The development-specific protein (Ssp1) from Sclerotinia sclerotiorum is encoded by a novel gene expressed exclusively in sclerotium tissues

Abstract

The gene encoding a development-specific protein (Ssp1) was identified; it previously was described as the major protein present in mature sclerotia of Sclerotinia sclerotiorum. To determine the developmental specificity of ssp1 gene expression in relation to protein accumulation we examined transcript and protein accumulation during various growth and development stages of the lifecycle. We found that ssp1 transcript accumulated exclusively within developing sclerotium tissue and not in any other examined stage of growth or development. In contrast high levels of Ssp1 protein were detectable by western blot and tandem mass spectrometry analyses in all stages of sclerotium as well as apothecium development. Immunolocalization further indicated that Ssp1 protein bodies were depleted from the sclerotium tissue surrounding the site of apothecium germination, but by this method Ssp1 was not detected in the apothecium. Together these findings suggest that Ssp1 is not metabolized during carpogenic germination, instead it is translocated from the sclerotium to the apothecium in an antigenically novel form. Outside the Sclerotiniaceae ssp1 homologs were found only from the sclerotium-forming Aspergillus species A. flavus and A. oryzae. Further studies concerning the regulation and function of this gene and its occurrence in other species have the potential to inform our understanding of sclerotium development and the evolution of sclerotia and other forms of fungal stroma.

• apothecium
• development
• fungus
• pathogen
• sclerotium
• stroma

We gratefully acknowledge Ms Karen Chamusco’s preparation of thin section tissue slides of sclerotia and Mr Cheng-Hua Huang and Ms Linley Smith for their assistance with phylogenetic analysis. Tandem mass spectrometry analysis and database queries were performed by Scott McClung at the University of Florida Interdisciplinary Center for Biotechnology Research proteomics core facility. We gratefully acknowledge Ms Ulla Benny for excellent laboratory support. This work was supported by USDA/NRICGP grant 9802312 from the United States Department of Agriculture and through an alumni fellowship awarded to ML from the University of Florida.