Background: While recent advances in next generation sequencing technologies have enabled researchers to
readily identify countless microbial species in soil, rhizosphere, and phyllosphere microbiomes, the biological
functions of the majority of these species are unknown. Functional studies are therefore urgently needed in order
to characterize the plethora of microorganisms that are being identified and to point out species that may be used
for biotechnology or plant protection. Here, we used a dual culture assay and growth analyses to characterise
yeasts (40 different isolates) and their antagonistic effect on 16 filamentous fungi; comprising plant pathogens,
antagonists, and saprophytes.
Results: Overall, this competition screen of 640 pairwise combinations revealed a broad range of outcomes,
ranging from small stimulatory effects of some yeasts up to a growth inhibition of more than 80% by individual
species. On average, yeasts isolated from soil suppressed filamentous fungi more strongly than phyllosphere yeasts
and the antagonistic activity was a species-/isolate-specific property and not dependent on the filamentous fungus
a yeast was interacting with. The isolates with the strongest antagonistic activity were Metschnikowia pulcherrima,
Hanseniaspora sp., Cyberlindnera sargentensis, Aureobasidium pullulans, Candida subhashii, and Pichia kluyveri. Among
these, the soil yeasts (C. sargentensis, A. pullulans, C. subhashii) assimilated and/or oxidized more di-, tri- and
tetrasaccharides and organic acids than yeasts from the phyllosphere. Only the two yeasts C. subhashii and M.
pulcherrima were able to grow with N-acetyl-glucosamine as carbon source.
Conclusions: The competition assays and physiological experiments described here identified known antagonists
that have been implicated in the biological control of plant pathogenic fungi in the past, but also little
characterised species such as C. subhashii. Overall, soil yeasts were more antagonistic and metabolically versatile
than yeasts from the phyllosphere. Noteworthy was the strong antagonistic activity of the soil yeast C. subhashii,
which had so far only been described from a clinical sample and not been studied with respect to biocontrol.
Based on binary competition assays and growth analyses (e.g., on different carbon sources, growth in root
exudates), C. subhashii was identified as a competitive and antagonistic soil yeast with potential as a novel
biocontrol agent against plant pathogenic fungi.