Format: ORAL

Erika Bellini1,2*, Carlo Sorce1, Andrea Andreucci1, Valentina Vitelli1, Alessandro Saba1,3, Mingai Li4, Claudio Varotto4, Luigi Sanità di Toppi1

1University of Pisa, Pisa, Italy; 2Sapienza University of Rome, Roma, Italy; 3Center for Instrument Sharing of the University of Pisa (CISUP), Pisa, Italy; 4Research and Innovation Centre, Fondazione E. Mach, Trento, Italy.

As with all living organisms, photoautotrophs face challenges in regulating metal homeostasis. Plants have evolved mechanisms to control the uptake and accumulation of both essential and non-essential metals, including chelation and sequestration by ligands such as phytochelatins (PCn). These thiol-peptides are synthesized by the phytochelatin synthase enzyme (PCS). PCn production is a widespread strategy for detoxifying harmful metal(loid)s, such as cadmium (Cd), mercury (Hg), lead (Pb), and arsenic (As). However, recent studies suggest that PCn and PCS roles may extend beyond detoxification of harmful metal(loid)s, to include responses to homeostatic needs and toxicity of essential metals, including copper (Cu), zinc (Zn), and iron (Fe). Yet, PCS function in early plant lineages remains largely underexplored. This study focuses on the PCS of the model liverwort Marchantia polymorpha L., namely MpPCS. Using a recombinant MpPCS protein, we confirmed that the MpPCS gene encodes a 530 amino acids polypeptide with a molecular mass of ~57 kDa. In vitro activity assays with Zn, Cu, and Fe demonstrated that MpPCS catalyzes a transpeptidation reaction, with activation effectiveness in the order of Zn, Cu, and, to a lesser extent, Fe. Gene expression and in vivo function of PCS were investigated under conditions of starvation, metal excess and physiological concentrations of Fe, Cu, and Zn, with a range of different exposure times. Metal-induced impacts were monitored through photochemical efficiency analysis using a chlorophyll fluorometer, offering insights into light absorption, trapping, photochemistry, electron transport, and PSII structure. Our results reveal that MpPCS plays a crucial role in detoxifying metal excess and regulating the homeostatic control of metal micronutrients. Thus, thiol-peptides such as glutathione and PCn have proven their efficiency in managing metal needs and detoxifying their excess. It is worth noting that PCn production appears to be regulated by enzyme activity rather than gene expression levels. Furthermore, this study highlights a potential novel extracellular detoxifying role for PCn and glutathione in M. polymorpha.