The development-specific protein (Ssp1) fromSclerotinia sclerotiorumis encoded by a novel gene expressed exclusively in sclerotium tissues

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 Sspl 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 Sspl was not detected in the apothecium. Together these findings suggest that Sspl 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.