The GATA-type IVb Zinc-finger Transcription Factor SsNsd1 Regulates Asexual-sexual Development and Appressoria Formation in Sclerotinia Sclerotiorum

The sclerotium, a multicellular structure composed of the compact aggregation of vegetative hyphae, is critical for the long-term survival and sexual reproduction of the plant-pathogenic fungus Sclerotinia sclerotiorum. The development and carpogenic germination of sclerotia are regulated by integrating signals from both environmental and endogenous processes. Here, we report the regulatory functions of the S. sclerotiorum GATA-type IVb zinc-finger transcription factor SsNsd1 in these processes. SsNsd1 is orthologous to the Aspergillus nidulans NsdD (never in sexual development) and the Neurospora crassa SUB-1 (submerged protoperithecia-1) proteins. Ssnsd1 gene transcript accumulation remains relatively low, but variable, during vegetative mycelial growth and multicellular development. Ssnsd1 deletion mutants (Δnsd1-KOs) produce phialides and phialospores (spermatia) excessively in vegetative hyphae and promiscuously within the interior medulla of sclerotia. In contrast, phialospore development occurs only on the sclerotium surface in the wild-type. Loss of SsNsd1 function affects sclerotium structural integrity and disrupts ascogonia formation during conditioning for carpogenic germination. As a consequence, apothecium development is abolished. The Ssnsd1 deletion mutants are also defective in the transition from hyphae to compound appressorium formation, resulting in a loss of pathogenicity on unwounded hosts. In sum, our results demonstrate that SsNsd1 functions in a regulatory role similar to its ascomycete orthologues in regulating sexual and asexual development. Further, SsNsd1 appears to have evolved as a regulator of pre-penetration infectious development required for the successful infection of its many hosts.