Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado, United States of America.
PLoS One. 2012;7(2):e32387. doi: 10.1371/journal.pone.0032387. Epub 2012 Feb 23.
Biogenically released isoprene plays important roles in both tropospheric photochemistry and plant metabolism. We performed a (13)CO(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 (13)C 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).
生物源异戊二烯在对流层光化学和植物代谢中都起着重要作用。我们使用质子转移反应质谱（PTR-MS）进行了（13）CO2 标记研究，以检查在不同大气 CO2 浓度下生长和测量的白杨（Populus × canescens）树中最近同化的光合作用产物转化为异戊二烯的动力学。这是第一项明确考虑大气 CO2 浓度变化对异戊二烯生物合成碳分配影响的研究。我们研究了两个质量片段中标记碳的比例随时间的变化，M41(+) 部分代表来自丙酮酸的底物，M69(+) 代表整个未标记的异戊二烯分子。我们观察到（13）C 掺入来自丙酮酸的异戊二烯碳的速度较慢，这与先前假设的叶绿体丙酮酸来源于细胞质磷酸烯醇丙酮酸（PEP）的起源一致。在亚大气 CO2（190 ppmv）下生长的树木的异戊二烯排放速率和 M41(+) 和 M69(+) 的标记速率几乎是在高 CO2（590 ppmv）下生长的树木的两倍。然而，它们也表现出完全标记的异戊二烯分子的比例最低。这些结果表明，在大气 CO2 供应减少的情况下，更多来自储存/旧碳源的碳参与异戊二烯生物合成，并且这种碳最有可能通过丙酮酸底物进入异戊二烯生物合成途径。我们提供了直接证据，表明在树木生长在亚大气 CO2 下时，需要增加异戊二烯生物合成途径所需的丙酮酸的可用性，从而动员质外体而不是叶绿体碳源来增加丙酮酸的可用性。
