Deficiency of the melanin biosynthesis genes CfSCD1impedes appressoria formation and disrupts Colletotrichum fructicolainfection 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 fructicolais a predominant pathogen responsible for global GLS outbreaks. C. fructicolainfects 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. fructicolais poorly understood up to nowadays. In the present study, we successfully identified the gene CfSCD1encoding scytalone dehydratase in C. fructicolaisolate 030206. The expression level of CfSCD1in 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 CfSCD1deletion mutant was generated to further elucidate gene function, revealing a significant reduction in melanin content in the ΔCfSCD1strain. The growth of ΔCfSCD1mycelium was inhibited by UV irradiation, and no regular appressorium formatted from the germinated conidia of ΔCfSCD1. In addition, the germinated conidia hyphae of ΔCfSCD1exhibited a loss in their ability to invade apple leaves. CfSCD1was identified as a crucial gene associated with conidial invasion, and its expression level may serve as an indicator of C. fructicolaisolate pathogenicity.