Evolution and functional differentiation of recently diverged phytochelatin synthase genes from Arundo donax L.

Phytochelatin synthases (PCS) play pivotal roles in the detoxification of heavy metals and metalloids in plants (1). Despite such relevance, little information on the evolution of recently duplicated PCS genes in plant species is available. Here we characterize the evolution and functional differentiation of three PCS genes from the giant reed (Arundo donax L.), a biomass/bioenergy crop with remarkable resistance to cadmium (Cd) and other heavy metals (2). Phylogenetic reconstruction with PCS genes from fully sequenced monocotyledonous genomes indicates (3) that the three A. donax PCS, namely AdPCS1-3, form a monophyletic clade (Fig. 1). AdPCS1-3 genes are expressed at low levels in many A. donax organs and, in particular, display different levels of Cdresponsive expression in roots (Fig. 2). Overexpression of AdPCS1-3 in Arabidopsis thaliana and yeast reproduces the phenotype of functional PCS genes (4). Mass-spectrometry analyses confirm that AdPCS1-3 are all functional enzymes, but also evidence significant differences in the amount of the phytochelatins synthesized (Fig. 3). Moreover, heterogeneous evolutionary rates characterize the AdPCS1-3 genes, indicative of relaxed natural selection (5). These results highlight the elevated functional differentiation of A. donax PCS genes from both a transcriptional and an enzymatic point of view, providing evidence of the high evolvability of PCS genes and of plant responsiveness to heavy metal stress.