Melanin synthesis gene Aapks contributes to appressorium formation, stress response, cell well integrity and virulence in Alternaria alternata

Melanin, as a common pigment, performs diverse biological function related to growth and development, environmental stress and pathogenicity in many pathogenic fungi. The polyketide synthase (Aapks) gene is a key gene in 1,8-dihydroxynaphthalene (DHN) melanin synthesis pathway, however, the regulatory roles of Aapks in pear fungal Alternaria alternata, on melanin synthesis, external stresses response, infection structure differentiation and pathogenicity are not clear. The pks gene was cloned and identified in A. alternata. Gene knockout experiment showed that ΔAapks mutant resulted in melanin deficiency and pink pigment production, reduced sporulation and abnormal spore and hyphae. Hypersensitivity to cell wall and oxidant stress factors were also found in ΔAapks, especially the impaired cell wall of ΔAapks was observed after CFW staining, and chitinase activity and related genes expression Aachs4 and Aachs6 were also decreased in ΔAapks. Interestingly, the ΔAapks mutant delayed infection structural differentiation induced by the different physicochemical signals. The virulence of ΔAapks strain was reduced after wound-inoculated pear fruit and non-wound-inoculated tobacco leaves. The above findings suggested the Aapks plays a vital regulatory role in response to stresses, infection structure formation, cell wall integrity and virulence in A. alternata by catalyzing the melanin synthesis.