Subcellular influences and metabolic regulation of chloroplast isoprene production

The emission of isoprene (2-methyl-1,3-butadiene) from the leaves of forest trees is a fundamental component of biosphere-atmosphere interactions, controlling many aspects of photochemistry in the lower atmosphere. The biochemical and cellular mechanisms involved in influencing leaf isoprene emission were investigated in this study. Isoprene-emitting species were found to have the highest potential for light-dependent production of dimethylallyl diphosphate (DMAPP), the immediate precursor of isoprene biosynthesis. In leaves of the isoprene-emitting agriforest species Populus deltoides, approximately 65--70% of the cellular DMAPP pool was chloroplastic in origin.; Exposure to elevated CO2 significantly reduced both isoprene emission rate and the cellular content of DMAPP in leaves and leaf protoplasts of this species. The suppression in emission under elevated CO2 occurred despite enhanced rates of photosynthesis, suggesting a significant role for non-photosynthetic processes in influencing isoprene emission rate. Intracellular metabolic competition for phosphoenolpyruvate (PEP) is identified as a possible control point in explaining the suppression of isoprene emission under elevated CO2.; A similar decrease in DMAPP content and isoprene emission rate was observed when P. deltoides saplings were grown with NO3 - as their sole nitrogen source, a treatment designed to perturb subcellular carbon availability. Growth with 8mM NO3- resulted in parallel reductions in both leaf isoprene emission rate and DMAPP content. Biochemical analyses suggest that foliar NO3 - assimilation reduces isoprene emission rate by competing for carbon skeletons (mediated by the enzyme PEP-carboxylase) within the cytosol.; Photosynthetically competent chloroplasts isolated from protoplasts of P. deltoides did not produce isoprene, indicating that chloroplast DMAPP biosynthesis and isoprene production likely requires the supply of cytosolic factors. However, isoprene production by protoplast extracts was significantly enhanced by the addition of the metabolite PEP.; When taken together, results from this study indicate that chloroplast DMAPP biosynthesis, and subsequent isoprene emission, may be strongly influenced by cytosolic processes; particularly those processes which impact the cytosolic availability of PEP.