If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Hemiterpenes Biosynthesis|
Expected Taxonomic Range: Embryophyta
Isoprene, a gaseous hydrocarbon, is emitted in substantial quantities by many different plant species from mosses to poplar trees [Sharkey08a]. Isoprene can affect atmospheric chemistry, and, under particular conditions, can contribute to ozone and aerosol formation in the atmosphere [Sharkey08a]. Ozone and aerosols both pose health threats to humans [Sharkey08a], and plant growth can also be reduced by elevated ozone levels [Karnosky07].
There are a few potential biological explanations for isoprene emissions by plants. Isoprene appears to help protect plants against sun flecks that can rapidly raise leaf temperatures [Sharkey01, Velikova05]. In addition, isoprene-emitting plants have increased tolerance to ozone and other reactive oxygen species (ROS) [Loreto01, Loreto01a, Affek02, Behnke09]. But, the molecular mechanisms of these isoprene-mediated stress responses are still under investigation. One hypothesis is that isoprene stabilizes lipid membranes [Siwko07].
Levels of isoprene emission are affected by plant developmental stage, circadian rhythms [Wilkinson06, Loivamaki07], and several environmental stimuli (reviewed in [Sharkey08a]). Notably, isoprene emissions rise in response to higher temperatures and are light-dependent [Sanadze69, Sharkey08a]. The level of isoprene synthase (IspS) activity is correlated with the levels of isoprene emission, indicating that this enzyme is a key regulator of isoprene production. Its activity seems to be largely determined by transcript abundance. Isoprene emissions can also be affected by the level of DMAPP substrate availability, the level of reducing power and ATP, competition with PEP carboxylase for CO2 [Loreto07], and other factors (reviewed in [Sharkey08a]).
Isoprene biosynthesis occurs in the chloroplasts and isoprene synthase can be detected in this organelle in a Populus tremula x Populus alba hybrid [Schnitzler05] but extra-chloroplastic sources of DMAPP can also be utilized [Loreto04].
Developing a better understanding of isoprene emissions is necessary for understanding plant development and for predicting atmospheric conditions in the short and long term.
Subpathways: methylerythritol phosphate pathway I
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