BACKGROUND: Septins are a highly conserved family of GTP-binding proteins 
involved in multiple cellular functions, including cell division and 
morphogenesis. Studies of septins in fungal cells underpin a clear correlation 
between septin-based structures and fungal morphology, providing clues to 
understand the molecular frame behind the varied morphologies found in fungal 
world.
METHODOLOGY/PRINCIPAL FINDINGS: Ustilago maydis genome has the ability to encode 
four septins. Here, using loss-of-function as well as GFP-tagged alleles of 
these septin genes, we investigated the roles of septins in the morphogenesis of 
this basidiomycete fungus. We described that septins in U. maydis could assemble 
into at least three different structures coexisting in the same cell: bud neck 
collars, band-like structures at the growing tip, and long septin fibers that 
run from pole to pole near the cell cortex. We also found that in the absence of 
septins, U. maydis cells lost their elongated shape, became wider at the central 
region and ended up losing their polarity, pointing to an important role of 
septins in the morphogenesis of this fungus. These morphological defects were 
alleviated in the presence of an osmotic stabilizer suggesting that absence of 
septins affected the proper formation of the cell wall, which was coherent with 
a higher sensitivity of septin defective cells to drugs that affect cell wall 
construction as well as exocytosis. As U. maydis is a phytopathogen, we analyzed 
the role of septins in virulence and found that in spite of the described 
morphological defects, septin mutants were virulent in corn plants.
CONCLUSIONS/SIGNIFICANCE: Our results indicated a major role of septins in 
morphogenesis in U. maydis. However, in contrast to studies in other fungal 
pathogens, in which septins were reported to be necessary during the infection 
process, we found a minor role of septins during corn infection by U. maydis.