The capability to synthesize phytochelatins and the presence of constitutive and functional phytochelatin synthases are ancestral (plesiomorphic) characters for basal land plants

Abstract

Bryophytes, a paraphyletic group which includes liverworts, mosses, and hornworts, have been stated as land plants that under metal stress (particularly cadmium) do not synthesize metal-binding peptides such as phytochelatins. Moreover, very little information is available to date regarding phytochelatin synthesis in charophytes, postulated to be the direct ancestors of land plants, or in lycophytes, namely very basal tracheophytes. In this study, it was hypothesized that basal land plants and charophytes have the capability to produce phytochelatins and possess constitutive and functional phytochelatin synthases. To verify this hypothesis, twelve bryophyte species (six liverworts, four mosses, and two hornworts), three charophytes, and two lycophyte species were exposed to 0-36 μM cadmium for 72 h, and then assayed for: (i) glutathione and phytochelatin quali-quantitative content by HPLC and mass spectrometry; (ii) the presence of putative phytochelatin synthases by western blotting; and (iii) in vitro activity of phytochelatin synthases. Of all the species tested, ten produced phytochelatins in vivo, while the other seven did not. The presence of a constitutively expressed and functional phytochelatin synthase was demonstrated in all the bryophyte lineages and in the lycophyte Selaginella denticulata, but not in the charophytes. Hence, current knowledge according to phytochelatins have been stated as being absent in bryophytes was therefore confuted by this work. It is argued that the capability to synthesize phytochelatins, as well as the presence of active phytochelatin synthases, are ancestral (plesiomorphic) characters for basal land plants.

Expression of Caenorhabditis elegans PCS in the AtPCS1‐deficient Arabidopsis thaliana cad1‐3 mutant separates the metal tolerance and non‐host resistance functions of phytochelatin synthases

“…The presence of PCS genes is not restricted to higher plants. Recent analyses demonstrated PCS protein expression in all bryophyte lineages as well as in lycophytes (Petraglia et al 2014). Systematic mapping of PCS-like sequences onto the 'Tree of life' revealed PCS occurrence in all major groups of eukaryotes (Clemens & Persoh 2009).…”

“…PCS genes are not only widely distributed in nature. They are also constitutively expressed in basically all cases investigated, which include organisms as diverse as the parasitic nematode Schistosoma mansoni (Ray & Williams 2011), the tunicate Ciona intestinalis (Franchi et al 2014), liverworts (Petraglia et al 2014) and A. thaliana (Vatamaniuk et al 2000). Both the widespread occurrence and the constitutive expression in all organs and developmental stages of the analysed species raised the question as to whether PCS have functions other than mediating the sequestration of nonessential metals, which most cells would hardly ever if at all be exposed to (Rea et al 2004;Clemens 2006).…”

Levels of Antioxidant Compound Glutathione in Moss from Industrial Areas

“…By contrast, when the value of PERI does not exceed 21, i.e., in SS with low levels of metal pollution, glutathione concentrations remain constant at a concentration equivalent to 40 µg g −1 (SS D1, D2, E1, and E2; Figure 1A), and could be involved in other cellular functions (e.g., synthesis, redox turnover, metabolism, and signalling), as suggested by Noctor et al [19]. This threshold is not unique for all moss species, and it is worth noting that glutathione concentration in moss that is grown in controlled conditions is very variable depending on the species, and an increase of glutathione was already observed in three out of four species of moss that were exposed to Cd [28].…”

“…Although cysteine-rich heavy metal-binding peptides, metallothioneins, organic acids, amino acids, and glutathione can carry out this function, in most eukaryotes it is generally carried out by PCs. However, in bryophytes it appears that the function is mainly carried out by glutathione [10,25,26,28]. Thus, mosses living in environments that are characterised by high levels of metals are expected to have higher concentrations of glutathione than mosses living in areas characterised by low levels of metals.…”

“…Researchers investigating the glutathione response in terrestrial and aquatic mosses have observed increased glutathione concentrations, however not PC concentrations, in mosses that have been exposed to high concentrations of Cd, Ni, and Pb [10][11][12]25,27]. Only Petraglia et al [28] showed the presence of PC in the moss Sphagnum palustre and in several liverworts that were exposed to Cd. These findings suggest that a specific biochemical response occurs in bryophytes, and that metals may be chelated by glutathione during their transport through the cytoplasm, as suggested by Bruns et al [10].…”