MAP Kinase FgHog1 and Importin β FgNmd5 Regulate Calcium Homeostasis in Fusarium graminearum

Maintaining cellular calcium (Ca(2+)) homeostasis is essential for many aspects of cellular life. The high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) pathway responsible for signal integration and transduction plays crucial roles in environmental adaptation, especially in the response to osmotic stress. Hog1 is activated by transient Ca(2+) increase in yeast, but the functions of the HOG pathway in Ca(2+) homeostasis are largely unknown. We found that the HOG pathway was involved in the regulation of Ca(2+) homeostasis in Fusarium graminearum, a devastating fungal pathogen of cereal crops. The deletion mutants of HOG pathway displayed increased sensitivity to Ca(2+) and FK506, and elevated intracellular Ca(2+) content. Ca(2+) treatment induced the phosphorylation of FgHog1, and the phosphorylated FgHog1 was transported into the nucleus by importin β FgNmd5. Moreover, the increased phosphorylation and nuclear accumulation of FgHog1 upon Ca(2+) treatment is independent of the calcineurin pathway that is conserved and downstream of the Ca(2+) signal. Taken together, this study reported the novel function of FgHog1 in the regulation of Ca(2+) homeostasis in F. graminearum, which advance the understanding of the HOG pathway and the association between the HOG and calcineurin pathways in fungi.