Fungal-Induced Formation of Auxin Maxima in Arabidopsis thaliana Roots

Phytohormones are crucial molecules for plant development and the interaction with microbes. This study focused on the phytohormone auxin (indole-3-acetic acid, IAA) and its role in the interaction of Arabidopsis thaliana (L.) Heynh. roots with two beneficial (Piriformospora indica and Mortierella hyalina) and two pathogenic (Alternaria brassicicola and Verticillium dahliae) root-colonizing fungi. Arabidopsis plants expressing the dual reporter construct DR5::EGFP-DR5v2::tdTomato allow visualization of auxin maxima during early stages of the plant–fungus interaction. Fluorescence microscopy was used to monitor changes in auxin levels and distribution patterns. We hereby demonstrate that only the beneficial P. indica activates the IAA reporter system. M. hyalina- but not P. indica-colonized roots accumulate jasmonates which might prevent the activation of the IAA reporter system. Additionally, both the necrotrophic fungus A. brassicicola and the biotrophic fungus V. dahliae completely inhibit the fluorescence emission from the IAA reporter system within 3–6 h. The results indicate that the reporter system responds to IAA accumulation in symbiotic roots, but the activation process might be controlled by a crosstalk with other phytohormones, such as jasmonates.