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Eukaryotic Cell, February 2007, p. 171-181, Vol. 6, No. 2
1535-9778/07/$08.00+0     doi:10.1128/EC.00326-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Two-Component Response Regulators Ssk1p and Skn7p Additively Regulate High-Osmolarity Adaptation and Fungicide Sensitivity in Cochliobolus heterostrophus{triangledown} ,{ddagger}

Kosuke Izumitsu, Akira Yoshimi,{dagger} and Chihiro Tanaka*

Laboratory of Environmental Mycoscience, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan

Received 12 October 2006/ Accepted 17 November 2006

Filamentous ascomycetous fungi possess many histidine kinases and two conserved response regulators, Ssk1p and Skn7p, in their two-component signaling systems. We previously reported that the fungus unique group III histidine kinase regulates high-osmolarity adaptation and iprodione/fludioxonil fungicide sensitivity by controlling the phosphorylation of Hog1-type mitogen-activated protein kinase (MAPK) in filamentous ascomycetes. Here, we have characterized the response regulator genes ChSsk1 and ChSkn7 in the southern corn leaf blight fungus Cochliobolus heterostrophus. Both ChSsk1- and ChSkn7-disrupted mutants showed little sensitivity to high-osmolarity stress and moderate resistance to the iprodione/fludioxonil fungicides. The phosphorylation of Hog1-type MAPK BmHog1p induced by high-osmolarity stress and fungicide treatments was only regulated by ChSsk1p, indicating that ChSkn7p has roles in high-osmolarity adaptation and fungicide sensitivity that are independent from the activation of BmHog1p. The Chssk1 Chskn7 double mutants clearly showed higher sensitivity to osmolar stress and higher resistance to fungicides than the single mutants. The dose responses of the double mutants fit well with those of the group III histidine kinase-deficient strain. These results suggest that in filamentous ascomycetes, the Ssk1- and Skn7-type response regulators control high-osmolarity adaptation and fungicide sensitivity additively with differential mechanisms under the regulation of the group III histidine kinase. This study provides evidence that filamentous fungi have a unique two-component signaling system that is different from that of yeast and is responsible for high-osmolarity adaptation and fungicide sensitivity.

* Corresponding author. Mailing address: Laboratory of Environmental Mycoscience, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan. Phone: 81 75 753 6311. Fax: 81 75 753 6312. E-mail: chihiro{at}kais.kyoto-u.ac.jp.

{triangledown} Published ahead of print on 8 December 2006.

{ddagger} Supplemental material for this article may be found at http://ec.asm.org/.

{dagger} Present address: New Industry Creation Hatchery Center (NICHe), Tohoku University, Aoba, Sendai 980-8579, Japan.

Eukaryotic Cell, February 2007, p. 171-181, Vol. 6, No. 2
1535-9778/07/$08.00+0     doi:10.1128/EC.00326-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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