Enzymatic activity and proteomic profile of class III peroxidases during sugarcane stem development

Biochemical Responses to the Long-Term Impact of Copper Sulfate (CuSO4) in Tobacco Plants

Metabolic changes under stress are often studied in short-term experiments, revealing rapid responses in gene expression, enzyme activity, and the amount of antioxidants. In a long-term experiment, it is possible to identify adaptive changes in both primary and secondary metabolism. In this study, we characterized the physiological state of tobacco plants and assessed the amount and spectrum of phenolic compounds and the lignification of axial organs under excess copper stress in a long-term experiment (40 days). Plants were treated with 100 and 300 μM CuSO4, as well as a control (Knop solution). Copper accumulation, the size and anatomical structure of organs, stress markers, and the activity of antioxidant enzymes were studied. Lignin content was determined with the cysteine-assisted sulfuric method (CASA), and the metabolite profile and phenolic spectrum were determined with UHPLC-MS and thin-layer chromatography (TLC). Cu2+ mainly accumulated in the roots and, to a lesser extent, in the shoots. Copper sulfate (100 μM) slightly stimulated stem and leaf growth. A higher concentration (300 μM) caused oxidative stress; H2O2 content, superoxide dismutase (SOD), and guaiacol peroxidase (GPOX) activity increased in roots, and malondialdehyde (MDA) increased in all organs. The deposition of lignin increased in the roots and stems compared with the control. The content of free phenolics, which could be used as substrates for lignification, declined. The proportions of ferulic, cinnamic, and p-coumaric acids in the hydrolysate of bound phenolics were higher, and they tended toward additional lignification. The metabolic profile changed in both roots and stems at both concentrations, and changed in leaves only at a concentration of 300 μM. Thus, changes in the phenolic spectrum and the enhanced lignification of cell walls in the metaxylem of axial (root and stem) organs in tobacco can be considered important metabolic responses to stress caused by excess CuSO4.

Genome-wide identification of the class III peroxidase gene family of sugarcane and its expression profiles under stresses

INTRODUCTION

Plant-specific Class III peroxidases (PRXs) play a crucial role in lignification, cell elongation, seed germination, and biotic and abiotic stresses.

METHODS

The class III peroxidase gene family in sugarcane were identified by bioinformatics methods and realtime fluorescence quantitative PCR.

RESULTS

Eighty-two PRX proteins were characterized with a conserved PRX domain as members of the class III PRX gene family in R570 STP. The ShPRX family genes were divided into six groups by the phylogenetic analysis of sugarcane, Saccharum spontaneum, sorghum, rice, and Arabidopsis thaliana. The analysis of promoter cis-acting elements revealed that most ShPRX family genes contained cis-acting regulatory elements involved in ABA, MeJA, light responsiveness, anaerobic induction, and drought inducibility. An evolutionary analysis indicated that ShPRXs was formed after Poaceae and Bromeliaceae diverged, and tandem duplication events played a critical role in the expansion of ShPRX genes of sugarcane. Purifying selection maintained the function of ShPRX proteins. SsPRX genes were differentially expressed in stems and leaves at different growth stages in S. spontaneum. However, ShPRX genes were differentially expressed in the SCMV-inoculated sugarcane plants. A qRT-PCR analysis showed that SCMV, Cd, and salt could specifically induce the expression of PRX genes of sugarcane.

DISCUSSION

These results help elucidate the structure, evolution, and functions of the class III PRX gene family in sugarcane and provide ideas for the phytoremediation of Cd-contaminated soil and breeding new sugarcane varieties resistant to sugarcane mosaic disease, salt, and Cd stresses.

Modulation of class III peroxidase pathways and phenylpropanoids in Arundo donax under salt and phosphorus stress

Arundo donax L. is an invasive species that has been recently employed for biomass production due to its well-known ability to colonize harsh environment. Based on previous observations, the present study investigated the potential role of phenylpropanoids and class III peroxidases to confer adaptation through biochemical and transcriptomic analysis in A. donax after Na⁺ and P excess supply, both in single stress and in combination, and after growth at low P level. The levels of hydrogen peroxide, flavonoids (i.e., quercetin, apigenin and kaempferol derivatives) and the activity of class III peroxidases, as well as the expression of several genes encoding for their enzymes involved in their biosynthesis, increased when Na⁺ was supplied in combination with P. These results suggest that those biomolecules are involved in the response of A. donax, to the presence of +Na and P in the soil. Moreover, even though at the sampling time no significant accumulation of lignin has been determined, the trend of accumulation of such metabolite and most of all the increase of several transcripts involved in its synthesis was found. This work for the first time indicates the need for further investigation devoted to elucidating whether the strengthening of cell walls via lignin synthesis is one of the mechanisms used by A. donax to adapt to harsh environments.

MeRAV5 promotes drought stress resistance in cassava by modulating hydrogen peroxide and lignin accumulation

Cassava, an important food and energy crop, is relatively more resistant to drought stress than other crops. However, the molecular mechanism underlying this resistance remains elusive. Herein, we report that silencing a drought stress-responsive transcription factor MeRAV5 significantly reduced drought stress resistance, with higher levels of hydrogen peroxide (H₂ O₂ ) and less lignin during drought stress. Yeast two-hybrid, pull down and bimolecular fluorescence complementation (BiFC) showed that MeRAV5 physically interacted with peroxidase (MePOD) and lignin-related cinnamyl alcohol dehydrogenase 15 (MeCAD15) in vitro and in vivo. MeRAV5 promoted the activities of both MePOD and MeCAD15 to affect H₂ O₂ and endogenous lignin accumulation respectively, which are important in drought stress resistance in cassava. When either MeCAD15 or MeRAV5 was silenced, or both were co-silenced, cassava showed lower lignin content and drought-sensitive phenotype, whereas exogenous lignin alkali treatment increased drought stress resistance and alleviated the drought-sensitive phenotype of these silenced cassava plants. This study documents that the modulation of H₂ O₂ and lignin by MeRAV5 is essential for drought stress resistance in cassava.

Applying Molecular Phenotyping Tools to Explore Sugarcane Carbon Potential

Sugarcane (Saccharum spp.), a C₄ grass, has a peculiar feature: it accumulates, gradient-wise, large amounts of carbon (C) as sucrose in its culms through a complex pathway. Apart from being a sustainable crop concerning C efficiency and bioenergetic yield per hectare, sugarcane is used as feedstock for producing ethanol, sugar, high-value compounds, and products (e.g., polymers and succinate), and bioelectricity, earning the title of the world's leading biomass crop. Commercial cultivars, hybrids bearing high levels of polyploidy, and aneuploidy, are selected from a large number of crosses among suitable parental genotypes followed by the cloning of superior individuals among the progeny. Traditionally, these classical breeding strategies have been favoring the selection of cultivars with high sucrose content and resistance to environmental stresses. A current paradigm change in sugarcane breeding programs aims to alter the balance of C partitioning as a means to provide more plasticity in the sustainable use of this biomass for metabolic engineering and green chemistry. The recently available sugarcane genetic assemblies powered by data science provide exciting perspectives to increase biomass, as the current sugarcane yield is roughly 20% of its predicted potential. Nowadays, several molecular phenotyping tools can be applied to meet the predicted sugarcane C potential, mainly targeting two competing pathways: sucrose production/storage and biomass accumulation. Here we discuss how molecular phenotyping can be a powerful tool to assist breeding programs and which strategies could be adopted depending on the desired final products. We also tackle the advances in genetic markers and mapping as well as how functional genomics and genetic transformation might be able to improve yield and saccharification rates. Finally, we review how "omics" advances are promising to speed up plant breeding and reach the unexplored potential of sugarcane in terms of sucrose and biomass production.

Breakthroughs in the discovery and use of different peroxidase isoforms of microbial origin

Peroxidases are classified as oxidoreductases and are the second largest class of enzymes applied in biotechnological processes. These enzymes are used to catalyze various oxidative reactions using hydrogen peroxide and other substrates as electron donors. They are isolated from various sources such as plants, animals and microbes. Peroxidase enzymes have versatile applications in bioenergy, bioremediation, dye decolorization, humic acid degradation, paper and pulp, and textile industries. Besides, peroxidases from different sources have unique abilities to degrade a broad range of environmental pollutants such as petroleum hydrocarbons, dioxins, industrial dye effluents, herbicides and pesticides. Ironically, unlike most biological catalysts, the function of peroxidases varies according to their source. For instance, manganese peroxidase (MnP) of fungal origin is widely used for depolymerization and demethylation of lignin and bleaching of pulp. While, horseradish peroxidase of plant origin is used for removal of phenols and aromatic amines from waste waters. Microbial enzymes are believed to be more stable than enzymes of plant or animal origin. Thus, making microbially-derived peroxidases a well-sought-after biocatalysts for versatile industrial and environmental applications. Therefore, the current review article highlights on the recent breakthroughs in the discovery and use of peroxidase isoforms of microbial origin at a possible depth.

An overview of the transcriptional responses of two tolerant and susceptible sugarcane cultivars to borer (Diatraea saccharalis) infestation

Diatraea saccharalis constitutes a threat to the sugarcane productivity, and obtaining borer tolerant cultivars is an alternative method of control. Although there are studies about the relationship between the interaction of D. saccharalis with sugarcane, little is known about the molecular and genomic basis of defense mechanisms that confer tolerance to sugarcane cultivars. Here, we analyzed the transcriptional profile of two sugarcane cultivars in response to borer attack, RB867515 and SP80-3280, which are considered tolerant and sensitive to the borer attack, respectively. A sugarcane genome and transcriptome were used for read mapping. Differentially expressed transcripts and genes were identified and termed to as DETs and DEGs, according to the sugarcane database adopted. A total of 745 DETs and 416 DEGs were identified (log₂|ratio| > 0.81; FDR corrected P value ≤ 0.01) after borer infestation. Following annotation of up- and down-regulated DETs and DEGs by similarity searches, the sugarcane cultivars demonstrated an up-regulation of jasmonic acid (JA), ethylene (ET), and defense protein genes, as well as a down-regulation of pathways involved in photosynthesis and energy metabolism. The expression analysis also highlighted that RB867515 cultivar is possibly more transcriptionally activated after 12 h from infestation than SP80-3280, which could imply in quicker responses by probably triggering more defense-related genes and mediating metabolic pathways to cope with borer attack.

Regulation of Lignin Biosynthesis by Post-translational Protein Modifications

Post-translational modification of proteins exerts essential roles in many biological processes in plants. The function of these chemical modifications has been extensively characterized in many physiological processes, but how these modifications regulate lignin biosynthesis for wood formation remained largely unknown. Over the past decade, post-translational modification of several proteins has been associated with lignification. Phosphorylation, ubiquitination, glycosylation, and S-nitrosylation of transcription factors, monolignol enzymes, and peroxidases were shown to have primordial roles in the regulation of lignin biosynthesis. The main discoveries of post-translational modifications in lignin biosynthesis are discussed in this review.

Deciphering the biological processes underlying tomato biomass production and composition

The huge amounts of biomass residues, remaining in the field after tomato fruits harvesting, can be utilized to produce bioenergy. A multiple level approach aimed to characterize two Solanum pennellii introgression lines (ILs), with contrasting phenotypes for plant architecture and biomass was carried out. The study of gene expression dynamics, microscopy cell traits and qualitative and quantitative cell wall chemical compounds variation enabled the discovery of key genes and cell processes involved biomass accumulation and composition. Enhanced biomass production observed in IL2-6 line is due to a more effective coordination of chloroplasts and mitochondria energy fluxes. Microscopy analysis revealed a higher number of cells and chloroplasts in leaf epidermis in the high biomass line whilst chemical measurements on the two lines pointed out striking differences in the cell wall composition and organization. Taken together, our findings shed light on the mechanisms underlying the tomato biomass production and processability.

Deposition of lignin in four species of Saccharum

We used primers designed on conserved gene regions of several species to isolate the most expressed genes of the lignin pathway in four Saccharum species. S. officinarum and S. barberi have more sucrose in the culms than S. spontaneum and S. robustum, but less polysaccharides and lignin in the cell wall. S. spontaneum, and S. robustum had the lowest S/G ratio and a lower rate of saccharification in mature internodes. Surprisingly, except for CAD, 4CL, and CCoAOMT for which we found three, two, and two genes, respectively, only one gene was found for the other enzymes and their sequences were highly similar among the species. S. spontaneum had the highest expression for most genes. CCR and CCoAOMT B presented the highest expression; 4CL and F5H showed increased expression in mature tissues; C3H and CCR had higher expression in S. spontaneum, and one of the CADs isolated (CAD B) had higher expression in S. officinarum. The similarity among the most expressed genes isolated from these species was unexpected and indicated that lignin biosynthesis is conserved in Saccharum including commercial varieties Thus the lignin biosynthesis control in sugarcane may be only fully understood with the knowledge of the promotor region of each gene.

Optimization of RT-PCR reactions in studies with genes of lignin biosynthetic route in Saccharum spontaneum

Saccharum spontaneum has been used for the development of energy cane a crop aimed to be used for the production of second-generation ethanol, or lignocellulosic ethanol. Lignin is a main challenge in the conversion of cell wall sugars into ethanol. In our studies to isolate the genes the lignin biosynthesis in S. spontaneum we have had great difficulty in RT-PCR reactions. Thus, we evaluated the effectiveness of different additives in the amplification of these genes. While COMT and CCoAOMT genes did not need any additives for other genes there was no amplification (HCT, F5H, 4CL and CCR) or the yield was very low (CAD and C4H). The application of supplementary cDNA was enough to overcome the non-specificity and low yield for C4H and C3H, while the addition of 0.04% BSA + 2% formamide was effective to amplify 4CL, CCR, F5H and CCR. HCT was amplified only by addition of 0.04% BSA + 2% formamide + 0.1 M trehalose and amplification of PAL was possible with addition of 2% of DMSO. Besides optimization of expression assays, the results show that additives can act independently or synergistically.

Identification, classification and transcriptional profiles of dirigent domain-containing proteins in sugarcane

Dirigent (DIR) proteins, encoded by DIR genes, are referred to as "dirigent" because they direct the outcome of the coupling of the monolignol coniferyl alcohol into (+) or (-) pinoresinol, the first intermediates in the enantiocomplementary pathways for lignan biosynthesis. DIR domain-containing or DIR-like proteins are, thus, termed for not having a clear characterization. A transcriptome- and genome-wide survey of DIR domain-containing proteins in sugarcane was carried out, in addition to phylogenetic, physicochemical and transcriptional analyses. A total of 120 non-redundant sequences containing the DIR domain were identified and classified into 64 groups according to phylogenetic and sequence alignment analyses. In silico analysis of transcript abundance showed that these sequences are expressed at low levels in leaves and genes in the same phylogenetic clade have similar expression patterns. Expression analysis of ShDIR1-like transcripts in the culm internodes of sugarcane demonstrates their abundance in mature internodes, their induction by nitrogen fertilization and their predominant expression in cells that have a lignified secondary cell wall, such as vascular bundles of young internodes and parenchymal cells of the pith of mature internodes. Due to the lack of information about the functional role of DIR in plants, a possible relationship is discussed between the ShDIR1-like transcriptional profile and cell wall development in parenchyma cells of sugarcane culm, which typically accumulates large amounts of sucrose. The number of genes encoding the DIR domain-containing proteins in sugarcane is intriguing and is an indication per se that these proteins may have an important metabolic role and thus deserve to be better studied.

Manipulation of ABA Content in Arabidopsis thaliana Modifies Sensitivity and Oxidative Stress Response to Dickeya dadantii and Influences Peroxidase Activity

The production of reactive oxygen species (ROS) is one of the first defense reactions induced in Arabidopsis in response to infection by the pectinolytic enterobacterium Dickeya dadantii. Previous results also suggest that abscisic acid (ABA) favors D. dadantii multiplication and spread into its hosts. Here, we confirm this hypothesis using ABA-deficient and ABA-overproducer Arabidopsis plants. We investigated the relationships between ABA status and ROS production in Arabidopsis after D. dadantii infection and showed that ABA status modulates the capacity of the plant to produce ROS in response to infection by decreasing the production of class III peroxidases. This mechanism takes place independently of the well-described oxidative stress related to the RBOHD NADPH oxidase. In addition to this weakening of plant defense, ABA content in the plant correlates positively with the production of some bacterial virulence factors during the first stages of infection. Both processes should enhance disease progression in presence of high ABA content. Given that infection increases transcript abundance for the ABA biosynthesis genes AAO3 and ABA3 and triggers ABA accumulation in leaves, we propose that D. dadantii manipulates ABA homeostasis as part of its virulence strategy.

Genome-Wide Identification of Sorghum bicolor Laccases Reveals Potential Targets for Lignin Modification

Laccase is a key enzyme in plant lignin biosynthesis as it catalyzes the final step of monolignols polymerization. Sweet sorghum [Sorghum bicolor (L.) Moench] is considered as an ideal feedstock for ethanol production, but lignin greatly limits the production efficiency. No comprehensive analysis on laccase has ever been conducted in S. bicolor, although it appears as the most promising target for engineering lignocellulosic feedstock. The aim of our work is to systematically characterize S. bicolor laccase gene family and to identify the lignin-specific candidates. A total of twenty-seven laccase candidates (SbLAC1-SbLAC27) were identified in S. bicolor. All SbLACs comprised the equivalent L1-L4 signature sequences and three typical Cu-oxidase domains, but exhibited diverse intron-exon patterns and relatively low sequence identity. They were divided into six groups by phylogenetic clustering, revealing potential distinct functions, while SbLAC5 was considered as the closest lignin-specific candidate. qRT-PCR analysis deciphered that SbLAC genes were expressed preferentially in roots and young internodes of sweet sorghum, and SbLAC5 showed high expression, adding the evidence that SbLAC5 was bona fide involved in lignin biosynthesis. Besides, high abundance of SbLAC6 transcripts was detected, correlating it a potential role in lignin biosynthesis. Diverse cis regulatory elements were recognized in SbLACs promoters, indicating putative interaction with transcription factors. Seven SbLACs were found to be potential targets of sbi-miRNAs. Moreover, putative phosphorylation sites in SbLAC sequences were identified. Our research adds to the knowledge for lignin profile modification in sweet sorghum.

Molecular Cloning, Characterization, and Expression Analysis of Lignin Genes from Sugarcane Genotypes Varying in Lignin Content

Sugarcane (Saccharum spp.) is one of the highest biomass-producing plant and the best lignocellulosic feedstock for ethanol production. To achieve more efficient conversion of biomass to ethanol, a better understanding of the main factors affecting biomass recalcitrance is needed. Therefore, with this objective, here, we report a systematic study on lignin content, deposition, identification, and cloning of genes involved in lignin biosynthesis and their differential expression in five sugarcane clones, EC11003, EC11010, IK 76-91, IK 76-99, and Co 86032. Lignin content among the clones varied from 26.87 to 23.19 % with the highest in the clone EC11010 and the lowest in high sugar Co86032. Lignin deposition studied through phloroglucinol staining of the cell walls implied that the sclerenchyma cells of the energy canes (EC11010 and EC11003) have more lignin deposition followed by the Erianthus (IK 76-91 and IK 76-99) clones whereas Co86032 has the minimum amount of lignin deposition. We cloned partial coding regions of important genes of lignification COMT (650 bp), CCR (332 bp), and PAL (650 bp) from Erianthus, wild relative of sugarcane followed by the expression analysis through real-time PCR. Differential expression analysis showed high level of expression for the three genes in the energy cane EC11010.

Sugarcane proteomics: An update on current status, challenges, and future prospects

Sugarcane is one of the most important commercial crops cultivated worldwide for the production of crystal sugar, ethanol, and other related by-products. Unlike other comparable monocots like sorghum, maize, and rice, sugarcane genome by virtue of its polyploidy nature remains yet to be fully deciphered. Proteomics-an established complementary tool to genomics is at its infancy in sugarcane as compared to the other monocots. However, with the surge in genomics research accomplished by next-generation sequencing platforms, sugarcane proteomics has gained momentum. This review summarizes the available literature from 1970 to 2014, which ensures a comprehensive coverage on sugarcane proteomics-a topic first of its kind to be reviewed. We herewith compiled substantial contributions in different areas of sugarcane proteomics, which include abiotic and biotic stresses, cell wall, organelle, and structural proteomics. The past decade has witnessed a paradigm shift in the pace with which sugarcane proteomics is progressing, as evident by the number of research publications. In addition to extensively reviewing the progress made thus far, we intend to highlight the scope in sugarcane proteomics, with an aspiration to instigate focused research on sugarcane to harness its full potential for the human welfare.

Large-Scale Transcriptome Analysis of Two Sugarcane Genotypes Contrasting for Lignin Content

Sugarcane is an important crop worldwide for sugar and first generation ethanol production. Recently, the residue of sugarcane mills, named bagasse, has been considered a promising lignocellulosic biomass to produce the second-generation ethanol. Lignin is a major factor limiting the use of bagasse and other plant lignocellulosic materials to produce second-generation ethanol. Lignin biosynthesis pathway is a complex network and changes in the expression of genes of this pathway have in general led to diverse and undesirable impacts on plant structure and physiology. Despite its economic importance, sugarcane genome was still not sequenced. In this study a high-throughput transcriptome evaluation of two sugarcane genotypes contrasting for lignin content was carried out. We generated a set of 85,151 transcripts of sugarcane using RNA-seq and de novo assembling. More than 2,000 transcripts showed differential expression between the genotypes, including several genes involved in the lignin biosynthetic pathway. This information can give valuable knowledge on the lignin biosynthesis and its interactions with other metabolic pathways in the complex sugarcane genome.

Water stress alters lignin content and related gene expression in two sugarcane genotypes

The lignin deposition in the stem of two sugarcane genotypes was assessed on exposure to water stress. The lignin content and the morphoanatomical characterization of the stem indicated that IACSP94-2094 plants are more lignified than those of IACSP95-5000 genotype, under normal water supply conditions, which was especially associated with higher lignin contents in the rind of mature internodes. Water deficit had negative impact on the biomass production, mostly with IACSP94-2094 plants, possibly due to stress severity or higher susceptibility of that genotype during the stem-lengthening phase. Water deficit led to significant alterations in the expression levels of lignin biosynthesis genes and led to an approximate 60% increase of lignin content in the rind of young internodes in both genotypes. It is concluded that the young rind region was more directly affected by water stress and, depending on the genotype, a higher lignin accumulation may occur in the stem, thus implying lower quality biomass for bioethanol production.

A model system to study the lignification process in Eucalyptus globulus

Recalcitrance of plant biomass is closely related to the presence of the phenolic heteropolymer lignin in secondary cell walls, which has a negative effect on forage digestibility, biomass-to-biofuels conversion and chemical pulping. The genus Eucalyptus is the main source of wood for pulp and paper industry. However, when compared to model plants such as Arabidopsis thaliana and poplar, relatively little is known about lignin biosynthesis in Eucalyptus and only a few genes were functionally characterized. An efficient, fast and inexpensive in vitro system was developed to study lignification in Eucalyptus globulus and to evaluate the potential role of candidate genes in this biological process. Seedlings were grown in four different conditions, in the presence or absence of light and with or without sucrose in the growth medium, and several aspects of lignin metabolism were evaluated. Our results showed that light and, to a lesser extent, sucrose induced lignin biosynthesis, which was followed by changes in S/G ratio, lignin oligomers accumulation and gene expression. In addition, higher total peroxidase activity and differential isoperoxidase profile were observed when seedlings were grown in the presence of light and sucrose. Peptide sequencing allowed the identification of differentially expressed peroxidases, which can be considered potential candidate class III peroxidases involved in lignin polymerization in E. globulus.

Mass spectrometry imaging: an expeditious and powerful technique for fast in situ lignin assessment in Eucalyptus

Plant biomass has been suggested as an alternative to produce bioethanol. The recalcitrance of plant biomass to convert cellulose into simpler carbohydrates used in the fermentation process is partially due to lignin, but the standard methods used to analyze lignin composition frequently use toxic solvents and are laborious and time-consuming. MS imaging was used to study lignin in Eucalyptus, since this genus is the main source of cellulose in the world. Hand-cut sections of stems of two Eucalyptus species were covered with silica and directly analyzed by matrix-assisted laser sesorption ionization (MALDI)-imaging mass spectrometry (MS). Information available in the literature about soluble lignin subunits and structures were used to trace their distribution in the sections and using a software image a relative quantification could be made. Matrixes routinely used in MALDI-imaging analysis are not satisfactory to analyze plant material and were efficiently substituted by thin layer chromatography (TLC) grade silica. A total of 22 compounds were detected and relatively quantified. It was also possible to establish a proportion between syringyl and guaiacyl monolignols, characteristic for each species. Because of the simple way that samples are prepared, the MALDI-imaging approach presented here can replace, in routine analysis, complex and laborious MS methods in the study of lignin composition.

Expression of SofLAC, a new laccase in sugarcane, restores lignin content but not S:G ratio of Arabidopsis lac17 mutant

Lignin is a complex phenolic heteropolymer deposited in the secondarily thickened walls of specialized plant cells to provide strength for plants to stand upright and hydrophobicity to conducting cells for long-distance water transport. Although essential for plant growth and development, lignin is the major plant cell-wall component responsible for biomass recalcitrance to industrial processing. Peroxidases and laccases are generally thought to be responsible for lignin polymerization, but, given their broad substrate specificities and large gene families, specific isoforms involved in lignification are difficult to identify. This study used a combination of co-expression analysis, tissue/cell-type-specific expression analysis, and genetic complementation to correlate a sugarcane laccase gene, SofLAC, to the lignification process. A co-expression network constructed from 37 cDNA libraries showed that SofLAC was coordinately expressed with several phenylpropanoid biosynthesis genes. Tissue-specific expression analysis by quantitative RT-PCR showed that SofLAC was expressed preferentially in young internodes and that expression levels decrease with stem maturity. Cell-type-specific expression analysis by in situ hybridization demonstrated the localization of SofLAC mRNA in lignifying cell types, mainly in inner and outer portions of sclerenchymatic bundle sheaths. To investigate whether SofLAC is able to oxidize monolignols during lignification, the Arabidopsis lac17 mutant, which has reduced lignin levels, was complemented by expressing SofLAC under the control of the Arabidopsis AtLAC17 promoter. The expression of SofLAC restored the lignin content but not the lignin composition in complemented lac17 mutant lines. Taken together, these results suggest that SofLAC participates in lignification in sugarcane.

Suspension cell culture as a tool for the characterization of class III peroxidases in sugarcane

Secreted class III peroxidases (EC 1.11.1.7) are implicated in a broad range of physiological processes throughout the plant life cycle. However, the unambiguous determination of the precise biological role of an individual class III peroxidase isoenzyme is still a difficult task due to genetic redundancy and broad substrate specificity in vitro. In addition, many difficulties are encountered during extraction and analysis of cell wall proteins. Since class III peroxidases are also secreted into the apoplast, the use of suspension cell cultures can facilitate isolation and functional characterization of individual isoforms. Here, we report on the characterization of class III peroxidases secreted in the spent medium of sugarcane suspension cell cultures. After treatment with specific inducers of cell wall lignification, peroxidases were isolated and activities assayed with guaiacol, syringaldazine and coniferyl alcohol. Enzymatic activity was not significantly different after treatments, regardless of the substrate, with the exception of methyl-jasmonate treatment, which led to a decreased guaiacol peroxidase activity. Remarkably, peroxidases isolated from the medium were capable of oxidizing syringaldazine, an analog to sinapyl alcohol, suggesting that sugarcane cultures can produce peroxidases putatively correlated to lignification. A proteomic approach using activity staining of 2-DE gels revealed a complex isoperoxidase profile, composed predominantly of cationic isoforms. Individual spots were excised and analyzed by LC-ESI-Q-TOF and homology-based search against the Sugarcane EST Database resulted in the identification of several proteins. Spatio-temporal expression pattern of selected genes was determined for validation of identified class III peroxidases that were preferentially expressed during sugarcane stem development.