Department of Biochemistry, University of Nebraska-Lincoln, East Campus, 68583-0718, Lincoln, NE, USA.
Planta. 1989 Aug;179(1):81-8. doi: 10.1007/BF00395774.
The assimilation of (14)CO2 into the C4 acids malate and aspartate by leaves of C3, C4 and C3-C4 intermediate Flaveria species was investigated near the CO2 compensation concentration Γ() in order to determine the potential role of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) in reducing photorespiration in the intermediates. Relative to air concentrations of CO2, the proportion of CO2 fixed by PEP carboxylase at Γ() increased in all six C3-C4 intermediate species examined. However, F. floridana J.R. Johnston and F. ramosissima Klatt were shown to be markedly less responsive to reduced external CO2, with only about a 1.6-fold enhancement of CO2 assimilation by PEP carboxylase, as compared to a 3.0- to 3.7-fold increase for the other C3-C4 species examined, namely, F. linearis Lag., F. anomala B.L. Robinson, F. chloraefolia A. Gray and F. pubescens Rydb. The C3 species F. pringlei Gandoger and F. cronquistii A.M. Powell exhibited a 1.5- and 2.9-fold increase in labeled malate and aspartate, respectively, at Γ(). Assimilation of CO2 by PEP carboxylase in the C4 species F. trinervia (Spreng.) C. Mohr, F. australasica Hook., and the C4-like species F. brownii A.M. Powell was relatively insensitive to subatmospheric levels of CO2. The interspecific variation among the intermediate Flaverias may signify that F. floridana and F. ramosissima possess a more C4-like compartmentation of PEP carboxylase and ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) between the mesophyll and bundle-sheath cells. Chasing recently labeled malate and aspartate with (12)CO2 for 5 min at Γ() resulted in an apparent turnover of 25% and 30% of the radiocarbon in these C4 acids for F. ramosissima and F. floridana, respectively. No substantial turnover was detected for F. linearis, F. anomala, F. chloraefolia or F. pubescens. With the exception of F. floridana and F. ramosissima, it is unlikely that enhanced CO2 fixation by PEP carboxylase at the CO2 compensation concentration is a major mechanism for reducing photorespiration in the intermediate Flaveria species. Moreover, these findings support previous related (14)CO2-labeling studies at air-levels of CO2 which indicated that F. floridana and F. ramosissima were more C4-like intermediate species. This is further substantiated by the demonstration that F. floridana PEP carboxylase, like the enzyme in C4 plants, undergoes a substantial activation (2.2-fold) upon illuminating dark-adapted green leaves. In contrast, light activation was not observed for the enzyme in F. linearis or F. chloraefolia.
我们在接近二氧化碳补偿浓度Γ()的条件下，研究了 C3、C4 和 C3-C4 中间型 Flaviria 属植物叶片将(14)CO2 同化到苹果酸和天冬氨酸中的情况，以便确定磷酸烯醇丙酮酸(PEP)羧化酶(EC 4.1.1.31)在降低中间型光合作用中的光呼吸方面的潜在作用。相对于空气中 CO2 的浓度，在所有六种被检测的 C3-C4 中间型物种中，PEP 羧化酶在 Γ()处固定的 CO2 比例都有所增加。然而，与其他被检测的 C3-C4 物种相比，Flaveria floridana J.R. Johnston 和 F. ramosissima Klatt 对降低的外部 CO2 的反应明显较弱，PEP 羧化酶同化 CO2 的能力仅增强了 1.6 倍，而其他被检测的 C3-C4 物种则增强了 3.0-3.7 倍，即 F. linearis Lag.、F. anomala B.L. Robinson、F. chloraefolia A. Gray 和 F. pubescens Rydb。C3 物种 F. pringlei Gandoger 和 F. cronquistii A.M. Powell 在 Γ()处分别有 1.5 倍和 2.9 倍的标记苹果酸和天冬氨酸的增加。在 C4 物种 F. trinervia (Spreng.) C. Mohr、F. australasica Hook. 和类似 C4 的物种 F. brownii A.M. Powell 中，PEP 羧化酶对亚大气水平的 CO2 的同化作用相对不敏感。中间型 Flaveria 种间的这种种间变异可能表明，F. floridana 和 F. ramosissima 在叶肉细胞和束鞘细胞之间具有更类似于 C4 的 PEP 羧化酶和核酮糖-1,5-二磷酸羧化酶/加氧酶(EC 4.1.1.39)的区室化。在 Γ()处用(12)CO2 对最近标记的苹果酸和天冬氨酸进行 5 分钟的追踪，结果表明，在 F. ramosissima 和 F. floridana 中，这些 C4 酸中约有 25%和 30%的放射性碳发生了明显的周转。在 F. linearis、F. anomala、F. chloraefolia 或 F. pubescens 中未检测到明显的周转。除了 F. floridana 和 F. ramosissima，在二氧化碳补偿浓度下通过 PEP 羧化酶增强 CO2 固定不太可能是降低中间型 Flaviria 属植物光合作用中光呼吸的主要机制。此外，这些发现支持了之前在空气水平 CO2 下进行的相关(14)CO2 标记研究，这些研究表明，F. floridana 和 F. ramosissima 是更类似于 C4 的中间型物种。这进一步证实了 F. floridana PEP 羧化酶，就像 C4 植物中的酶一样，在光照暗适应的绿色叶片时会发生显著的激活(2.2 倍)。相比之下，在 F. linearis 或 F. chloraefolia 中未观察到酶的光激活。
