The APSES transcription factor Vst1 is a key regulator of development in microsclerotium- and resting mycelium-producing Verticillium species

Abstract

Plant pathogens of the genus Verticillium pose a threat to many important crops worldwide. They are soil-borne fungi which invade the plant systemically, causing wilt symptoms. We functionally characterized the APSES family transcription factor Vst1 in two Verticillium species, V. dahliae and V. nonalfalfae, which produce microsclerotia and melanized hyphae as resistant structures, respectively. We found that, in V. dahliae Avstl strains, microsclerotium biogenesis stalled after an initial swelling of hyphal cells and cultures were never pigmented. In V. nonalfalfae Avst7, melanized hyphae were also absent. These results suggest that Vst1 controls melanin biosynthesis independent of its role in morphogenesis. The absence of vstl also had a great impact on sporulation in both species, affecting the generation of the characteristic verticillate conidiophore structure and sporula-tion rates in liquid medium. In contrast with these key roles in development, Vst1 activity was dispensable for virulence. We performed a microarray analysis comparing global transcription patterns of wild-type and Avstl in V. dahliae. G-protein/cyclic adenosine monophosphate (G-protein/cAMP) signalling and mitogen-activated protein kinase (MAPK) cascades are known to regulate fungal morphogenesis and virulence. The microarray analysis revealed a negative interaction of Vst1 with G-protein/ cAMP signalling and a positive interaction with MAPK signalling. This analysis also identified Rho signalling as a potential regulator of morphogenesis in V. dahliae, positively interacting with Vst1. Furthermore, it exposed the association of secondary metabolism and development in this species, identifying Vst1 as a potential co-regulator of both processes. Characterization of the putative Vst1 targets identified in this study will aid in the dissection of specific aspects of development.