The Verticillium dahliae Vta3 genetic network and hydrophobins are required in tomato xylem sap for plant disease
by Isabel Maurus
Date of Examination:2023-02-10
Date of issue:2023-03-27
Advisor:Prof. Dr. Gerhard H. Braus
Referee:Prof. Dr. Gerhard H. Braus
Referee:Prof. Dr. Volker Lipka
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Abstract
English
The ascomycete Verticillium dahliae is a pathogenic fungus for a variety of important crops and spends long phases of its life cycle in the vascular xylem system of particular host plants. Growth in this unique and nutrient-poor niche requires adaptation processes. The transcription factor Vta3 (Verticillium transcription activator of adhesion 3) is an important regulator of V. dahliae development, plant root colonization and pathogenicity. RNA sequencing of the V. dahliae wild-type strain cultured in tomato xylem sap was performed to identify genes that may be important for plant vascular colonization or disease symptom induction. The wild-type transcriptome was compared with that of the VTA3 deletion strain to identify Vta3-dependent genetic networks required for growth in natural plant xylem sap. Over 1,500 fungal transcripts were identified to be significantly more abundant in cells grown in xylem sap compared with pectin-rich medium. An intact VTA3 gene is required for wild-type transcription of over 1,000 genes. This genetic network includes candidate genes for proteins with predicted functions in virulence and morphogenesis such as Egh16-like virulence factor 1 (Elv1) or Master transcription factor 1 (Mtf1). Transcription of ELV1, which is induced in the presence of VTA3, is specifically important for virulence without affecting growth or spore production of V. dahliae. The presence of VTA3 hampers the action of Mtf1, which elicits plant immune responses and is required for the late stages of disease progression. Mtf1 promotes the formation of resting structures at the end of the infection cycle, allowing the fungus to survive in the soil and re-infect suitable host plants in the next growing season. Transcription of 85 of the identified genes was strongly induced (log2(fold change) ≥ 4) in cells grown in xylem sap. Strikingly, among these candidates are four (VDH1, VDH2, VDH4 and VDH5) of the five hydrophobin-encoding genes (VDH1-5) annotated in V. dahliae JR2. Apart from being structurally distinct from other described hydrophobins, Vdh4 and Vdh5 are specifically important for full disease development in tomato plants by acting at later stages of colonization. These properties also make Vdh4 and Vdh5 functionally unique hydrophobins, as they are simultaneously dispensable for the development of fungal structures. In conclusion, Elv1, Mtf1 and the Vdh4/5 hydrophobins were identified as important molecular factors for the late stage of disease progression in tomato plants. They all represent potential new targets to combat the spread of the pathogen V. dahliae in fields.
Keywords: Plant fungal pathogen; Fungal genetics; Gene expression; Xylem; Virulence factors; Transcriptional control; Verticillium dahliae; RNA sequencing; Hydrophobin; Microsclerotia