Distinct and essential roles of bZIP transcription factors in stress response and pathogenesis in Alternaria alternata

The ability to cope with environmental abiotic stress and biotic stress is crucial for the survival of plants and microorganisms, which enable them to occupy multiple niches in the environment. Previous studies have shown that transcription factors play crucial roles in regulating various biological processes including multiple stress tolerance and response in eukaryotes. This work identified multiple critical transcription factor genes, metabolic pathways and gene ontology (GO) terms related to abiotic stress response were broadly activated by analyzing the transcriptome of phytopathogenic fungus Alternaria alternata un- der metal ions stresses, oxidative stress, salt stresses, and host-pathogen interaction. We determined the biological functions and regulatory roles of the bZIP transcriptional factor (TF) genes in the phytopathogenic fungus A. alternata by analyzing targeted gene deletion mutants. Morphological analysis provides evidence that bZIPs including Gcn4, MeaB, Atf1, Hac1 and Ada1 are required for morphogenesis as the colony morphology of these gene deletion mutants was significantly different from that of the wild-type. In addition, bZIPs are involved in the resistance to multiple stresses such as oxidative stress (Ada1, Yap1, MetR) and virulence (Hac1, MetR, Yap1, Ada1) at varying degrees. Transcriptome data demonstrated that the inactivation of bZIPs (Hac1, Atf1, Ada1 and Yap1) significantly affected many genes in multiple critical metabolism pathways and gene ontology (GO) terms. Moreover, the ΔHac1 mutants displayed reduced aerial hypha and are hypersensitivity to endoplasmic reticulum disruptors such as tunicamycin and dithiothreitol. Transcriptome analysis showed that inactivation of Hac1 significantly affected the proteasome process and its downstream unfolded protein binding, indicating that Hac1 participates in the endoplasmic reticulum stress response through the conserved unfolded protein response. Taken together, our findings identified many crucial transcription factor genes and pathways related to cell development, abiotic stress response and pathogenesis, and expand our understanding of how microbial pathogens utilize these genes to deal with environmental stresses and achieve successful infection in the host plant.