Pteris vittata L. is able to counteract the toxicity induced by cadmium combined with arsenic

Pteris vittata L. (Chinese brake fern) is the first fern to be identified as a hyperaccumulator of the semimetal arsenic (As). The fern can accumulate more than 27000 mg As kg 1 dry weight in its fronds and for this reason it has been proposed to remove As from contaminated soil and water. Due to excessive agricultural and industrial practices, frequently As is released into the environment with other toxic metals, such as Cadmium (Cd). Both Cd and As are a threat for human health and ecosystems due to their accumulation and long permanence in the soil, in the food chain and locally in drinking water. The toxicity of Cd and As, for plants and animals, is due to their high reactivity with biological molecules and to their similarity with essential ions that they can replace altering cellular homeostasis. The plants easily adsorb these elements through their root apparatus. In plant cells, Cd and As cause oxidative stress reducing glutathione (GSH) level with severe consequences at all organism level. For this reason there is a need to find a natural system able to remove As and Cd efficiently. Lately, it was demonstrated that the fern is able to absorb and accumulate higher levels of Cd and As when exposed to both elements in comparison with when exposed to single pollutants. The interaction between Cd and As in the soil may have synergic or antagonistic effects on uptake and accumulation of the two ions in the plant organs. In the plant cells, Cd and As induce the synthesis of small peptides called phytochelatins (PCs), with a general structure (g-Glu-Cys)n-Gly (n = 2 11). These peptides are able to form complexes with metals and semimetals (such as Cd and As) in order to reduce their toxicity by sequestering them in the vacuole. The fern also produces PCs after As exposure. Today the response of Pteris vittata to Cd and As combined toxicity requires further investigation because it is unknown whether the fern uses the PCs as detoxification system when exposed to both pollutants and if other strategies are implemented. Thus,the aim of this research was to investigate if and how the fern is able to counteract metal and semimetal toxicity by analyzing the formation of phytochelatin complexes and the morphogenic responses when exposed simultaneously to Cd and As. The plant response was evaluated by analyzing Cd and As accumulation in roots and fronds, by measuring the plant biomass, by analyzing histologically the fronds and evaluating the formation of PC-As complexes. Furthermore the exudates extruded from the fronds were chemically analyzed to assess the contribution of the extrusion mechanism to the metals detoxification in the fern. The results show that plant morphology is strongly affected by Cd presence combined with As. In addition their simultaneous exposure increases the semimetal uptake while reduces the metal one. The exudates extruded from fronds are composed by both Cd and As. Furthermore in our results, the fern exposed to As and Cd together forms different As-PC complexes in the roots in comparison with the fronds. In conclusion, we can state that Pteris vittata activates strategies that helps it respond to the additive toxicity of Cd combined with As.