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 (ISPS). 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 (JA) 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 JA-mediated defense signaling by isoprene may trigger growth-defense trade-off leading to variations in the growth response. Our data support a role for isoprene as a signaling molecule.