Crassulacean acid metabolism. A plastic photosynthetic adaptation to arid environments
Crassulacean acid metabolism. A plastic photosynthetic adaptation to arid environments
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DOI：
10.1104/pp.010818
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发表时间：
2001-12-01
期刊：
影响因子：
7.4
通讯作者：
Cushman, JC
中科院分区：
文献类型：
--
作者：
Cushman, JC
景天科酸代谢（CAM）是光合作用碳固定的重要阐述，其允许含有叶绿体的细胞最初在夜间使用胞质溶胶中的磷酸烯醇式丙酮酸羧化酶（PEPC）固定CO2。这导致形成C4有机酸（通常是苹果酸），储存在液泡中。随后白天这些有机酸在关闭的气孔后面脱羧产生内部CO2源，其被叶绿体中的Rubisco再同化。这种内部CO2的再固定通过传统的光合碳还原循环产生碳水化合物。因此，CAM涉及的碳固定模式在时间上的分离，而在C4植物中发现的空间分离。对夜间酸化过程的第一次认识可以追溯到罗马人，他们注意到某些多肉植物在早上比晚上尝起来更苦（罗利，1978）。然而，直到世纪早期，才正式描述了肉质植物进行夜间CO2固定或在夜间酸化光合组织并在白天还原它们的能力（de Saussure，1804; Heyne，1815）。CAM一词是为了感谢Heyne对景天科多汁植物Bryophyllum calycinum的观察而创造的。由于这些早期的描述，已经实现了CAM循环的生物化学反应序列的详细描述（兰森和托马斯，1960）、不同CAM物种之间的途径中的生物化学变化的复杂性以及环境对其的调节（Osmond，1978; Ting，1985）。最初的夜间CO2固定PEPC发生时，气孔是开放的，蒸腾水分损失低。白天的CO2释放促进气孔关闭，并将CO2集中在Rubisco周围，抑制其加氧酶活性，从而使光呼吸最小化。这种CO2浓缩策略的净效应是CAM植物在可比条件下表现出比C3和C4植物高几倍的水分利用效率（WUE）率（Drennan和Nobel，2000）。因此，CAM通常（但不完全）与栖息在极端干旱环境（如沙漠）、季节性供水的半干旱地区（如地中海气候）或间歇性供水的栖息地（如热带附生栖息地）的植物有关。其中最值得注意的是商业上或园艺上重要的植物，如菠萝（Ananas comosus）、龙舌兰（Agave subsp.）仙人掌（仙人掌科）和兰花（兰科）。CAM还与各种解剖学或形态学特征相关，这些解剖学或形态学特征使水分损失最小化，包括厚的表皮、低的表面积与体积比、具有增强的水储存能力（即多汁性）的大细胞和液泡以及减少的气孔大小和/或频率。在过去的20年里，深入的生理生态研究表明，CAM存在于约7%的维管植物物种中，比C4物种的百分比大得多（Winter和Smith，1996 a）。CAM在33个分类学上不同的科中的广泛分布（Smith和Winter，1996）表明CAM很可能在不同科中甚至在单个科中多次独立进化（Griffiths，1989; Ehleringer和蒙森，1993; Pilon-Smits等，1996年）。最近使用PEPC序列信息的系统发育重建提供了更令人信服的支持。
Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO2 initially at night using phosphoenolpyruvate carboxylase (PEPC) in the cytosol. This leads to the formation of C4 organic acids (usually malate), which are stored in the vacuole. Subsequent daytime decarboxylation of these organic acids behind closed stomata creates an internal CO2 source that is reassimilated by Rubisco in the chloroplast. The refixation of this internal CO2 generates carbohydrates via the conventional photosynthetic carbon reduction cycle. Thus, CAM involves a temporal separation of carbon fixation modes in contrast to the spatial separation found in C4 plants. The first recognition of the nocturnal acidification process can be traced to the Romans, who noted that certain succulent plants taste more bitter in the morning than in the evening (Rowley, 1978). However, formal descriptions of the ability of succulent plants to conduct nocturnal CO2 fixation or to acidify photosynthetic tissues at night and deacidify them during the day did not appear until the early 19th century (de Saussure, 1804; Heyne, 1815). The term CAM was coined to give credit to Heyne’s observations that were made using Bryophyllum calycinum, a succulent member of the Crassulaceae. Since these early descriptions, a detailed account of the sequence of biochemical reactions of the CAM cycle (Ranson and Thomas, 1960), the complexity of the biochemical variations in the pathway among different CAM species, and its regulation by the environment have been achieved (Osmond, 1978; Ting, 1985). Initial nocturnal CO2 fixation by PEPC occurs when stomata are open and transpirational water losses are low. CO2 release during the day promotes stomatal closure and concentrates CO2 around Rubisco, suppressing its oxygenase activity, thereby minimizing photorespiration. The net effect of this CO2-concentrating strategy is that CAM plants exhibit water use efficiency (WUE) rates severalfold higher than C3 and C4 plants under comparable conditions (Drennan and Nobel, 2000). Thus, CAM is typically, although not exclusively, associated with plants that inhabit extremely arid environments (eg deserts), semi-arid regions with seasonal water availability (eg Mediterranean climates), or habitats with intermittent water supply (eg tropical epiphytic habitats). Most notable among these are commercially or horticulturally important plants such as pineapple (Ananas comosus), agave (Agave subsp.), cacti (Cactaceae), and orchids (Orchidaceae). CAM is also correlated with various anatomical or morphological features that minimize water loss, including thick cuticles, low surface-to-volume ratios, large cells and vacuoles with enhanced water storage capacity (ie succulence), and reduced stomatal size and/or frequency.The selective advantage of high WUE likely accounts for the extensive diversification and speciation among CAM plants principally in water-limited environments. Intensive ecophysiological studies over the last 20 years have documented that CAM is present in approximately 7% of vascular plant species, a much larger percentage than the percentage of C4 species (Winter and Smith, 1996a). The widespread distribution of CAM among 33 taxonomically diverse families (Smith and Winter, 1996) suggests that CAM most likely evolved independently on numerous occasions in different families and even within individual families (Griffiths, 1989; Ehleringer and Monson, 1993; Pilon-Smits et al., 1996). More recent phylogenetic reconstructions using PEPC sequence information have provided more convincing support …