Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721.
Mycologia. 2003 May-Jun;95(3):388-98.
Fungal endophytes inhabit healthy tissues of all terrestrial plant taxa studied to date and are diverse and abundant in leaves of tropical woody angiosperms. Studies have demonstrated that plant location and leaf age influence density of endophyte infection in leaves of tropical forest trees. However, ecological factors underlying these observations have not been explored in detail. Here, we establish that foliar endophytes of a tropical tree (Theobroma cacao, Malvaceae) are transmitted horizontally and that endophyte-free seedlings can be produced for experimental manipulation by protecting aerial tissues from surface wetting. At Barro Colorado Island, Panama, we used transects of endophyte-free seedlings to determine the importance of several factors (canopy cover, abundance of aerial and epiphytic propagules, leaf age, leaf chemistry, leaf toughness and duration of exposure to viable air spora) in shaping colonization by endophytic fungi. Endophytes colonized leaves of T. cacao more rapidly beneath the forest canopy than in cleared sites, reflecting local abundance of aerial and epiphytic propagules. The duration of exposure, rather than absolute leaf age, influenced endophyte infection, whereas leaf toughness and chemistry had no observed effect. Endophytes isolated from mature T. cacao grew more rapidly on media containing leaf extracts of T. cacao than on media containing extracts from other co-occurring tree species, suggesting that interspecific differences in leaf chemistry influence endophyte assemblages. Together, these data allow us to identify factors underlying patterns of endophyte colonization within healthy leaves of this tropical tree.
真菌内生菌栖息于所有迄今研究过的陆地植物类群的健康组织中，在热带木本被子植物的叶片中种类繁多且丰富。研究表明，植物的位置和叶片的年龄会影响热带森林树木叶片中内生菌感染的密度。然而，这些观察结果背后的生态因素尚未得到详细探讨。在这里，我们证实了一种热带树木（可可树，锦葵科）叶片内生菌是通过水平传播的，并且可以通过保护气生组织免受表面润湿来产生无内生菌的幼苗，以便进行实验操作。在巴拿马的巴罗科罗拉多岛，我们使用无内生菌幼苗的横断面对几个因素（树冠覆盖、气生和附生繁殖体的丰度、叶片年龄、叶片化学性质、叶片韧性和暴露于有活力的空气孢子的持续时间）在塑造内生真菌定殖方面的重要性进行了确定。内生菌在森林树冠下比在清理过的地方更快地定殖在可可树的叶片上，反映了气生和附生繁殖体的局部丰度。暴露时间而不是绝对的叶片年龄影响内生菌感染，而叶片韧性和化学性质则没有观察到影响。从成熟的可可树上分离出的内生菌在含有可可树叶片提取物的培养基上比在含有其他共存树种叶片提取物的培养基上生长得更快，这表明叶片化学性质的种间差异会影响内生菌组合。这些数据共同使我们能够确定这种热带树木健康叶片内生菌定殖模式的基础因素。
