Fig. 1: Conceptual diagram of the three experimental treatments.

In the Permeable treatment, a permeable barrier separates donor and receiver plants; the dark septate endophyte (DSE) is added to the donor chamber and can cross the barrier to colonize the receiver plant. In the Impermeable treatment, an impermeable barrier separates the chambers; the DSE is added to the donor side but cannot reach the receiver plant. In the Axenic treatment (no culturable microbes), a permeable barrier is used, but the DSE is not introduced, so neither plant associates with a DSE. See Supplementary Fig. 1 for barrier design details. Box dimensions: 24.5 × 9.7 × 8.2 cm (length × width × height).