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

Abstract

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 startingpoint 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.

Bryophytes and heavy metals: a review

“…During that time, however, an increase of GSH content, primarily under Cd treatment, could be observed. The findings of this study and the studies performed by Carginale et al (2004) and Degola et al (2014) showed that intracellular Cd is primarily stored in the vacuoles of bryophyte cells. Here, high amounts of S and P were also observed, which led to the conclusion that one of the dominant mechanisms of bryophyte tolerance to heavy metals, at least to Cd, is the formation of cytoplasmatic GSH/Cd complexes and their subsequent transport into vacuoles, where they can be degraded and the Cd accumulated as phosphate.…”

“…More importantly, it has also been found that most of the intracellular Cd is bound to the thiol-rich compounds of similar weight such as phytochelatins, indicating that, as in other plants, these compounds may constitute the principal mechanism for heavy metal sequestration. Further, Degola et al (2014) have unambiguously confirmed that the compounds found in the L. cruciata are phytochelatins and that some of them (such as PC2) may constantly be present in bryophyte cells managing the homeostasis of the micronutrients. However, testing the effects of different heavy metals on the induction of phytochelatin synthesis, and comparing the results of this study with those from a study of Arabidopsis thaliana (L.) Heynh., led to the conclusion that bryophyte PCs and their synthases have a narrower function, involved only in the regulation of the Fe/Zn homeostasis and detoxification of Cd.…”

“…In the liverwort Lunularia cruciata (L.) Dumort. treated with Cd, Degola et al (2014) also found numerous small vacuoles containing electron-dense precipitates, which were absent in the control plants. The additional analysis showed that Cd and sulphur co-localized in these vacuoles indicating an important role of sulphates in the sequestration of intracellular heavy metals.…”

“…The formed complexes between the heavy metal ions and PCs can then be 9 transported into the vacuole, decreasing the concentration of metals in the cytoplasm and protecting the plants from their deleterious effects (Yadav 2010). These mechanisms also operate in bryophytes as has been demonstrated by Carginale et al (2004) and Degola et al (2014) in studies on the liverwort Lunularia cruciata exposed to cadmium stress. The results have shown that Cd is accumulated in the vacuoles of the Cd-stressed liverwort and that this is accompanied by an increase of sulphur concentration in this organelle.…”

“…In this sense, bryophytes, especially mosses, are very important (Markert andWeckert 1989, Boquete et al 2014). Bryophytes were the first green plants to colonize the terrestrial environment (Nickrent et al 2000), and as such had to evolve mechanisms to cope with the much greater amounts of heavy metals present on land than in the water (Degola et al 2014). These mechanisms resulted in the ability of many bryophytes to be consistent colonizers of metal contaminated 3 environments (Shaw et al 1989), or to accumulate large amounts of heavy metals, in extremely polluted areas without any visible negative effect on their growth and development (Sassmann et al 2010).…”