The Alternaria alternata Mip1/RAPTOR Mediates Virulence by Regulating Toxin Production and Autophagy.

Abstract

The necrotrophic pathogen Alternaria alternata produces a host-selective toxin to attack its host plants. This study characterized the crucial function of the Mip1/RAPTOR ortholog (AaMip1) in toxin production and autophagy formation. AaMip1 physically interacts with the Target of Rapamycin (Tor) protein. In response to nitrogen starvation and H₂O₂, AaMip1 binds to Tor and triggers autophagy and oxidative stress detoxification. Deleting the AaMip1 gene resulted in a ΔAaMip1 strain that increased sensitivity to various oxidants, decreased the expression of two oxidative-stress-response genes, AaYap1 and AaNoxA, and had lower catalase activity than the wild type. ΔAaMip1 produced lower levels of ACT toxin than the wild type after a 7-day incubation; however, ΔAaMip1 produced tricycloalternarene mycotoxins but not ACT after 21 days. The reduction of ΔAaMip1 virulence in the host plant is due to low ACT production, defective H₂O₂ detoxification, impaired autophagy, and slow growth during invasion. However, AaMip1 plays a negative role in maintaining cell wall integrity and lipid body accumulation. ΔAaMip1 had thicker cell walls and emitted brighter red fluorescence after staining with the cell-wall disrupting agents Congo red and calcofluor white. ΔAaMip1 was more resistant to these compounds than the wild type under nutrient-rich conditions. The observed defects in the ΔAaMip1 were restored in the complementation (CP) strain after re-expressing a functional copy of AaMip1. This study increases our understanding of how A. alternata deals with toxic ROS, triggers autophagy formation, maintains normal cell wall integrity, and regulates toxin metabolism via the AaMip1-mediated signaling pathways.