Department of Biochemistry and Microbiology, Rutgers University, 08903, New Brunswick, NJ, USA.
Planta. 1979 Jan;145(1):13-23. doi: 10.1007/BF00379923.
The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO2 in leaves of C4 plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C3 and C4 Panicum species. CO2 evolution by leaf slices of P. bisulcatum, a C3 species, fed glycolate or glycine was light-independent and O2-sensitive. The C4 P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO2 in the dark but not in the light. In C4 species, dark CO2 evolution was abolished by the addition of phosphoenolpyruvate (PEP)(4). The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO2 efflux by C4 leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O2 uptake by the C4 leaf slices or on glycolate and glycine metabolism in C3 leaf slices. The rate of photorespiratory CO2 evolution in the C3 Panicum species was 3 times higher than that observed with the C4 species. The ratio of glycolate-dependent CO2 evolution to O2 uptake in both groups was 1:2. Isolated C4 mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C3 mesophyll protoplasts and C4 bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O2-sensitive manner, and the addition of PEP or maleate had no effect. C4 bundle sheath- and C3-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C4 bundle sheath- and C3-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C4 mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C4 plants bundle sheath cells are the primary site of O2-sensitive photorespiratory CO2 evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO2 before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO2 pump and reassimilation of photorespiratory CO2 are discussed in relation to the apparent lack of photorespiration in leaves of C4 species.
通过对 C3 和 C4 潘克氏属植物的薄片、不同叶细胞类型和分离的线粒体进行平行抑制剂实验，研究了 C4 植物叶片中甘醇酸盐和甘氨酸代谢的潜力以及光呼吸 CO2 的再固定机制。C3 种双穗雀稗的叶片薄片在光下呼吸时消耗 CO2，且对 O2 敏感，当添加甘醇酸盐或甘氨酸时，CO2 的释放与光无关。当用甘醇酸盐或甘氨酸喂养 C4 的 P. maximum 和 P. miliaceum 叶片薄片时，叶片在黑暗中会释放 CO2，但在光照下不会。在 C4 物种中，添加磷酸烯醇丙酮酸（PEP）（4）可使暗呼吸 CO2 逸出。光照下，马来酸的添加也导致 C4 叶片薄片的光呼吸 CO2 流出。然而，PEP 和马来酸对 C4 叶片薄片依赖甘醇酸盐的 O2 摄取或 C3 叶片薄片的甘醇酸盐和甘氨酸代谢均没有影响。C3 潘克氏属植物的光呼吸 CO2 释放速率比 C4 物种高 3 倍。两组的依赖甘醇酸盐的 CO2 释放与 O2 摄取的比值均为 1：2。分离的 C4 叶肉原生质体或其线粒体不能代谢甘醇酸盐或甘氨酸。然而，C3 叶肉原生质体和 C4 束鞘链都能以非光依赖、O2 敏感的方式代谢甘醇酸盐和甘氨酸，并且添加 PEP 或马来酸没有影响。C4 束鞘和 C3 线粒体都能够氧化甘氨酸。这种氧化与线粒体电子传递链相连，与三个磷酸化位点偶联，并对电子传递抑制剂敏感。C4 束鞘和 C3 线粒体甘氨酸脱羧作用被草酰乙酸刺激，NAD 没有影响。相比之下，从 C4 叶肉细胞中分离的线粒体不能氧化或脱羧添加的甘氨酸。结果表明，在 C4 植物叶片中，束鞘细胞是 O2 敏感的光呼吸 CO2 释放的主要部位，而存在于叶肉细胞中的 PEP 羧化酶在 CO2 逸出叶片之前具有有效固定 CO2 的潜力。讨论了 PEP 羧化酶介导的 CO2 泵和光呼吸 CO2 再同化的相对作用，与 C4 物种叶片中明显缺乏光呼吸有关。
