An evolutionary survey on the intracellular response to heavy metals in plants

The enzyme phytochelatin synthase (PCS) is a γ-glutamylcysteine dipeptidyl transpeptidase constitutively expressed in the majority of plants, but also in other eukaryota and some cyanobacteria. In the presence of metal(loid)s such as cadmium, lead, mercury, arsenic and excesses of copper and zinc, PCS is promptly activated, leading to the production of phytochelatins (PCn), thiol-peptide compounds, able to chelate these metals and compartmentalise them inside the vacuole, with a consequent detoxification of the cell environment. Nonetheless, since PCS is ubiquitously and constitutively expressed in plants, even in the complete absence of toxic metal(loid)s, it is postulated that this enzyme could have primordial function(s) other than mere metal detoxification. This hypothesis is supported by the evidence that plants, able to hyper-accumulate toxic metal(loid)s, in no way show an adaptative increase of PCn synthesis. Thus, a relevant primary function performed by PCS might be the regulation of the physiological requirements of Fe(II) and Fe(III), since this element is and has always been widely present in all environments, even though it has serious solubility and bioavailability problems for the vast majority of organisms. To validate this idea, I intend to study the involvement of PCS enzyme in the homeostatic balance of Fe using as model organism the liverwort Marchantia polymorpha, an ancient bryophyte interesting for its pivotal position in land plant phylogeny and known for the capability to accumulate metals. This investigation may possibly be carried out using other early bryophytes and tracheophytes. Furthermore, other elements could be studied, such as zinc, in possible cross-homeostasis with iron, as well as copper, but also toxic metals such as cadmium, the main inducer of PCn, and arsenic, present in inorganic forms such as arsenate and arsenite. Due the multi-tasking nature of PCS enzyme (for example its hypothesized function in callose deposition and in auxin metabolism of the model-plant Arabidopsis thaliana), it seems therefore promising to investigate the involvement of PCS in function(s) other than detoxification of toxic metals.