13C labelling reveals chloroplastic and extra-chloroplastic pools of dimethylallyl pyrophosphate and their contribution to isoprene formation.

Isoprene emitted from plants is made in chloroplasts from dimethylallyl pyrophosphate (DMAPP). Leaves of Populus nigra and Phragmites australis exposed to 13CO2 for 15 min emitted isoprene that was about 90% 13C, but DMAPP isolated from those leaves was only 28% and 36% 13C, respectively. The labeled DMAPP is likely to represent chloroplastic DMAPP contributing to isoprene emission. A substantial 13C labeling was also found in both emission and DMAPP pool of low-emitting, young leaves of Phragmites. This confirms that low emission of young leaves is not caused by absence of chloroplastic DMAPP but rather by enzyme characteristics. Avery low 13C labeling was found in the DMAPP pool and in the residual isoprene emission of leaves previously fed with fosmidomycin to inhibit isoprene formation. This shows that fosmidomycin is a very effective inhibitor of the chloroplastic biosynthetic pathway of isoprene synthesis, that the residual isoprene is formed from extra-chloroplastic sources, and that chloroplastic and extrachloroplastic pathways are not cross-linked, at least following inhibition of the chloroplastic pathway. Refixation of unlabeled respiratory CO2 in the light may explain incomplete labeling of isoprene emission, as we found a good association between these two parameters.