How useful is the mutualism-parasitism continuum of arbuscular mycorrhizal functioning?
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A recent review in this journal puts forward the premise that our recent studies have resulted in our questioning the validity of the so-called mutualism-parasitism continuum of functioning of arbuscular mycorrhizas. This premise is incorrect and appears largely to result from a misunderstanding of terminology.
Scope and conclusions
We clarify a comment in one of our publications that influenced the previous review, which contains several statements that do not accurately represent our views. Our research has overturned not the continuum concept itself, but some past ideas about the balance of resources traded between AM fungi and plants. Of course, we recognize that outcomes of AM symbiosis in relation to the non-mycorrhizal (NM) state are strongly influenced by many environmental factors. Nevertheless, underlying resource trade is always a key determinant of costs and benefits of the symbiosis for both partners. In this context, we address uncertainties and contradictory ideas about mechanisms, causes, effects and outcomes in AM symbioses that occur in the literature, and issues of relevance of research at different scales. We also discuss semantics that can cause confusion. Finally, we assess how useful the mutualism-parasitism continuum is for design of hypothesis-driven experiments to disentangle the complex interactions that determine growth of AM plants, i.e. the so-called emergent properties.
KeywordsArbuscular mycorrhizas Benefits Costs Mycorrhizal growth response Resource trade Mutualism Parasitism
Mycorrhizal growth response
We thank the editors of Plant and Soil for inviting us to produce this contribution, clarify the aims of our research, and explore further some important conceptual issues raised by Johnson and Graham (2012). We are grateful to members of the Adelaide group whose studies have contributed to ideas in “Smiths” papers, to our colleague Iver Jakobsen, to our other co-authors, and not least to other colleagues who are interested in the physiology of AM symbiosis and the extent to which it can be validly extrapolated to higher scales of research.
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