Promotion of Arabidopsis Immune Responses by a Rhizosphere Fungus Via Supply of Pipecolic Acid to Plants and Selective Augment of Phytoalexins

The ascomycete insect pathogenic fungi such as Metarhizium species have been demonstrated with the abilities to form the rhizosphere or endophytic relationships with different plants for nutrient exchanges. In this study, after the evident infeasibility of bacterial disease development in the boxed sterile soils, we established a hydroponic system for the gnotobiotic growth of Arabidopsis thaliana with the wild-type and transgenic strain of Metarhizium robertsii. The transgenic fungus could produce a high amount of pipecolic acid (PIP), a pivotal plant-immune-stimulating metabolite. Fungal inoculation experiments showed that M. robertsii could form a non-selective rhizosphere relationship with Arabidopsis. Similar to the PIP uptake by plants after exogenous application, PIP level increased in Col-0 and could be detected in the PIP-non-producing Arabidopsis mutant (ald1) after fungal inoculations, indicating that plants can absorb the PIP produced by fungi. The transgenic fungal strain had a better efficacy than the wild type to defend plants against the bacterial pathogen and aphid attacks. Contrary to ald1, fmo1 plants could not be boosted to resist bacterial infection after treatments. After fungal inoculations, the phytoalexins camalexin and aliphatic glucosinolate were selectively increased in Arabidopsis via both PIP-dependent and -independent ways. This study unveils the potential mechanism of the fungus-mediated beneficial promotion of plant immunity against biological stresses. The data also highlight the added values of M. robertsii to plants beyond the direct suppression of insect pest populations.