Effect of environmental factors on polyketide synthase gene expression, usnic acid biosynthesis, and ground cover of a lichen-fungus, Cladonia uncialis

Lichens are composed of symbiotic slow-growing organisms and are often exposed to extreme microenvironmental conditions, resulting in the production of unique secondary metabolites. One of the most commonly produced secondary metabolites is usnic acid, which is thought to be produced by two genes. The objectives of the present study were to compare polyketide synthase (PKS) gene expression and usnic acid concentration in Cladonia uncialis (L.) Weber ex F.H. Wigg., with two environmental factors. Seventy-five lichen samples were collected from three locations in Newfoundland, Canada, using a strip transect method (×5 transects, ×5 quadrats). Usnic acid concentration was measured using the liquid chromatography tandem mass-spectrometric method and gene expression of two PKS genes (methylphloracetophenone oxidase (MPAO) and methylphloracetophenone synthase (MPAS)) was examined using quantitative real-time PCR. The results showed that percent ground cover of C. uncialis was affected by soil pH level but not soil moisture, and usnic acid concentration was not affected by either soil pH or soil moisture. MPAO gene expression level was significantly affected by soil pH level but not soil moisture, and MPAS gene expression level was not affected by either soil pH level or soil moisture. There was no significant relationship between MPAS and MPAO gene expression levels and usnic acid concentration. These findings suggest that soil pH may be important for the production of usnic acid by C. uncialis but the genes involved require further study.