Universität zu Köln

Petti, Gabriella Cosmo ORCID: 0000-0001-9250-7746 (2026). Homologous antimicrobial proteins in plants and fungi: conserved mechanisms shaping the plant microbiota. PhD thesis, Universität zu Köln.

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Abstract

Plants form close association with a diverse and dynamic community of microorganisms, including bacteria, fungi, protists, nematodes, and viruses, collectively referred to as their microbiota. These microorganisms form complex associations with their hosts and play critical roles in plant health. It has therefore been proposed that plants and their microbiota function together as a single ecological unit, the holobiont. The plant microbiota plays an important role in plant defence, influencing the outcome of plant-pathogen interactions. Microbiota members can suppress pathogens directly through competition, antibiosis, and nutrient sequestration, or indirectly by priming host immune responses, thereby enhancing disease resistance. Together, these mechanisms expand the plant capacity to withstand invading pathogens. To establish themselves within the host, plant pathogens secrete effector proteins. Initially defined as modulators of host immunity, effectors are now known to serve broader roles in microbial self-protection as well as in the manipulation of host physiology. Increasing evidence indicates that some pathogens deploy effectors with antibacterial activity to inhibit microbial competitors and reshape host-associated communities, thereby promoting pathogen establishment and persistence within their niche. The studies presented in this thesis aim to deepen our understanding of antimicrobial effector proteins by elucidating the mechanisms of action and ecological functions of selected effectors in fungal pathogens and their homologs in other organisms. In this PhD thesis, I describe the first Verticillium dahliae effector protein identified with antifungal activity, VdAMP3. This effector belongs to the cysteine-stabilized αβ defensin family, an ancient antimicrobial protein family. V. dahliae secretes VdAMP3 during the formation of resting structures in senescing plant tissues, where it modulates the host mycobiota by suppressing fungal competitors and safeguarding the development of its resting structures. V. dahliae also secretes the bactericidal effector VdAve1 to promote niche colonization by inhibiting bacterial antagonists. In this thesis, I present the structural and functional characterization of VdAve1. I demonstrate that VdAve1 binds lipoteichoic acid, a key component of Gram-positive bacterial cell walls, and disrupts microbial membranes in a charge-dependent manner. Moreover, its antimicrobial activity extends beyond bacteria, as VdAve1 also inhibits eukaryotic microbes. Finally, I show that plant homologs of VdAve1, known as plant natriuretic peptides (PNPs), from diverse plant species as well as the the plant-derived PNP horizontally acquired by the whitefly Bemisia tabaci, exhibit both antibacterial and antifungal activities. Loss of PNP genes in Arabidopsis thaliana alters bacterial, but not fungal, phyllosphere communities, suggesting that PNPs are involved in microbiota manipulation.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Petti, Gabriella Cosmo gab.petti@gmail.com https://orcid.org/0000-0001-9250-7746 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-801015
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Ehemalige Fakultäten, Institute, Seminare > Faculty of Mathematics and Natural Sciences
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Plant-pathogen interactions UNSPECIFIED Antimicrobial proteins UNSPECIFIED Verticillium dahliae UNSPECIFIED
Date of oral exam:	26 January 2026
Referee:	Name Academic Title Thomma, Bart Prof. Dr. Thomas, Kroj Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/80101