Isolation and preliminary characterization of the phytochelatin synthase from the liverwort Marchantia polymorpha

The enzyme phytochelatin synthase (PCS) is a ubiquitous enzyme in land plants, and it is responsible for the synthesis of phytochelatins (PC). Previous studies suggest that PCS likely plays a pivotal role in the response to metal homeostatic needs and toxicity in all tracheopytes, but until now only a few handful investigations have been addressed to functional characterization of PCS from bryophytes. In the study performed here, we cloned the only PCS gene present in the genome of the model liverwort Marchantia polymorpha. The gene (MpPCS) codes for a predicted protein of 530 aminoacids with an overall identity of 49% to Arabidopsis thaliana PCS1 protein. In line with all other known PCS, also MpPCS has a highly conserved N-terminal domain and a poorly conserved C-terminus encompassing approximately half of the protein. Overexpression of MpPCS in a mutant yeast strain susceptible to heavy metal toxicity increases its tolerance to both cadmium and excess zinc. We are now going to verify, by mass-spectrophotometry, if significantly higher amounts of PCs are synthesized both in cadmium-exposed M. polymorpha gametophytes and in the overexpressor yeast strain, compared respectively with metal-untreated gametophytes and yeast transformed with an empty vector, thus demonstrating that the liverwort possesses the ability of producing PCs and that the encoded enzyme is indeed a functional PCS. Even more so, we want to demonstrate that, when overexpressed in Arabidopsis thaliana cad1-3 mutant (which is lacking PCS1), MpPCS can rescue the mutant phenotype of increased susceptibility to cadmium, indicating a remarkable evolutionary conservation of PCS function between A. thaliana and M. polymorpha.