Two-Component Response Regulators Ssk1p and Skn7p Additively Regulate High-Osmolarity Adaptation and Fungicide Sensitivity in Cochliobolus heterostrophus

Abstract

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 highosmolarity 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 twocomponent signaling system that is different from that of yeast and is responsible for high-osmolarity adaptation and fungicide sensitivity.

Two-Component Signaling System in Filamentous Fungi and the Mode of Action of Dicarboximide and Phenylpyrrole Fungicides

“…Mutants deficient in the Tco1 (also called CnNIK1) gene coding a Group III HisK in this fungus show strong phenylpyrrole resistance but do not become hyper-osmosensitive. Tco2, a HisK belonging to a class specific to this fungus, is believed to control the hyperosmotic stress response of this fungus (Bahn et al, 2006 (Yoshimi et al, 2003;Izumitsu et al, 2007). However, Hik1 gene disruptants in M. grisea are highly sensitive to hyperosmotic stress with sorbitol, but not with certain solutes (KCl, NaCl and glycerol).…”

“…However, the specific responses that these proteins govern have only been characterized in certain yeast species, not in filamentous fungi (Bahn et al, 2006;Cottarel 1997;Krems et al, 1996;Nakamichi et al, 2003;Posas et al, 1996;Singh et al, 2004). The role of the response regulators in the filamentous fungus was first characterized in a study using C. heterostrophus RR disruptant (Izumitsu et al, 2007). Ssk1 mutants of C. heterostrophus showed increased sensitivity to hyperosmotic stress and moderate dicarboximide and phenylpyrrole resistance, implying that Ssk1 plays a role in osmotic adaptation and fungicide sensitivity.…”

Characterization of the NikA Histidine Kinase Implicated in the Phosphorelay Signal Transduction ofAspergillus nidulans, with Special Reference to Fungicide Responses

“…13) This pathogen also has two RRs, named ChSsk1p (an ortholog of SskA) and ChSkn7p (an ortholog of SrrA), both of which are additively implicated in fungicide sensitivity. Hence the view presented in this study might be generally applicable to other filamentous ascomycetes, many of which commonly have orthologs of NikA, SskA, and SrrA, as well as HogA.…”

Characterization of NikA Histidine Kinase and Two Response Regulators with Special Reference to Osmotic Adaptation and Asexual Development inAspergillus nidulans

“…20,21) The C. heterostrophus RRs, ChSsk1 and ChSkn7 (SrrA ortholog), have also been reported to be responsible for the growth-inhibitory effects of fungicides. 22) Hence, the HK-RR (NikA-SskA/SrrA) system, in which NikA-HK phosphorylates the RRs SskA and SrrA, with YpdA as the intermediate, appears to play a crucial role in the fungicide response among filamentous fungi. 12) Nevertheless, these fungicides are not natural stimuli, although they are derivatives of anti-fungal bacterial metabolites.…”