Xylem parenchyma cells deliver the H2O2 necessary for lignification in differentiating xylem vessels

Modified atmosphere packaging maintains stem quality of Chinese flowering cabbage by restraining postharvest lignification and ROS accumulation

In this study, the impact of modified atmosphere packaging (MAP) on quality, lignin biosynthesis, reactive oxygen species (ROS) metabolism, and microstructures of stem in Chinese flowering cabbages was investigated. Compared with control, MAP treatment retained higher content of protein, total soluble solid, and vitamin C, while lower weight loss rate, carbon dioxide (CO2) production rate, electrolyte leakage, firmness and hollowing of stems. Lignin content in MAP-treated stems was 1.23-fold higher than that of control stems on the twelfth day. Moreover, MAP treatment inhibited the increasing in cell wall thickness by inhibiting activities of lignin biosynthesis-related enzymes. In addition, MAP suppressed ROS contents, while enhanced levels of ascorbic acid and reduced glutathione through promoting activities of antioxidant enzymes. The above results suggest that maintaining stems quality of Chinese flowering cabbages through MAP treatment is related to prevent lignin accumulation around the vascular tissue and enhance antioxidant capacity.

Unraveling Quinoa (Chenopodium quinoa Willd.) Defense Against Downy Mildew (Peronospora variabilis): Comparative Molecular Analysis of Resistant "Hualhuas" and Susceptible "Real" Cultivars

Quinoa (Chenopodium quinoa Willd.) is a new, promising non-conventional useful crop; however, its susceptibility to downy mildew, caused by Peronospora variabilis, is a key obstacle limiting its productivity in Egypt. Identifying and utilizing resistant quinoa cultivars appear to be reliable and cost-efficient strategies for controlling downy mildew, particularly in resource-limited farmers' fields. This study aimed to evaluate the differential resistance of the Peruvian "Hualhuas" and Bolivian "Real" quinoa cultivars to P. variabilis infection under laboratory conditions to provide precise insight into their basic defense mechanism(s). Inoculated "Hualhuas" plants displayed complete resistance against P. variabilis, with no visible symptoms (incompatible reaction), while those of "Real" plants revealed high susceptibility (compatible reaction), with typical downy mildew lesions on their leaf surfaces. Disease incidence reached about 66% in the inoculated "Real" plants, with most inoculated leaves having lesions of grades 4 and 5 covering up to 90% of their leaf surfaces. Susceptibility indices reached up to 66% in the inoculated "Real" plants. Resistance to P. variabilis observed in the "Hualhuas" plants may have been largely attributed to elevated endogenous H2O2 levels, increased peroxidase (POX) activity and abundance, enhanced phenylalanine ammonia-lyase (PAL) activity and expression, as well as the upregulation of the pathogen-related protein 10 gene (PR-10). The results of this study indicate that the quinoa cultivar "Hualhuas" not only is a promising candidate for sustainable control of quinoa downy mildew but also, through a deep understanding of its molecular resistance mechanisms, would provide a possible route to enhance downy mildew resistance in other genotypes.

A new oxidative pathway of nitric oxide production from oximes in plants

Nitric oxide (NO) is an essential reactive oxygen species and a signal molecule in plants. Although several studies have proposed the occurrence of oxidative NO production, only reductive routes for NO production, such as the nitrate (NO-3) -upper-reductase pathway, have been evidenced to date in land plants. However, plants grown axenically with ammonium as the sole source of nitrogen exhibit contents of nitrite and NO3-, evidencing the existence of a metabolic pathway for oxidative production of NO. We hypothesized that oximes, such as indole-3-acetaldoxime (IAOx), a precursor to indole-3-acetic acid, are intermediate oxidation products in NO synthesis. We detected the production of NO from IAOx and other oximes catalyzed by peroxidase (POD) enzyme using both 4-amino-5-methylamino-2',7'-difluorescein fluorescence and chemiluminescence. Flavins stimulated the reaction, while superoxide dismutase inhibited it. Interestingly, mouse NO synthase can also use IAOx to produce NO at a lower rate than POD. We provided a full mechanism for POD-dependent NO production from IAOx consistent with the experimental data and supported by density functional theory calculations. We showed that the addition of IAOx to extracts from Medicago truncatula increased the in vitro production of NO, while in vivo supplementation of IAOx and other oximes increased the number of lateral roots, as shown for NO donors, and a more than 10-fold increase in IAOx dehydratase expression. Furthermore, we found that in vivo supplementation of IAOx increased NO production in Arabidopsis thaliana wild-type plants, while prx33-34 mutant plants, defective in POD33-34, had reduced production. Our data show that the release of NO by IAOx, as well as its auxinic effect, explain the superroot phenotype. Collectively, our study reveals that plants produce NO utilizing diverse molecules such as oximes, POD, and flavins, which are widely distributed in the plant kingdom, thus introducing a long-awaited oxidative pathway to NO production in plants. This knowledge has essential implications for understanding signaling in biological systems.

Cell-wall microdomain remodeling controls crucial developmental processes

Plant cell walls display cellular and subcellular specificities. At the subcellular level, wall regional territories with specific compositions are necessary for macroscopic developmental processes. These regional specificities were named differently throughout the years, and are unified here under the term 'cell-wall microdomains' that define the local composition and organization of wall polymers underlying territories of wall loosening and/or softening or stiffening. We review the occurrence and developmental role of wall microdomains in different cell types. We primarily focus on the contribution of two categories of wall-remodeling molecular actors: fine-tuning of homogalacturonan (HG; pectin) demethylesterification patterns and two classes of oxidoreductases [class III peroxidases (CIII PRXs) and laccases (LACs)], but we also highlight two different molecular scaffolds recently identified for positioning specific CIII PRXs.

A vacuolar hexose transport is required for xylem development in the inflorescence stem

In Angiosperms, the development of the vascular system is controlled by a complex network of transcription factors. However, how nutrient availability in the vascular cells affects their development remains to be addressed. At the cellular level, cytosolic sugar availability is regulated mainly by sugar exchanges at the tonoplast through active and/or facilitated transport. In Arabidopsis (Arabidopsis thaliana), among the genes encoding tonoplastic transporters, SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTER 16 (SWEET16) and SWEET17 expression has been previously detected in the vascular system. Here, using a reverse genetics approach, we propose that sugar exchanges at the tonoplast, regulated by SWEET16, are important for xylem cell division as revealed in particular by the decreased number of xylem cells in the swt16 mutant and the accumulation of SWEET16 at the procambium-xylem boundary. In addition, we demonstrate that transport of hexoses mediated by SWEET16 and/or SWEET17 is required to sustain the formation of the xylem secondary cell wall. This result is in line with a defect in the xylem cell wall composition as measured by Fourier-transformed infrared spectroscopy in the swt16swt17 double mutant and by upregulation of several genes involved in secondary cell wall synthesis. Our work therefore supports a model in which xylem development partially depends on the exchange of hexoses at the tonoplast of xylem-forming cells.

Production of reactive oxygen species by PuRBOHF is critical for stone cell development in pear fruit

The production of reactive oxygen species (ROS) by NADPH oxidase, which is also referred to as respiratory burst oxidase homolog (RBOH), affects several processes in plants. However, the role of RBOHs in cell wall lignification is not well understood. In this study, we show that PuRBOHF, an RBOH isoform, plays an important role in secondary wall formation in pear stone cells. ROS were closely associated with lignin deposition and stone cell formation according to microscopy data. In addition, according to the results of an in situ hybridization analysis, the stage-specific expression of PuRBOHF was higher in stone cells than in cells of other flesh tissues. Inhibitors of RBOH activity suppressed ROS accumulation and stone cell lignification in pear fruit. Moreover, transient overexpression of PuRBOHF caused significant changes in the amount of ROS and lignin that accumulated in pear fruit and flesh calli. We further showed that PuMYB169 regulates PuRBOHF expression, while PuRBOHF-derived ROS induces the transcription of PuPOD2 and PuLAC2. The findings of this study indicate that PuRBOHF-mediated ROS production, which is regulated by a lignin-related transcriptional network, is essential for monolignol polymerization and stone cell formation in pear fruit.

Mitochondria: Key Organelles Accelerating Cell Wall Material Accumulation in Juice Sacs of Pummelo (Citrus grandis L. Osbeck) Fruits during Postharvest Storage

Granulation is a physiological disorder of juice sacs in citrus fruits, which develops through secondary cell wall formation. However, the synergistic changes in the cytoplasm of juice sac cells remain largely unknown. This study investigated the dynamic ultrastructure of juice sacs of “Guanxi” pummelo fruits by transmission electron microscopy and determined their cell wall material, soluble sugar, and organic acid contents. The results showed that lignin and hemicellulose are accumulated in juice sacs isolated from dorsal vascular bundles, while lignin and cellulose contribute to the granulation of juice sacs isolated from septal vascular bundles. The significant differences in lignin, cellulose, and hemicellulose contents between the two types of juice sacs began to be observed at 30 days of storage. Fructose levels were elevated in juice sacs isolated from the dorsal vascular bundles from 10 to 60 days. Sucrose contents significantly decreased in juice sacs isolated from the septal vascular bundles from 30 to 60 days. Meanwhile glucose, citric acid, and malic acid contents exhibited no apparent changes in both types of juice sacs. Based on the comprehensive analysis of the ultrastructure of both types of juice sacs, it was clearly found that plasma membrane ruptures induce cell wall material synthesis in intracellular spaces; however, cell wall substance contents did not significantly increase until the number of mitochondria sharply increased. In particular, sucrose contents began to decrease significantly just after the mitochondria amount largely increased in juice sacs isolated from the septal vascular bundles, indicating that mitochondria play a key role in regulating carbon source sugar partitioning for cell wall component synthesis.

Protein Carbonylation: Emerging Roles in Plant Redox Biology and Future Prospects

Plants are sessile in nature and they perceive and react to environmental stresses such as abiotic and biotic factors. These induce a change in the cellular homeostasis of reactive oxygen species (ROS). ROS are known to react with cellular components, including DNA, lipids, and proteins, and to interfere with hormone signaling via several post-translational modifications (PTMs). Protein carbonylation (PC) is a non-enzymatic and irreversible PTM induced by ROS. The non-enzymatic feature of the carbonylation reaction has slowed the efforts to identify functions regulated by PC in plants. Yet, in prokaryotic and animal cells, studies have shown the relevance of protein carbonylation as a signal transduction mechanism in physiological processes including hydrogen peroxide sensing, cell proliferation and survival, ferroptosis, and antioxidant response. In this review, we provide a detailed update on the most recent findings pertaining to the role of PC and its implications in various physiological processes in plants. By leveraging the progress made in bacteria and animals, we highlight the main challenges in studying the impacts of carbonylation on protein functions in vivo and the knowledge gap in plants. Inspired by the success stories in animal sciences, we then suggest a few approaches that could be undertaken to overcome these challenges in plant research. Overall, this review describes the state of protein carbonylation research in plants and proposes new research avenues on the link between protein carbonylation and plant redox biology.

Molecular mechanisms for magnesium-deficiency-induced leaf vein lignification, enlargement and cracking in Citrus sinensis revealed by RNA-Seq

Citrus sinensis (L.) Osbeck seedlings were fertigated with nutrient solution containing 2 [magnesium (Mg)-sufficiency] or 0 mM (Mg-deficiency) Mg(NO3)2 for 16 weeks. Thereafter, RNA-Seq was used to investigate Mg-deficiency-responsive genes in the veins of upper and lower leaves in order to understand the molecular mechanisms for Mg-deficiency-induced vein lignification, enlargement and cracking, which appeared only in the lower leaves. In this study, 3065 upregulated and 1220 downregulated, and 1390 upregulated and 375 downregulated genes were identified in Mg-deficiency veins of lower leaves (MDVLL) vs Mg-sufficiency veins of lower leaves (MSVLL) and Mg-deficiency veins of upper leaves (MDVUL) vs Mg-sufficiency veins of upper leaves (MSVUL), respectively. There were 1473 common differentially expressed genes (DEGs) between MDVLL vs MSVLL and MDVUL vs MSVUL, 1463 of which displayed the same expression trend. Magnesium-deficiency-induced lignification, enlargement and cracking in veins of lower leaves might be related to the following factors: (i) numerous transciption factors and genes involved in lignin biosynthesis pathways, regulation of cell cycle and cell wall metabolism were upregulated; and (ii) reactive oxygen species, phytohormone and cell wall integrity signalings were activated. Conjoint analysis of proteome and transcriptome indicated that there were 287 and 56 common elements between DEGs and differentially abundant proteins (DAPs) identified in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, and that among these common elements, the abundances of 198 and 55 DAPs matched well with the transcript levels of the corresponding DEGs in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, indicating the existence of concordances between protein and transcript levels.

PIRIN2 suppresses S-type lignin accumulation in a noncell-autonomous manner in Arabidopsis xylem elements

PIRIN (PRN) genes encode cupin domain-containing proteins that function as transcriptional co-regulators in humans but that are poorly described in plants. A previous study in xylogenic cell cultures of Zinnia elegans suggested a role for a PRN protein in lignification. This study aimed to identify the function of Arabidopsis (Arabidopsis thaliana) PRN proteins in lignification of xylem tissues. Chemical composition of the secondary cell walls was analysed in Arabidopsis stems and/or hypocotyls by pyrolysis-gas chromatography/mass spectrometry, 2D-nuclear magnetic resonance and phenolic profiling. Secondary cell walls of individual xylem elements were chemotyped by Fourier transform infrared and Raman microspectroscopy. Arabidopsis PRN2 suppressed accumulation of S-type lignin in Arabidopsis stems and hypocotyls. PRN2 promoter activity and PRN2:GFP fusion protein were localised specifically in cells next to the vessel elements, suggesting a role for PRN2 in noncell-autonomous lignification of xylem vessels. Accordingly, PRN2 modulated lignin chemistry in the secondary cell walls of the neighbouring vessel elements. These results indicate that PRN2 suppresses S-type lignin accumulation in the neighbourhood of xylem vessels to bestow G-type enriched lignin composition on the secondary cell walls of the vessel elements. Gene expression analyses suggested that PRN2 function is mediated by regulation of the expression of the lignin-biosynthetic genes.

Allies or Enemies: The Role of Reactive Oxygen Species in Developmental Processes of Black Cottonwood (Populus trichocarpa)

In contrast to aboveground organs (stems and leaves), developmental events and their regulation in underground organs, such as pioneer and fine roots, are quite poorly understood. The objective of the current study was to achieve a better understanding of the physiological and molecular role of reactive oxygen species (ROS) and ROS-related enzymes in the process of stem and pioneer root development in black cottonwood (Populus trichocarpa), as well as in the senescence of leaves and fine roots. Results of a transcriptomic analysis revealed that primary/secondary growth and senescence are accompanied by substantial changes in the expression of genes related to oxidative stress metabolism. We observed that some mechanisms common for above- and under-ground organs, e.g., the expression of superoxide dismutase (SOD) genes and SOD activity, declined during stems' and pioneer roots' development. Moreover, the localization of hydrogen peroxide (H₂O₂) and superoxide (O₂•^-) in the primary and secondary xylem of stems and pioneer roots confirms their involvement in xylem cell wall lignification and the induction of programmed cell death (PCD). H₂O₂ and O₂•^- in senescing fine roots were present in the same locations as demonstrated previously for ATG8 (AuTophaGy-related) proteins, implying their participation in cell degradation during senescence, while O₂•^- in older leaves was also localized similarly to ATG8 in chloroplasts, suggesting their role in chlorophagy. ROS and ROS-related enzymes play an integral role in the lignification of xylem cell walls in Populus trichocarpa, as well as the induction of PCD during xylogenesis and senescence.

ROS and oxidative burst: Roots in plant development

Reactive oxygen species (ROS) are widely generated in various redox reactions in plants. In earlier studies, ROS were considered toxic byproducts of aerobic metabolism. In recent years, it has become clear that ROS act as plant signaling molecules that participate in various processes such as growth and development. Several studies have elucidated the roles of ROS from seed germination to senescence. However, there is much to discover about the diverse roles of ROS as signaling molecules and their mechanisms of sensing and response. ROS may provide possible benefits to plant physiological processes by supporting cellular proliferation in cells that maintain basal levels prior to oxidative effects. Although ROS are largely perceived as either negative by-products of aerobic metabolism or makers for plant stress, elucidating the range of functions that ROS play in growth and development still require attention.

Modulatory Role of Reactive Oxygen Species in Root Development in Model Plant of Arabidopsis thaliana

Reactive oxygen species (ROS), a type of oxygen monoelectronic reduction product, have a higher chemical activity than O₂. Although ROS pose potential risks to all organisms via inducing oxidative stress, indispensable role of ROS in individual development cannot be ignored. Among them, the role of ROS in the model plant Arabidopsis thaliana is deeply studied. Mounting evidence suggests that ROS are essential for root and root hair development. In the present review, we provide an updated perspective on the latest research progress pertaining to the role of ROS in the precise regulation of root stem cell maintenance and differentiation, redox regulation of the cell cycle, and root hair initiation during root growth. Among the different types of ROS, O₂ ^•- and H₂O₂ have been extensively investigated, and they exhibit different gradient distributions in the roots. The concentration of O₂ ^•- decreases along a gradient from the meristem to the transition zone and the concentration of H₂O₂ decreases along a gradient from the differentiation zone to the elongation zone. These gradients are regulated by peroxidases, which are modulated by the UPBEAT1 (UPB1) transcription factor. In addition, multiple transcriptional factors, such as APP1, ABO8, PHB3, and RITF1, which are involved in the brassinolide signaling pathway, converge as a ROS signal to regulate root stem cell maintenance. Furthermore, superoxide anions (O₂ ^•-) are generated from the oxidation in mitochondria, ROS produced during plasmid metabolism, H₂O₂ produced in apoplasts, and catalysis of respiratory burst oxidase homolog (RBOH) in the cell membrane. Furthermore, ROS can act as a signal to regulate redox status, which regulates the expression of the cell-cycle components CYC2;3, CYCB1;1, and retinoblastoma-related protein, thereby controlling the cell-cycle progression. In the root maturation zone, the epidermal cells located in the H cell position emerge to form hair cells, and plant hormones, such as auxin and ethylene regulate root hair formation via ROS. Furthermore, ROS accumulation can influence hormone signal transduction and vice versa. Data about the association between nutrient stress and ROS signals in root hair development are scarce. However, the fact that ROBHC/RHD2 or RHD6 is specifically expressed in root hair cells and induced by nutrients, may explain the relationship. Future studies should focus on the regulatory mechanisms underlying root hair development via the interactions of ROS with hormone signals and nutrient components.

Silicon increases cell wall thickening and lignification in rice (Oryza sativa) root tip under excess Fe nutrition

Iron (Fe) as a micronutrients and silicon (Si) as a cell wall element are important in plant cell wall extension and integrity. While the interaction of exogenous Si and excess Fe on root cell wall modifications is known, the effects of these nutritional parameters on the spatial changes in the activities of genes and/or enzymes involved in the lignification of root cell walls are not well studied. Thus, these parameters were investigated in the root apical part (AP) and basal part (BP) of rice (Oryza sativa L.) plants supplied with and without Si (1.5 mM) under normal (10 mg/L) and excess Fe (150 mg/L) nutrition for 7 days. Beside growth retardation, excess Fe increased the activities of phenylalanine ammonia lyase (PAL), superoxide dismutase and NADPH-oxidase and PAL and cell wall peroxidase (POD) genes expression, along with the increased phenols and H₂O₂ contents in the root AP. Furthermore, the increased thickening of endodermal, exodermal and metaxylem cell walls in the root AP by excess Fe was attributed to the enhanced POD activity. POD expression, endodermal and exodermal cell wall thickenings were not affected by excess Fe in the root BP. Si application under excess Fe exaggerated the effects of excess Fe on root cell wall thickening, increased POD activity but reduced H₂O₂ content in the root AP. Thus, Si application under excess Fe nutrition promotes earlier initiation of lignin polymerization closer to and toward the root tip and hence restricts the entry of excess Fe into the plant.

Deciphering hydrogen peroxide-induced signalling towards stress tolerance in plants

Plants encounter a variety of adverse environmental conditions, such as high salinity, drought, extreme heat/cold and heavy metals contamination (abiotic stress) or attack of various pathogens (biotic stress). These detrimental environmental factors enhanced the ROS production such as singlet oxygen (¹O₂), superoxide (O₂ ^•-), hydrogen peroxide (H₂O₂) and hydroxyl radicals (OH^•). ROS are highly reactive and directly target several cellular molecules and metabolites, which lead to severe cellular dysfunction. Plants respond to oxidative damages by activating antioxidant machinery to trigger signalling cascades for stress tolerance. H₂O₂ signalling balances the plant metabolism through cross-talk with other signals and plant hormones during growth, development and stress responses. H₂O₂ facilitates the regulation of different stress-responsive transcription factors (TFs) including NAC, Zinc finger, WRKY, ERF, MYB, DREB and bZIP as both upstream and downstream events during stress signalling. The present review focuses on the biological synthesis of the H₂O₂ and its effect on the upregulation of kinase genes and stress related TFs for imparting stress tolerance.

Reactive oxygen species and reactive carbonyl species constitute a feed-forward loop in auxin signaling for lateral root formation

In auxin-stimulated roots, production of reactive oxygen species (ROS) via the hormone-induced activation of respiratory burst oxidase homologous NADPH oxidases facilitates lateral root (LR) formation. In this study, in order to verify that ROS can modulate auxin signaling, we examined the involvement of the lipid peroxide-derived agents known as reactive carbonyl species (RCS) in LR formation. When auxin was added to Arabidopsis thaliana roots, the levels of RCS, for example acrolein, 4-hydroxynonenal and crotonaldehyde, were increased prior to LR formation. Addition of the carbonyl scavenger carnosine suppressed auxin-induced LR formation. Addition of RCS to the roots induced the expression of the auxin-responsive DR5 promoter and the TIR1, IAA14, ARF7, LBD16 and PUCHI genes and facilitated LR formation without increasing the endogenous auxin level. DR5 and LBD16 were activated in the LR primordia. The auxin signaling-deficient mutants arf7 arf19 and slr-1 did not respond - and tir1 afb2 appeared to show a poor response - to RCS. When given to the roots RCS promoted the disappearance of the AXR3NT-GUS fusion protein, i.e. the degradation of the auxin/indole-3-acetic acid protein, as did auxin. These results indicate that the auxin-induced production of ROS and their downstream products RCS modulate the auxin signaling pathway in a feed-forward manner. RCS are key agents that connect the ROS signaling and the auxin signaling pathways.

Biotechnological Potential of LSD1, EDS1, and PAD4 in the Improvement of Crops and Industrial Plants

Lesion Simulating Disease 1 (LSD1), Enhanced Disease Susceptibility (EDS1) and Phytoalexin Deficient 4 (PAD4) were discovered a quarter century ago as regulators of programmed cell death and biotic stress responses in Arabidopsis thaliana. Recent studies have demonstrated that these proteins are also required for acclimation responses to various abiotic stresses, such as high light, UV radiation, drought and cold, and that their function is mediated through secondary messengers, such as salicylic acid (SA), reactive oxygen species (ROS), ethylene (ET) and other signaling molecules. Furthermore, LSD1, EDS1 and PAD4 were recently shown to be involved in the modification of cell walls, and the regulation of seed yield, biomass production and water use efficiency. The function of these proteins was not only demonstrated in model plants, such as Arabidopsis thaliana or Nicotiana benthamiana, but also in the woody plant Populus tremula x tremuloides. In addition, orthologs of LSD1, EDS1, and PAD4 were found in other plant species, including different crop species. In this review, we focus on specific LSD1, EDS1 and PAD4 features that make them potentially important for agricultural and industrial use.

Hydrogen Peroxide Prompted Lignification Affects Pathogenicity of Hemi-biotrophic Pathogen Bipolaris sorokiniana to Wheat

Spot blotch caused by Bipolaris sorokiniana has spread to more than 9 million ha of wheat in the warm, humid areas of the Eastern Gangetic Plains (EGP) of South Asia and is a disease of major concern in other similar wheat growing regions worldwide. Differential lignin content in resistant and susceptible genotypes and its association with free radicals such as hydrogen peroxide (H₂O₂), superoxide (O₂ ^-) and hydroxyl radical (OH^-) were studied after inoculation under field conditions for two consecutive years. H₂O₂ significantly influenced lignin content in flag leaves, whereas there was a negative correlation among lignin and H₂O₂ to the Area Under Disease Progress Curve (AUDPC). The production of H₂O₂ was higher in the resistant genotypes than susceptible ones. The O₂ ^- and OH^- positively correlated with AUDPC but negatively with lignin content. This study illustrates that H₂O₂ has a vital role in prompting lignification and thereby resistance to spot blotch in wheat. We used cluster analysis to separate the resistant and susceptible genotypes by phenotypic and biochemical traits. H₂O₂ associated lignin production significantly reduced the number of appressoria and penetration pegs. We visualized the effect of lignin in disease resistance using differential histochemical staining of tissue from resistant and susceptible genotypes, which shows the variable accumulation of hydrogen peroxide and lignin around penetration sites.

An integrated approach to highlight biological responses of Pisum sativum root to nano-TiO2 exposure in a biosolid-amended agricultural soil

This study focused on crop plant response to a simultaneous exposure to biosolid and TiO₂ at micro- and nano-scale, being biosolid one of the major sink of TiO₂ nanoparticles released into the soil environment. We settled an experimental design as much as possible realistic, at microcosm scale, using the crop Pisum sativum. This experimental design supported the hypotheses that the presence of biosolid in the farming soil might influence plant growth and metabolism and that, after TiO₂ spiking, the different dimension and crystal forms of TiO₂ might be otherwise bioavailable and differently interacting with the plant system. To test these hypotheses, we have considered different aspects of the response elicited by TiO₂ and biosolid at cellular and organism level, focusing on the root system, with an integrative approach. In our experimental conditions, the presence of biosolid disturbed plant growth of P. sativum, causing cellular damages at root level, probably through mechanisms not only oxidative stress-dependent but also involving altered signalling processes. These disturbances could depend on non-humified compounds and/or on the presence of toxic elements and of nanoparticles in the biosolid-amended soil. The addition of TiO₂ particles in the sludge-amended soil, further altered plant growth and induced oxidative and ultrastructural damages. Although non typical dose-effect response was detected, the most responsiveness treatments were found for the anatase crystal form, alone or mixed with rutile. Based on ultrastructural observations, we could hypothesise that the toxicity level of TiO₂ nanoparticles may depend on the cell ability to isolate nanoparticles in subcellular compartments, avoiding their interaction with organelles and/or metabolic processes. The results of the present work suggest reflections on the promising practice of soil amendments and on the use of nanomaterials and their safety for food plants and living organisms.

Identification and Characterization of Menadione and Benzethonium Chloride as Potential Treatments of Pierce's Disease of Grapevines

Xylella fastidiosa infects a wide range of plant hosts and causes Pierce's disease (PD) of grapevines. The type 1 multidrug resistance (MDR) efflux system is essential for pathogenicity and survival of bacterial pathogens in planta. X. fastidiosa, with a single MDR system, is significantly more vulnerable to inhibition by small-molecule treatments than most bacterial pathogens that typically carry redundant MDR systems. A high-throughput cell viability assay using a green fluorescent protein-marked strain of X. fastidiosa Temecula 1 was developed to screen two Prestwick combinatorial small-molecule libraries of drugs and phytochemicals (1,600 chemicals in total) approved by the Food and Drug Administration and European Medicines Agency for cell growth inhibition. The screens revealed 215 chemicals that inhibited bacterial growth by >50% at 50 µM concentrations. Seven chemicals proved to lyse X. fastidiosa cells at 25 µM, including four phytochemicals. Menadione (2-methyl-1,4-naphthoquinone, vitamin K) from the phytochemical library and benzethonium chloride (a topical disinfectant) from the chemical library both showed significant bactericidal activity against X. fastidiosa. Both menadione and benzethonium chloride foliar spray (15 and 5 mM, respectively) and soil drench (5 and 25 mM, respectively) treatments were equally effective in reducing PD symptoms by 54 to 59% and revealed that the effects of both chemical treatments became systemic. However, menadione was phytotoxic when applied as a foliar spray at effective concentrations, causing significant loss of photosynthetic capacity.

Evolutionary and functional analysis of the plant-specific NADPH oxidase gene family in Brassica rapa L

NADPH oxidases (NOXs) have been known as respiratory burst oxidase homologues (RBOHs) in plants. To characterize the evolutionary relationships and functions of RBOHs in Brassica rapa, 134 RBOH homologues were identified from 13 plant species, including 14 members (namely BrRBOH01-14) from B. rapa. There presented 47 gene-pairs among 14 BrRBOHs and other RBOHs, consisting of five pairs within B. rapa, and 15 pairs between B. rapa and Arabidopsis thaliana. Together with phylogenetic analysis, the results suggested that whole-genome duplication might have played an important role in BrRBOH gene expansion, and these duplication events occurred after the divergence of the eudicot and the monocot lineages examined. Furthermore, gene expression of RBOHs in both A. thaliana and B. rapa were assayed via qRT-PCR. An RBOH gene, BrRBOH13 in B. rapa, was transformed into wild-type Arabidopsis plants. The transgenic lines with the overexpressed level of BrRBOH13 conferred to be more tolerant to heavy metal lead (0.05 mM) than wild-type plants. Overall, this integrated analysis at genome-wide level has provided some information on the evolutionary relationships among plant-specific NOXs and the coordinated diversification of gene structure and function in B. rapa.

The Casparian strip-one ring to bring cell biology to lignification?

Lignin research has long been motivated by the outstanding importance of wood for human societies. The annual, non-woody Arabidopsis thaliana, has nevertheless contributed greatly to our understanding of lignification, due to its unrivalled genetic resources. Arabidopsis is also great for cell and developmental biology, allowing precise imaging and tracking of cell types. Root endodermis differentiation involves the precise lignification of the Casparian Strip, as an apoplastic barrier; while barrier damage triggers a less localized, compensatory lignification. Transcriptional reprogramming and peptide-induced signalling emerge as promising tools for the study of endodermal lignification. We argue that endodermis lignification is an attractive model complementary to equally powerful, cellular xylem differentiation systems, as it might better represent the restricted - often localized - lignification seen in non-vascular cells.

Insights into the molecular regulation of monolignol-derived product biosynthesis in the growing hemp hypocotyl

BACKGROUND

Lignin and lignans are both derived from the monolignol pathway. Despite the similarity of their building blocks, they fulfil different functions in planta. Lignin strengthens the tissues of the plant, while lignans are involved in plant defence and growth regulation. Their biosyntheses are tuned both spatially and temporally to suit the development of the plant (water conduction, reaction to stresses). We propose to study the general molecular events related to monolignol-derived product biosynthesis, especially lignin. It was previously shown that the growing hemp hypocotyl (between 6 and 20 days after sowing) is a valid system to study secondary growth and the molecular events accompanying lignification. The present work confirms the validity of this system, by using it to study the regulation of lignin and lignan biosynthesis. Microscopic observations, lignin analysis, proteomics, together with in situ laccase and peroxidase activity assays were carried out to understand the dynamics of lignin synthesis during the development of the hemp hypocotyl.

RESULTS

Based on phylogenetic analysis and targeted gene expression, we suggest a role for the hemp dirigent and dirigent-like proteins in lignan biosynthesis. The transdisciplinary approach adopted resulted in the gene- and protein-level quantification of the main enzymes involved in the biosynthesis of monolignols and their oxidative coupling (laccases and class III peroxidases), in lignin deposition (dirigent-like proteins) and in the determination of the stereoconformation of lignans (dirigent proteins).

CONCLUSIONS

Our work sheds light on how, in the growing hemp hypocotyl, the provision of the precursors needed to synthesize the aromatic biomolecules lignin and lignans is regulated at the transcriptional and proteomic level.

Tolerance to oxidative stress is required for maximal xylem colonization by the xylem-limited bacterial phytopathogen, Xylella fastidiosa

Bacterial plant pathogens often encounter reactive oxygen species (ROS) during host invasion. In foliar bacterial pathogens, multiple regulatory proteins are involved in the sensing of oxidative stress and the activation of the expression of antioxidant genes. However, it is unclear whether xylem-limited bacteria, such as Xylella fastidiosa, experience oxidative stress during the colonization of plants. Examination of the X. fastidiosa genome uncovered only one homologue of oxidative stress regulatory proteins, OxyR. Here, a knockout mutation in the X. fastidiosa oxyR gene was constructed; the resulting strain was significantly more sensitive to hydrogen peroxide (H2 O2 ) relative to the wild-type. In addition, during early stages of grapevine infection, the survival rate was 1000-fold lower for the oxyR mutant than for the wild-type. This supports the hypothesis that grapevine xylem represents an oxidative environment and that X. fastidiosa must overcome this challenge to achieve maximal xylem colonization. Finally, the oxyR mutant exhibited reduced surface attachment and cell-cell aggregation and was defective in biofilm maturation, suggesting that ROS could be a potential environmental cue stimulating biofilm development during the early stages of host colonization.

Drought and exogenous abscisic acid alter hydrogen peroxide accumulation and differentially regulate the expression of two maize RD22-like genes

Increased biosynthesis of abscisic acid (ABA) occurs in plants in response to water deficit, which is mediated by changes in the levels of reactive oxygen species such as H₂O₂. Water deficit and ABA induce expression of some RD22-like proteins. This study aimed to evaluate the effect of water deficit and exogenous ABA (50 µM ABA applied every 24 hours for a total of 72 hours) on H₂O₂ content in Zea mays (maize) and to characterise genes encoding two putative maize RD22-like proteins (designated ZmRD22A and ZmRD22B). The expression profiles of the two putative maize RD22-like genes in response to water deficit and treatment with ABA were examined in leaves. In silico analyses showed that the maize RD22-like proteins share domain organisation with previously characterized RD22-like proteins. Both water deficit and exogenous ABA resulted in increased H₂O₂ content in leaves but the increase was more pronounced in response to water deficit than to exogenous ABA. Lignin content was not affected by exogenous ABA, whereas it was decreased by water deficit. Expression of both RD22-like genes was up-regulated by drought but the ZmRD22A gene was not influenced by exogenous ABA, whereas ZmRD22B was highly responsive to exogenous ABA.

Xylogenesis in zinnia (Zinnia elegans) cell cultures: unravelling the regulatory steps in a complex developmental programmed cell death event

MAIN CONCLUSION

Physiological and molecular studies support the view that xylogenesis can largely be determined as a specific form of vacuolar programmed cell death (PCD). The studies in xylogenic zinnia cell culture have led to many breakthroughs in xylogenesis research and provided a background for investigations in other experimental models in vitro and in planta . This review discusses the most essential earlier and recent findings on the regulation of xylem elements differentiation and PCD in zinnia and other xylogenic systems. Xylogenesis (the formation of water conducting vascular tissue) is a paradigm of plant developmental PCD. The xylem vessels are composed of fused tracheary elements (TEs)-dead, hollow cells with patterned lignified secondary cell walls. They result from the differentiation of the procambium and cambium cells and undergo cell death to become functional post-mortem. The TE differentiation proceeds through a well-coordinated sequence of events in which differentiation and the programmed cellular demise are intimately connected. For years a classical experimental model for studies on xylogenesis was the xylogenic zinnia (Zinnia elegans) cell culture derived from leaf mesophyll cells that, upon induction by cytokinin and auxin, transdifferentiate into TEs. This cell system has been proven very efficient for investigations on the regulatory components of xylem differentiation which has led to many discoveries on the mechanisms of xylogenesis. The knowledge gained from this system has potentiated studies in other xylogenic cultures in vitro and in planta. The present review summarises the previous and latest findings on the hormonal and biochemical signalling, metabolic pathways and molecular and gene determinants underlying the regulation of xylem vessels differentiation in zinnia cell culture. Highlighted are breakthroughs achieved through the use of xylogenic systems from other species and newly introduced tools and analytical approaches to study the processes. The mutual dependence between PCD signalling and the differentiation cascade in the program of TE development is discussed.

OxyR-regulated catalase CatB promotes the virulence in rice via detoxifying hydrogen peroxide in Xanthomonas oryzae pv. oryzae

BACKGROUND

To facilitate infection, Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice, needs to degrade hydrogen peroxide (H₂O₂) generated by the host defense response via a mechanism that is mediated by the transcriptional regulator OxyR. The catalase (CAT) gene catB has previously been shown to belong to the OxyR regulon in Xoo. However, its expression patterns and function in H₂O₂ detoxification and bacterial pathogenicity on rice remain to be elucidated.

RESULTS

The catB gene encodes a putative catalase and is highly conserved in the sequenced strains of Xanthomonas spp. β-galactosidase analysis and electrophoretic mobility shift assays (EMSA) showed that OxyR positively regulated the transcription of catB by directly binding to its promoter region. The quantitative real-time PCR (qRT-PCR) assays revealed that the expression levels of catB and oxyR were significantly induced by H₂O₂. Deletion of catB or oxyR drastically impaired bacterial viability in the presence of extracellular H₂O₂ and reduced CAT activity, demonstrating that CatB and OxyR contribute to H₂O₂ detoxification in Xoo. In addition, ΔcatB and ΔoxyR displayed shorter bacterial blight lesions and reduced bacterial growth in rice compared to the wild-type stain, indicating that CatB and OxyR play essential roles in the virulence of Xoo.

CONCLUSIONS

Transcription of catB is enhanced by OxyR in response to exogenous H₂O₂. CatB functions as an active catalase that is required for the full virulence of Xoo in rice.

RBOH-mediated ROS production facilitates lateral root emergence in Arabidopsis

Lateral root (LR) emergence represents a highly coordinated process in which the plant hormone auxin plays a central role. Reactive oxygen species (ROS) have been proposed to function as important signals during auxin-regulated LR formation; however, their mode of action is poorly understood. Here, we report that Arabidopsis roots exposed to ROS show increased LR numbers due to the activation of LR pre-branch sites and LR primordia (LRP). Strikingly, ROS treatment can also restore LR formation in pCASP1:shy2-2 and aux1 lax3 mutant lines in which auxin-mediated cell wall accommodation and remodeling in cells overlying the sites of LR formation is disrupted. Specifically, ROS are deposited in the apoplast of these cells during LR emergence, following a spatiotemporal pattern that overlaps the combined expression domains of extracellular ROS donors of the RESPIRATORY BURST OXIDASE HOMOLOGS (RBOH). We also show that disrupting (or enhancing) expression of RBOH in LRP and/or overlying root tissues decelerates (or accelerates) the development and emergence of LRs. We conclude that RBOH-mediated ROS production facilitates LR outgrowth by promoting cell wall remodeling of overlying parental tissues.

Summary: Reactive oxygen species promote cell wall remodeling of cells overlying the sites of lateral root formation, thereby contributing to lateral root emergence in Arabidopsis.

Control of root growth and development by reactive oxygen species

Reactive oxygen species (ROS) are relatively simple molecules that exist within cells growing in aerobic conditions. ROS were originally associated with oxidative stress and seen as highly reactive molecules that are injurious to many cell components. More recently, however, the function of ROS as signal molecules in many plant cellular processes has become more evident. One of the most important functions of ROS is their role as a plant growth regulator. For example, ROS are key molecules in regulating plant root development, and as such, are comparable to plant hormones. In this review, the molecular mechanisms of ROS that are mainly associated with plant root growth are discussed. The molecular links between root growth regulation by ROS and other signals will also be briefly discussed.

Reactive Oxygen Species (ROS): Beneficial Companions of Plants' Developmental Processes

Reactive oxygen species (ROS) are generated inevitably in the redox reactions of plants, including respiration and photosynthesis. In earlier studies, ROS were considered as toxic by-products of aerobic pathways of the metabolism. But in recent years, concept about ROS has changed because they also participate in developmental processes of plants by acting as signaling molecules. In plants, ROS regulate many developmental processes such as cell proliferation and differentiation, programmed cell death, seed germination, gravitropism, root hair growth and pollen tube development, senescence, etc. Despite much progress, a comprehensive update of advances in the understanding of the mechanisms evoked by ROS that mediate in cell proliferation and development are fragmentry and the matter of ROS perception and the signaling cascade remains open. Therefore, keeping in view the above facts, an attempt has been made in this article to summarize the recent findings regarding updates made in the regulatory action of ROS at various plant developmental stages, which are still not well-known.

The cell biology of lignification in higher plants

BACKGROUND

Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying.

SCOPE

Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level.

CONCLUSIONS

The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.

Lignification: different mechanisms for a versatile polymer

Lignins are cell wall phenolic polymers resulting from monolignol radical coupling. They have characteristically high diversity in their structures which is a direct consequence of the versatile character of the lignification mechanisms discussed in this review. We will relate the latest discoveries regarding the main participants involved in lignin deposition in various tissues. Lignification is often described as a cell autonomous event occurring progressively in all cell wall layers during lignifying cell life and stopping with the cell death. However, recent data combined to old data from studies of tree lignification and zinnia cultures challenged these entrenched views and showed that the lignification process is cell-type dependent and can involve neighboring cells. Therefore, we consider recent data on cell-autonomous and non-cell autonomous lignification processes. We conclude that the role of lignins still need to be assessed during plant development and that control of polymerization/lignin deposition remains elusive and need to be investigated.

Cooperative lignification of xylem tracheary elements

The development of xylem tracheary elements (TEs)--the hydro-mineral sap conducting cells--has been an evolutionary breakthrough to enable long distance nutrition and upright growth of vascular land plants. To allow sap conduction, TEs form hollow laterally reinforced cylinders by combining programmed cell death and secondary cell wall formation. To ensure their structural resistance for sap conduction, TE cell walls are reinforced with the phenolic polymer lignin, which is deposited after TE cell death by the cooperative supply of monomers and other substrates from the surrounding living cells.

SPINDLY, ERECTA, and its ligand STOMAGEN have a role in redox-mediated cortex proliferation in the Arabidopsis root

Reactive oxygen species (ROS) are harmful to all living organisms and therefore they must be removed to ensure normal growth and development. ROS are also signaling molecules, but so far little is known about the mechanisms of ROS perception and developmental response in plants. We here report that hydrogen peroxide induces cortex proliferation in the Arabidopsis root and that SPINDLY (SPY), an O-linked glucosamine acetyltransferase, regulates cortex proliferation by maintaining cellular redox homeostasis. We also found that mutation in the leucine-rich receptor kinase ERECTA and its putative peptide ligand STOMAGEN block the effect of hydrogen peroxide on root cortex proliferation. However, ERECTA and STOMAGEN are expressed in the vascular tissue, whereas extra cortex cells are produced from the endodermis, suggesting the involvement of intercellular signaling. SPY appears to act downstream of ERECTA, because the spy mutation still caused cortex proliferation in the erecta mutant background. We therefore have not only gained insight into the mechanism by which SPY regulates root development but also uncovered a novel pathway for ROS signaling in plants. The importance of redox-mediated cortex proliferation as a protective mechanism against oxidative stress is also discussed.

Xylan deposition and lignification in the multi-layered cell walls of phloem fibres in Mallotus japonicus (Euphorbiaceae)

Phloem fibres in Mallotus japonicus Müll. Arg. were found to have a multi-layered structure that is S1 + S2 + n(G + L), where a non-lignified gelatinous layer (G) and a lignified layer (L) are formed alternately and n indicates the number of repetitions of these two layers. The aim of this study was to determine the process of xylan deposition and lignification in the multi-layered cell walls of phloem fibres. The formation process of the multi-layered structure of secondary phloem fibres was examined by light microscopy, ultraviolet microscopy and transmission electron microscopy. The distribution of glucuronoxylan was examined by immunoelectron microscopy. The activity of peroxidase was also determined using metal-enhanced diaminobenzidine substrates. Immunolabelling of glucuronoxylan occurred in lignified cell wall layers, except in the compound middle lamella (CML), i.e., the S1, S2 and L layers but not the G layers. Change in immunolabelling density suggests that xylan deposition in these lignified layers occurs appositionally, i.e., xylan is deposited into the lignified layers directly and not by a penetrative mechanism, and deposition does not occur after the layers are fully deposited. Peroxidase activity was found in CML including cell corners during S2 layer formation, then in developing G layers during G layer formation. Peroxidase activity was also found in the thin L layers that formed recently and was not found in the L layers already present. Xylan labelling was not found in the thin L layers that formed recently but did occur in L layers that developed earlier. Lignification of the S1 and S2 layers continued during the formation of the G layers, whereas in the L layers it finished just after deposition of the L layer.

Phenylcoumaran benzylic ether reductase prevents accumulation of compounds formed under oxidative conditions in poplar xylem

Phenylcoumaran benzylic ether reductase (PCBER) is one of the most abundant proteins in poplar (Populus spp) xylem, but its biological role has remained obscure. In this work, metabolite profiling of transgenic poplar trees downregulated in PCBER revealed both the in vivo substrate and product of PCBER. Based on mass spectrometry and NMR data, the substrate was identified as a hexosylated 8-5-coupling product between sinapyl alcohol and guaiacylglycerol, and the product was identified as its benzyl-reduced form. This activity was confirmed in vitro using a purified recombinant PCBER expressed in Escherichia coli. Assays performed on 20 synthetic substrate analogs revealed the enzyme specificity. In addition, the xylem of PCBER-downregulated trees accumulated over 2000-fold higher levels of cysteine adducts of monolignol dimers. These compounds could be generated in vitro by simple oxidative coupling assays involving monolignols and cysteine. Altogether, our data suggest that the function of PCBER is to reduce phenylpropanoid dimers in planta to form antioxidants that protect the plant against oxidative damage. In addition to describing the catalytic activity of one of the most abundant enzymes in wood, we provide experimental evidence for the antioxidant role of a phenylpropanoid coupling product in planta.

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.

New insights into pioneer root xylem development: evidence obtained from Populus trichocarpa plants grown under field conditions

BACKGROUND AND AIMS

Effective programmed xylogenesis is critical to the structural framework of the plant root system and its central role in the acquisition and long-distance transport of water and nutrients. The process of xylem differentiation in pioneer roots under field conditions is poorly understood. In this study it is hypothesized that xylogenesis, an example of developmental programmed cell death (PCD), in the roots of woody plants demonstrates a clearly defined sequence of events resulting in cell death. A comprehensive analysis was therefore undertaken to identify the stages of xylogenesis in pioneer roots from procambial cells to fully functional vessels with lignified cell walls and secondary cell wall thickenings.

METHODS

Xylem differentiation was monitored in the pioneer roots of Populus trichocarpa at the cytological level using rhizotrons under field conditions. Detection and localization of the signalling molecule nitric oxide (NO) and hydrogen peroxide (H2O2) was undertaken and a detailed examination of nuclear changes during xylogenesis was conducted. In addition, analyses of the expression of genes involved in secondary cell wall synthesis were performed in situ.

KEY RESULTS

The primary event in initially differentiating tracheary elements (TEs) was a burst of NO in thin-walled cells, followed by H2O2 synthesis and the appearance of TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling)-positive nuclei. The first changes in nuclear structure were observed in the early stages of xylogenesis of pioneer roots, prior to lignification; however, the nucleus was detectable under transmission electron microscopy in differentiating cells until the stage at which vacuole integrity was maintained, indicating that their degradation was slow and prolonged. The subsequent sequence of events involved secondary cell wall formation and autophagy. Potential gene markers from the cinnamyl alcohol dehydrogenase (CAD) gene family that were related to secondary wall synthesis were associated with primary xylogenesis, showing clear expression in cells that undergo differentiation into TEs and in the thin-walled cells adjacent to the xylem pole.

CONCLUSIONS

The early events of TE formation during pioneer root development are described, together with the timing of xylogenesis from signalling via NO, through secondary cell wall synthesis and autophagy events that are initiated long before lignification. This is the first work describing experiments conducted in planta on roots under field conditions demonstrating that the process of xylogenesis in vivo might be gradual and complex.

OxyR and SoxR modulate the inducible oxidative stress response and are implicated during different stages of infection for the bacterial phytopathogen Pantoea stewartii subsp. stewartii

Reactive oxygen species (ROS) from a variety of sources are often encountered by invading plant pathogens during the infection process. Pantoea stewartii subsp. stewartii, the etiological agent of Stewart's wilt, is a serious bacterial pathogen of sweet corn that colonizes both the apoplast and xylem tissues in which ROS are produced. The P. stewartii genome predicts the presence of two redox-sensing transcriptional regulators, OxyR and SoxR, which both activate gene expression in response to oxidative stress. ROS exposure in the form of hydrogen peroxide and the superoxide-generating compound paraquat initiates an induced stress response through OxyR and SoxR that includes activation of the ROS-detoxifying enzymes alkyl hydroperoxide reductase and superoxide dismutase. P. stewartii ΔsoxR was more sensitive to paraquat and was compromised in the ability to form water-soaked lesions, while ΔoxyR was more sensitive to hydrogen peroxide treatment and was deficient in exopolysaccharide production and the elicitation of wilting symptoms. This demonstrates that both SoxR and OxyR play an important role in virulence in the different niches that P. stewartii colonize during the infection process.

Transcriptome sequencing and profiling of expressed genes in cambial zone and differentiating xylem of Japanese cedar (Cryptomeria japonica)

BACKGROUND

Forest trees have ecological and economic importance, and Japanese cedar has highly valued wood attributes. Thus, studies of molecular aspects of wood formation offer practical information that may be used for screening and forward genetics approaches to improving wood quality.

RESULTS

After identifying expressed sequence tags in Japanese cedar tissue undergoing xylogenesis, we designed a custom cDNA microarray to compare expression of highly regulated genes throughout a growing season. This led to identification of candidate genes involved both in wood formation and later cessation of growth and dormancy. Based on homology to orthologous protein groups, the genes were assigned to functional classes. A high proportion of sequences fell into functional classes related to posttranscriptional modification and signal transduction, while transcription factors and genes involved in the metabolism of sugars, cell-wall synthesis and lignification, and cold hardiness were among other classes of genes identified as having a potential role in xylem formation and seasonal wood formation.

CONCLUSIONS

We obtained 55,051 unique sequences by next-generation sequencing of a cDNA library prepared from cambial meristem and derivative cells. Previous studies on conifers have identified unique sequences expressed in developing xylem, but this is the first comprehensive study utilizing a collection of expressed sequence tags for expression studies related to xylem formation in Japanese cedar, which belongs to a different lineage than the Pinaceae. Our characterization of these sequences should allow comparative studies of genome evolution and functional genetics of wood species.

Temporal analysis of poplar woody root response to bending stress

Temperate perennial woody plants use different environmental signals to coordinate their growth and development in relation to seasonal changes. Preliminary evidences suggest that, even during dormancy, plants maintain effective metabolic activities and molecular mechanisms ensuring them an eventual recording of mechanical loads during winter times. Despite their great importance for productivity and survival, plant biology investigations have poorly characterized the root growth cycle and its response to environmental stresses. In this study, we describe the proteomic changes occurring over the time in poplar root either in the absence or in response to a bending stress; corresponding expression of cell cycle regulator and auxin transporter genes was also evaluated by reverse transcription polymerase chain reaction analysis. Our results confirm previous evidences on the effect of the bending stress on the anticipation of root growth resumption, providing additional insights on a temporal modulation of various plant metabolic processes involved in dormancy break, growth resumption and stress response in the bent root; these events seem related to the differential compression and tension force distribution occurring over the plant taproot.

Cell walls and the developmental anatomy of the Brachypodium distachyon stem internode

While many aspects of plant cell wall polymer structure are known, their spatial and temporal distribution within the stem are not well understood. Here, we studied vascular system and fiber development, which has implication for both biofuel feedstock conversion efficiency and crop yield. The subject of this study, Brachypodium distachyon, has emerged as a grass model for food and energy crop research. Here, we conducted our investigation using B. distachyon by applying various histological approaches and Fourier transform infrared spectroscopy to the stem internode from three key developmental stages. While vascular bundle size and number did not change over time, the size of the interfascicular region increased dramatically, as did cell wall thickness. We also describe internal stem internode anatomy and demonstrate that lignin deposition continues after crystalline cellulose and xylan accumulation ceases. The vascular bundle anatomy of B. distachyon appears to be highly similar to domesticated grasses. While the arrangement of bundles within the stem is highly variable across grasses, B. distachyon appears to be a suitable model for the rind of large C4 grass crops. A better understanding of growth and various anatomical and cell wall features of B. distachyon will further our understanding of plant biomass accumulation processes.

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.

Non-cell-autonomous postmortem lignification of tracheary elements in Zinnia elegans

Postmortem lignification of xylem tracheary elements (TEs) has been debated for decades. Here, we provide evidence in Zinnia elegans TE cell cultures, using pharmacological inhibitors and in intact Z. elegans plants using Fourier transform infrared microspectroscopy, that TE lignification occurs postmortem (i.e., after TE programmed cell death). In situ RT-PCR verified expression of the lignin monomer biosynthetic cinnamoyl CoA reductase and cinnamyl alcohol dehydrogenase in not only the lignifying TEs but also in the unlignified non-TE cells of Z. elegans TE cell cultures and in living, parenchymatic xylem cells that surround TEs in stems. These cells were also shown to have the capacity to synthesize and transport lignin monomers and reactive oxygen species to the cell walls of dead TEs. Differential gene expression analysis in Z. elegans TE cell cultures and concomitant functional analysis in Arabidopsis thaliana resulted in identification of several genes that were expressed in the non-TE cells and that affected lignin chemistry on the basis of pyrolysis-gas chromatography/mass spectrometry analysis. These data suggest that living, parenchymatic xylem cells contribute to TE lignification in a non-cell-autonomous manner, thus enabling the postmortem lignification of TEs.

Symplasmic, long-distance transport in xylem and cambial regions in branches of Acer pseudoplatanus (Aceraceae) and Populus tremula x P. tremuloides (Salicaceae)

PREMISE OF THE STUDY

The picture of how long-distance transport proceeds in trees is still far from being complete. Beside the apoplasmic pathway, transport undoubtedly also takes place within the system of living cells in the secondary xylem and cambial region. Because detailed, thorough studies of the symplasmic routes in woody branches, using direct localization with fluorescent tracers, had not been done, here we focused on the main routes of long-distance symplasmic transport in xylem and cambial tissues and analyzed in detail tracer distribution in the rays on the extended cambial surface in branches of Acer pseudoplatanus and Populus tremula ×P. tremuloides.

METHODS

Fluorescent tracers were loaded into branches through the vascular system, then their distribution in xylem and cambial regions was analyzed.

KEY RESULTS

Tracer signal was present in the symplast of axial and radial xylem parenchyma cells and in both types of cambial cells. The living cells of xylem parenchyma and of the cambium were symplasmically interconnected via xylem rays. Tracer distribution in rays was uneven on the extended cambial surface; cambial regions with intensively or sparsely dyed rays alternated along the vertical axis of analyzed branches.

CONCLUSIONS

Symplasmic, long-distance transport is present between the living cells of xylem and the cambial region in woody branches. The uneven distribution of fluorescent tracers in cambial rays along the stems is surprising and suggests the presence of an intrinsic pattern caused by an unknown mechanism.

Lignification in poplar tension wood lignified cell wall layers

The lignification process in poplar tension wood lignified cell wall layers, specifically the S(1) and S(2) layers and the compound middle lamella (CML), was analysed using ultraviolet (UV) and transmission electron microscopy (TEM). Variations in the thickness of the gelatinous layer (G-layer) were also measured to clarify whether the lignified cell wall layers had completed their lignification before the deposition of G-layers, or, on the contrary, if lignification of these layers was still active during G-layer formation. Observations using UV microscopy and TEM indicated that both UV absorbance and the degree of potassium permanganate staining increased in the CML and S(1) and S(2) layers during G-layer formation, suggesting that the lignification of these lignified layers is still in progress during G-layer formation. In the context of the cell-autonomous monolignol synthesis hypothesis, our observations suggest that monolignols must go through the developing G-layer during the lignification of CML and the S(1) and S(2) layers. The alternative hypothesis of external synthesis (in the rays) does not require that monolignols go through the G-layer before being deposited in the CML, or the S(1) and S(2) layers. Interestingly, the previous observation of lignin in the poplar G-layer was not confirmed with the microscopy techniques used in the present study.

The peach (Prunus persica L. Batsch) genome harbours 10 KNOX genes, which are differentially expressed in stem development, and the class 1 KNOPE1 regulates elongation and lignification during primary growth

The KNOTTED-like (KNOX) genes encode homeodomain transcription factors and regulate several processes of plant organ development. The peach (Prunus persica L. Batsch) genome was found to contain 10 KNOX members (KNOPE genes); six of them were experimentally located on the Prunus reference map and the class 1 KNOPE1 was found to link to a quantitative trait locus (QTL) for the internode length in the peach×Ferganensis population. All the KNOPE genes were differentially transcribed in the internodes of growing shoots; the KNOPE1 mRNA abundance decreased progressively from primary (elongation) to secondary growth (radial expansion). During primary growth, the KNOPE1 mRNA was localized in the cortex and in the procambium/metaphloem zones, whereas it was undetected in incipient phloem and xylem fibres. KNOPE1 overexpression in the Arabidopsis bp4 loss-of-function background (35S:KNOPE1/bp genotype) restored the rachis length, suggesting, together with the QTL association, a role for KNOPE1 in peach shoot elongation. Several lignin biosynthesis genes were up-regulated in the bp4 internodes but repressed in the 35S:KNOPE1/bp lines similarly to the wild type. Moreover, the lignin deposition pattern of the 35S:KNOPE1/bp and the wild-type internodes were the same. The KNOPE1 protein was found to recognize in vitro one of the typical KNOX DNA-binding sites that recurred in peach and Arabidopsis lignin genes. KNOPE1 expression was inversely correlated with that of lignin genes and lignin deposition along the peach shoot stems and was down-regulated in lignifying vascular tissues. These data strongly support that KNOPE1 prevents cell lignification by repressing lignin genes during peach stem primary growth.