The development-specific ssp1 and ssp2 genes of Sclerotinia sclerotiorum encode lectins with distinct yet compensatory regulation

The Ssp1 development-specific protein is the most abundant soluble protein in sclerotia and apothecia of Sclerotinia sclerotiorum. Although closely associated with these developmental stages, the functions of the Ssp1 protein and its paralog, Ssp2, are not known. In this study, protein structure prediction analysis revealed that Ssp1 and Ssp2 are structurally similar to fucose-specific lectins. In an effort to understand the function of these abundant, development-specific proteins, a homokaryotic ssp1 deletion mutant was generated. The resulting mutant (Δssp1) displays a wild-type growth and development phenotype in culture but produces approximately 50% fewer sclerotia in cultures supplemented with hygromycin. Genetic complementation with a wild-type copy of ssp1 restores normal sclerotium formation in the presence of hygromycin. This suggests that Ssp1 might play a role in resistance to glycoside-containing antibiotics encountered in the environment. Although a slight delay in carpogenic germination was observed, no additional effects of ssp1 loss-of-function were found in regards to apothecial morphology or fecundity. When the expression of ssp2 was examined in the Δssp1 mutant, it was found to be expressed earlier in sclerotial development and its encoded protein accumulated to higher levels in both sclerotia and apothecia. These findings suggest regulatory compensation for loss of Ssp1 coupled with potential functional redundancy among lectins accumulating in sclerotia and apothecia.