From cyanobacteria to land plants: is the phytochelatin synthase enzyme necessary for both detoxification and homeostasis of metals?

The phytochelatin synthase (PCS) is a ubiquitous enzyme in land plants, and it is involved in the biosynthesis of peculiar thiol-peptide compounds, the so-called phytochelatins (PCn). PCn production represents indeed a widespread strategy addressed to detoxification of several harmful metal(loid)s, e.g. cadmium, mercury, lead, silver, arsenic, etc. However, previous sporadic studies carried out in angiosperms (especially in the model-plant Arabidopsis thaliana) suggest that PCS and PCn might also play a role in regulating the intracellular homeostatic needs of metal micronutrients, mainly zinc and copper. The possible involvement of the PCS in a primeval physiological phenomenon such as the homeostatic control of metal micronutrients might be also supported by the widespread and constitutive presence of this enzyme not only in angiosperms, but also in various early plants, including some bryophytes, charophytes, marine and freshwater algae, some fungi, lichens, and even in certain animal species. Moreover, PCS-like proteins, which share significant sequence homologies with plant PCSs, were identified in some photoautotrophic prokaryotes, such as the cyanobacterium Nostoc sp. PCC 7120. In fact, proteins encoded by the cyanobacterial genes seem to be primitive forms of PCS and represent an early stage in the enzyme evolution in photoautotrophic organisms. Thus, in the study here presented we focused our efforts on the evolutionary relevance of PCS, by investigating its activity and function in various phylogenetically-relevant groups of photoautotrophic organisms. The first hypothesis we verified was the following: are the PCSs expressed by the organisms under investigation all able to synthesize PCn through a full transpeptidation reaction? Accordingly, in vivo and in vitro PCS activities were evaluated by HPLC-ESI-MS-MS in three cyanobacterial strains (Nostoc sp. PCC 7120, Geitlerinema sp. PCC 7407 and Gloeobacter violaceous PCC 7421), in the model-liverwort Marchantia polymorpha, in the cosmopolitan moss Leptodictyum riparium, and in the model-angiosperm Arabidopsis thaliana. In all species investigated, the results demonstrated that PCSs – even the prokaryotic ones – were able to catalyze a full transpeptidation reaction. This finding evidences also a remarkable evolutionary conservation of the PCS function among different photoautotrophic organisms. The second important mechanistic point we are currently investigating is the following: why do so many photoautotrophic organisms express active PCSs even in the complete absence of toxic metal(loid)s? Consistently, we are studying in all the above-mentioned species if the PCS enzyme is involved in the homeostatic control of metallic micronutrients, such as iron(II)/(III), zinc and copper, both in conditions of total starvation and in the presence of physiological concentrations of these essential metals.