Phytochelatin synthase de-regulation in Marchantia polymorpha indicates cadmium detoxification as its primary ancestral function in land plants and provides a novel visual bioindicator for detection of this metal

Bryophytes are a large group of plants commonly used as bioindicators of metal and metalloid pollution. However, very little is known about the bryophyte genes responsible for metal(loid) detoxification, a knowledge that could provide novel tools for environmental monitoring applications.By genetic transformation and genome editing, in this study we obtained lines of the model bryophyte Marchantia polymorpha with de-regulated activity of the phytochelatin synthase (MpPCS) enzyme, responsible for the biosynthesis of metal(loid) chelators phytochelatins (PCn). Lack of PCn causes hypersensitivity to cadmium but has only subtle effects on sensitivity to excess of other highly toxic metals and the metalloid arsenic in M. polymorpha. Besides, our results indicate that MpPCS has a minor role in the maintenance of essential metals like zinc.As liverworts are sister to tracheophytes, these results suggest that the primary ancestral function of PCS genes in the common ancestor of all land plants may have been the detoxification of the non-essential cadmium ion. Hypersensitivity to cadmium further suggests that the Mppcs mutants could become useful bioindicators to specifically detect environmental contaminations of cadmium through direct visual assessment of plant growth and pigmentation.