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Eukaryotic Cell, December 2002, p. 954-966, Vol. 1, No. 6
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.6.954-966.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

ras2 Controls Morphogenesis, Pheromone Response, and Pathogenicity in the Fungal Pathogen Ustilago maydis

Nancy Lee and James W. Kronstad*

Biotechnology Laboratory, Department of Microbiology and Immunology, and Faculty of Agricultural Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

Received 17 September 2002/ Accepted 1 October 2002

Ustilago maydis, a pathogen of maize, is a useful model for the analysis of mating, pathogenicity, and the morphological transition between budding and filamentous growth in fungi. As in other fungi, these processes are regulated by conserved signaling mechanisms, including the cyclic AMP (cAMP)/protein kinase A (PKA) pathway and at least one mitogen-activated protein kinase (MAP kinase) pathway. A current challenge is to identify additional factors that lie downstream of the cAMP pathway and that influence morphogenesis in U. maydis. In this study, we identified suppressor mutations that restored budding growth to a constitutively filamentous mutant with a defect in the gene encoding a catalytic subunit of PKA. Complementation of one suppressor mutation unexpectedly identified the ras2 gene, which is predicted to encode a member of the well-conserved ras family of small GTP-binding proteins. Deletion of the ras2 gene in haploid cells altered cell morphology, eliminated pathogenicity on maize seedlings, and revealed a role in the production of aerial hyphae during mating. We also used an activated ras2 allele to demonstrate that Ras2 promotes pseudohyphal growth via a MAP kinase cascade involving the MAP kinase kinase Fuz7 and the MAP kinase Ubc3. Overall, our results reveal an additional level of crosstalk between the cAMP signaling pathway and a MAP kinase pathway influenced by Ras2.

* Corresponding author. Mailing address: Biotechnology Laboratory, Department of Microbiology and Immunology, and Faculty of Agricultural Sciences, University of British Columbia, Vancouver, B.C. V6T 1Z3, Canada. Phone: (604) 822-4732. Fax: (604) 822-2114. E-mail: kronstad{at}interchange.ubc.ca.

Eukaryotic Cell, December 2002, p. 954-966, Vol. 1, No. 6
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.6.954-966.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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