A Virulence Factor from Sclerotinia sclerotiorum Targets the Host Chloroplast Proteins to Promote Infection

Chloroplasts are not only places for photosynthesis, but also participate in plant immunity and are important targets of pathogens. Pathogens secrete chloroplast-targeted proteins (CTPs) that disrupt host immunity and promote infection. <i>Sclerotinia sclerotiorum</i> (Lib.) de Bary is a phytopathogenic fungus with a broad host range. However, little is known about the pathogenic mechanisms underlying this wide host range. In this study, we investigated the role of Chloroplast-Targeted Protein 1 (SsCTP1) secreted by <i>S. sclerotiorum</i> in pathogenesis, which inhibits plant immunity and promotes pathogen infections. <i>SsCTP1</i> was highly up-regulated during the early stages of <i>S. sclerotiorum</i> infection in various hosts, and its transient expression in <i>Nicotiana benthamiana</i> revealed that it was predominantly localized within chloroplasts. Mutants with <i>SsCTP1</i> deletion exhibited a similar growth rate and colony morphology to the wild type, but significantly reduced pathogenicity in various hosts. Moreover, SsCTP1 inhibited chitin-induced callose deposition and defense gene expression, and enhanced sensitivity to <i>S. sclerotiorum</i> in <i>N. benthamiana</i>. Similarly, transgenic <i>Arabidopsis thaliana</i> overexpressing SsCTP1 displayed an increased susceptibility to <i>S. sclerotiorum</i>. Furthermore, two host proteins that interact with SsCTP1, Coproporphyrinogen-III oxidase (GmCPX), and shikimate kinase 2 (GmSKL2) were identified by screening the soybean cDNA library, and these interactions were confirmed in vivo. Importantly, the silencing of <i>NbCPX</i> by virus-induced gene silencing enhanced <i>N. benthamiana</i> resistance to <i>S. sclerotiorum</i>. Our results indicate that SsCTP1 is an important pathogenic factor that contributes to the wide host range of <i>S. sclerotiorum</i> and may inhibit plant immunity by targeting the chloroplast proteins GmCPX and GmSKL2, which are ubiquitous in host plants.