In order to survive in nutrient-limited habitats, microorganisms have evolved both mechanisms of cooperation and competition. The sum of all associated microorganisms in a habitat is called microbiome. Such microbial communities form integral networks which are permanently disturbed by biotic and abiotic factors. This requires constant adaptation and recalibration of microbial interactions within the community. So-called microbial "hubs" play a key role in understanding the stability of such dynamic networks. This term describes microorganisms that play a central role within a network and are therefore of overriding importance for the formation of a microbial community. Hubs, whose presence is essential for the formation and stability of a species community, are called key organisms or keystone species, whereby keystoneness is a measure for measuring disproportionate effects of individual organisms on the community. Recent findings indicate that antagonistic microorganisms in particular, or their negative interactions, have a stabilizing effect on microbial communities.

In wild populations of the model plant Arabidopsis thaliana, we have identified oomycetes of the genus Albugo as the central hub. However, this hub is decisively influenced by two yeasts belonging to the basidiomycetes. While the genus Dioszegia promotes the growth of Albugo laibachii on plants, a yeast of the genus Moesziomyces inhibits it. Furthermore, we could show in preliminary work that Moesziomyces also inhibits several isolates of the bacterial microbiome of the phyllosphere of A. thaliana. From these observations we conclude that yeasts belonging to the class of ustilaginomycetes like Moesziomyces have a central function in the formation of the microbial community of A. thaliana and thus represent keystone species. In order to test this hypothesis, we have started to establish a comprehensive yeast collection of A. thaliana leaves. For Moesziomyces, we have laid the groundwork for functional genetic analyses by genome sequencing and annotation, and by establishing a transformation system.

To analyse the interactions of ustilaginomycetes yeasts within the microbiome, we will record the microbial populations associated with A. thaliana in natural habitats over time and reconstruct them in directed networks. By combining the reconstruction of microbial networks, genome analysis of identified keystone isolates, and comprehensive transcriptional analysis of Moesziomyces in distinct microbial interactions, we will identify functionally relevant candidate yeast genes. Using reverse genetic approaches, we will produce modified yeasts to study the influence of identified genes in synthetic microbial communities and thus gain mechanistic insights into the molecular basis of microbial networks.

The DECRyPT Yeast project forms part of the DFG priority programme: Deconstuction and Reconstruction of the Plant Microbiota, DECRyPT

The central scientific objectives of this priority programme (SPP) are to obtain deep and potentially predictive insights into plant-microbiota associations and to develop pioneering reductionist approaches towards a molecular understanding of plant microbiota functions. This SPP will elucidate genetic factors underlying plant microbiota establishment, test presumed community adaptation in ecological contexts and define community-associated emergent properties.