Yeasts on Tree Gum: Taxonomic, Genetic and Phenotypic Diversity.

Depending on their composition, exudates oozing from trees can support or inhibit the growth of microorganisms. Resins released by conifers have a strong antimicrobial effect. Sugar-rich saps are usually colonised by diverse microorganisms including yeasts. In this study, the yeasts associated with the polysaccharide-containing gums of cherry trees were examined. 227 strains were isolated from gums of 53 abandoned road-side and orchard sour cherry trees in 20 locations. 12 ascomycetous and 19 basidiomycetous species were found among the isolates. All grew very poorly on laboratory media containing gum as the only source of nutrients. The high taxonomic diversity and poor growth of the isolates on gum-based media indicate that the cherry-tree gum is unlikely to be permanently colonised by a characteristic yeast community. Strains of 16 species showed antifungal antagonism. The Metschnikowia pulcherrima isolates killed the cells of the test organisms by releasing pulcherriminic acid. These isolates showed high intragenomic barcode (D1/D2 domain and ITS) diversity due to single-nucleotide dimorphism (ambiguous nucleotides) in many sites. In the D1/D2 domain, the ambiguous sites were concentrated in four segments that correspond to the back-folding strands of the stems of the D1 and D2 loops of the secondary structure of the LSU rRNA.