Ancient duplication and functional differentiation of phytochelatin synthases is conserved in plant genomes

Despite the paramount importance in metal(loid) detoxification by phytochelatin synthase (PCS) genes, no comprehensive analysis of their evolutionary patterns has been carried out in land plants in general and in crops in particular. A phylogenetic large-scale analysis of gene duplication in angiosperms was carried out followed by in vitro recombinant protein assays as well as complementation analysis (growth, thiol-peptides, elements) of Arabidopsis cad1-3 mutant with four representative PCS genes from two model crop species, Malus domestica and Medicago truncatula. We uncovered a so far undetected ancient tandem duplication (D duplication) spanning the whole core eudicotyledon radiation. Complementation with PCS genes from both D-subclades from M. domestica and M. truncatula displayed clear in vivo conservation of the differences between D1 and D2 paralogous proteins in plant growth, phytochelatin and glutathione pools, as well as element contents under heavy metal(loid) stress. In vitro recombinant PCS analysis identified analogous patterns of differentiation, showing a higher activity of D2 PCS genes, so far largely overlooked, compared to their paralogs from the D1 clade. This suggests that in many other crop species where the duplication is present, the D2 copy might play a significant role in metal(loid) detoxification. The retention of both PCS paralogs and of their functional features for such long divergence time suggests that PCS copy number could be constrained by functional specialization and/or gene dosage sensitivity. These results uncover the patterns of PCS evolution in plant genomes and of functional specialization of their paralogs in the genomes of two important model crops.