Isoprene: An antioxidant itself or a molecule with multiple regulatory functions in plants?

Isoprene (C5H8) is a small lipophilic, volatile organic compound (VOC), synthesized inchloroplasts of plants through the photosynthesis-dependent 2-C-methyl-D-erythritol 4-phosphate(MEP) pathway. Isoprene-emitting plants are better protected against thermal and oxidative stressesbut only about 20% of the terrestrial plants are able to synthesize isoprene. Many studies have beenperformed to understand the still elusive isoprene protective mechanism. Isoprene reacts with, andquenches, many harmful reactive oxygen species (ROS) like singlet oxygen (1O2). A role for isopreneas antioxidant, made possible by its reduced state and conjugated double bonds, has been oftensuggested, and sometimes demonstrated. However, as isoprene is present at very low concentrationscompared to other molecules, its antioxidant role is still controversial. Here we review updatedevidences on the function(s) of isoprene, and outline contrasting indications on whether isoprene isan antioxidant directly scavenging ROS, or a membrane strengthener, or a modulator of genomic,proteomic and metabolomic profiles (perhaps as a secondary effect of ROS removal) eventuallyleading to priming of antioxidant plant defenses, or a signal of stress for neighbor plants alike otherVOCs, or a hormone-like molecule, controlling the metabolic flux of other hormones made by theMEP pathway, or acting itself as a growth and development hormone