Humboldt review: Revisiting plant isoprene emission: From atmospheric chemistry to plant stress resilience

The emission of isoprene from plants was first discovered in the 1950s but was relatively unknown in the plant science community until the 1990s. Isoprene is the five-carbon branched molecule that is the root member of the extensive family of isoprenoids. The amount of isoprene from plants exceeds all other hydrocarbon inputs to the atmosphere. Plant-emitted isoprene can affect ozone formation and often increases (but can decrease) growth of aerosols (particles in the atmosphere). The rate of isoprene emission is estimated using empirical or mechanistic modeling has been used to predict global emissions. Beyond its atmospheric role, isoprene can improve plant resilience to biotic and abiotic stress, likely through interactions with transcriptional networks that regulate plant growth and defense. Isoprene has all the properties of the five compounds classically described as plant hormones. These and an additional four molecules are now called small molecule plant growth regulators, and we propose that isoprene also belongs to this group. Plants previously thought to lack the capacity for isoprene emission have now been found that make isoprene in response to leaf damage. This discovery suggests that many plants once classified as non-emitters likely have the capacity to emit isoprene under specific conditions. This review summarizes past and current understanding of the biosynthesis and regulatory mechanisms, atmospheric significance, and physiological roles of isoprene emitted from plants. Environmental cues such as elevated temperature, drought, wounding, and herbivory enhance isoprene formation and release. Isoprene emission in plants originates in the chloroplast via the methylerythritol phosphate (MEP) pathway, where dimethylallyl diphosphate (DMADP) serves as the precursor. Emitted isoprene contributes to multiple physiological and ecological functions, including improved thermotolerance and deterring herbivore. Isoprene also interacts with atmospheric oxidants, influencing atmospheric chemistry and climate processes. • Isoprene is the major natural source of atmospheric hydrocarbons • Plant isoprene emission exceeds all other sources including anthropogenic sources • Isoprene participates in atmospheric chemistry and can exacerbate ozone formation • Mechanistic and empirical models exist to predict isoprene emission globally • Isoprene improves plant resilience in the face of biotic and abiotic stress