Abstract
Auxin (indole-3-acetic acid, IAA) is an important phytohormone involved in root growth and development. Root-interacting beneficial and pathogenic fungi utilize auxin and its target genes to manipulate the performance of their hosts for their own needs. In order to follow and visualize auxin effects in fungi-colonized Arabidopsis roots, we used the dual auxin reporter construct DR5::EGFP-DR5v2::tdTomato and fluorescence microscopy as well as LC-MS-based phytohormone analyses. We demonstrate that the beneficial endophytic fungi Piriformospora indica and Mortierella hyalina produce and accumulate IAA in their mycelia, in contrast to the phytopathogenic biotrophic fungus Verticillium dahliae and the necrotrophic fungus Alternaria brassicicola. Within 3 h after exposure of Arabidopsis roots to the pathogens, the signals of the auxin-responsive reporter genes disappeared. When exposed to P. indica, significantly higher auxin levels and stimulated expression of auxin-responsive reporter genes were detected both in lateral root primordia and the root elongation zone within 1 day. Elevated auxin levels were also present in the M. hyalina/Arabidopsis root interaction, but no downstream effects on auxin-responsive reporter genes were observed. However, the jasmonate level was strongly increased in the colonized roots. We propose that the lack of stimulated root growth upon infection with M. hyalina is not caused by the absence of auxin, but an inhibitory effect mediated by high jasmonate content.
Highlights
We addressed the hypothesis that interactions of Arabidopsis roots with pathogenic and beneficial fungi are accompanied by different activation of auxin-responsive genes during very early phases of contact
To test the expression of the auxin reporters to exogenously applied indole-acetic acid (IAA), the roots of 12 day-old Arabidopsis seedlings were incubated with increasing IAA concentrations
Incubation with the lowest applied concentration of 5 μM IAA resulted in a significant increase in both fluorescence signals which were extended to the rhizodermal cell layers (Figure 2B)
Summary
Auxin plays a central role for root growth and participates in many aspects of root development, including cell elongation, differentiation (Rahman et al, 2007), lateral root, and root hair formation (Masucci and Schiefelbein, 1994, 1996; Pitts et al, 1998; Reed et al, 1998; Casimiro et al, 2001; Bhalerao et al, 2002), and gravitropic responses (Rashotte et al, 2000; Sukumar et al, 2009). Auxin responses are coupled to antagonistic and synergistic interactions with salicylic acid (SA)- and jasmonic acid (JA)-mediated defenses, respectively (Hoffmann et al, 2011; Naseem et al, 2015; Huang et al, 2017). Jasmonates participate in the regulation of primary root growth and reproductive development, thereby interacting with auxins. Crosstalk can occur at multiple levels, including hormone perception, since indole-acetic acid (IAA) and JA-isoleucine (JAIle) are perceived by SCF E3-ligases through the interaction of IAA- and JA-related regulators of gene expression and the modulation of each other’s homeostasis (Hoffmann et al, 2011; Naseem et al, 2015). In the presence of IAA, the SKP1-CULLIN1-F-BOX TIR1 ubiquitin ligase complex binds to AUXIN/IAAs and triggers their degradation (Calderon-Villalobos et al, 2010)
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