Deficiency of the melanin biosynthesis genes CfSCD1 impedes appressoria formation and disrupts Colletotrichum fructicola infection on apple

Apple Glomerella leaf spot (GLS) is a severe foliar disease of apples trees, prevalent in several countries including China, Brazil, and United States. Colletotrichum fructicola is a predominant pathogen responsible for global GLS outbreaks. C. fructicola infects apple leaves through penetration pegs that are generated from melanized appressoria, and the melanization process is crucial for their proper functioning. Scytalone dehydratase is a key enzyme in melanin biosynthesis. The role of melanin in the biology and pathogenicity of C. fructicola is poorly understood up to nowadays. In the present study, we successfully identified the gene CfSCD1 encoding scytalone dehydratase in C. fructicola isolate 030206. The expression level of CfSCD1 in germinated conidia exhibited a remarkable increase to approximately tenfold at 6 hpi, followed by a decrease to around five-fold at 12 hpi, and then stabilized from 12 to 24 hpi. The appressoria exhibited a complete morphology at 6 hpi, followed by deep melanization at 12 hpi and subsequent production of primary hyphae from the appressoria at 24 hpi. A CfSCD1 deletion mutant was generated to further elucidate gene function, revealing a significant reduction in melanin content in the ΔCfSCD1 strain. The growth of ΔCfSCD1 mycelium was inhibited by UV irradiation, and no regular appressorium formatted from the germinated conidia of Δ CfSCD1 . In addition, the germinated conidia hyphae of Δ CfSCD1 exhibited a loss in their ability to invade apple leaves. CfSCD1 was identified as a crucial gene associated with conidial invasion, and its expression level may serve as an indicator of C. fructicola isolate pathogenicity.