Parietin in the tolerant lichen Xanthoria parietina (L.) Th. Fr. increases protection of Trebouxia photobionts from cadmium excess

Secondary metabolites of lichens can be involved in production of chelates with heavy metals. We hypothesized that parietin plays important role in protection of photobiont cells in Xanthoria parietina from an excess of cadmium ions. Two types of X. parietina lichen thalli, natural with presence of secondary metabolite parietin (p+) as well as without parietin (p?) were exposed to different doses of cadmium (up to 300 μmol g^?1 dw). Based on determination of the total and intracellular Cd-accumulation, ergosterol and thiobarbituric acid reactive substances (TBARS) content did not show statistically significant differences in the response of both types of thalli (p+ and p?). However, a stronger toxic effect of the highest Cd-dose on photosynthetic pigment content and chlorophyll a fluorescence was observed in the parietin-depleted thalli. The protective role of parietin against Cd excess was better supported and concluded from the differences observed in the production of non-protein thiol compounds (cysteine, glutathione and phytochelatins) involved in Cd detoxification. In the p+ thalli Cys content was stable but GSH content slightly decreased in the studied Cd range, while in the p? thalli these compounds were completely absent at high Cd doses. At Cd doses higher than 37.5 μmol Cd g^?1 dw, toxic to both types of X. parietina thalli, Cys and GSH contents were significantly higher in p+ than in p? thalli. Also, the photobiont partner in the p+ thalli was better protected of the metal exposition, and able to produce phytochelatins (PCs) over the whole range of metal, while in the p? thalli the production was completely inhibited at 75 μmol Cd g^?1 dw and higher concentrations, together with the inhibition of cysteine (Cys) and reduced glutathione (GSH) production. The obtained results indicate that the parietin layer is a natural barrier decreasing Cd access to algal cells in X. parietina. Comparison of PCs production appeared to be the most sensitive marker for estimation of Cd availability to photobiont in the symbiotic system.