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图1. 陆地生态系统中植物-微生物互惠共生的演化。a. 在登陆后不久, 陆地植物与内生菌根真菌(AMF)的祖先形成了共生关系(Strullu-Derrien et al, 2014)。图中展示了单个根尖(左, 浅绿色的表皮根细胞和浅棕色的皮层细胞)和AMF菌丝是如何形成细胞内结构的。b. 早期森林出现于木本维管植物占优势的早泥盆纪(Willis & McElwain, 2014)。c. 陆地植物与外生菌根真菌(EMF) (Cairney, 2000; Martin et al, 2016)建立稳定共生关系。EMF是能够分解木质素的腐生真菌的后代, 帮助植物获取有机质中被锁住的养分。d. 受环境条件变化的选择, 早白垩纪出现了有花植物, 是植物适应性创新的一个里程碑(Willis & McElwain, 2014)。e. 植物创新的另一个里程碑事件发生在地下,白垩纪-古近纪界线不久之后(Werner et al, 2015), 植物与固氮菌形成了互惠共生关系。这些细菌(居住在橙色的细胞中)可以打开氮气的三键, 为植物供应可利用的氮。共生关系a、c、e用红色箭头指向地质时间, 地质史关键事件b、d则使用蓝色箭头。该图修改自Lu和Hedin (2019)的图1。a、c、e图作者为孙漪南, b图来自布朗大学的Andrew Lesile, d图来自作者本人。
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Fig. 1. Evolution of plant-microbe mutualism in terrestrial ecosystems. a. Land plants formed associations with early ancestors of arbuscular mycorrhizal fungi (AMF) soon after the plant’s colonization of terrestrial ecosystems (Strullu-Derrien et al, 2014). The schematic illustrates how an individual root tip (left, showing epidermal root cells in light green and cortical cells in light brown) and AMF mycelial forms intracellular structure. AMF hyphae is magnitude thinner than even the thinnest plant roots, allowing them superb ability to access soil resources from the porous soil matrix. b. The early forest emerged in the early Devonian after woody vascular plants gained dominance (Willis & McElwain, 2014). c. Land plants formed associations with ectomycorrhizal fungi (EMF) (Cairney, 2000), the descendant of wood-decaying fungi, aiding plants in accessing nutrients that otherwise would be locked into organic matter. d. Selected by the changing environmental condition, flowering plants emerged during the early Cretaceous as a milestone for plants’ adaptive innovation (Willis & McElwain, 2014). e. Another milestone for plant innovation happened belowground, shortly after the Cretaceous-Paleogene boundary Werner et al (2015), with plants forming mutualistic associations with nitrogen-fixing bacteria. These bacteria (housed in these orange cells) can breakdown the triple bond of N2 gas and supply plants with plant available forms of nitrogen. The geological timings of mutualistic relationships a, c, e are indicated by red arrows, while that of geological events b, d by blue arrows. This figure is modified based on Figure 1 in Lu & Hedin (2019). Illustration in a, c, e, from Yinan Sun, b from Andrew Lesile of Brown University, and d from the author.
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