title: Temperature response of isoprene emission in vivo reflects a combined effect of substrate limitations and isoprene synthase activity: a kinetic analysis.
abstract: The responses of isoprene emission rate to temperature are characterized by complex time-dependent behaviors that are currently not entirely understood. To gain insight into the temperature dependencies of isoprene emission, we studied steady-state and transient responses of isoprene emission from hybrid aspen (Populus tremula × Populus tremuloides) leaves using a fast-response gas-exchange system coupled to a proton-transfer reaction mass spectrometer. A method based on postillumination isoprene release after rapid temperature transients was developed to determine the rate constant of isoprene synthase (IspS), the pool size of its substrate dimethylallyldiphosphate (DMADP), and to separate the component processes of the temperature dependence of isoprene emission. Temperature transients indicated that over the temperature range 25°C to 45°C, IspS was thermally stable and operated in the linear range of its substrate DMADP concentration. The in vivo rate constant of IspS obeyed the Arrhenius law, with an activation energy of 42.8 kJ mol(-1). In contrast, steady-state isoprene emission had a significantly lower temperature optimum than IspS and higher activation energy. The reversible temperature-dependent decrease in the rate of isoprene emission between 35°C and 44°C was caused by decreases in DMADP concentration, possibly reflecting reduced pools of energetic metabolites generated in photosynthesis, particularly of ATP. Strong control of isoprene temperature responses by the DMADP pool implies that transient temperature responses under fluctuating conditions in the field are driven by initial DMADP pool size as well as temperature-dependent modifications in DMADP pool size during temperature transients. These results have important implications for the development of process-based models of isoprene emission.
authors.affiliation: Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia.
authors.forename: Bahtijor
authors.initials: B
authors.lastname: Rasulov
authors.forename: Katja
authors.initials: K
authors.lastname: Hüve
authors.forename: Irina
authors.initials: I
authors.lastname: Bichele
authors.forename: Agu
authors.initials: A
authors.lastname: Laisk
authors.forename: Ulo
authors.initials: U
authors.lastname: Niinemets
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000378:metabolism*; D002070:Butadienes / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D045782:Hemiterpenes / Q000378:metabolism*; D007700:Kinetics; D010420:Pentanes / Q000378:metabolism*; D010788:Photosynthesis; D032107:Populus / Q000201:enzymology / Q000378:metabolism*; D013696:Temperature
chemical_list: D002070:Butadienes; D045782:Hemiterpenes; D010420:Pentanes; C005059:isoprene; D019883:Alkyl and Aryl Transferases; C093854:isoprene synthase