Abstract
Verticillium dahliae, a ubiquitous phytopathogenic fungus, forms resting structures, known as microsclerotia that play crucial roles in Verticillium wilt diseases. VdHog1, a mitogen-activated protein kinase (MAPK), controls microsclerotia formation, virulence, and stress response in V. dahliae. In this study, we present detailed evidence that the conserved upstream component of VdHog1, VdPbs2, is a key regulator of microsclerotia formation, oxidative stress and fungicide response and plant virulence in V. dahliae. We identified VdPbs2, homologous to the yeast MAPK kinase Pbs2. Similar to the VdHog1 deletion mutant, VdPbs2 deletion strains exhibited delayed melanin synthesis and reduced formation of microsclerotia. When exposed to stresses, VdPbs2 mutants were more sensitive than the wild type to osmotic agents and peroxide, but more resistant to inhibitors of cell wall synthesis and some fungicides. Finally, VdPbs2 deletion mutants exhibited reduced virulence on smoke tree and tobacco seedlings. When taken together, we implicate that VdPbs2 and VdHog1 function in a cascade that regulates microsclerotia formation and virulence, but not all VdHog1 dependent functions are VdPbs2 regulated. This study thus provides novel insights into the signal transduction mechanisms that regulate microsclerotia formation and pathogenesis in this fungus.
Highlights
The mitogen-activated protein kinase (MAPK) signaling pathways are involved in integrating multiple extracellular and intracellular signals to regulate transcription of specific genes that help the cell adapt to the conditions in eukaryotic cells (Gustin et al, 1998; Widmann et al, 1999)
We investigated the role of VdPbs2 in the development of microsclerotia and pathogenicity in V. dahliae
Similar to the VdHog1 deletion mutant, VdPbs2 deletion mutants exhibited reduced microsclerotia formation, heightened sensitivity to osmotic stress, enhanced resistance to chemicals that interfered with cell wall synthesis and attenuated virulence on seedlings of smoke trees and tobacco
Summary
The mitogen-activated protein kinase (MAPK) signaling pathways are involved in integrating multiple extracellular and intracellular signals to regulate transcription of specific genes that help the cell adapt to the conditions in eukaryotic cells (Gustin et al, 1998; Widmann et al, 1999). Upon stress (osmotic, oxidative, acid and heat, etc), the high osmolarity glycerol (HOG) pathway is activated and the stressactivated MAPK Hog is phosphorylated (Brewster and Gustin, 2014). This pathway is initiated by two upstream branches, Sln and Sho, and they converge at the Pbs MAPKK and are able to activate Pbs, which phosphorylates the MAPK Hog (Brewster et al, 1993; O’Rourke and Herskowitz, 2004; Roman et al, 2007). HOG pathway plays an important and somewhat specialized role in sensing stress conditions and activating gene expression, enabling the cell to resist the toxic effects of stress, survive and grow under adverse conditions (Gustin et al, 1998; Widmann et al, 1999)
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