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Plants synthesize thousands of volatile organic compounds (VOCs) (see Glossary), which mainly include isoprene (C5, hemiterpene) and monoterpenes (C10) from the methylerythritol-4-phosphate pathway (MEP) in chloroplasts (Figure 1). Sesquiterpenes and diterpenes are less volatile and are not discussed here. Global emissions are estimated to range from 300 to 440 Tg C·year–1 for isoprene and 90 to 100 Tg C·year–1 for monoterpenes [1., 2., 3.]. Biosynthesis of isoprenoids consumes 1–2% of carbon fixed by photosynthesis under normal conditions and >10% under stress, especially under high temperature [4., 5., 6.]. In nature, the highest emissions of isoprenoids are also detected during the hottest months of the year, as a result of an increase in the abundance of precursors, related enzyme activities, gene expression, and diffusion rates (of storage monoterpenoids) 

Isoprene is the most abundant nonmethane biogenic hydrocarbon emitted by some plants, mostly trees. It plays critical roles in atmospheric chemistry by contributing to ozone and aerosol formation. Isoprene also benefits plants, particularly under stress, through its signaling roles. Legume crops like soybean were thought to have evolutionarily lost isoprene synthase (ISPS) and are typically considered nonemitters. Here, we report that damage to soybean leaves by wounding or burning triggered a burst of isoprene emission from the undamaged part of the leaves. In silico analysis identified intactISPSgenes in the soybean genome, with features similar to known ISPSs. Protein made from these gene sequences catalyzed isoprene production in the presence of dimethylallyl diphosphate. Isoprene emission in soybeans was linked to reduced photosynthesis rates and stomatal conductance. Metabolomic analysis showed that leaf damage caused a surge in glyceraldehyde 3-phosphate and pyruvate levels, leading to an increase of most of the methylerythritol 4-phosphate pathway metabolites. 

Isoprene, emitted by some plants, deters insect herbivory. However, the associated biochemical and physiological responses that confer herbivory resistance remain unknown. We used engineered isoprene-emitting (IE) and non-emitting (NE) control tobacco plants to interpret isoprene-mediated defense against herbivory in plants. Hornworm larvae raised on IE plants exhibited stunted growth compared to those raised on NE plants. Worms preferred to feed on NE rather than IE leaves, indicating deterrent effects of isoprene on insect feeding. Worm feeding induced a greater increase in jasmonic acid (JA), a crucial hormone for insect resistance, in IE leaves compared to that in NE leaves. Assimilation rates were stably maintained in IE plants, suggesting a protective role of isoprene in preserving photosynthetic efficiency during insect herbivory. Wound-induced increase in isoprene emission correlated with the elevation of key metabolites of the isoprene biosynthesis pathway. Our results highlight JA-priming functions of isoprene and provide insights into isoprene-mediated defense against insect herbivory. 

In recent years, anthropogenic activities and climate change have significantly increased exposure of plants to environmental stresses (single or multiple) and pollutants, with negative consequences for the survival and productivity of vegetation. Plants may activate an armament of defenses against stresses. Isoprene, the most abundant biogenic volatile organic compound (BVOC) emitted by plants, is supposed to have a direct or indirect antioxidant role by quenching reactive oxygen species (ROS) or by reprogramming gene expression for antioxidant activation. On the other hand, isoprene is involved in the chemistry of troposphere, further contributing to a build up of pollutants when mixing with anthropogenic gases. In this review, we summarize present knowledge on the impact of air and soil pollutants on isoprene emission by plants, indicating possible feedback and feedforward mechanisms that may affect whole ecosystem functioning and evolution of plant species. 

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 CO₂concentration; 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 CO₂to identify why isoprene emission is reduced under high CO₂. We found that hydroxymethylbutenyl diphosphate (HMBDP) was increased and dimethylallyl diphosphate (DMADP) decreased at high CO_2.This implies that high CO₂impeded 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 CO₂on 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. 

When isotopes of carbon are fed to photosynthesizing leaves, metabolites of the Calvin–Benson cycle (CBC) are rapidly labeled initially, but then the rate of labeling slows considerably, raising questions about the integration of the CBC within leaf metabolism. We have used 2-h time courses of labeling of Camelina sativa leaf metabolites to test models of 12 C washout when the CO 2 source is rapidly switched to 13 CO 2 . Fitting exponential functions to the time course of CBC metabolites, we found evidence for three temporally distinct processes contributing to the labeling but none for metabolically inactive pools. We next modeled the data of all metabolites by 13 C isotopically nonstationary metabolic flux analysis, testing a variety of flux networks. In the model that best explains measured data, three processes determine CBC metabolite labeling. First is fixation of incoming 13 CO 2 ; second is dilution by weakly labeled carbon in cytosolic glucose reentering the CBC following oxidative pentose phosphate pathway reactions, which forms a shunt bypassing much of the CBC. Third, very weakly labeled carbon from the vacuole further dilutes the labeling. This model predicts the shunt proceeds at about 5% of the rate of net CO 2 fixation and explains the three phases of labeling. In showing the interconnection of three compartments, we have drawn a more complete picture of how carbon moves through photosynthetic metabolism in a way that integrates the CBC, cytosolic sugar pools, glucose-6-phosphate shunt, and vacuolar sugars into a single system. 

Summary Building sustainable platforms to produce biofuels and specialty chemicals has become an increasingly important strategy to supplement and replace fossil fuels and petrochemical‐derived products. Terpenoids are the most diverse class of natural products that have many commercial roles as specialty chemicals. Poplar is a fast growing, biomassdense bioenergy crop with many species known to produce large amounts of the hemiterpene isoprene, suggesting an inherent capacity to produce significant quantities of other terpenes. Here we aimed to engineer poplar with optimized pathways to produce squalene, a triterpene commonly used in cosmetic oils, a potential biofuel candidate, and the precursor to the further diversified classes of triterpenoids and sterols. The squalene production pathways were either re‐targeted from the cytosol to plastids or co‐produced with lipid droplets in the cytosol. Squalene and lipid droplet co‐production appeared to be toxic, which we hypothesize to be due to disruption of adventitious root formation, suggesting a need for tissue specific production. Plastidial squalene production enabled up to 0.63 mg/g fresh weight in leaf tissue, which also resulted in reductions in isoprene emission and photosynthesis. These results were also studied through a technoeconomic analysis, providing further insight into developing poplar as a production host.