Minisymposium 3: Global Change

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Abs # M0303: Cytosolic regulation of chloroplastic isoprene production: reconciling the ups and downs of isoprene emission in response to elevated CO₂

Presenter:	Rosenstiel, Todd N.       Contact Presenter
Authors	Rosenstiel, Todd N. (A) (D)  Fall, Ray  (C) (A)  Monson, Russell K. (B) (C)
Affiliations:	(A): University of Colorado, CIRES (B): University of Colorado, Dept. of Ecology and Evolutionary Biology (C): University of Colorado, Dept. of Chemistry and Biochemistry (D): Portland State University, Dept. of Biology

It is well known that the capacity to synthesize and emit isoprene in the leaves of many trees is controlled by the activity of the chloroplastic enzyme isoprene synthase. The expression of this enzyme appears to scale positively with the expression of photosynthetic capacity. However, the connections between photosynthesis and isoprene synthase activity cannot soley explain the decrease in isoprene emissions observed when plants are grown at elevated atmospheric CO₂ concentrations. In past studies we have shown that cytosolic processes that utilize phosphoenol pyruvate (PEP) have the potential to limit the rate of isoprene emission from the leaves of poplar trees, especially when exposed to elevated atmospheric CO₂ concentration. In more recent studies, we have exposed poplar trees to enhanced CO₂ concentration to the roots, as opposed to the shoots. This treatment caused the trees to shift their allocation of nitrate reductase activity and PEP carboxylase activity from the leaves to the roots, enhancing leaf PEP availability and increasing both the substrate available for isoprene biosynthesis (DMAPP) and isoprene emission rate. However, this increase in isoprene emission rate occurred despite a pronounced reduction in the activity and amount of isoprene synthase in the leaves. This is the first report of an increase in isoprene emission rate, despite a reduction in isoprene synthase expression, and it confirms the important role of substrate supply in regulating isoprene biosynthesis in vivo, especially in elevated atmospheric CO₂.