The DENN Protein SMD1 Forms a Signalling Hub With Rab GTPase YPT1 to Regulate Diverse Biological Processes in Phytopathogenic Fungi and Serves as a HIGS Target for Crop Protection.

Sclerotinia sclerotiorum is a notorious necrotrophic plant pathogen that causes severe yield losses in many important crops. Despite its profound economic impact, the molecular mechanisms underpinning its development and virulence processes remain largely elusive. In this study, we characterized a mutant with reduced virulence through a forward genetic screen. Genetic analysis confirmed that mutations in a gene encoding a DENN (Differentially Expressed in Normal cells and Neoplasia) domain-containing protein SsSMD1 (Sclerotia Maturation Deficient 1) led to impaired sclerotia formation and reduced virulence. SsSMD1 affects cell wall integrity and stress tolerance. Notably, SsSMD1 seems to act as a Guanine Nucleotide Exchange Factor (GEF) that interacts with the Rab GTPase SsYPT1. While the knockout of SsYPT1 was lethal, a single amino acid substitution (A136D) in SsYPT1 significantly impaired vegetative growth, sclerotia formation, and virulence. Similarly, knockout analysis in different phytopathogenic fungi revealed that SMD1 homologues were essential for their growth and virulence. We employed host-induced gene silencing (HIGS) targeting SsSMD1 and SsYPT1, which effectively attenuated the virulence of S. sclerotiorum in Nicotiana benthamiana and Arabidopsis thaliana. Collectively, our findings demonstrate that the DENN domain-containing protein is indispensable for development and virulence in major phytopathogenic fungi, and the SsSMD1-SsYPT1 modules can be used as practical HIGS targets against necrotrophic fungal pathogens.