Abstract
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.
Highlights
Lichens are intimate symbioses, which result from the association between fungi and green algae or cyanobacteria
The slow growth rates of most lichens and prolonged exposure to environmental factors are thought to explain the protective function and production of large quantities of secondary metabolites (Rundel 1978; Solhaug and Gauslaa 1996; Thomas and Nash 2008); little is known about the effect of environmental conditions on percent cover of the lichen thallus, which allows for more secondary metabolite production, and the regulation of secondary metabolite production
The percent ground cover of C. uncialis was greater in the Birchy Lake site compared to the Northern Peninsula and Port aux Basques sites
Summary
Lichens are intimate symbioses, which result from the association between fungi (mycobionts) and green algae or cyanobacteria (photobionts). The large number of known secondary metabolites in lichens, over 1050 (Stocker-Wörgötter 2008), may comprise up to 20% of the dry weight of a lichen thallus (Shukla et al 2010), making them attractive for further investigation The majority of these metabolites are thought to provide protection from extreme or prolonged environmental conditions such as sunlight (Rao and LeBlanc 1965; Rundel 1978; Solhaug and Gauslaa 1996; Nybakken et al 2004; McEvoy et al 2006; Asplund et al 2017), herbivory (Asplund et al 2010), allelochemicals (Rundel 1978; Romagni et al 2004), excessive moisture changes (Palmer and Friedmann 1990; Culberson and Armaleo 1992), acid tolerance and pH changes (Hauck and Ju€rgens 2008), and heavy metal accumulation (Białonska and Dayan 2005; Hauck et al 2009). The slow growth rates of most lichens and prolonged exposure to environmental factors are thought to explain the protective function and production of large quantities of secondary metabolites (Rundel 1978; Solhaug and Gauslaa 1996; Thomas and Nash 2008); little is known about the effect of environmental conditions on percent cover of the lichen thallus, which allows for more secondary metabolite production, and the regulation of secondary metabolite production
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