Looking at the present to gain knowledge on the past: what might be the primeval function of the phytochelatin synthase enzyme?

The phytochelatin synthase (PCS) enzyme is constitutively expressed in the majority of plants, other eukaryotes and certain cyanobacteria. In the presence of metal(loid)s (i.e., Cd, Pb, Hg, As; excess Cu and Zn) eukaryotic PCS is activated and produces the so-called phytochelatins, able to bind and compartmentalise these elements inside the vacuole. The constitutive expression of PCS in the plant clade, even in the absence of toxic metal(loid)s, would lead us to postulate other functions of this enzyme, besides toxic metal detoxification; this hypothesis is also supported by the fact that metal hyperaccumulation in plants do not depend on a “super-synthesis” of phytochelatins as far as their metal hypertolerance is concerned. Iron (Fe) has always been widely present in all environments, but at the same time its solubility and bioavailability poses serious problems for the vast majority of organisms. Thus, we hypothesise that PCS in plants and cyanobacteria might possess a primeval and ubiquitarian function geared towards control of physiological requirements of Fe(II) and Fe(III), perhaps in “cross-homeostasis” with Zn. To this end, our work deals with PCS identification and characterisation in freshwater green algae and basal land plants, namely the charophytes Nitella mucronata and Chara vulgaris, the liverworts Lunularia cruciata and Marchantia polymorpha, the lycophytes Selaginella kraussiana, S. denticulata and S. moellendorffii, and some strains of cyanobacteria (Geitlerinema sp., Gloeobacter violaceus, Nostoc sp.). Functional characterisation of the PCS has been carried out, focusing in particular on the potential post-translational controls of the enzyme activity in the presence/absence of physiological concentrations of Fe and Zn, as well as after exposure to toxic metal(loid)s, such as Cd and As. The results would allow us to achieve comparative characterisations of ancestral PCSs and further clarify their functions in early plants and cyanobacteria.