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Abstract

The causative agent of the corn smut disease Ustilago maydis infects its host plant Zea mays by specialized infection structures, so-called appressoria, which are formed upon perception of chemical and physical stimuli on the leave surface. During the colonization process U. maydis secretes effector proteins that help to establish a biotrophic interaction. These effector proteins harbor an N-terminal hydrophobic secretion signal that targets them to the classical secretory pathway. In recent years, however, the existence of unconventionally secreted proteins has been uncovered which reach the extracellular space independently of the classical ER-Golgi system. In the present study the non-specific lipid transfer protein Scp2 (sterol carrier protein 2) of U. maydis was analyzed, which was identified as a putative candidate for unconventional protein secretion. Scp2 lacks a classical N-terminal signal peptide but exhibits a peroxisomal targeting signal (PTS1). A quantitative real-time PCR approach revealed that scp2 is up-regulated during early stages of plant colonization. Microscopic analyses demonstrated that the ability of scp2 deletion strains to form appressoria on artificial surfaces was significantly decreased. Furthermore, deletion of scp2 caused a virulence defect that appeared to result from a reduced efficiency of plant cuticle penetration. These defects are unlikely to result from deficiency in peroxisomal β- oxidation. In contrast to scp2 deletion strains, the infection of maize plants with a strain overexpressing scp2 under the cmu1 promoter triggered strong plant defense reactions. Two Scp2 paralogs were shown to localize in peroxisomes but deletion of the respective genes revealed no effect on U. maydis virulence. With the help of colony secretion assays it was demonstrated that small amounts of Scp2 are unconventionally secreted. The export of Scp2 via the classical ER-Golgi route, however, could not complement the virulence phenotype of the scp2 mutant strain, suggesting that the virulence defect is unconnected to the extracellular population of the protein. Surprisingly, peroxisomes and lipid droplets in the scp2 deletion strains displayed an altered distribution during filamentation on parafilm and on the plant surface. Based on these results, it is proposed that Scp2 affects appressorium development by influencing the distribution of peroxisomes and lipid droplets and thus constitutes a novel player in plant surface penetration.