Septins regulate virulence in Verticillium dahliae and differentially contribute to microsclerotial formation and stress responses

Abstract

Septin proteins play a role in the formation of hyphal septa as well as in the division of nuclei, cytoskeletal organization, and cell morphogenesis in filamentous fungi. Herein, we investigated the functions of four septin-coding genes (VdSep3, VdSep4, VdSep5, and VdSep6) in the plant pathogenic fungus Verticillium dahliae. Microsclerotial formation was positively regulated by VdSep4 and VdSep6, whereas VdSep3 and VdSep5 had no effect on microsclerotial development but their deletion slightly reduced melanin production. Deletion of VdSep4 or VdSep5, but not VdSep3, resulted in hypersensitivity to high-temperature stress. Deletion of VdSep4 led to increased benomyl sensitivity whereas deletion of VdSep3 showed increased benomyl resistance. The previously reported roles of VdSep3 and VdSep5 in virulence were confirmed. Disruption of each of the four septin-coding genes led to reduction in penetration peg formation and hyphal expansion into plant cells. Loss of VdSep3 or VdSep4 increased sensitivity to reactive oxygen species (ROS) and reactive nitrogen species (RNS) stress, whereas VdSep6 played a role in RNS stress response but not in ROS stress response. In addition, all the septin gene deletion mutants showed an abnormality in chitin distribution but varied in their responses to several stresses examined. Taken together, our results indicate that members of the septin family in V. dahliae play different roles in regulating microsclerotial development, melanin synthesis, and stress responses, while they are all required for full virulence of the fungus.