A Novel Predicted Nonclassical Secreted Protein, MfHS1, Contributes to Environmental Fitness and Pathogenicity of Monilinia fructicola

Monilinia fructicola, the most widely distributed species among the Monilinia genus globally, causes blossom blight, twig canker, and fruit rot on Rosaceae fruits. Despite previous studies, limitations still exist regarding virulence of M. fructicola. In this study, we identified a gene significantly upregulated during the early stages of infection. Bioinformatics analysis revealed that this gene encoded a protein containing the HAD_SAK_1 domain (abbreviated as HS1), and we named it MfHS1. Knockout and complemented transformants were generated and evaluated for environmental fitness. Results revealed that MfHS1 was involved in the regulation of osmotic stress and cell wall integrity. Additionally, microscopic observations showed that MfHS1 participated in the differentiation process of hyphal tips. Virulence assays indicated that the knockout transformant ΔMfHS1 exhibited significantly reduced virulence. Considering that MfHS1 is predicted as a nonclassical secreted protein, it was transiently expressed in Nicotiana benthamiana leaves, and the induced plant cell death was observed, indicating that MfHS1 might trigger plant defense responses, e.g., programmed cell death, and supply nutrients to necrotic pathogens, thus aiding host infection. This study offers a new perspective for further understanding the pathogenic mechanisms of M. fructicola and developing control strategies.