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 CO_2. 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.

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