A Network of Phosphate Starvation and Immune-Related Signaling and Metabolic Pathways Controls the Interaction between and the Beneficial Fungus .

The beneficial root-colonizing fungus mediates plant growth promotion (PGP) upon phosphate (Pi) starvation in . This activity is dependent on the Trp metabolism of the host, including indole glucosinolate (IG) hydrolysis. Here, we show that resolves several Pi starvation-induced molecular processes in the host, one of which is the downregulation of auxin signaling in germ-free plants, which is restored in the presence of the fungus. Using CRISPR/Cas9 genome editing, we generated an triple mutant lacking three homologous nitrilases (NIT1 to NIT3) that are thought to link IG-hydrolysis products with auxin biosynthesis. Retained -induced PGP in mutant plants demonstrated that this metabolic connection is dispensable for the beneficial activity of the fungus. This suggests that either there is an alternative metabolic link between IG-hydrolysis products and auxin biosynthesis, or restores auxin signaling independently of IG metabolism. We show that , similar to pathogenic microorganisms, triggers immune pathways that rely on IG metabolism as well as salicylic acid and ethylene signaling. Analysis of IG-deficient mutants revealed that these metabolites are, indeed, important for control of in planta growth: however, enhanced biomass does not necessarily negatively correlate with PGP. We show that Pi deficiency enables more efficient colonization of by , possibly due to the MYC2-mediated repression of ethylene signaling and changes in the constitutive IG composition in roots.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.