Abstract
Phytochelatin synthases (PCSs) play essential roles in detoxification of a broad range of heavy metals in plants and other organisms. Until now, however, no PCS gene from liverworts, the earliest branch of land plants and possibly the first one to acquire a PCS with a C-terminal domain, has been characterized. In this study, we isolated and functionally characterized the first PCS gene from a liverwort, Marchantia polymorpha (MpPCS). MpPCS is constitutively expressed in all organs examined, with stronger expression in thallus midrib. The gene expression is repressed by Cd2+ and Zn2+. The ability of MpPCS to increase heavy metal resistance in yeast and to complement cad1-3 (the null mutant of the Arabidopsis ortholog AtPCS1) proves its function as the only PCS from M. polymorpha. Site-directed mutagenesis of the most conserved cysteines of the C-terminus of the enzyme further uncovered that two twin-cysteine motifs repress, to different extents, enzyme activation by heavy metal exposure. These results highlight an ancestral function of the PCS elusive C-terminus as a regulatory domain inhibiting enzyme overactivation by essential and non-essential heavy metals. The latter finding may be relevant for obtaining crops with decreased root to shoot mobility of cadmium, thus preventing its accumulation in the food chain.
Highlights
Plants are sessile organisms, they have evolved diverse defense mechanisms such as accumulation and detoxification of different metals to adapt to environmental stresses related to the mineral composition of soil
A single copy of Phytochelatin synthases (PCSs) is present in the M. polymorpha genome, in contrast to the two copies found in Arabidopsis and many other higher plant species (Filiz et al, 2019)
Decades of studies have shown that PCn are constitutively present in various plant lineages and play critical roles for detoxification/homeostasis of a wide range of heavy metals (Degola et al, 2014; Kühnlenz et al, 2016; Fontanini et al, 2018)
Summary
They have evolved diverse defense mechanisms such as accumulation and detoxification of different metals to adapt to environmental stresses related to the mineral composition of soil. Some heavy metals such as zinc (Zn), copper (Cu), and iron (Fe) are essential for plant growth and development, as they are cofactors in protein structural and catalytic components, mediating ligand interactions and redox reactions (Giles et al, 2003; Olson et al, 2013; Schmidt and Husted, 2019). Phytochelatins (PCn) are among the most important and studied chelators for heavy metal detoxification (Clemens, 2019)
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