The beneficial root-colonizing fungus Mortierella hyalina promotes the aerial growth of Arabidopsis and activates calcium-dependent responses that restrict Alternaria brassicae-induced disease development in roots

The endophytic fungus Mortierella hyalina colonizes the roots of Arabidopsis thaliana and stimulates growth and biomass production of the aerial parts but not of roots. An exudate fraction from the fungus induces rapid and transient cytoplasmic Ca²⁺elevation in the roots. The Ca²⁺ response does not require the well-characterized (co)receptors BAK1, CERK1, and FLS2 for pathogen-associated molecular patterns, and the Ca²⁺ channels GLR-2.4, GLR-2.5, and GLR-3.3 or the vacuolar TWO PORE CHANNEL1, which might be involved in cytoplasmic Ca²⁺ elevation. We isolated an ethyl-methane-sulfonate–induced Arabidopsis mutant that is impaired in this Ca²⁺ response. The roots of the mutant are impaired in M. hyalina–mediated suppression of immune responses after Alternaria brassicae infection, i.e., jasmonate accumulation, generation of reactive oxygen species, as well as the activation of jasmonate-related defense genes. Furthermore, they are more colonized by M. hyalina than wild-type roots. We propose that the mutant gene product is involved in a Ca²⁺-dependent signaling pathway activated by M. hyalina to suppress immune responses in Arabidopsis roots.