Hyphal exploration strategies and habitat modification of an arbuscular mycorrhizal fungus in microengineered soil chips

Abstract

Arbuscular mycorrhizal fungi (AMF) are considered ecosystem engineers, however, the exact mechanisms by which they modify and influence their immediate surroundings are largely unknown and difficult to study in soil. In this study, we used microfluidic chips, simulating artificial soil structures, to study foraging strategies and habitat modification of Rhizophagus irregularis in symbiotic state associated to carrot roots. Our results suggest that AMF hyphae forage over long distances in void spaces, prefer straight over tortuous passages, anastomose and show strong inducement of branching when encountering obstacles. We observed bi-directional vesicle transport inside active hyphae and documented strategic allocation of biomass within the mycelium e.g., truncated hyphal growth and cytoplasm retraction from inefficient paths. We found R. irregularis able to modify pore-spaces in the chips by producing irregularly shaped spores that filled up pores. We suggest that studying AMF hyphal behaviour in spatial settings can explain phenomena reported at bulk scale such as AMF modification of water retention in soils. The use of microfluidic soil chips in AMF research opens up novel opportunities to under very controlled conditions study ecophysiology and interactions of the mycelium with both biotic and abiotic factors.