Format: ORAL

Luigi Sanita di Toppi

Department of Biology, University of Pisa, Italy

My contribution will deal, in an evolutionary perspective, with traditional and new roles of the phytochelatin synthase enzyme (PCS), the functions of which still appear shrouded in a degree of mystery. I will illustrate and discuss the five main key-points that characterize this peculiar multi-tasking enzyme in plants: 1) PCS is a papain-like Clan CA cysteine peptidase; in particular, the eukaryotic PCS is a gamma-glutamylcysteine-dipeptidyl-(trans)peptidase. 2) PCS is constitutively expressed in plants, algae, lichen chlorophyte photobionts, as well as in other eukaryotes, such as Fungi, some Animalia and Amoebozoa, SAR (especially Bacillariophyceae and Phaeophyta) and, more rarely, Excavata. Moreover, prokaryotic PCSs, which possess significant sequence homologies with those of plants, have been identified in cyanobacteria and in some proteobacteria. 3) Through its transpeptidasic products – i.e., the metal(loid)-binding oligopeptides named phytochelatins (PCn) – PCS detoxifies toxic heavy metals and metalloids (e.g., cadmium, lead, mercury, arsenic, etc.), but can also help regulate the homeostatic needs of metal micronutrients such as iron, zinc, and copper. 4) Being a peptidase (see point 1), PCS can also hydrolyze GSH and GS-conjugated xenobiotics in the cytosolic environment through the cleavage of glycine from GSH. The GS-conjugation capacity of plant PCSs is a feature also detected in prokaryotic PCSs, and the breakdown of GS-conjugates may accordingly be considered a primigenial function of PCS, perhaps older than the biosynthesis of PCn. 5) Lastly, a yet limited number of experimental works is increasingly highlighting the importance of PCS in plant defence against phytopathogens, essentially in terms of microbe-triggered-immunity. Advances in the overall area of knowledge on PCS may well help clarify the evolutionary history and function(s) of this ubiquitous and partly mysterious “multi-tasking” enzyme.