Cell polarity and cell morphogenesis in Ustilago maydis

Localization of positional cues to distinct cell domains is critical for the generation of cell polarity, cell morphogenesis, septum positioning, and organelle distribution. We are interested in understanding how positional cues govern cell morphogenesis and nuclear position in Ustilago maydis, a member of the Basidiomycota. U. maydis exhibits a yeast-like and a filamentous form. Additional morphologies are generated by interaction with its hosts maize (Zea mays) and teozintle (Zea mays subsp. parviglumis and subsp. mexicana), suggesting that plant signals modulate fungal morphogenesis. Thus, U. maydis provides excellent opportunities to understand how positional cues control polarized growth in the different morphologies.

We identified genes for cell polarity and cell morphogenesis using a genetic screen to isolate mutants with altered colony and cell morphology. One of the genes identified codes for a protein with similarity to fission yeast Tea4, a SH3 protein that determines the axis of polarized growth (Martin et al., 2005). U. maydis tea4 codes for a multidomain protein of 1684 amino acid residues. The presence of some domains only in Tea4 homologues in the Basidiomycota, suggests that Tea4 performs additional functions in this group of fungi. Another gene identified codes for a protein of 1698 amino acid residues with similarity to S. pombe Tea1, a Kelch domain protein, that is a key determinant of directionality of polarized growth (Mata and Nurse, 1997). UmTea1 is a multidomain protein. Tea1 homologues in the Ascomycota and Basidiomycota contain Kelch repeats located near the amino terminus of the protein. Analysis of the Umtea4 and Umtea1 null phenotype indicates that UmTea4 and UmTea1 are important positional markers for polarized growth. We also uncovered novel roles for Tea4 and Tea1 in other processes that contribute to the generation of normal cell shape in U. maydis. Here we describe the results of this analysis.