental factors on terpenoid emissions, especially soil moisture and nitrogen content, which should be quantified in emission models to improve the accuracy of estimation. In China, it is necessary to conduct more accurate measurements for local plants in different regions using the dynamic enclosure technique to establish an accurate local emission rate database for dominant tree species. This will help improve the accuracy of both national and global emission inventories. This review provides a comprehensive understanding of terpenoid emissions as well as prospects for detailed research to accurately describe terpenoid emission characteristics worldwide and in China.

CO2 -responsiveness of leaf isoprene emission: Why do species differ?

Leaf isoprene emission rate, I, decreases with increasing atmospheric CO₂ concentration with major implications for global change. There is a significant interspecific variability in [CO₂ ]-responsiveness of I, but the extent of this variation is unknown and its reasons are not understood. We hypothesized that the magnitude of emission reduction reflects the size and changeability of precursor pools responsible for isoprene emission (dimethylallyl diphosphate, DMADP and 2-methyl-erythritol 2,4-cyclodiphosphate, MEcDP). Changes in I and intermediate pool sizes upon increase of [CO₂ ] from 400 to 1500 μmol/mol were studied in nine woody species spanning boreal to tropical ecosystems. I varied 10-fold, total substrate pool size 37-fold and the ratio of DMADP/MEcDP pool sizes 57-fold. At higher [CO₂ ], I was reduced on average by 65%, but [CO₂ ]-responsiveness varied an order of magnitude across species. The increase in [CO₂ ] resulted in concomitant reductions in both substrate pools. The variation in [CO₂ ]-responsiveness across species scaled with the reduction in pool sizes, the substrate pool size supported and the share of DMADP in total substrate pool. This study highlights a major interspecific variation in [CO₂ ]-responsiveness of isoprene emission and conclusively links this variation to interspecific variability in [CO₂ ] effects on substrate availability and intermediate pool size.

Heat priming improved heat tolerance of photosynthesis, enhanced terpenoid and benzenoid emission and phenolics accumulation in Achillea millefolium

The mechanism of heat priming, triggering alteration of secondary metabolite pathway fluxes and pools to enhance heat tolerance is not well understood. Achillea millefolium is an important medicinal herbal plant, rich in terpenoids and phenolics. In this study, the potential of heat priming treatment (35°C for 1 hr) to enhance tolerance of Achillea plants upon subsequent heat shock (45°C for 5 min) stress was investigated through recovery (0.5-72 hr). The priming treatment itself had minor impacts on photosynthesis, led to moderate increases in the emission of lipoxygenase (LOX) pathway volatiles and isoprene, and to major elicitation of monoterpene and benzaldehyde emissions in late stages of recovery. Upon subsequent heat shock, in primed plants, the rise in LOX and reduction in photosynthetic rate (A) was much less, stomatal conductance (g_s ) was initially enhanced, terpene emissions were greater and recovery of A occurred faster, indicating enhanced heat tolerance. Additionally, primed plants accumulated higher contents of total phenolics and condensed tannins at the end of the recovery. These results collectively indicate that heat priming improved photosynthesis upon subsequent heat shock by enhancing g_s and synthesis of volatile and non-volatile secondary compounds with antioxidative characteristics, thereby maintaining the integrity of leaf membranes under stress.

Protein expression plasticity contributes to heat and drought tolerance of date palm

Climate change is increasing the frequency and intensity of warming and drought periods around the globe, currently representing a threat to many plant species. Understanding the resistance and resilience of plants to climate change is, therefore, urgently needed. As date palm (Phoenix dactylifera) evolved adaptation mechanisms to a xeric environment and can tolerate large diurnal and seasonal temperature fluctuations, we studied the protein expression changes in leaves, volatile organic compound emissions, and photosynthesis in response to variable growth temperatures and soil water deprivation. Plants were grown under controlled environmental conditions of simulated Saudi Arabian summer and winter climates challenged with drought stress. We show that date palm is able to counteract the harsh conditions of the Arabian Peninsula by adjusting the abundances of proteins related to the photosynthetic machinery, abiotic stress and secondary metabolism. Under summer climate and water deprivation, these adjustments included efficient protein expression response mediated by heat shock proteins and the antioxidant system to counteract reactive oxygen species formation. Proteins related to secondary metabolism were downregulated, except for the P. dactylifera isoprene synthase (PdIspS), which was strongly upregulated in response to summer climate and drought. This study reports, for the first time, the identification and functional characterization of the gene encoding for PdIspS, allowing future analysis of isoprene functions in date palm under extreme environments. Overall, the current study shows that reprogramming of the leaf protein profiles confers the date palm heat- and drought tolerance. We conclude that the protein plasticity of date palm is an important mechanism of molecular adaptation to environmental fluctuations.

Responses of isoprene emission and photochemical efficiency to severe drought combined with prolonged hot weather in hybrid Populus

Isoprene emissions have been considered as a protective response of plants to heat stress, but there is limited information of how prolonged heat spells affect isoprene emission capacity, particularly under the drought conditions that often accompany hot weather. Under combined long-term stresses, presence of isoprene emission could contribute to the maintenance of the precursor pool for rapid synthesis of essential isoprenoids to repair damaged components of leaf photosynthetic apparatus. We studied changes in leaf isoprene emission rate, photosynthetic characteristics, and antioxidant enzyme activities in two hybrid Populus clones, Nanlin 1388 (relatively high drought tolerance) and Nanlin 895 (relatively high thermotolerance) that were subjected to long-term (30 d) soil water stress (25% versus 90% soil field capacity) combined with a natural heat spell (day-time temperatures of 35-40 °C) that affected both control and water-stressed plants. Unexpectedly, isoprene emissions from both the clones were similar and the overall effects of drought on the emission characteristics were initially minor; however, treatment effects and clonal differences increased with time. In particular, the isoprene emission rate only increased slightly in the Nanlin 895 control plants after 15 d of treatment, whereas it decreased by more than 5-fold in all treatment × clone combinations after 30 d. The reduction in isoprene emission rate was associated with a decrease in the pool size of the isoprene precursor dimethylallyl diphosphate in all cases at 30 d after the start of treatment. Net assimilation rate, stomatal conductance, the openness of PSII centers, and the effective quantum yield all decreased, and non-photochemical quenching and catalase activity increased in both control and water-stressed plants. Contrary to the hypothesis of protection of leaf photosynthetic apparatus by isoprene, the data collectively indicated that prolonged stress affected isoprene emissions more strongly than leaf photosynthetic characteristics. This primarily reflected the depletion of isoprene precursor pools under long-term severe stress.

Shoot Characterization of Isoprene and Ocimene-Emitting Transgenic Arabidopsis Plants under Contrasting Environmental Conditions

Isoprenoids are among the most abundant biogenic volatile compounds (VOCs) emitted by plants, and mediate both biotic and abiotic stress responses. Here, we provide for the first time a comparative analysis of transgenic Arabidopsis lines constitutively emitting isoprene and ocimene. Transgenic lines and Columbia-0 (Col-0) Arabidopsis were characterized under optimal, water stress, and heat stress conditions. Under optimal conditions, the projected leaf area (PLA), relative growth rate, and final dry weight were generally higher in transgenics than Col-0. These traits were associated to a larger photosynthetic capacity and CO2 assimilation rate at saturating light. Isoprene and ocimene emitters displayed a moderately higher stress tolerance than Col-0, showing higher PLA and gas-exchange traits throughout the experiments. Contrasting behaviors were recorded for the two overexpressors under water stress, with isoprene emitters showing earlier stomatal closure (conservative behavior) than ocimene emitters (non-conservative behavior), which might suggest different induced strategies for water conservation and stress adaptation. Our work indicates that (i) isoprene and ocimene emitters resulted in enhanced PLA and biomass under optimal and control conditions and that (ii) a moderate stress tolerance is induced when isoprene and ocimene are constitutively emitted in Arabidopsis, thus providing evidence of their role as a potential preferable trait for crop improvement.

Influence of Brevibacterium linens RS16 on foliage photosynthetic and volatile emission characteristics upon heat stress in Eucalyptus grandis

Heat stress induces secondary metabolic changes in plants, channeling photosynthetic carbon and energy, away from primary metabolic processes, including, growth. Use of ACC (1-aminocyclopropane-1-carboxylate) deaminase containing plant growth promoting bacteria (PGPB) in conferring heat resistance in plants and the role of PGPB, in altering net carbon assimilation, constitutive and stress volatile emissions has not been studied yet. We exposed leaves of Eucalyptus grandis inoculated and non-inoculated with PGPB Brevibacterium linens RS16 to two levels of heat stress (37 °C and 41 °C for 5 min) and quantified temporal changes in foliage photosynthetic characteristics and volatile emission rates at 0.5 h, day 1 and day 5 after the stress application. Heat stress resulted in immediate reductions in dark-adapted photosystem II (PSII) quantum yield (F_v/F_m), net assimilation rate (A), stomatal conductance to water vapor (g_s), and enhancement of stress volatile emissions, including enhanced emissions of green leaf volatiles (GLV), mono- and sesquiterpenes, light weight oxygenated volatile organic compounds (LOC), geranyl-geranyl diphosphate pathway volatiles (GGDP), saturated aldehydes, and benzenoids, with partial recovery by day 5. Changes in stress-induced volatiles were always less in leaves inoculated with B. linens RS16. However, net assimilation rate was enhanced by bacterial inoculation only in the 37 °C treatment and overall reduction of isoprene emissions was observed in bacterially-treated leaves. Principal component analysis (PCA), correlation analysis and partial least squares discriminant analysis (PLS-DA) indicated that different stress applications influenced specific volatile organic compounds. In addition, changes in the expression analysis of heat shock protein 70 gene (DnaK) gene in B. linens RS16 upon exposure to higher temperatures further indicated that B. linens RS16 has developed its own heat resistance mechanism to survive under higher temperature regimes. Taken together, this study demonstrates that foliar application of ACC deaminase containing PGPB can ameliorate heat stress effects in realistic biological settings.

Isoprene: New insights into the control of emission and mediation of stress tolerance by gene expression

Isoprene is a volatile compound produced in large amounts by some, but not all, plants by the enzyme isoprene synthase. Plants emit vast quantities of isoprene, with a net global output of 600 Tg per year, and typical emission rates from individual plants around 2% of net carbon assimilation. There is significant debate about whether global climate change resulting from increasing CO2 in the atmosphere will increase or decrease global isoprene emission in the future. We show evidence supporting predictions of increased isoprene emission in the future, but the effects could vary depending on the environment under consideration. For many years, isoprene was believed to have immediate, physical effects on plants such as changing membrane properties or quenching reactive oxygen species. Although observations sometimes supported these hypotheses, the effects were not always observed, and the reasons for the variability were not apparent. Although there may be some physical effects, recent studies show that isoprene has significant effects on gene expression, the proteome, and the metabolome of both emitting and nonemitting species. Consistent results are seen across species and specific treatment protocols. This review summarizes recent findings on the role and control of isoprene emission from plants.

Emission of constitutive isoprene, induced monoterpenes, and other volatiles under high temperatures in Eucalyptus camaldulensis: A 13 C labelling study

Eucalypts are major emitters of biogenic volatile organic compounds (BVOCs), especially volatile isoprenoids. Emissions and incorporation of ¹³ C in BVOCs were measured in Eucalyptus camaldulensis branches exposed to rapid heat stress or progressive temperature increases, in order to detect both metabolic processes and their dynamics. Isoprene emission increased and photosynthesis decreased with temperatures rising from 30°C to 45°C, and an increasing percentage of unlabelled carbon was incorporated into isoprene in heat-stressed leaves. Intramolecular labelling was also incomplete in isoprene emitted by heat-stressed leaves, suggesting increasing contribution of respiratory (and possibly also photorespiratory) carbon. At temperature above 45°C, a drop of isoprene emission was mirrored by the appearance of unlabelled monoterpenes, green leaf volatiles, methanol, and ethanol, indicating that the emission of stored volatiles was mainly induced by cellular damage. Emission of partially labelled acetaldehyde was also observed at very high temperatures, suggesting a double source of carbon, with a large unlabelled component likely transported from roots and associated to the surge of transpiration at very high temperatures. Eucalypt plantations cover large areas worldwide, and our findings may dramatically change forecast and modelling of future BVOC emissions at planetary level, especially considering climate warming and frequent heat waves.

Reassimilation of Leaf Internal CO2 Contributes to Isoprene Emission in the Neotropical Species Inga edulis Mart

Isoprene (C5H8) is a hydrocarbon gas emitted by many tree species and has been shown to protect photosynthesis under abiotic stress. Under optimal conditions for photosynthesis, ~70%–90% of carbon used for isoprene biosynthesis is produced from recently assimilated atmospheric CO2. While the contribution of alternative carbon sources that increase with leaf temperature and other stresses have been demonstrated, uncertainties remain regarding the biochemical source(s) of isoprene carbon. In this study, we investigated leaf isoprene emissions (Is) from neotropical species Inga edulis Mart. as a function of light and temperature under ambient (450 µmol m−2 s−1) and CO2-free (0 µmol m−2 s−1) atmosphere. Is under CO2-free atmosphere showed light-dependent emission patterns similar to those observed under ambient CO2, but with lower light saturation point. Leaves treated with the photosynthesis inhibitor DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to produce detectable Is in normal light under a CO2-free atmosphere. While strong temperature-dependent Is were observed under CO2-free atmosphere in the light, dark conditions failed to produce detectable Is even at the highest temperatures studied (40 °C). Treatment of leaves with 13C-labeled sodium bicarbonate under CO2-free atmosphere resulted in Is with over 50% containing at least one 13C atom. Is under CO2-free atmosphere and standard conditions of light and leaf temperature represented 19% ± 7% of emissions under ambient CO2. The results show that the reassimilation of leaf internal CO2 contributes to Is in the neotropical species I. edulis. Through the consumption of excess photosynthetic energy, our results support a role of isoprene biosynthesis, together with photorespiration, as a key tolerance mechanism against high temperature and high light in the tropics.

Isoprene Acts as a Signaling Molecule in Gene Networks Important for Stress Responses and Plant Growth

Isoprene synthase converts dimethylallyl diphosphate to isoprene and appears to be necessary and sufficient to allow plants to emit isoprene at significant rates. Isoprene can protect plants from abiotic stress but is not produced naturally by all plants; for example, Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum) do not produce isoprene. It is typically present at very low concentrations, suggesting a role as a signaling molecule; however, its exact physiological role and mechanism of action are not fully understood. We transformed Arabidopsis with a Eucalyptus globulus isoprene synthase The regulatory mechanisms of photosynthesis and isoprene emission were similar to those of native emitters, indicating that regulation of isoprene emission is not specific to isoprene-emitting species. Leaf chlorophyll and carotenoid contents were enhanced by isoprene, which also had a marked positive effect on hypocotyl, cotyledon, leaf, and inflorescence growth in Arabidopsis. By contrast, leaf and stem growth was reduced in tobacco engineered to emit isoprene. Expression of genes belonging to signaling networks or associated with specific growth regulators (e.g. gibberellic acid that promotes growth and jasmonic acid that promotes defense) and genes that lead to stress tolerance was altered by isoprene emission. Isoprene likely executes its effects on growth and stress tolerance through direct regulation of gene expression. Enhancement of jasmonic acid-mediated defense signaling by isoprene may trigger a growth-defense tradeoff leading to variations in the growth response. Our data support a role for isoprene as a signaling molecule.

Alternative Carbon Sources for Isoprene Emission

Isoprene and other plastidial isoprenoids are produced primarily from recently assimilated photosynthates via the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. However, when environmental conditions limit photosynthesis, a fraction of carbon for MEP pathway can come from extrachloroplastic sources. The flow of extrachloroplastic carbon depends on the species and on leaf developmental and environmental conditions. The exchange of common phosphorylated intermediates between the MEP pathway and other metabolic pathways can occur via plastidic phosphate translocators. C1 and C2 carbon intermediates can contribute to chloroplastic metabolism, including photosynthesis and isoprenoid synthesis. Integration of these metabolic processes provide an example of metabolic flexibility, and results in the synthesis of primary metabolites for plant growth and secondary metabolites for plant defense, allowing effective use of environmental resources under multiple stresses.

Small-scale indirect plant responses to insect herbivory could have major impacts on canopy photosynthesis and isoprene emission

Insect herbivores cause substantial changes in the leaves they attack, but their effects on the ecophysiology of neighbouring, nondamaged leaves have never been quantified in natural canopies. We studied how winter moth (Operophtera brumata), a common herbivore in temperate forests, affects the photosynthetic and isoprene emission rates of its host plant, the pedunculate oak (Quercus robur). Through a manipulative experiment, we measured leaves on shoots damaged by caterpillars or mechanically by cutting, or left completely intact. To quantify the effects at the canopy scale, we surveyed the extent and patterns of leaf area loss in the canopy. Herbivory reduced photosynthesis both in damaged leaves and in their intact neighbours. Isoprene emission rates significantly increased after mechanical leaf damage. When scaled up to canopy-level, herbivory reduced photosynthesis by 48 ± 10%. The indirect effects of herbivory on photosynthesis in undamaged leaves (40%) were much more important than the direct effects of leaf area loss (6%). If widespread across other plant-herbivore systems, these findings suggest that insect herbivory has major and previously underappreciated influences in modifying ecosystem carbon cycling, with potential effects on atmospheric chemistry.

Biochemical characterization of isoprene synthase from Ipomoea batatas

The bio-production process of isoprene, an essential chemical used in industry, is strongly limited by isoprene synthase. In our previous work, relatively high isoprene production was observed with isoprene synthase from Ipomoea batatas (IspS_ib). In this work the biochemical properties of IspS_ib were analyzed and compared with those of isoprene synthase from Populus alba (IspS_pa) and other species. Firstly, IspS_ib and IspS_pa were expressed, purified, and identified by SDS-PAGE and western blot analysis. Secondly, pH and temperature dependence of IspS_ib were performed and an optimum pH of 8.6 and an optimum temperature of 42 °C were resulted. Mg²⁺ with optimum concentration of 56 mM was proved to be needed for enzyme activation. In addition, in vivo and in vitro study of the thermostabilities of IspS_ib and IspS_pa were performed. The enzyme activity of IspS_ib and IspS_pa dropped very rapidly after incubation at 30 °C; almost 80% enzyme activity of IspS_ib was lost after 20 min of incubation. Moreover, the Michaelis-Menten constant was measured. IspS_ib showed a lower K_m, 0.2 mM, and a higher k_cat, 0.37 s^-1, as compared with IspS_pa. The high catalytic efficiency, which was reflected by the high k_cat/K_m ratio, indicates that IspS_ib is a good candidate for the bio-isoprene production, while its thermal instability remains as a challenge. Enzyme engineering efforts, such as direction evolution or semi-rational evolution, are planned for further research.

Contribution of volatile organic compound fluxes to the ecosystem carbon budget of a poplar short-rotation plantation

Biogenic volatile organic compounds (BVOCs) are major precursors of both ozone and secondary organic aerosols (SOA) in the troposphere and represent a non-negligible portion of the carbon fixed by primary producers, but long-term ecosystem-scale measurements of their exchanges with the atmosphere are lacking. In this study, the fluxes of 46 ions corresponding to 36 BVOCs were continuously monitored along with the exchanges of mass (carbon dioxide and water vapor) and energy (sensible and latent heat) for an entire year in a poplar (Populus) short-rotation crop (SRC), using the eddy covariance methodology. BVOC emissions mainly consisted of isoprene, acetic acid, and methanol. Total net BVOC emissions were 19.20 kg C ha^-1 yr^-1, which represented 0.63% of the net ecosystem exchange (NEE), resulting from -23.59 Mg C ha^-1 yr^-1 fixed as CO ₂ and 20.55 Mg C ha^-1 yr^-1 respired as CO ₂ from the ecosystem. Isoprene emissions represented 0.293% of NEE, being emitted at a ratio of 1 : 1709 mol isoprene per mol of CO ₂ fixed. Based on annual ecosystem-scale measurements, this study quantified for the first time that BVOC carbon emissions were lower than previously estimated in other studies (0.5-2% of NEE) on poplar trees. Furthermore, the seasonal and diurnal emission patterns of isoprene, methanol, and other BVOCs provided a better interpretation of the relationships with ecosystem CO ₂ and water vapor fluxes, with air temperature, vapor pressure deficit, and photosynthetic photon flux density.

In Planta Recapitulation of Isoprene Synthase Evolution from Ocimene Synthases

Isoprene is the most abundant biogenic volatile hydrocarbon compound naturally emitted by plants and plays a major role in atmospheric chemistry. It has been proposed that isoprene synthases (IspS) may readily evolve from other terpene synthases, but this hypothesis has not been experimentally investigated. We isolated and functionally validated in Arabidopsis the first isoprene synthase gene, AdoIspS, from a monocotyledonous species (Arundo donax L., Poaceae). Phylogenetic reconstruction indicates that AdoIspS and dicots isoprene synthases most likely originated by parallel evolution from TPS-b monoterpene synthases. Site-directed mutagenesis demonstrated invivo the functional and evolutionary relevance of the residues considered diagnostic for IspS function. One of these positions was identified by saturating mutagenesis as a major determinant of substrate specificity in AdoIspS able to cause invivo a dramatic change in total volatile emission from hemi- to monoterpenes and supporting evolution of isoprene synthases from ocimene synthases. The mechanism responsible for IspS neofunctionalization by active site size modulation by a single amino acid mutation demonstrated in this study might be general, as the very same amino acidic position is implicated in the parallel evolution of different short-chain terpene synthases from both angiosperms and gymnosperms. Based on these results, we present a model reconciling in a unified conceptual framework the apparently contrasting patterns previously observed for isoprene synthase evolution in plants. These results indicate that parallel evolution may be driven by relatively simple biophysical constraints, and illustrate the intimate molecular evolutionary links between the structural and functional bases of traits with global relevance.

Isoprene research - 60 years later, the biology is still enigmatic

Isoprene emission is a major component of biosphere-atmosphere interactions. It is the single largest source of non-methane hydrocarbon in the atmosphere. The first report of isoprene emission from plants was published in 1957 by Professor Guivi Sanadze. While humans have smelled the monoterpene hydrocarbons made by coniferous trees since their earliest migrations, only in 1957 did the world became aware that other trees make a type of hydrocarbon in even greater amounts but one to which the human nose is much less sensitive. For this 60th anniversary of the first report of isoprene emission from leaves, we trace the discovery and development of the research field, highlighting some of the most seminal observations and theoretical interpretations. This is not an exhaustive review, and many important papers are not cited, but we hope it will be of general interest to read how research in this field developed, how new observations forced us to reevaluate our theories about the significance of isoprene biosynthesis to plant physiology and adaptation and how scientific serendipity can sometimes drive a topic forward.

Isoprene Responses and Functions in Plants Challenged by Environmental Pressures Associated to Climate Change

The functional reasons for isoprene emission are still a matter of hot debate. It was hypothesized that isoprene biosynthesis evolved as an ancestral mechanism in plants adapted to high water availability, to cope with transient and recurrent oxidative stresses during their water-to-land transition. There is a tight association between isoprene emission and species hygrophily, suggesting that isoprene emission may be a favorable trait to cope with occasional exposure to stresses in mesic environments. The suite of morpho-anatomical traits does not allow a conservative water use in hygrophilic mesophytes challenged by the environmental pressures imposed or exacerbated by drought and heat stress. There is evidence that in stressed plants the biosynthesis of isoprene is uncoupled from photosynthesis. Because the biosynthesis of isoprene is costly, the great investment of carbon and energy into isoprene must have relevant functional reasons. Isoprene is effective in preserving the integrity of thylakoid membranes, not only through direct interaction with their lipid acyl chains, but also by up-regulating proteins associated with photosynthetic complexes and enhancing the biosynthesis of relevant membrane components, such as mono- and di-galactosyl-diacyl glycerols and unsaturated fatty acids. Isoprene may additionally protect photosynthetic membranes by scavenging reactive oxygen species. Here we explore the mode of actions and the potential significance of isoprene in the response of hygrophilic plants when challenged by severe stress conditions associated to rapid climate change in temperate climates, with special emphasis to the concomitant effect of drought and heat. We suggest that isoprene emission may be not a good estimate for its biosynthesis and concentration in severely droughted leaves, being the internal concentration of isoprene the important trait for stress protection.

Monoterpene emissions in response to long-term night-time warming, elevated CO2 and extended summer drought in a temperate heath ecosystem

Monoterpenes emitted from plants have an important role in atmospheric chemistry through changing atmospheric oxidative capacity, forming new particles and secondary organic aerosols. The emission rates and patterns can be affected by changing climate. In this study, emission responses to six years of climatic manipulations (elevated CO₂, extended summer drought and night-time warming) were investigated in a temperate semi-natural heath ecosystem. Samples for monoterpene analysis were collected in seven campaigns during an entire growing season (April-November, 2011). The results showed that the temperate heath ecosystem was a considerable source of monoterpenes to the atmosphere, with the emission averaged over the 8month measurement period of 21.7±6.8μgm^-2groundareah^-1 for the untreated heath. Altogether, 16 monoterpenes were detected, of which the most abundant were α-pinene, δ-3-carene and limonene. The emissions of these three compounds were positively correlated with light, chamber temperature and litter abundance, but negatively correlated with soil temperature. Elevated CO₂ tended to decrease the average monoterpene emissions by 40% over the whole growing season, and significantly reduced emissions in August. Extended summer drought significantly decreased the emission right after the drought treatment period, but also in the late growing season. Night-time warming significantly increased the total emissions (mainly α-pinene) in April, and tended to mitigate the decrease caused by drought. The inhibition effects of elevated CO₂ on emissions were diminished when the treatment was combined with drought or warming. The emission responses to different treatments were not explained by vegetation changes, and the monoterpene emission profile was only moderately related to plant species coverage. The emission responses to these long-term climate manipulations varied over the growing season (with strong correlation with litter abundance) and the observed antagonistic effects in the combined treatments underlie the importance of long-term studies with multiple factors acting in concert.

Temperature controls on the basal emission rate of isoprene in a tropical tree Ficus septica: exploring molecular regulatory mechanisms

Isoprene emission from plants is very sensitive to environmental temperature both at short-term and long-term scales. Our previous study demonstrated suppression of isoprene emission by cold temperatures in a high emitting tropical tree Ficus septica and revealed a strong correlation of emission to isoprene synthase (IspS) protein levels. When challenged with decreasing daily temperatures from 30 to 12 °C, F. septica completely stopped isoprene emission at 12 °C, only to recover on the second day after re-exposure to 30 °C. Here, we explored this regulation of isoprene emission in response to environmental temperature by a comprehensive analysis of transcriptome data, gene expressions and metabolite pools of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. MEP pathway genes and metabolites dynamics did not support substrate-level limitations as major control over observed basal emission, but transcriptome data, network inferences and putative regulatory elements on IspS promoter suggested transcriptional regulation of IspS gene through circadian rhythm and phytohormone signalling processes. Expression levels of 29 genes involved in these pathways were examined by quantitative real-time PCR. We propose that temperature controls over basal isoprene emission at a time-scale of hours to few days are regulated by phytohormone-mediated transcriptional modulation of IspS gene under synchronization by the circadian clock.

Gene expression analysis of disabled and re-induced isoprene emission by the tropical tree Ficus septica before and after cold ambient temperature exposure

Isoprene is the most abundant type of nonmethane, biogenic volatile organic compound in the atmosphere, and it is produced mainly by terrestrial plants. The tropical tree species Ficus septica Burm. F. (Rosales: Moraceae) has been shown to cease isoprene emissions when exposed to temperatures of 12 °C or lower and to re-induce isoprene synthesis upon subsequent exposure to temperatures of 30 °C or higher for 24 h. To elucidate the regulation of genes underlying the disabling and then induction of isoprene emission during acclimatization to ambient temperature, we conducted gene expression analyses of F. septica plants under changing temperature using quantitative real-time polymerase chain reaction and western blotting. Transcription levels were analyzed for 17 genes that are involved in metabolic pathways potentially associated with isoprene biosynthesis, including isoprene synthase (ispS). The protein levels of ispS were also measured. Changes in transcription and protein levels of the ispS gene, but not in the other assessed genes, showed identical temporal patterns to isoprene emission capacity under the changing temperature regime. The ispS protein levels strongly and positively correlated with isoprene emission capacity (R(2) = 0.92). These results suggest that transcriptional regulation of ispS gave rise to the temporal variation in isoprene emission capacity in response to changing temperature.

Impact of increasing heat waves on U.S. ozone episodes in the 2050s: Results from a multimodel analysis using extreme value theory

We develop a statistical model using extreme value theory to estimate the 2000-2050 changes in ozone episodes across the United States. We model the relationships between daily maximum temperature (Tmax) and maximum daily 8-hour average (MDA8) ozone in May-September over 2003-2012 using a Point Process (PP) model. At ~20% of the sites, a marked decrease in the ozone-temperature slope occurs at high temperatures, defined as ozone suppression. The PP model sometimes fails to capture ozone-Tmax relationships, and so we refit the ozone-Tmax slope using logistic regression and a Generalized Pareto Distribution model. We then apply the resulting hybrid-EVT model to projections of Tmax from an ensemble of downscaled climate models. Assuming constant anthropogenic emissions at the present level, we find an average increase of 2.3 days a-1 in ozone episodes (> 75 ppbv) across the United States by the 2050s, with a change of +3-9 days a-1 at many sites.

Controls of the quantum yield and saturation light of isoprene emission in different-aged aspen leaves

Leaf age alters the balance between the use of end-product of plastidic isoprenoid synthesis pathway, dimethylallyl diphosphate (DMADP), in prenyltransferase reactions leading to synthesis of pigments of photosynthetic machinery and in isoprene synthesis, but the implications of such changes on environmental responses of isoprene emission have not been studied. Because under light-limited conditions, isoprene emission rate is controlled by DMADP pool size (SDMADP ), shifts in the share of different processes are expected to particularly strongly alter the light dependency of isoprene emission. We examined light responses of isoprene emission in young fully expanded, mature and old non-senescent leaves of hybrid aspen (Populus tremula x P. tremuloides) and estimated in vivo SDMADP and isoprene synthase activity from post-illumination isoprene release. Isoprene emission capacity was 1.5-fold larger in mature than in young and old leaves. The initial quantum yield of isoprene emission (αI ) increased by 2.5-fold with increasing leaf age primarily as the result of increasing SDMADP . The saturating light intensity (QI90 ) decreased by 2.3-fold with increasing leaf age, and this mainly reflected limited light-dependent increase of SDMADP possibly due to feedback inhibition by DMADP. These major age-dependent changes in the shape of the light response need consideration in modelling canopy isoprene emission.

Aerosol particulate matter in the Baltimore metropolitan area: Temporal variation over a six-year period

This study investigates the sources of fine particulate matter (aerodynamic diameter ≤2.5 μm; PM(2.5)) composition for the Baltimore, Maryland, metropolitan area, covering a 6-year period (2008-2013). Data obtained from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS) were used for the identification of eight chemical speciation clusters (factors), which, as a percentage of the average concentration, were identified as secondary sulfate (31.9%), secondary nitrate (14.3%), gasoline (17.4%), diesel (10.1%), soil (4.0%), biomass burning (11%), marine aerosol (4.1%), and industrial processing (7.2%). The results show predominant influence from vehicle emissions transiting major highways I-695 and I-95 located in the vicinity of the sampling site. Strong influence on PM2.5 mass from biomass burning was found in the first 2 years (2008-2009) due to particulate matter remnants from forest fire events in North Carolina and a strong contribution in 2013 that was due mainly to wood burning during winter. Sulfate, nitrate, soil, and marine aerosol fractions registered very low variability over the 6-year period analyzed. In addition, this study shows a significant reduction in particulate matter from industrial origins after a major industrial source in Baltimore shut down. The results obtained from Baltimore were compared with those from the Beltsville, Maryland, sampling station located 25 miles south of Baltimore for 2011 and 2012, where good agreement was found for most of the factors.

IMPLICATIONS

This paper presents the first long-term aerosol speciation analysis in a Mid-Atlantic United States metropolitan area, which is essential for the air quality management agencies in order to revise regulations and reduce human exposure to adverse air quality conditions. The results suggest that although a declining trend in the overall PM2.5 was observed, no significant tendency was observed in the identified sources besides exceptional events such as the impact of wildfires on local air quality and downward contribution from industrial fraction of PM(2.5) after the Steel Mill at Sparrows Point closure in 2012.

Biochemical characterization of an isoprene synthase from Campylopus introflexus (heath star moss)

Each year, plants emit terragram quantities of the reactive hydrocarbon isoprene (2-methyl-1,3-butadiene) into the earth's atmosphere. In isoprene-emitting plants, the enzyme isoprene synthase (ISPS) catalyzes the production of isoprene from the isoprenoid intermediate dimethylallyl diphosphate (DMADP). While isoprene is emitted from all major classes of land plants, to date ISPSs from angiosperms only have been characterized. Here, we report the identification and initial biochemical characterization of a DMADP-dependent ISPS from the isoprene-emitting bryophyte Campylopus introflexus (heath star moss). The partially-purified C. introflexus ISPS (CiISPS) exhibited a Km for DMADP of 0.37 ± 0.28 mM, a pH optimum of 8.6 ± 0.5, and a temperature optimum of 40 ± 3 °C in vitro. Like ISPSs from angiosperms, the CiISPS required the presence of a divalent cation. However, unlike angiosperm ISPSs, the CiISPS utilized Mn(2+) preferentially over Mg(2+). Efforts are currently underway in our laboratory to further purify the CiISPS and clone the cDNA sequence encoding this novel enzyme. Our discovery of the first bryophyte ISPS paves the way for future studies concerning the evolutionary origins of isoprene emission in land plants and may help generate new bryophyte model systems for physiological and biochemical research on plant isoprene function.

Facing the Future: Effects of Short-Term Climate Extremes on Isoprene-Emitting and Nonemitting Poplar

Isoprene emissions from poplar (Populus spp.) plantations can influence atmospheric chemistry and regional climate. These emissions respond strongly to temperature, [CO2], and drought, but the superimposed effect of these three climate change factors are, for the most part, unknown. Performing predicted climate change scenario simulations (periodic and chronic heat and drought spells [HDSs] applied under elevated [CO2]), we analyzed volatile organic compound emissions, photosynthetic performance, leaf growth, and overall carbon (C) gain of poplar genotypes emitting (IE) and nonemitting (NE) isoprene. We aimed (1) to evaluate the proposed beneficial effect of isoprene emission on plant stress mitigation and recovery capacity and (2) to estimate the cumulative net C gain under the projected future climate. During HDSs, the chloroplastidic electron transport rate of NE plants became impaired, while IE plants maintained high values similar to unstressed controls. During recovery from HDS episodes, IE plants reached higher daily net CO2 assimilation rates compared with NE genotypes. Irrespective of the genotype, plants undergoing chronic HDSs showed the lowest cumulative C gain. Under control conditions simulating ambient [CO2], the C gain was lower in the IE plants than in the NE plants. In summary, the data on the overall C gain and plant growth suggest that the beneficial function of isoprene emission in poplar might be of minor importance to mitigate predicted short-term climate extremes under elevated [CO2]. Moreover, we demonstrate that an analysis of the canopy-scale dynamics of isoprene emission and photosynthetic performance under multiple stresses is essential to understand the overall performance under proposed future conditions.

Isoprenoids and phenylpropanoids are part of the antioxidant defense orchestrated daily by drought-stressed Platanus × acerifolia plants during Mediterranean summers

The hypothesis was tested that isoprenoids and phenylpropanoids play a prominent role in countering photooxidative stress, following the depletion of antioxidant enzyme activity in plants exposed to severe drought stress under high solar irradiance and high temperatures. Platanus × acerifolia, a high isoprene-emitting species, was drought-stressed during summer (WS) and compared with unstressed controls (WW). Water relations and photosynthetic parameters were measured under mild, moderate, and severe drought stress conditions. Volatile and nonvolatile isoprenoids, antioxidant enzymes, and phenylpropanoids were measured with the same time course, but in four different periods of the day. Drought severely inhibited photosynthesis, whereas it did not markedly affect the photochemical machinery. Isoprene emission and zeaxanthin concentration were higher in WS than in WW leaves, particularly at mild and moderate stresses, and during the hottest hours of the day. The activities of catalase and ascorbate peroxidase steeply declined during the day, while the activity of guaiacol peroxidase and the concentration of quercetin increased during the day, peaking in the hottest hours in both WW and WS plants. Our experiment reveals a sequence of antioxidants that were used daily by plants to orchestrate defense against oxidative stress induced by drought and associated high light and high temperature. Secondary metabolites seem valuable complements of antioxidant enzymes to counter oxidative stress during the hottest daily hours.

Bisphosphonate inhibitors reveal a large elasticity of plastidic isoprenoid synthesis pathway in isoprene-emitting hybrid aspen

Recently, a feedback inhibition of the chloroplastic 1-deoxy-D-xylulose 5-phosphate (DXP)/2-C-methyl-D-erythritol 4-phosphate (MEP) pathway of isoprenoid synthesis by end products dimethylallyl diphosphate (DMADP) and isopentenyl diphosphate (IDP) was postulated, but the extent to which DMADP and IDP can build up is not known. We used bisphosphonate inhibitors, alendronate and zoledronate, that inhibit the consumption of DMADP and IDP by prenyltransferases to gain insight into the extent of end product accumulation and possible feedback inhibition in isoprene-emitting hybrid aspen (Populus tremula × Populus tremuloides). A kinetic method based on dark release of isoprene emission at the expense of substrate pools accumulated in light was used to estimate the in vivo pool sizes of DMADP and upstream metabolites. Feeding with fosmidomycin, an inhibitor of DXP reductoisomerase, alone or in combination with bisphosphonates was used to inhibit carbon input into DXP/MEP pathway or both input and output. We observed a major increase in pathway intermediates, 3- to 4-fold, upstream of DMADP in bisphosphonate-inhibited leaves, but the DMADP pool was enhanced much less, 1.3- to 1.5-fold. In combined fosmidomycin/bisphosphonate treatment, pathway intermediates accumulated, reflecting cytosolic flux of intermediates that can be important under strong metabolic pull in physiological conditions. The data suggested that metabolites accumulated upstream of DMADP consist of phosphorylated intermediates and IDP. Slow conversion of the huge pools of intermediates to DMADP was limited by reductive energy supply. These data indicate that the DXP/MEP pathway is extremely elastic, and the presence of a significant pool of phosphorylated intermediates provides an important valve for fine tuning the pathway flux.

Acclimation of isoprene emission and photosynthesis to growth temperature in hybrid aspen: resolving structural and physiological controls

Acclimation of foliage to growth temperature involves both structural and physiological modifications, but the relative importance of these two mechanisms of acclimation is poorly known, especially for isoprene emission responses. We grew hybrid aspen (Populus tremula x P. tremuloides) under control (day/night temperature of 25/20 °C) and high temperature conditions (35/27 °C) to gain insight into the structural and physiological acclimation controls. Growth at high temperature resulted in larger and thinner leaves with smaller and more densely packed chloroplasts and with lower leaf dry mass per area (MA). High growth temperature also led to lower photosynthetic and respiration rates, isoprene emission rate and leaf pigment content and isoprene substrate dimethylallyl diphosphate pool size per unit area, but to greater stomatal conductance. However, all physiological characteristics were similar when expressed per unit dry mass, indicating that the area-based differences were primarily driven by MA. Acclimation to high temperature further increased heat stability of photosynthesis and increased activation energies for isoprene emission and isoprene synthase rate constant. This study demonstrates that temperature acclimation of photosynthetic and isoprene emission characteristics per unit leaf area were primarily driven by structural modifications, and we argue that future studies investigating acclimation to growth temperature must consider structural modifications.

How light, temperature, and measurement and growth [CO2] interactively control isoprene emission in hybrid aspen

Plant isoprene emissions have been modelled assuming independent controls by light, temperature and atmospheric [CO2]. However, the isoprene emission rate is ultimately controlled by the pool size of its immediate substrate, dimethylallyl diphosphate (DMADP), and isoprene synthase activity, implying that the environmental controls might interact. In addition, acclimation to growth [CO2] can shift the share of the control by DMADP pool size and isoprene synthase activity, and thereby alter the environmental sensitivity. Environmental controls of isoprene emission were studied in hybrid aspen (Populus tremula × Populus tremuloides) saplings acclimated either to ambient [CO2] of 380 μmol mol(-1) or elevated [CO2] of 780 μmol mol(-1). The data demonstrated strong interactive effects of environmental drivers and growth [CO2] on isoprene emissions. Light enhancement of isoprene emission was the greatest at intermediate temperatures and was greater in elevated-[CO2]-grown plants, indicating greater enhancement of the DMADP supply. The optimum temperature for isoprene emission was higher at lower light, suggesting activation of alternative DMADP sinks at higher light. In addition, [CO2] inhibition of isoprene emission was lost at a higher temperature with particularly strong effects in elevated-[CO2]-grown plants. Nevertheless, DMADP pool size was still predicted to more strongly control isoprene emission at higher temperatures in elevated-[CO2]-grown plants. We argue that interactive environmental controls and acclimation to growth [CO2] should be incorporated in future isoprene emission models at the level of DMADP pool size.

A fully integrated isoprenoid emissions model coupling emissions to photosynthetic characteristics

The lack of a mechanistic basis has hampered modelling isoprene emission responses to environmental drivers, in particular the simulation of isoprene emissions under different CO₂ concentrations. Here, we advance previous semi-mechanistic model formulations by introducing a model that explicitly links electron availability for other purpose than carbon assimilation (or available energy for secondary metabolism processes; supply-constraint) and enzyme activity (capacity-constraint) to emissions. We furthermore investigate the sensitivity of the model to variations in photosynthetic and emission-specific parameters. By comparing species-specific simulations with experimental data, we demonstrate that differences in photosynthetic characteristics can explain inter-species differences in emissions. Interestingly, the seasonal development of emissions could also be explained to some degree by the change in energy supply from photosynthesis throughout the season. In addition, we show that the principal responses are not limited to isoprene but can be formulated to describe the emission of other light-dependent volatile species. The proposed model is suitable for implementation into regional and global models, particularly those that already provide species-specific photosynthesis estimates.

The future of isoprene emission from leaves, canopies and landscapes

Isoprene emission from plants plays a dominant role in atmospheric chemistry. Predicting how isoprene emission may change in the future will help predict changes in atmospheric oxidant, greenhouse gas and secondary organic aerosol concentrations in the future atmosphere. At the leaf-scale, an increase in isoprene emission with increasing temperature is offset by a reduction in isoprene emission rate caused by increased CO₂. At the canopy scale, increased leaf area index in elevated CO₂ can offset the reduction in leaf-scale isoprene emission caused by elevated CO₂. At the landscape scale, a reduction in forest coverage may decrease, while forest fertilization and community composition dynamics are likely to cause an increase in the global isoprene emission rate. Here we review the potential for changes in the isoprene emission rate at all of these scales. When considered together, it is likely that these interacting effects will result in an increase in the emission of the most abundant plant volatile, isoprene, from the biosphere to the atmosphere in the future.

Cytokinin modulates proteomic, transcriptomic and growth responses to temperature shocks in Arabidopsis

As sessile organisms, plants must sense environmental conditions and adjust their growth and development processes accordingly, through adaptive responses regulated by various internal factors, including hormones. A key environmental factor is temperature, but temperature-sensing mechanisms are not fully understood despite intense research. We investigated proteomic responses to temperature shocks (15 min cold or heat treatments) with and without exogenous applications of cytokinin in Arabidopsis. Image and mass spectrometric analysis of the two-dimensionally separated proteins detected 139 differentially regulated spots, in which 148 proteins were identified, most of which have not been previously linked to temperature perception. More than 70% of the temperature-shock response proteins were modulated by cytokinin, mostly in a similar manner as heat shock. Data mining of previous transcriptomic datasets supported extensive interactions between temperature and cytokinin signalling. The biological significance of this finding was tested by assaying an independent growth response of Arabidopsis seedlings to heat stress: hypocotyl elongation. This response was strongly inhibited in mutants with deficiencies in cytokinin signalling or endogenous cytokinin levels. Thus, cytokinins may directly participate in heat signalling in plants. Finally, large proportions of both temperature-shock and cytokinin responsive proteomes co-localize to the chloroplast, which might therefore host a substantial proportion of the temperature response machinery.

A model of plant isoprene emission based on available reducing power captures responses to atmospheric CO₂

We present a unifying model for isoprene emission by photosynthesizing leaves based on the hypothesis that isoprene biosynthesis depends on a balance between the supply of photosynthetic reducing power and the demands of carbon fixation. We compared the predictions from our model, as well as from two other widely used models, with measurements of isoprene emission from leaves of Populus nigra and hybrid aspen (Populus tremula × P. tremuloides) in response to changes in leaf internal CO2 concentration (C(i)) and photosynthetic photon flux density (PPFD) under diverse ambient CO2 concentrations (C(a)). Our model reproduces the observed changes in isoprene emissions with C(i) and PPFD, and also reproduces the tendency for the fraction of fixed carbon allocated to isoprene to increase with increasing PPFD. It also provides a simple mechanism for the previously unexplained decrease in the quantum efficiency of isoprene emission with increasing C(a). Experimental and modelled results support our hypothesis. Our model can reproduce the key features of the observations and has the potential to improve process-based modelling of isoprene emissions by land vegetation at the ecosystem and global scales.