Abstract
Mycoparasitic species of the fungal genus Trichoderma are potent antagonists able to combat plant pathogenic fungi by direct parasitism. An essential step in this mycoparasitic fungus-fungus interaction is the detection of the fungal host followed by activation of molecular weapons in the mycoparasite by host-derived signals. The Trichoderma atroviride MAP kinase Tmk1, a homolog of yeast Fus3/Kss1, plays an essential role in regulating the mycoparasitic host attack, aerial hyphae formation and conidiation. However, the transcription factors acting downstream of Tmk1 are hitherto unknown. Here we analyzed the functions of the T. atroviride Ste12 transcription factor whose orthologue in yeast is targeted by the Fus3 and Kss1 MAP kinases. Deletion of the ste12 gene in T. atroviride not only resulted in reduced mycoparasitic overgrowth and lysis of host fungi but also led to loss of hyphal avoidance in the colony periphery and a severe reduction in conidial anastomosis tube formation and vegetative hyphal fusion events. The transcription of several orthologues of Neurospora crassa hyphal fusion genes was reduced upon ste12 deletion; however, the Δste12 mutant showed enhanced expression of mycoparasitism-relevant chitinolytic and proteolytic enzymes and of the cell wall integrity MAP kinase Tmk2. Based on the comparative analyses of Δste12 and Δtmk1 mutants, an essential role of the Ste12 transcriptional regulator in mediating outcomes of the Tmk1 MAPK pathway such as regulation of the mycoparasitic activity, hyphal fusion and carbon source-dependent vegetative growth is suggested. Aerial hyphae formation and conidiation, in contrast, were found to be independent of Ste12.
Highlights
Mycoparasitic species of the fungal genus Trichoderma are potent biocontrol agents and promising substitutes for chemical fungicides as they attack and parasitize plant pathogens, such as Rhizoctonia spp., Phythium spp., Botrytis cinerea und Fusarium spp. [1]
Characteristics of the T. atroviride Ste12 homologue The aim of this study was to characterize the role of the Ste12 transcription factor, which is assumed to act as a central component of the mycoparasitism-relevant signaling pathway involving the Tmk1 MAPK in T. atroviride
Similar to Ste12 proteins from other filamentous fungi, T. atroviride Ste12 contains an N-terminally located homeodomain-like (STE) motif which is presumed to be involved in DNA binding [39] and two distinct C-terminal C2H2 zinc finger domains which are distinguishing features of Ste12-like proteins of filamentous fungi [15]
Summary
Mycoparasitic species of the fungal genus Trichoderma are potent biocontrol agents and promising substitutes for chemical fungicides as they attack and parasitize plant pathogens, such as Rhizoctonia spp., Phythium spp., Botrytis cinerea und Fusarium spp. [1]. In the yeast Saccharomyces cerevisiae, mating and filamentous growth are controlled by the Fus and Kss MAPKs, respectively [10] Despite their distinct activation mechanisms and signaling output, both MAPKs target the homeodomain transcription factor Ste, which acts as a central node in both mating and invasive growth and that is under complex regulation by several regulatory proteins and co-factors being tightly controlled by each MAPK. Studies from several phytopathogenic fungi, including appressorium- and nonappressorium-forming pathogens, necrotrophs and biotrophs, revealed a conserved role of the Pmk MAPK pathway for regulating plant infection with respective deletion mutants being affected in pathogenicity-related processes such as appressorium formation, penetration hyphae differentiation, root attachment and the production of plant cell wall-degrading enzymes (reviewed in [9,14]). Concordant with the model of Ste being targeted by the Fus3/Kss homologous Pmk1-type MAP kinase, ste12deficient mutants of several phytopathognic fungi are either nonpathogenic or suffer from strongly attenuated virulence (reviewed in [11,15])
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