The putative role of fumarprotocetraric acid in the manganese tolerance of the lichen Lecanora conizaeoides

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Hauck & Huneck (2007) and Hauck et al. (2007) established that lichen secondary chemistry is related to metal absorption in the apoplast of lichens. Depsidones occurring in the medulla of the Mn²⁺ and Cu²⁺-sensitive epiphytic lichen Hypogymnia physodes were found to reduce the absorption of Mn²⁺ and Cu²⁺ at cation exchange sites (Hauck & Huneck 2007). Several lichen substances, belonging to the depsidones, depsides, anthraquinones, or pulvinic acid derivatives, are known to absorb Fe³⁺ (Engstrom et al. 1980; Hauck et al. 2007). Occurrence of such lichen substances in lichens of Fe-poor sites, but rarity in lichens from Fe-rich sites suggests that they promote the intracellular uptake of Fe³⁺ (Hauck et al. 2007). In Acarosporion sinopicae lichens specialized on Fe-rich rock and slag, only two efficient Fe³⁺ absorbers are known, viz. the depsidone norstictic acid and the pulvinic acid derivative rhizocarpic acid. Their occurrence in Acarosporion sinopicae lichens is attributed to the fact that they reduce the absorption of Fe²⁺ at cation exchange sites despite their high affinity to Fe³⁺ (Hauck et al. 2007). The observations by Engstrom et al. (1980), Hauck & Huneck (2007) and Hauck et al. (2007), which were made in experiments with isolated lichen substances, suggest that these secondary metabolites control metal homeostasis in lichens by reducing the absorption of selected metal ions in the apoplast and by promoting the uptake of Fe³⁺.