Lichens respond to heavy metals by phytochelatin synthesis

The main aims of this paper are to investigate: if widespread lichen species (Xanthoria parietina, Physconia grisea and Physcia adscendens, collected in Italy and Poland) exposed to cadmium (Cd), lead (Pb) and zinc (Zn) synthesize phytochelatins; whether phytochelatins are synthesized by the heterotrophic fungus (mycobiont) or by the photosynthetic alga (photobiont), or by both of the partners; and if there are significant differences in phytochelatin production in the same lichen species growing at different latitudes. In the lichens investigated, Cd, Pb and Zn induced the biosynthesis of phytochelatins (PC2, PC3 and PC4) and some DES-GLYCIL derivatives. However, only the photobiont partners (the green microalgae of the genus Trebouxia) were capable of phytochelatin production and, by contrast, aposymbiotically grown mycobionts only produced glutathione. The Cd-exposed lichens grown in Italy produced significantly larger amounts of phytochelatins than the same species grown in Poland. An opposite trend was observed in Pb-exposed lichens. The trebouxioid lichens may gain an ecological advantage from their capability to counter heavy metal stress with prompt phytochelatin synthesis.