Control of isoprene emission from plants and bacteria

Isoprene is a volatile compound produced in large amounts by some, but not all, plants by the enzyme isoprene synthase. Plants emit vast quantities of isoprene, with a net global output of 600 Tg per year, and typical emission rates from individual plants are around 2% of net carbon assimilation in the light. Isoprene is also an important economic compound, as it is a precursor to synthetic fragrances, pharmaceuticals, and rubber. Despite being studied since the discovery of isoprene emission from plants in 1954, the reason why plants make so much isoprene is unknown. My graduate work presented in this thesis focuses on the joint control of isoprene emission by the methylerythritol 4-phosphate (MEP) pathway, which produces the precursor dimethylallyl diphosphate (DMADP), and by isoprene synthase. I found that the changes in isoprene emission from plants due to changes in CO2 and temperature are not linked to electron transport and can occur independently of changes in carbon assimilation. This disproves several modern models of isoprene emission and my data predict that contrary to current models, isoprene emission will increase in future climate conditions. While changes in electron transport should lead to changes in the MEP pathway, there are changes in DMADP concentration that are not linked to electron transport. I propose that calcium signaling caused by wounding or high CO2 leading to post-translational modification of isoprene synthase may explain some of the changes in isoprene emission. I report changes in isoprene synthase itself under increased temperature that are necessary to understand how isoprene emission will change under future atmospheric conditions. In collaboration with the Kerfeld lab, we encapsulated isoprene synthase in bacterial microcompartments, which may improve synthetic isoprene production by channeling DMADP directly into isoprene synthase. We also produced a modified isoprene synthase and methylbutenol synthase that has improved activity. I profile MEP pathway metabolites and isoprene synthase under changing environmental conditions and in transgenic emitting and non-emitting species to further our understanding of the joint control of isoprene emission by the MEP pathway and isoprene synthase.