Downregulation of the basic peroxidase isoenzyme from Zinnia elegans by gibberellic acid

Overexpression of ZePrx in Nicotiana tabacum Affects Lignin Biosynthesis Without Altering Redox Homeostasis

Class III plant peroxidases (Prxs) are involved in the oxidative polymerization of lignins. Zinnia elegans Jacq. Basic peroxidase (ZePrx) has been previously characterized as capable of catalyzing this reaction in vitro and the role in lignin biosynthesis of several of its Arabidopsis thaliana homologous has been previously confirmed. In the present work, ZePrx was overexpressed in Nicotiana tabacum to further characterize its function in planta with particular attention to its involvement in lignin biosynthesis. Since Prxs are known to alter ROS levels by using them as electron acceptor or producing them in their catalytic activity, the impact of this overexpression in redox homeostasis was studied by analyzing the metabolites and enzymes of the ascorbate-glutathione cycle. In relation to the modification induced by ZePrx overexpression in lignin composition and cellular metabolism, the carbohydrate composition of the cell wall as well as overall gene expression through RNA-Seq were analyzed. The obtained results indicate that the overexpression of ZePrx caused an increase in syringyl lignin in cell wall stems, suggesting that ZePrx is relevant for the oxidation of sinapyl alcohol during lignin biosynthesis, coherently with its S-peroxidase nature. The increase in the glucose content of the cell wall and the reduction of the expression of several genes involved in secondary cell wall biosynthesis suggests the occurrence of a possible compensatory response to maintain cell wall properties. The perturbation of cellular redox homeostasis occurring as a consequence of ZePrx overexpression was kept under control by an increase in APX activity and a reduction in ascorbate redox state. In conclusion, our results confirm the role of ZePrx in lignin biosynthesis and highlight that its activity alters cellular pathways putatively aimed at maintaining redox homeostasis.

Identification and function analyses of senescence-associated WRKYs in wheat

Leaf senescence is a positive, highly regulated, complex process, and transcription factors play important roles in the regulation of this process. We identified and characterized 116 WRKYs from the wheat genome database. Thirteen TaWRKYs were confirmed as senescence-associated genes. We focused on TaWRKY7, which is up-regulated in the natural leaf senescence process. TaWRKY7 is expressed in different tissues of wheat and is localized in the nucleus. It shows transcriptional activation activity in yeast cells. The ectopic over-expression of TaWRKY7 in Arabidopsis (Arabidopsis thaliana) significantly promoted early leaf senescence under darkness treatment and prevented leaf moisture losses. TaWRKY7 played important roles in the senescence process and was involved in abiotic stress responses. Our transcriptomic and genetic studies on WRKYs suggest that WRKY transcription factors are a type of vital regulator in leaf senescence in wheat (Triticum aestivum L.).

Lignin biosynthesis in wheat (Triticum aestivum L.): its response to waterlogging and association with hormonal levels

BACKGROUND

Lignin is an important structural component of plant cell wall that confers mechanical strength and tolerance against biotic and abiotic stressors; however it affects the use of biomass such as wheat straw for some industrial applications such as biofuel production. Genetic alteration of lignin quantity and quality has been considered as a viable option to overcome this problem. However, the molecular mechanisms underlying lignin formation in wheat biomass has not been studied. Combining molecular and biochemical approaches, the present study investigated the transcriptional regulation of lignin biosynthesis in two wheat cultivars with varying lodging characteristics and also in response to waterlogging. It also examined the association of lignin level in tissues with that of plant hormones implicated in the control of lignin biosynthesis.

RESULTS

Analysis of lignin biosynthesis in the two wheat cultivars revealed a close association of lodging resistance with internode lignin content and expression of 4-coumarate:CoA ligase1 (4CL1), p-coumarate 3-hydroxylase1 (C3H1), cinnamoyl-CoA reductase2 (CCR2), ferulate 5-hydroxylase2 (F5H2) and caffeic acid O-methyltransferase2 (COMT2), which are among the genes highly expressed in wheat tissues, implying the importance of these genes in mediating lignin deposition in wheat stem. Waterlogging of wheat plants reduced internode lignin content, and this effect is accompanied by transcriptional repression of three of the genes characterized as highly expressed in wheat internode including phenylalanine ammonia-lyase6 (PAL6), CCR2 and F5H2, and decreased activity of PAL. Expression of the other genes was, however, induced by waterlogging, suggesting their role in the synthesis of other phenylpropanoid-derived molecules with roles in stress responses. Moreover, difference in internode lignin content between cultivars or change in its level due to waterlogging is associated with the level of cytokinin.

CONCLUSION

Lodging resistance, tolerance against biotic and abiotic stresses and feedstock quality of wheat biomass are closely associated with its lignin content. Therefore, the findings of this study provide important insights into the molecular mechanisms underlying lignin formation in wheat, an important step towards the development of molecular tools that can facilitate the breeding of wheat cultivars for optimized lignin content and enhanced feedstock quality without affecting other lignin-related agronomic benefits.

Hormonal regulation of secondary cell wall formation

Secondary cell walls (SCWs) have critical functional importance but also constitute a high proportion of the plant biomass and have high application potential. This is true mainly for the lignocellulosic constituents of the SCWs in xylem vessels and fibres, which form a structured layer between the plasma membrane and the primary cell wall (PCW). Specific patterning of the SCW thickenings contributes to the mechanical properties of the different xylem cell types, providing the plant with mechanical support and facilitating the transport of solutes via vessels. In the last decade, our knowledge of the basic molecular mechanisms controlling SCW formation has increased substantially. Several members of the multi-layered regulatory cascade participating in the initiation and transcriptional regulation of SCW formation have been described, and the first cellular components determining the pattern of SCW at the subcellular resolution are being uncovered. The essential regulatory role of phytohormones in xylem development is well known and the molecular mechanisms that link hormonal signals to SCW formation are emerging. Here, we review recent knowledge about the role of individual plant hormones and hormonal crosstalk in the control over the regulatory cascades guiding SCW formation and patterning. Based on the analogy between many of the mechanisms operating during PCW and SCW formation, recently identified mechanisms underlying the hormonal control of PCW remodelling are discussed as potentially novel mechanisms mediating hormonal regulatory inputs in SCW formation.

Interplay of sugar, light and gibberellins in expression of Rosa hybrida vacuolar invertase 1 regulation

Our previous findings showed that the expression of the Rosa hybrida vacuolar invertase 1 gene (RhVI1) was tightly correlated with the ability of buds to grow out and was under sugar, gibberellin and light control. Here, we aimed to provide an insight into the mechanistic basis of this regulation. In situ hybridization showed that RhVI1 expression was localized in epidermal cells of young leaves of bursting buds. We then isolated a 895 bp fragment of the promoter of RhVI1. In silico analysis identified putative cis-elements involved in the response to sugars, light and gibberellins on its proximal part (595 bp). To carry out functional analysis of the RhVI1 promoter in a homologous system, we developed a direct method for stable transformation of rose cells. 5' deletions of the proximal promoter fused to the uidA reporter gene were inserted into the rose cell genome to study the cell's response to exogenous and endogenous stimuli. Deletion analysis revealed that the 468 bp promoter fragment is sufficient to trigger reporter gene activity in response to light, sugars and gibberellins. This region confers sucrose- and fructose-, but not glucose-, responsive activation in the dark. Inversely, the -595 to -468 bp region that carries the sugar-repressive element (SRE) is required to down-regulate the RhVI1 promoter in response to sucrose and fructose in the dark. We also demonstrate that sugar/light and gibberellin/light act synergistically to up-regulate β-glucuronidase (GUS) activity sharply under the control of the 595 bp pRhVI1 region. These results reveal that the 127 bp promoter fragment located between -595 and -468 bp is critical for light and sugar and light and gibberellins to act synergistically.

Arabidopsis thaliana peroxidases involved in lignin biosynthesis: in silico promoter analysis and hormonal regulation

Phytohormones such as auxins, cytokinins, and brassinosteroids, act by means of a signaling cascade of transcription factors of the families NAC, MYB, AP2 (APETALA2), MADS and class III HD (homeodomain) Zip, regulating secondary growth. When the hormonal regulation of Zinnia elegans peroxidase (ZePrx), an enzyme involved in lignin biosynthesis, was studied, it was found that this peroxidase is sensitive to a plethora of hormones which control xylem lignification. In a previous study we sought Arabidopsis thaliana homologues to ZePrx. Peroxidases 4, 52, 49 and 72 are the four peroxidases that fulfill the restrictive conditions that a peroxidase involved in lignification must have. In the present study, we focus our attention on hormonal regulation in order to establish the minimal structural and regulatory elements contained in the promoter region which an AtPrx involved in lignification must have. The results indicate that of the four peroxidases selected in our previous study, the one most likely to be homologous to ZePrx is AtPrx52. The results suggest that hormones such as auxins, cytokinins and BRs directly regulate AtPrx52, and that the AtPrx52 promoter may be the target of the set of transcription factors (NAC, MYB, AP2 and class I and III HD Zip) which are up-regulated by these hormones during secondary growth. In addition, the AtPrx52 promoter contains multiple copies of all the putative cis-elements (the ACGT box, the OCS box, the OPAQ box, the L1BX, the MYCL box and the W box) known to confer regulation by NO and H2O2.

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.

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.