KU Leuven, Department of Biosystems, Division of Crop Biotechnics, Research group for Sustainable Crop Production & Protection, Campus Geel, Kleinhoefstraat, Geel, Belgium.
Ghent University, Department of Data Analysis and Mathematical Modelling, Coupure links, Gent, Belgium.
J Exp Bot. 2019 Jun 28;70(12):3269-3281. doi: 10.1093/jxb/erz170.
Crassulacean acid metabolism (CAM) is a major adaptation of photosynthesis that involves temporally separated phases of CO2 fixation and accumulation of organic acids at night, followed by decarboxylation and refixation of CO2 by the classical C3 pathway during the day. Transitory reserves such as soluble sugars or starch are degraded at night to provide the phosphoenolpyruvate (PEP) and energy needed for initial carboxylation by PEP carboxylase. The primary photosynthetic pathways in CAM species are well known, but their integration with other pathways of central C metabolism during different phases of the diel light-dark cycle is poorly understood. Gas exchange was measured in leaves of the CAM orchid Phalaenopsis 'Edessa' and leaves were sampled every 2 h during a complete 12-h light-12-h dark cycle for metabolite analysis. A hierarchical agglomerative clustering approach was employed to explore the diel dynamics and relationships of metabolites in this CAM species, and compare these with those in model C3 species. High levels of 3-phosphoglycerate (3PGA) in the light activated ADP-glucose pyrophosphorylase, thereby enhancing production of ADP-glucose, the substrate for starch synthesis. Trehalose 6-phosphate (T6P), a sugar signalling metabolite, was also correlated with ADP-glucose, 3PGA and PEP, but not sucrose, over the diel cycle. Whether or not this indicates a different function of T6P in CAM plants is discussed. T6P levels were low at night, suggesting that starch degradation is regulated primarily by circadian clock-dependent mechanisms. During the lag in starch degradation at dusk, carbon and energy could be supplied by rapid consumption of a large pool of aconitate that accumulates in the light. Our study showed similarities in the diel dynamics and relationships between many photosynthetic metabolites in CAM and C3 plants, but also revealed some major differences reflecting the specialized metabolic fluxes in CAM plants, especially during light-dark transitions and at night.
景天酸代谢（CAM）是光合作用的一种主要适应机制，涉及到二氧化碳固定和有机酸积累的时间分离阶段，夜间进行，白天通过经典的 C3 途径进行脱羧和再固定。夜间，可溶性糖或淀粉等暂存物会降解，为磷酸烯醇丙酮酸（PEP）的初始羧化提供所需的 PEP 羧化酶和能量。CAM 物种的主要光合作用途径是众所周知的，但它们在日长明暗循环的不同阶段与中央 C 代谢的其他途径的整合还知之甚少。在一个完整的 12 小时光照-12 小时黑暗周期中，对 CAM 兰花蝴蝶兰“Edessa”的叶片进行了气体交换测量，并每隔 2 小时采样一次，用于代谢物分析。采用层次聚类方法探讨了这种 CAM 物种的日动态变化和代谢物之间的关系，并将其与模型 C3 物种进行了比较。在光照下，3-磷酸甘油酸（3PGA）水平升高激活了 ADP-葡萄糖焦磷酸化酶，从而增强了 ADP-葡萄糖的产生，这是淀粉合成的底物。海藻糖 6-磷酸（T6P）是一种糖信号代谢物，也与 ADP-葡萄糖、3PGA 和 PEP 相关，但与蔗糖无关，在日周期中都是如此。是否这表明 T6P 在 CAM 植物中具有不同的功能，还需要进一步讨论。夜间 T6P 水平较低，表明淀粉降解主要受到生物钟依赖机制的调节。在黄昏时淀粉降解的滞后期间，大量在光下积累的顺乌头酸可以快速消耗，为碳和能量提供供应。我们的研究表明，CAM 和 C3 植物中许多光合作用代谢物的日动态变化和关系具有相似性，但也揭示了一些主要差异，反映了 CAM 植物特有的代谢通量，尤其是在光暗转换和夜间。
