Lignins result from the oxidative polymerization of three hydroxycinnamyl (p-coumaryl, coniferyl, and sinapyl) alcohols in a reaction mediated by peroxidases. The most important of these is the cationic peroxidase from Zinnia elegans (ZePrx), an enzyme considered to be responsible for the last step of lignification in this plant. Bibliographical evidence indicates that the arabidopsis peroxidase 72 (AtPrx72), which is homolog to ZePrx, could have an important role in lignification. For this reason, we performed a bioinformatic, histochemical, photosynthetic, and phenotypical and lignin composition analysis of an arabidopsis knock-out mutant of AtPrx72 with the aim of characterizing the effects that occurred due to the absence of expression of this peroxidase from the aspects of plant physiology such as vascular development, lignification, and photosynthesis. In silico analyses indicated a high homology between AtPrx72 and ZePrx, cell wall localization and probably optimal levels of translation of AtPrx72. The histochemical study revealed a low content in syringyl units and a decrease in the amount of lignin in the atprx72 mutant plants compared to WT. The atprx72 mutant plants grew more slowly than WT plants, with both smaller rosette and principal stem, and with fewer branches and siliques than the WT plants. Lastly, chlorophyll a fluorescence revealed a significant decrease in ΦPSII and q L in atprx72 mutant plants that could be related to changes in carbon partitioning and/or utilization of redox equivalents in arabidopsis metabolism. The results suggest an important role of AtPrx72 in lignin biosynthesis. In addition, knock-out plants were able to respond and adapt to an insufficiency of lignification.

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Zinnia elegans constitutes one of the most useful model systems for studying xylem differentiation, which simultaneously involves secondary cell wall synthesis, cell wall lignification, and programmed cell death. Likewise, the in vitro culture system of Z. elegans has been the best characterized as the differentiation of mesophyll cells into tracheary elements allows study of the biochemistry and physiology of xylogenesis free from the complexity that heterogeneous plant tissues impose. Moreover, Z. elegans has emerged as an excellent plant model to study the involvement of peroxidases in cell wall lignification. This is due to the simplicity and duality of the lignification pattern shown by the stems and hypocotyls, and to the basic nature of the peroxidase isoenzyme. This protein is expressed not only in hypocotyls and stems but also in mesophyll cells transdifferentiating into tracheary elements. Therefore, not only does this peroxidase fulfil all the catalytic requirements to be involved in lignification overcoming all restrictions imposed by the polymerization step, but also its expression is inherent in lignification. In fact, its basic nature is not exceptional since basic peroxidases are differentially expressed during lignification in other model systems, showing unusual and unique biochemical properties such as oxidation of syringyl moieties. This review focuses on the experiments which led to a better understanding of the lignification process in Zinnia, starting with the basic knowledge about the lignin pattern in this plant, how lignification takes place, and how a sole basic peroxidase with unusual catalytic properties is involved and regulated by hormones, H2O2, and nitric oxide.

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The tolerance of the metallophyte Silene vulgaris, a plant suitable for the phytostabilisation of metal(loid)contaminated soils, to arsenic (As), mercury (Hg) and cadmium (Cd) was evaluated in a semi-hydroponic culture system under controlled environmental conditions. The appearance of oxidative stress, alteration of photochemical processes and modification of biothiol content were studied as physiological parameters of metal(loid) stress in plants treated with 0, 6 and 30 mM (As, Hg or Cd) for 7 days. In spite of the metal(loid) excluder behaviour of S. vulgaris, Cd was translocated to the aerial part of the plant at a higher rate than Hg or As. The major toxic effects were observed in roots, where lipid peroxidation was increased in a dose-dependent manner. Redox enzymes such as glutathione reductase (GR) were severely inhibited by Hg, whereas GR was overexpressed. The accumulation of Cd produced a remarkable production of phytochelatins (PCs) in roots, whereas Hg and As led to modest PCs synthesis. There was a severe loss of chlorophyll content in Cd-treated plants, accompanied with a significant decrease in photosystem II efficiency (WPSII) and photochemical quenching (qP). Similar negative effects were observed in Hg-and Asexposed plants, but to a lesser degree. The exposure to the highest dose of each toxic element (30 mM) caused depletion of the light harvesting complex b1 protein. In conclusion, specific stress signatures to each metal(loid) were observed, with As being the least toxic element, suggesting that different mechanisms of tolerance were exerted. These results could be applied in future experiments to select tolerant ecotypes to optimize the phytostabilisation of metal(loid) multipolluted soils.