Fig. 1. Experimental system (a microcosm) employed in this study.

The system was prepared in a large Petri dish (not shown here). Inside that dish, a small rhizobox (A), made of a small Petri dish and delimited from the rest of the system with a 42 µm nylon mesh, contained the Ri T-DNA transformed Cichorium intybus roots, either non-mycorrhizal or mycorrhizal. Mycorrhizal fungal hyphae growing out of the mycorrhizal roots through the mesh colonised the MSR medium [35] filling the large dish volume (B) and eventually reached the labelling compartment (C). The labelling compartment was made of another small Petri dish and was filled with a modified (nitrogen-free) MSR medium, with (Experiment 1) or without (Experiment 2) sucrose and with or without an added ¹⁵N-labelled nitrogen source (either mineral or organic) and various bacteria combined or not combined with a protist grazer (more details in supplementary Fig. S4).