On-line analysis of the ¹³CO₂ labeling of leaf isoprene suggests multiple subcellular origins of isoprene precursors

Abstract.

Isoprene (2-methyl-1,3-butadiene) is the most abundant biogenic hydrocarbon released from vegetation, and there is continuing interest in understanding its biosynthesis from photosynthetic precursors in leaf chloroplasts. We used on-line proton-transfer-reaction mass spectrometry (PTR–MS) to observe the kinetics of ¹³C-labeling of isoprene following exposure to ¹³CO₂ and then the loss of ¹³C after a return to normal ¹²CO₂ in oak (Quercus agrifolia Nee) and cottonwood (Populus deltoides Barr.) leaves. Assignments of labeled isoprene species were verified by gas chromatography–mass spectrometry. For the first time, it was possible to observe the half-lives of individually ¹³C-labeled isoprene species during these transitions, and to trace some of the label to a C3 fragment that contained the two isoprene carbons derived from pyruvate via the deoxyxylulose-5-phosphate (DOXP) pathway. At steady state (under ¹³CO₂), approximately 80% of isoprene carbon was labeled, with fully labeled isoprene as the major species (approx. 60%). The source of the unlabeled C is suggested to be extrachloroplastic, but not from photorespiratory carbon. After a transfer to ¹²CO₂, ¹³C-labeling persisted in one isoprene carbon for several hours; this persistence was much more pronounced in (i) leaves inhibited by fosmidomycin, a specific inhibitor of the DOXP pathway, and (ii) in sun leaves which have higher ratios of soluble sugars to starch. From the mass 41–44 fragment data, and labeling predicted from the DOXP pathway in chloroplasts, precursors may arise from cytosolic pyruvate/phosphoenolpyruvate equivalents transported into the chloroplast; this idea was supported by an indirect measure of pyruvate labeling. Other sources of cytosolic isoprene precursors (i.e. dimethylallyl diphosphate or pentose phosphate) could not be excluded. The data obtained shed light on the half-lives of photosynthetic metabolites, exchanges of carbon between cellular pools, and suggest multiple origins of isoprene precursors in leaves.