The TOR signaling pathway regulates vegetative development, aflatoxin biosynthesis, and pathogenicity in Aspergillus flavus

The target of rapamycin (TOR) signaling pathway is highly conserved and plays a crucial role in diverse biological processes in eukaryotes. However, the underlying mechanism of the TOR pathway in A. flavus remains elusive. In this study, we identified and characterized seven genes encoding various components of the TOR pathway in A. flavus, and investigated their biological function. The FK506-binding protein Fkbp3 and its lysine succinylation are important for aflatoxin production and rapamycin resistance. As a significant downstream effector molecule of the TorA kinase, the Sch9 kinase regulates aflatoxin B 1 (AFB 1) synthesis, osmotic and calcium stress response in A. flavus, and this regulation is mediated through its STKc, STKX domains, and the ATP binding site at K340. We also showed that the Sch9 kinase may have a regulatory impact on the high-osmolarity glycerol (HOG) signaling pathway. TipA, the other downstream component of the TorA kinase, plays a significant role in regulating sclerotia formation and cell wall stress response in A. flavus. The members of the TapA-phosphatase complexes, SitA and Ppg1, are crucial for various biological processes in A. flavus, including vegetative growth, sclerotia formation, AFB 1 biosynthesis, and pathogenicity. Furthermore, we showed that SitA and Ppg1 are involved in regulating lipid droplets (LDs) biogenesis and cell wall integrity (CWI) signaling pathways. In addition, another phosphatase complex, Nem1/Spo7, plays critical roles in hyphal development, conidiation, aflatoxin, and lipid droplets biogenesis. This study provides an important insight into the regulatory network of the TOR signaling pathway and the molecular mechanism of aflatoxin biosynthesis in A. flavus.