Functional characterization of the Ustilago maydis virulence gene scp2
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 effec...
|Online Access:||PDF Full Text|
No Tags, Be the first to tag this record!
|Summary:||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
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.|