Mycorrhizal-induced growth depression in plants

As plant mutualists, one would not expect arbuscular mycorrhizal fungi (AMF) to cause growth depression of their host plants. The mechanism responsible for negative effects of AMF is still debated and so here we review the possible abiotic and biotic reasons for AMF-induced growth depression in plants: 1) The Phytocentric explanations, include: a) AMF and non-mycotrophic plants, b) different growth stages of plants. 2) The Mycocentric explanations, include: a) Low effective AMF species, b) The existence of vesicles, c) Genetic variability of AMF, and d) Geographic origin of AMF. 3) Unbalanced C-for-nutrient-trade, involving both partners and 4) Indirect effects of other organisms. We note deficiencies in previous studies and suggest improvements in experimental designs such as the use of realistic mixtures of AM fungal species, and growing plants in mixtures in field situations, rather than single pot studies, with and without fungi. Determining whether and how AM fungi cheat on their hosts will enable a better understanding of their roles in natural communities and their use as biofertilizers in agriculture.