Effect of Environmental Change on Secondary Metabolite Production in Lichen-Forming Fungi

The production and regulation of secondary metabolites in non-lichenized fungi, mainly ascomycetes, has been reviewed by a number of authors with an emerging understanding of the biosynthesis and the pathways involved in regulation (Keller et al., 2005; Yu & Keller 2005; and others). However, lichenized fungi make up almost half of all known ascomycetes (Kirk et al., 2001) and are known to produce over 800 secondary metabolites, most of which are unique to lichenized fungi. Many of these compounds have bioactive properties (Huneck, 1999) and some studies have shown or suggested that secondary metabolite production is influenced by changes in culture conditions, which might be regarded as environmental changes. Intense investigation of the changes in production of these unique bioactive secondary metabolites from lichen fungi have been hampered by problems associated with isolating and growing cultures of lichen fungi. Lichens have been studied for more than two centuries as morphological entities but experimental lichenology has remained a nearly unexplored scientific field for many decades because of the slow growing nature of lichens. Thomas (1939 in Stocker-Worgotter, 2001) reported the first successful resynthesis of Cladonia pyxidata. Since the 1970’s, one major goal of experimental lichenology has been the improvement and optimization of culture conditions of lichen fungi. Culture techniques for lichen fungi have improved in recent years allowing for further research on these challenging organisms. Therefore, with greater access to cultures of lichen symbionts and progression of knowledge of non-lichenized fungi, studies are just beginning to accumulate on genes involved in production of secondary metabolites from lichen fungi; and the effects of the environment on the expression of these genes by observations in ecological studies, and through experimentation by manipulating culture conditions. Fungal secondary metabolism is covered by extensive body of literature (see Bennett & Ciegler, 1983). Secondary metabolism is not required for survival and its products are dispensible whereas primary metabolism is essential for survival with anabolic and catabolic activities to maintain life. Secondary metabolites are chemically diverse but are produced from a few key intermediates of primary metabolism, and are generally categorized by the intermediates from which they are produced. Bennett and Ciegler (1983) summarize six categories of secondary metabolites derived from different primary intermediates. Although fungal secondary metabolites are extensive, they are generally