Contribution of various carbon sources toward isoprene biosynthesis in poplar leaves mediated by altered atmospheric co ₂ concentrations

Biogenically released isoprene plays important roles in both tropospheric photochemistry and plant metabolism. We performed a ¹³CO ₂-labeling study using proton-transfer-reaction mass spectrometry (PTR-MS) to examine the kinetics of recently assimilated photosynthate into isoprene emitted from poplar (Populus × canescens) trees grown and measured at different atmospheric CO ₂ concentrations. This is the first study to explicitly consider the effects of altered atmospheric CO ₂ concentration on carbon partitioning to isoprene biosynthesis. We studied changes in the proportion of labeled carbon as a function of time in two mass fragments, M41 ⁺, which represents, in part, substrate derived from pyruvate, and M69 ⁺, which represents the whole unlabeled isoprene molecule. We observed a trend of slower ¹³C incorporation into isoprene carbon derived from pyruvate, consistent with the previously hypothesized origin of chloroplastic pyruvate from cytosolic phosphenolpyruvate (PEP). Trees grown under sub-ambient CO ₂ (190 ppmv) had rates of isoprene emission and rates of labeling of M41 ⁺ and M69 ⁺ that were nearly twice those observed in trees grown under elevated CO ₂ (590 ppmv). However, they also demonstrated the lowest proportion of completely labeled isoprene molecules. These results suggest that under reduced atmospheric CO ₂ availability, more carbon from stored/older carbon sources is involved in isoprene biosynthesis, and this carbon most likely enters the isoprene biosynthesis pathway through the pyruvate substrate. We offer direct evidence that extra-chloroplastic rather than chloroplastic carbon sources are mobilized to increase the availability of pyruvate required to up-regulate the isoprene biosynthesis pathway when trees are grown under sub-ambient CO ₂.