Department of Plant Biology, Michigan State University, Michigan 48824, USA.
Department of Energy-Plant Research Laboratory, Michigan State University, Michigan 48824, USA.
Plant Physiol. 2021 May 27;186(1):297-314. doi: 10.1093/plphys/kiab076.
Respiration in the light (RL) releases CO2 in photosynthesizing leaves and is a phenomenon that occurs independently from photorespiration. Since RL lowers net carbon fixation, understanding RL could help improve plant carbon-use efficiency and models of crop photosynthesis. Although RL was identified more than 75 years ago, its biochemical mechanisms remain unclear. To identify reactions contributing to RL, we mapped metabolic fluxes in photosynthesizing source leaves of the oilseed crop and model plant camelina (Camelina sativa). We performed a flux analysis using isotopic labeling patterns of central metabolites during 13CO2 labeling time course, gas exchange, and carbohydrate production rate experiments. To quantify the contributions of multiple potential CO2 sources with statistical and biological confidence, we increased the number of metabolites measured and reduced biological and technical heterogeneity by using single mature source leaves and quickly quenching metabolism by directly injecting liquid N2; we then compared the goodness-of-fit between these data and data from models with alternative metabolic network structures and constraints. Our analysis predicted that RL releases 5.2 μmol CO2 g-1 FW h-1 of CO2, which is relatively consistent with a value of 9.3 μmol CO2 g-1 FW h-1 measured by CO2 gas exchange. The results indicated that ≤10% of RL results from TCA cycle reactions, which are widely considered to dominate RL. Further analysis of the results indicated that oxidation of glucose-6-phosphate to pentose phosphate via 6-phosphogluconate (the G6P/OPP shunt) can account for >93% of CO2 released by RL.
在光照下呼吸（RL）会释放光合作用叶片中的 CO2，这是一种独立于光呼吸发生的现象。由于 RL 会降低净碳固定，因此了解 RL 可以帮助提高植物的碳利用效率和作物光合作用模型。尽管 RL 早在 75 年前就被发现，但它的生化机制仍不清楚。为了确定导致 RL 的反应，我们对油料作物和模式植物荠蓝（Camelina sativa）的光合作用源叶进行了代谢通量作图。我们使用中心代谢物的同位素标记模式在 13CO2 标记时间过程、气体交换和碳水化合物产生速率实验中进行通量分析。为了用统计和生物学置信度量化多个潜在 CO2 来源的贡献，我们增加了测量的代谢物数量，并通过使用单个成熟源叶和直接注入液态 N2 快速猝灭代谢来减少生物学和技术异质性；然后，我们比较了这些数据与具有替代代谢网络结构和约束的模型数据之间的拟合优度。我们的分析预测 RL 会释放 5.2 μmol CO2 g-1 FW h-1 的 CO2，这与通过 CO2 气体交换测量的 9.3 μmol CO2 g-1 FW h-1 值相对一致。结果表明，RL 来自三羧酸循环反应的比例≤10%，而三羧酸循环反应被广泛认为主导 RL。对结果的进一步分析表明，通过 6-磷酸葡萄糖（G6P/OPP 支路）氧化葡萄糖-6-磷酸到戊糖磷酸可以解释 RL 释放的 CO2 的>93%。
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