Loreto Francesco, Centritto Mauro, Barta Csengele, Calfapietra Carlo, Fares Silvano, Monson Russell K
Consiglio Nazionale delle Ricerche (CNR), Istituto di Biologia Agroambientale e Forestale, Rome, Italy.
Plant Cell Environ. 2007 May;30(5):662-9. doi: 10.1111/j.1365-3040.2007.01648.x.
In past studies, it was hypothesized that reductions in chloroplast isoprene emissions at high atmospheric CO(2) concentrations were caused by competition between cytosolic and mitochondrial processes for the same substrate, possibly phosphoenolpyruvate (PEP). We conducted field and laboratory experiments using leaves of white poplar (Populus alba L.) to identify whether an inverse relationship occurs between the dark respiration rate (a mitochondrial process) and the isoprene emission rate. Field experiments that were carried out in a free-air CO(2)-enriched (FACE) facility showed no clear effect of elevated CO(2) on either isoprene emission rate or respiration rate by leaves. In young, not yet fully expanded leaves, low isoprene emission and high dark respiration rates were measured in both ambient and elevated CO(2). In these leaves, isoprene emission was inversely correlated with dark respiration. It is possible to interpret from these results that, in young leaves, high rates of growth respiration compete with isoprene biosynthesis for the same substrate. However, it is also possible that the negative correlation reflects the contrasting reductions in growth respiration and increases in expression of the enzyme isoprene synthase at this final stage of leaf maturation. In contrast to our observations on young leaves, respiration rate and isoprene emission rate were positively correlated in older, fully expanded leaves (8 and 11 from apex). A positive correlation was also found between respiration rate and isoprene emission rate when these parameters were modulated using different ozone exposure, growth light intensity, growth temperature and exposure to different leaf temperatures in laboratory experiments. These data show that competition for substrate between isoprene biosynthesis and leaf respiration does not determine the rate of isoprene emission in most circumstances that affect both processes. A negative correlation was observed across all experiments between isoprene emission rate and the activity of phosphoenolpyruvate carboxylase (PEPc), a cytosolic enzyme that competes with isoprene biosynthesis for substrate. The cytosolic metabolite, PEP, occurs at a metabolic branch point from which substrate flows into three processes: (1) the production of pyruvate for mitochondrial respiration, (2) the production of oxaloacetate (OAA) by PEPc for anabolic support of mitochondrial respiration and (3) transport into the chloroplast to support chloroplastic demands for pyruvate, including isoprenoid biosynthesis. The results of our observations suggest that only the second process competes for substrate with isoprenoid synthesis, while the partitioning of PEP between mitochondrial respiration and chloroplast isoprenoid biosynthesis is controlled in a way that retains balance in substrate demand.
在过去的研究中，曾有假设认为，在高大气二氧化碳浓度下叶绿体异戊二烯排放减少是由于胞质和线粒体过程对同一底物（可能是磷酸烯醇式丙酮酸，PEP）的竞争所致。我们使用白杨（Populus alba L.）叶片进行了田间和实验室实验，以确定暗呼吸速率（一种线粒体过程）与异戊二烯排放速率之间是否存在反比关系。在自由空气二氧化碳富集（FACE）设施中进行的田间实验表明，二氧化碳浓度升高对叶片的异戊二烯排放速率或呼吸速率均无明显影响。在幼嫩、尚未完全展开的叶片中，无论是在环境二氧化碳浓度还是升高的二氧化碳浓度下，都测量到了低异戊二烯排放和高暗呼吸速率。在这些叶片中，异戊二烯排放与暗呼吸呈负相关。从这些结果可以推断，在幼叶中，高生长呼吸速率与异戊二烯生物合成竞争同一底物。然而，也有可能这种负相关反映了在叶片成熟的最后阶段，生长呼吸的对比性降低和异戊二烯合酶表达的增加。与我们对幼叶的观察结果相反，在较老、完全展开的叶片（从叶尖起第8片和第11片）中，呼吸速率与异戊二烯排放速率呈正相关。在实验室实验中，当使用不同的臭氧暴露、生长光强、生长温度以及不同的叶片温度处理来调节这些参数时，呼吸速率与异戊二烯排放速率之间也发现了正相关。这些数据表明，在影响这两个过程的大多数情况下，异戊二烯生物合成与叶片呼吸之间对底物的竞争并不能决定异戊二烯的排放速率。在所有实验中，均观察到异戊二烯排放速率与磷酸烯醇式丙酮酸羧化酶（PEPc，一种与异戊二烯生物合成竞争底物的胞质酶）的活性呈负相关。胞质代谢物PEP出现在一个代谢分支点，底物从该点流入三个过程：（1）产生丙酮酸用于线粒体呼吸，（2）由PEPc产生草酰乙酸（OAA）用于支持线粒体呼吸的合成代谢，（3）转运到叶绿体中以支持叶绿体对丙酮酸（包括类异戊二烯生物合成）的需求。我们的观察结果表明，只有第二个过程与类异戊二烯合成竞争底物，而PEP在线粒体呼吸和叶绿体类异戊二烯生物合成之间的分配是以保持底物需求平衡的方式进行控制的。
