Origin, evolution, and future of isoprene and nitric oxide interactions within leaves.

Photolytic generation of nitric oxide (NO), isoprene, and reactive oxygen species (ROS) predated life on Earth (~4 Bya). However, isoprene-ROS-NO interactions became relevant to climate chemistry around 50 Mya, after aquatic and terrestrial ecosystems became dominated by isoprene-emitting diatoms and angiosperms. Today, NO and NO2 (together referred to as NOx) are dangerous biogenic gaseous atmospheric pollutants. In plants, NO, with its multiple sources and sinks, acts as a secondary messenger that regulates development at low doses and induces cell death at high doses. Likewise, biogenic isoprene is a putative antioxidant and hormone 'enabler' that hastens plant (and leaf) growth, reproduction, and improves plant tolerance to transient abiotic stresses. Using examples from controlled chamber-simulation and field studies of isoprene oxidation, we discuss the likely nature and extent of isoprene oxidation within-leaves. We argue that isoprene-NO interactions vary greatly among plant species, driven by differences in isoprene emission and nitrate assimilation (i.e., NO sink strength) rates, ROS availability, and isoprene to free-NO ratio. In a warmer and CO2-fertilized future climate, antagonism between isoprene and NO within leaf will likely occur in a NO-rich (relative to present) environment, yielding greater proportion of isoprene oxidation products, and inducing major changes in NO-mediated growth and stress responses.