Two Verticillium dahliae MAPKKKs, VdSsk2 and VdSte11, have distinct roles in pathogenicity, microsclerotial formation, and stress adaptation

Authors: YU, JUN - Beijing Forestry University; LI, TIANYU - Beijing Forestry University; TIAN, LONGYAN - Beijing Forestry University; TANG, CHEN - Beijing Forestry University; Klosterman, Steven; TIAN, CHENGMING - Beijing Forestry University; WANG, YONGLIN - Beijing Forestry University

Submitted to: mSphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2019
Publication Date: 7/10/2019
Citation: Yu, J., Li, T., Tian, L., Tang, C., Klosterman, S.J., Tian, C., Wang, Y. 2019. Two Verticillium dahliae MAPKKKs, VdSsk2 and VdSte11, have distinct roles in pathogenicity, microsclerotial formation, and stress adaptation. mSphere. 4(4). https://doi.org/10.1128/mSphere.00426-19.
DOI: https://doi.org/10.1128/mSphere.00426-19

Interpretive Summary: Verticillium dahliae is a soilborne fungus, and the cause of Verticillium wilt diseases on over 200 plant species worldwide, including many agriculturally important crops. The darkly pigmented resting structures produced by V. dahliae are called microsclerotia, and these structures contribute to the long term survival of the pathogen and there is not much known on the genetic basis of how the microsclerotia are formed. In this work we identified the roles of two genes in microsclerotia production in V. dahliae. Both of the genes encode proteins that are involved in perceiving upstream signals in the cell and signaling to downstream cellular components that lead to the activation of genes that lead to the formation microsclerotia. One of the signaling mutant strains was also nonpathogenic because it could not penetrate a plant surface, as demonstrated in this study with onion skin, whereas the other signaling mutant was defective in mounting appropriate responses and survival in response to stress. The results of this current study contribute to increasing collective insights on proteins and genes involved in microsclerotia formation and fungal survival, and thus some of the proteins involved in these processes may be important targets to develop alternative disease control measures.

Technical Abstract: The plant pathogenic fungus Verticillium dahliae causes destructive vascular wilt diseases on over 200 plant species worldwide. The melanized microsclerotia produced by this fungus are crucial for its disease cycle. Previous reports have shown that the VdPbs2-VdHog1 kinase module plays key roles in microsclerotia formation, stress responses, and virulence in this fungus. The MAP kinase kinase kinase (MAPKKK) responsible for the module activation remains less understood. In this study, two MAPKKKs homologous to Ssk22p and Ste11p, which link the Pbs2p-Hog1p module by phosphorylation in yeast, were identified in the genome of V. dahliae. Genetic and biochemical analyses demonstrated that VdSsk22 can phosphorylate VdHog1, but not VdSte11, in V. dahliae. Both 'VdSsk22 and 'VdSte11 strains showed severe defects in microsclerotia formation and melanin biosynthesis, but the relative importance of these two genes in microsclerotial development was discernable. Deletion of VdSsk22, but not VdSte11, affected responses to osmotic stress, fungicidal response, and cell wall stressors. The 'VdSsk22 mutant exhibited a significant reduction in virulence, while the 'VdSte11 strain was nonpathogenic due to failure to penetrate and form hyphopodia. Finally, coupled with the finding that the expression of the calcineurin-responsive zinc finger transcription factor VdCrz1 was misregulated in the 'VdSsk22/VdPbs2/VdHog1 mutants, we postulate that the HOG signaling pathway is involved in calcium response and functions upstream of VdCrz1. Taken together, these data provide insights into functions of VdSsk22 and VdSte11 in pathogenicity, stress responses, and microsclerotial development in V. dahliae.