From Zinnia to Arabidopsis: approaching the involvement of peroxidases in lignification

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.

Bioinformatic and functional characterization of the basic peroxidase 72 from Arabidopsis thaliana involved in lignin biosynthesis

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.

The promoter region of the Zinnia elegans basic peroxidase isoenzyme gene contains cis-elements responsive to nitric oxide and hydrogen peroxide

NO and H2O2 are important biological messengers in plants. They are formed during xylem differentiation in Zinnia elegans and apparently play important roles during the xylogenesis. To ascertain the responsiveness of the Z. elegans peroxidase (ZePrx) to these endogenous signals, the effects of NO and H2O2 on ZePrx were studied. The results showed that ZePrx is up-regulated by NO and H2O2, as confirmed by RT-qPCR, and that its promoter contains multiple copies of all the putative cis-elements (ACGT box, OCS box, OPAQ box, L1BX, MYCL box and W box) known to confer regulation by NO and H2O2. Like other OCS elements, the OCS element of ZePrx contains the sequence TACG that is recognized by OBF5, a highly conserved bZIP transcription factor, and the 10 bp sequence, ACAaTTTTGG, which is recognized by OBP1, a Dof domain protein that binds down-stream the OCS element. Furthermore, the ZePrx OCS element is flanked by two CCAAT-like boxes, and encloses one auxin-responsive ARFAT element and two GA3-responsive Pyr boxes. Results also showed that ZePrx may be described as the first protein to be up-regulated by NO and H2O2, whose mRNA contains several short-longevity conferring elements, such as a downstream (DST) sequence analogous to the DSTs contained in the highly unstable SAUR transcripts. The presence of these regulatory elements strongly suggests that ZePrx is finely regulated, as one may expect from an enzyme that catalyzes the last irreversible step of the formation of lignins, the major irreversible sink for the photosynthetically fixed CO2.

Hormonal regulation of the basic peroxidase isoenzyme from Zinnia elegans

Xylem differentiation in plants is under strict hormonal regulation. Auxins and cytokinins, together with brassinosteroids (BRs), appear to be the main hormones controlling vascular differentiation. In this report, we study the effect of these hormones on the basic peroxidase isoenzyme from Zinnia elegans (ZePrx), an enzyme involved in lignin biosynthesis. Results showed that auxins and cytokinins induce ZePrx, similarly to the way in which they induce seedling secondary growth (in particular, metaxylem differentiation). Likewise, the exogenous application of BR reduces the levels of ZePrx, in a similar way to their capacity to inhibit seedling secondary growth. Consistent with this notion, the exogenous application of BR reverses the auxin/cytokinin-induced ZePrx expression, but has no effect on the auxin/cytokinin-induced secondary growth. This differential hormonal response is supported by the analysis of the ZePrx promoter, which contains (a) cis-elements directly responsive to these hormones and (b) cis-elements targets of the plethora of transcription factors, such as NAC, MYB, AP2, MADS and class III HD Zip, which are up-regulated during the auxin- and cytokinin-induced secondary growth. Taken together, these results suggest that ZePrx is directly and indirectly regulated by the plethora of hormones that control xylem differentiation, supporting the role of ZePrx in xylem lignification.

Analysis of the soluble cell wall proteome of gymnosperms

We analyzed the cell wall proteome of lignifying suspension cell cultures (SCCs) from four gymnosperms that differ in evolution degree. This analysis showed the presence of "peptide sequence tags" (PSTs) corresponding to glucan endo-1,3-beta-D-glucosidase, xyloglucan-endotrans-glucosylase/hydrolase, chitinases, thaumatin-like proteins and proteins involved in lignin/lignan biosynthesis, such as dirigent-like proteins and peroxidases. Surprisingly, and given the abundance of peroxidases in the cell wall proteome of these gymnosperms, PSTs corresponding to peroxidases were only detected in tryptic fragments of the cell wall proteome of Cycas revoluta. The current lack of knowledge regarding C. revoluta peroxidases led us to purify, characterize and partially sequence the peroxidases responsible for lignin biosynthesis in this species. This yielded three peroxidase-enriched fractions: CrPrx 1, CrPrx 2 and CrPrx 3. Analyses of tryptic peptides of CrPrx 2 (32kDa) and CrPrx 3 (26kDa) suggest that CrPrx 3 arises from CrPrx 2 by protein truncation, and that CrPrx 3 apparently constitutes a post-translational modification of CrPrx 2. That CrPrx 2 and CrPrx 3 are apparently the same enzyme was also deduced from the similarity between the k(cat) shown by both peroxidases for the three monolignols. These results emphasize the analogies between the cell wall proteome of gymnosperms and angiosperms, the complexity of the peroxidase proteome, and the difficulties involved in establishing fine structure-function relationships.

Structural motifs of syringyl peroxidases are conserved during angiosperm evolution

The most distinctive variation in the monomer composition of lignins in vascular land plants is that between the two main groups of seed plants. Thus, whereas gymnosperm (softwood) lignins are typically composed of guaiacyl (G) units, angiosperm (hardwood) lignins are largely composed of similar levels of G and syringyl (S) units. However, there are some studies that suggest that certain angiosperm peroxidases are unable to oxidize sinapyl alcohol, and a coniferyl alcohol shuttle has been proposed for oxidizing S units during the biosynthesis of lignins. With this in mind, a screening of the presence of S peroxidases in angiosperms (including woody species and forages) was performed. Contrarily to what might be expected, the intercellular washing fluids from lignifying tissues of 25 woody, herbaceous, and shrub species, belonging to both monocots and dicotyledons, all showed both S peroxidase activities and basic peroxidase isoenzymes analogous, with regard the isoelectric point, to the Zinnia elegans basic peroxidase isoenzyme, the only S peroxidase that has been fully characterized. These results led to the protein database in the search for homologies between angiosperm peroxidases and a true eudicot S peroxidase, the Z. elegans peroxidase. The findings showed that certain structural motifs of S peroxidases are conserved within the first 15 million years of angiosperm history, because they are found in peroxidases from the two major lineages of flowering plants, eumagnoliids and eudicotyledons, of note being the presence of these peroxidases in Amborella and Nymphaeales, which represent the first stages of angiosperm evolution. These phylogenetic studies also suggest that guaiacyl peroxidases apparently constitute the most "evolved state" of the plant peroxidase family evolution.

Characterization of the last step of lignin biosynthesis in Zinnia elegans suspension cell cultures

The last step of lignin biosynthesis in Zinnia elegans suspension cell cultures (SCCs) catalyzed by peroxidase (ZePrx) has been characterized. The k(3) values shown by ZePrx for the three monolignols revealed that sinapyl alcohol was the best substrate, and were proportional to their oxido/reduction potentials, signifying that these reactions are driven exclusively by redox thermodynamic forces. Feeding experiments demonstrate that cell wall lignification in SCCs is controlled by the rate of supply of H(2)O(2). The results also showed that sites for monolignol beta-O-4 cross-coupling in cell walls may be saturated, suggesting that the growth of the lineal lignin macromolecule is not infinite.

Fractionation of thylakoid membranes into grana and stroma thylakoids

The chloroplasts contain an extensive system of internal membranes or thylakoids in which all the light-harvesting and energy-transducing processes of the photosynthesis are located. Thylakoids are differentiated into stacked membrane regions (or grana thylakoids) and nonstacked membranes (or stroma thylakoids), each with a specialized structure and function. Both kinds of thylakoids can be separated by detergent-based methods or mechanical fragmentation such as sonication. We describe the fractionation of thylakoid membranes into grana and stroma thylakoids by treatment with the detergent digitonin and successive ultracentrifugation of the resultant vesicles. After their separation, the thylakoid fractions retain electron transport and enzymatic activities and are characterized using various parameters. The stroma thylakoids have higher chlorophyll a/chlorophyll b and protein/total chlorophyll ratios, and greater photosystem I and NADH dehydrogenase activities than the grana thylakoids. In the conditions used and on a protein basis of total thylakoids, the yield of stroma thylakoids is 5%, which is considerable taking into account that the stroma thylakoids are a minor component of total thylakoids.

Association of ferredoxin-NADP oxidoreductase with the chloroplastic pyridine nucleotide dehydrogenase complex in barley leaves

Barley (Hordeum vulgare L.) leaves were used to isolate and characterize the chloroplast NAD(P)H dehydrogenase complex. The stroma fraction and the thylakoid fraction solubilized with sodium deoxycholate were analyzed by native polyacrylamide gel electrophoresis, and the enzymes detected with NADH and nitroblue tetrazolium were electroeluted. The enzymes electroeluted from band S from the stroma fraction and from bands T1 (ET1) and T2 from the thylakoid fraction solubilized with sodium deoxycholate had ferredoxin-NADP oxidoreductase (FNR; EC 1.18.1.2) and NAD(P)H-FeCN oxidoreductase (NAD[P]H-FeCNR) activities. Their NADPH-FeCNR activities were inhibited by 2'-monophosphoadenosine-5'-diphosphoribose and by enzyme incubation with p-chloromercuriphenylsulfonic acid (p-CMPS), NADPH, and p-CMPS plus NADPH. They presented Michaelis constant NADPH values that were similar to those of FNRs from several sources. Their NADH-FeCNR activities, however, were not inhibited by 2'-monophosphoadenosine-5'-diphosphoribose but were weakly inhibited by enzyme incubation with NADH, p-CMPS, and p-CMPS plus NADH. We found that only ET1 contained two polypeptides of 29 and 35 kD, which reacted with the antibodies raised against the mitochondrial complex I TYKY subunit and the chloroplast ndhA gene product, respectively. However, all three enzymes contained two polypeptides of 35 and 53 kD, which reacted with the antibodies raised against barley FNR and the NADH-binding 51-kD polypeptide of the mitochondrial complex I, respectively. The results suggest that ET1 is the FNR-containing thylakoidal NAD(P)H dehydrogenase complex.

Chloroplast polypeptides synthesized by leaf segments and isolated chloroplasts during senescence in barley

Starting from senescent barley (Hordeum vulgare L. cv Hassan) leaf segments receiving light and hormone treatments affecting senescence, the plastid polypeptides synthesized by isolated chloroplasts and by leaf segments were analyzed by radiolabelling followed SDS-PAGE and fluorography. Among 20 to 30 polypeptides detected, a few were specifically synthesized (by chloroplasts and/or leaf segments) after each senescence treatment. Apparently, the polypeptides labelled in assays with isolated chloroplasts are truly synthesized in vivo, because most of them were also labelled in assays with leaf segments. The comparison of polypeptide profiles, for every senescence treatment, after labelling with isolated chloroplasts or leaf segments, suggests that most plastid polypeptides synthesized during senescence are coded in plastid DNA.

Phytochrome and hormone control of polypeptides synthesized by chloroplasts of senescent barley leaves

To identify the polypeptides involved in the mechanism of leaf senescence, light-driven protein synthesis was assayed with chloroplasts isolated from barley leaf segments incubated during 20 h under different light and hormone treatments affecting senescence. The radioactive products were analyzed by SDS-PAGE and fluorography. The synthesis of some polypeptides was stimulated by ABA (66, 44, 30, 22, 20, kDa) and ethylene (66, 50, 48, 44, kDa) which accelerate senescence. Kinetin and red light (in an effect mediated by phytochrome), which retard senescence, inhibited the synthesis of some polypeptides (50, 48, 37, kDa) and stimulated the synthesis of others (54, 32, kDa). Probably phytochrome and hormones control senescence by affecting the synthesis of specific polypeptides.

Correlated responses to selection for wing length in allozyme systems of Drosophila melanogaster

Significant changes of genotypic structure in 20 lines selected for wing length are detected by analysis of the allelic frequencies of several enzyme loci (XDH, LAP-D, EST-6, 1-APH, ADH, α-GPDH). These changes are not haphazard but a consequence of the effects of selection on the genetic structure of the population, since replicate lines always behave in a parallel way. The changes are larger in the lines selected for short wings, in which the genetic variability decreases considerably. This decrease is the result of selection for homozygosity, detected at the allozyme loci, but most probably reflects homozygosity of more or less extended chromosomal segments. Selection for wing length, especially for short wings, favoured recombinants of the initial founder chromosomes. Only in the 1-APH and the EST-6 loci, separated by 11.7 centimorgans on the genetic map, do the alleles linked in the founder lines change in parallel in the control and long wing lines. The correlated response in the allozyme allele frequencies cannot be accounted for by a direct influence of the allozymes on the variability in wing length. The changes in the EST-6, 1-APH and perhaps in the LAP-D, can be explained by a direct effect of natural selection on the allozyme loci, probably in interaction with the effect of selection for wing length on linked loci. This last effect seems to be the main factor contributing to the change detected in the XDH locus.