The root nodule-arbuscular mycorrhizae-plant pathogen relationship

摘要

Terrestrial vascular plants live simultaneously in two vastly different worlds. The aerial parts are subject to wind, rain, u.v. irradiation and other abiotic climatic features, and microbial insults are primarily restricted to potential pathogens or non-pathogenic epiphytes. In contrast, the subterranean roots are part of an extraordinarily complex biotic system containing a microbial "soup" of saprophytes, parasites and mutualists that can directly or indirectly provide benefit or harm to the plant. Because of the different microbial environments that they inhabit, it seems reasonable to assume that aerial and subterranean plant parts have been subject to different selective pressures with respect to interactions with microbes [8]. Although mutualistic microbial associations with aerial plant parts exist, such as those in grasses involving fungal endophytes, they are relatively rare among plant families, and decades of study have revealed that shoots and leaves are well adapted to initiate rapid and usually effective defence responses against microbial invasion. In contrast, the potential of micro-organisms to provide subterranean structures with mineral nutrients has led to the early evolution and almost ubiquitous occurrence of mutualistic fungal mycorrhizal associations [13, 14]. Theretbre, from their evolutionary inception, roots have had to devise ways of distinguishing between beneficial microbes, particularly fungi, and potential microbial parasites.