A Cd/Fe/Zn-Responsive Phytochelatin Synthase is Constitutively Present in the Ancient Liverwort Lunularia cruciata (L.) Dumort

Lunularia cruciata occupies a very basal position in the phylogenetic tree of liverworts, which in turn have been recognized as a very early clade of land plants. It would therefore seem appropriate to take L. cruciata as the starting point for investigating character evolution in plants' metal(loid) response. One of the strongest evolutionary pressures for land colonization by plants has come from potential access to much greater amounts of nutritive ions from surface rocks, compared to water. This might have resulted in the need to precisely regulate trace element homeostasis and to minimize the risk of exposure to toxic concentrations of certain metals, prompting the evolution of a number of response mechanisms, such as synthesis of phytochelatins, metal(loid)-binding thiol-peptides. Accordingly, if the ability to synthesize phytochelatins and the occurrence of an active phytochelatin synthase are traits present in a basal liverwort species, and have been even reinforced in 'modern' tracheophytes, e.g. Arabidopsis thaliana, then such traits would presumably have played an essential role in plant fitness over time. Hence, we demonstrated here that: (i) L. cruciata compartmentalizes cadmium in the vacuoles of the phototosynthetic parenchyma by means of a phytochelatin-mediated detoxification strategy, and possesses a phytochelatin synthase that is activated by cadmium and homeostatic concentrations of iron(II) and zinc; and (ii) A. thaliana phytochelatin synthase displays a higher and broader response to several metal(loid)s [namely: cadmium, iron(II), zinc, copper, mercury, lead, arsenic(III)] than L. cruciata phytochelatin synthase.