Root hair initiation is coupled to a highly localized increase of xyloglucan endotransglycosylase action in Arabidopsis roots

Spatiotemporal distribution of reactive oxygen species production, delivery, and use in Arabidopsis root hairs

Fluorescent selective probes for reactive oxygen species (ROS) detection in living cells are versatile tools for the documentation of ROS production in plant developmental or stress reactions. We employed high-resolution live-cell imaging and semiquantitative analysis of Arabidopsis (Arabidopsis thaliana) stained with CM-H2DCFDA, CellROX Deep Red, and Amplex Red for functional characterization of the spatiotemporal mode of ROS production, delivery, and utilization during root hair formation. Cell viability marker fluorescein diacetate served as a positive control for dye loading and undisturbed root hair tip growth after staining. Using a colocalization analysis with subcellular molecular markers and two root hair mutants with similar phenotypes of nonelongating root hairs, but with contrasting reasons for this impairment, we found that: (i) CM-H2DCFDA is a sensitive probe for ROS generation in the cytoplasm, (ii) CellROX Deep Red labels ROS in mitochondria, (iii) Amplex Red labels apoplastic ROS and mitochondria and shows high selectivity to root hairs, (iv) the root hair defective 2-1 (rhd2-1) mutant with nonfunctional NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG PROTEIN C/ROOT HAIR-DEFECTIVE 2 (AtRBOHC/RHD2) has a low level of CM-H2DCFDA-reactive ROS in cytoplasm and lacks Amplex Red-reactive ROS in apoplast, and (v) the ACTIN2-deficient deformed root hairs1-3 (der1-3) mutant is not altered in these aspects. The sensitivity of CellROX Deep Red was documented by discrimination between larger ROS-containing mitochondria and small, yet ROS-free premature mitochondria in the growing tip of root hairs. We characterized spatial changes in ROS production and compartmentalization induced by external ROS modulators, ethylene precursor 1-aminocyclopropane-1-carboxylic acid, and ionophore valinomycin. This dynamic and high-resolution study of ROS production and utilization opens opportunities for precise speciation of particular ROS involved in root hair formation.

Phycicoccus sonneraticus sp. nov., a Novel Endophytic Actinobacterium Isolated from the Bark of Sonneratia apetala

A novel endophytic actinobacterial strain, designated MQZ13P-5T, was isolated from a piece of bark of Sonneratia apetala, collected from Guangxi Zhuang Autonomous Region, China. This strain was Gram-stain positive, aerobic, non-spore-forming, non-motile and rod-shaped. Comparative 16S rRNA gene sequence analysis showed that strain MQZ13P-5T was related to the genus Phycicoccus with exhibiting the highest similarity (98.0%) to Phycicoccus endophyticus IP6SC6T. The phylogenetic trees based on 16S rRNA gene sequences and core genes indicated that strain MQZ13P-5T belonged to the genus Phycicoccus and could not be assigned to any described species. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain MQZ13P-5T and type strains of Phycicoccus species were less than 84% and 27%, respectively, below the thresholds for species delineation. This strain showed chemotaxonomic and phenotypic properties consistent with its classification in the genus Phycicoccus. Based on the taxonomic data, strain MQZ13P-5T should represent a novel species of the genus Phycicoccus, for which the name Phycicoccus sonneraticus sp. nov. is proposed, with the type strain MQZ13P-5T (= CGMCC 1.18744T = JCM 34337T).

Transcriptome analysis showed that tomato-rootstock enhanced salt tolerance of grafted seedlings was accompanied by multiple metabolic processes and gene differences

Introduction

Grafting is a commonly used cultural practice to counteract salt stress and is especially important for vegetable production. However, it is not clear which metabolic processes and genes are involved in the response of tomato rootstocks to salt stress.

Methods

To elucidate the regulatory mechanism through which grafting enhances salt tolerance, we first evaluated the salt damage index, electrolyte permeability and Na⁺ accumulation in tomato (Solanum lycopersicum L.) leaves of grafted seedlings (GSs) and nongrafted seedlings (NGSs) subjected to 175 mmol·L^{- 1} NaCl for 0-96 h, covering the front, middle and rear ranges.

Results

Compared with the NGS, the GSs were more salt tolerant, and the Na⁺ content in the leaves decreased significantly. Through transcriptome sequencing data analysis of 36 samples, we found that GSs exhibited more stable gene expression patterns, with a lower number of DEGs. WRKY and PosF21 transcription factors were significantly upregulated in the GSs compared to the NGSs. Moreover, the GSs presented more amino acids, a higher photosynthetic index and a higher content of growth-promoting hormones. The main differences between GSs and NGSs were in the expression levels of genes involved in the BR signaling pathway, with significant upregulation of XTHs. The above results show that the metabolic pathways of "photosynthetic antenna protein", "amino acid biosynthesis" and "plant hormone signal transduction" participate in the salt tolerance response of grafted seedlings at different stages of salt stress, maintaining the stability of the photosynthetic system and increasing the contents of amino acids and growth-promoting hormones (especially BRs). In this process, the transcription factors WRKYs, PosF21 and XTHs might play an important role at the molecular level.

Discussion

The results of this study demonstrates that grafting on salt tolerant rootstocks can bring different metabolic processes and transcription levels changes to scion leaves, thereby the scion leaves show stronger salt tolerance. This information provides new insight into the mechanism underlying tolerance to salt stress regulation and provides useful molecular biological basis for improving plant salt resistance.

Shank-localized cell wall growth contributes to Arabidopsis root hair elongation

Root hairs are highly elongated tubular extensions of root epidermal cells with a plethora of physiological functions, particularly in establishing the root-rhizosphere interface. Anisotropic expansion of root hairs is generally thought to be exclusively mediated by tip growth-a highly controlled apically localized secretion of cell wall material-enriched vesicles that drives the extension of the apical dome. Here we show that tip growth is not the only mode of root hair elongation. We identified events of substantial shank-localized cell wall expansion along the polar growth axis of Arabidopsis root hairs using morphometric analysis with quantum dots. These regions expanded after in vivo immunolocalization using cell wall-directed antibodies and appeared as distinct bands that were devoid of cell wall labelling. Application of a novel click chemistry-enabled galactose analogue for pulse chase and real-time imaging allowed us to label xyloglucan, a major root hair glycan, and demonstrate its de novo deposition and enzymatic remodelling in these shank regions. Our data reveal a previously unknown aspect of root hair growth in which both tip- and shank-localized dynamic cell wall deposition and remodelling contribute to root hair elongation.

Root hair growth from the pH point of view

Root hairs are tubular outgrowths of epidermal cells that increase the root surface area and thereby make the root more efficient at absorbing water and nutrients. Their expansion is limited to the root hair apex, where growth is reported to take place in a pulsating manner. These growth pulses coincide with oscillations of the apoplastic and cytosolic pH in a similar way as has been reported for pollen tubes. Likewise, the concentrations of apoplastic reactive oxygen species (ROS) and cytoplasmic Ca²⁺ oscillate with the same periodicity as growth. Whereas ROS appear to control cell wall extensibility and opening of Ca²⁺ channels, the role of protons as a growth signal in root hairs is less clear and may differ from that in pollen tubes where plasma membrane H⁺-ATPases have been shown to sustain growth. In this review, we outline our current understanding of how pH contributes to root hair development.

The XTH Gene Family in Schima superba: Genome-Wide Identification, Expression Profiles, and Functional Interaction Network Analysis

Xyloglucan endotransglucosylase/hydrolase (XTH), belonging to glycoside hydrolase family 16, is one of the key enzymes in plant cell wall remodeling. Schima superba is an important timber and fireproof tree species in southern China. However, little is known about XTHs in S. superba. In the present study, a total of 34 SsuXTHs were obtained, which were classified into three subfamilies based on the phylogenetic relationship and unevenly distributed on 18 chromosomes. Furthermore, the intron-exon structure and conserved motif composition of them supported the classification and the members belonging to the same subfamily shared similar gene structures. Segmental and tandem duplication events did not lead to SsuXTH gene family expansion, and strong purifying selection pressures during evolution led to similar structure and function of SsuXTH gene family. The interaction network and cis-acting regulatory elements analysis revealed the SsuXTH expression might be regulated by multiple hormones, abiotic stresses and transcription factors. Finally, expression profiles and GO enrichment analysis showed most of the tandem repeat genes were mainly expressed in the phloem and xylem and they mainly participated in glycoside metabolic processes through the transfer and hydrolysis of xyloglucan in the cell wall and then regulated fiber elongation.

Xyloglucan endotransglucosylase/hydrolase genes LcXTH4/7/19 are involved in fruitlet abscission and are activated by LcEIL2/3 in litchi

Organ abscission in plants requires the hydrolysis of cell wall components, mainly including celluloses, pectins, and xyloglucans. However, how the genes that encode those hydrolytic enzymes are regulated and their function in abscission remains unclear. Previously we revealed that two cellulase genes LcCEL2/8 and two polygalacturonase genes LcPG1/2 were responsible for the degradation of celluloses and pectins, respectively, during fruitlet abscission in litchi. Here, we further identified three xyloglucan endotransglucosylase/hydrolase genes (LcXTH4, LcXTH7, LcXTH19) that are also involved in this process. Nineteen LcXTHs, named LcXTH1-19, were identified in the litchi genome. Transcriptome data and qRT-PCR confirmed that LcXTH4/7/19 were significantly induced at the abscission zone (AZ) during fruitlet abscission in litchi. The GUS reporter driven by each promoter of LcXTH4/7/19 was specifically expressed at the floral abscission zone of Arabidopsis, and importantly ectopic expression of LcXTH19 in Arabidopsis resulted in precocious floral organ abscission. Moreover, electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter analysis showed that the expression of LcXTH4/7/19 could be directly activated by two ETHYLENE INSENSITIVE 3-like (EIL) transcription factors LcEIL2/3. Collectively, we propose that LcXTH4/7/19 are involved in fruitlet abscission, and LcEIL2/3-mediated transcriptional regulation of diverse cell wall hydrolytic genes is responsible for this process in litchi.

Elongation of wood fibers combines features of diffuse and tip growth

Xylem fibers are highly elongated cells that are key constituents of wood, play major physiological roles in plants, comprise an important terrestrial carbon reservoir, and thus have enormous ecological and economic importance. As they develop, from fusiform initials, their bodies remain the same length while their tips elongate and intrude into intercellular spaces. To elucidate mechanisms of tip elongation, we studied the cell wall along the length of isolated, elongating aspen xylem fibers and used computer simulations to predict the forces driving the intercellular space formation required for their growth. We found pectin matrix epitopes (JIM5, LM7) concentrated at the tips where cellulose microfibrils have transverse orientation, and xyloglucan epitopes (CCRC-M89, CCRC-M58) in fiber bodies where microfibrils are disordered. These features are accompanied by changes in cell wall thickness, indicating that while the cell wall elongates strictly at the tips, it is deposited all over fibers. Computer modeling revealed that the intercellular space formation needed for intrusive growth may only require targeted release of cell adhesion, which allows turgor pressure in neighboring fiber cells to 'round' the cells creating spaces. These characteristics show that xylem fibers' elongation involves a distinct mechanism that combines features of both diffuse and tip growth.

Alternative Oxidase Inhibition Impairs Tobacco Root Development and Root Hair Formation

Alternative oxidase (AOX) is the terminal oxidase of the mitochondrial respiratory electron transport chain in plant cells and is critical for the balance of mitochondrial hemostasis. In this study, the effect of inhibition of AOX with different concentrations of salicylhydroxamic acid (SHAM) on the tobacco root development was investigated. We show here that AOX inhibition significantly impaired the development of the main root and root hair formation of tobacco. The length of the main root of SHAM-treated tobacco was significantly shorter than that of the control, and no root hairs were formed after treatment with a concentration of 1 mM SHAM or more. The transcriptome analysis showed that AOX inhibition by 1 mM SHAM involved in the regulation of gene expression related to root architecture. A total of 5,855 differentially expressed genes (DEGs) were obtained by comparing SHAM-treated roots with control. Of these, the gene expression related to auxin biosynthesis and perception were significantly downregulated by 1 mM SHAM. Similarly, genes related to cell wall loosening, cell cycle, and root meristem growth factor 1 (RGF1) also showed downregulation on SHAM treatment. Moreover, combined with the results of physiological measurements, the transcriptome analysis demonstrated that AOX inhibition resulted in excessive accumulation of reactive oxygen species in roots, which further induced oxidative damage and cell apoptosis. It is worth noting that when indoleacetic acid (20 nM) and dimethylthiourea (10 mM) were added to the medium containing SHAM, the defects of tobacco root development were alleviated, but to a limited extent. Together, these findings indicated that AOX-mediated respiratory pathway plays a crucial role in the tobacco root development, including root hair formation.

Xyloglucan endotransglycosylase/hydrolase increases tightly-bound xyloglucan and chain number but decreases chain length contributing to the defense response that Glycine max has to Heterodera glycines

The Glycine max xyloglucan endotransglycosylase/hydrolase (EC 2.4.1.207), GmXTH43, has been identified through RNA sequencing of RNA isolated through laser microdissection of Heterodera glycines-parasitized root cells (syncytia) undergoing the process of defense. Experiments reveal that genetically increasing XTH43 transcript abundance in the H. glycines-susceptible genotype G. max[Williams 82/PI 518671] decreases parasitism. Experiments presented here show decreasing XTH43 transcript abundance through RNA interference (RNAi) in the H. glycines-resistant G. max[Peking/PI 548402] increases susceptibility, but it is unclear what role XTH43 performs. The experiments presented here show XTH43 overexpression decreases the relative length of xyloglucan (XyG) chains, however, there is an increase in the amount of those shorter chains. In contrast, XTH43 RNAi increases XyG chain length. The experiments show that XTH43 has the capability to function, when increased in its expression, to limit XyG chain extension. This outcome would likely impair the ability of the cell wall to expand. Consequently, XTH43 could provide an enzymatically-driven capability to the cell that would allow it to limit the ability of parasitic nematodes like H. glycines to develop a feeding structure that, otherwise, would facilitate parasitism. The experiments presented here provide experimentally-based proof that XTHs can function in ways that could be viewed as being able to limit the expansion of the cell wall.

Three highly acidic Equisetum XTHs differ from hetero-trans-β-glucanase in donor substrate specificity and are predominantly xyloglucan homo-transglucosylases

Transglycanases are enzymes that remodel the primary cell wall in plants, potentially loosening and/or strengthening it. Xyloglucan endotransglucosylase (XET; EC 2.4.1.207), ubiquitous in land plants, is a homo-transglucanase activity (donor, xyloglucan; acceptor, xyloglucan) exhibited by XTH (xyloglucan endotransglucosylase/hydrolase) proteins. By contrast, hetero-trans-β-glucanase (HTG) is the only known enzyme that is preferentially a hetero-transglucanase. Its two main hetero-transglucanase activities are MLG : xyloglucan endotransglucosylase (MXE) and cellulose : xyloglucan endotransglucosylase (CXE). HTG is highly acidic and found only in the evolutionarily isolated genus of fern-allies, Equisetum. We now report genes for three new highly acidic HTG-related XTHs in E. fluviatile (EfXTH-A, EfXTH-H and EfXTH-I). We expressed them heterologously in Pichia and tested the encoded proteins' enzymic activities to determine whether their acidity and/or their Equisetum-specific sequences might confer high hetero-transglucanase activity. Untransformed Pichia was found to secrete MLG-degrading enzyme(s), which had to be removed for reliable MXE assays. All three acidic EfXTHs exhibited very predominantly XET activity, although low but measurable hetero-transglucanase activities (MXE and CXE) were also detected in EfXTH-H and EfXTH-I. We conclude that the extremely high hetero-transglucanase activities of Equisetum HTG are not emulated by similarly acidic Equisetum XTHs that share up to 55.5% sequence identity with HTG.

Flavonols regulate root hair development by modulating accumulation of reactive oxygen species in the root epidermis

Reactive oxygen species (ROS) are signaling molecules produced by tissue-specific respiratory burst oxidase homolog (RBOH) enzymes to drive development. In Arabidopsis thaliana, ROS produced by RBOHC was previously reported to drive root hair elongation. We identified a specific role for one ROS, H₂O₂, in driving root hair initiation and demonstrated that localized synthesis of flavonol antioxidants control the level of H₂O₂ and root hair formation. Root hairs form from trichoblast cells that express RBOHC and have elevated H₂O₂ compared with adjacent atrichoblast cells that do not form root hairs. The flavonol-deficient tt4 mutant has elevated ROS in trichoblasts and elevated frequency of root hair formation compared with the wild type. The increases in ROS and root hairs in tt4 are reversed by genetic or chemical complementation. Auxin-induced root hair initiation and ROS accumulation were reduced in an rbohc mutant and increased in tt4, consistent with flavonols modulating ROS and auxin transport. These results support a model in which localized synthesis of RBOHC and flavonol antioxidants establish patterns of ROS accumulation that drive root hair formation.

Comparative Proteomics Profiling Illuminates the Fruitlet Abscission Mechanism of Sweet Cherry as Induced by Embryo Abortion

Sweet cherry (Prunus avium L.) is a delicious nutrient-rich fruit widely cultivated in countries such as China, America, Chile, and Italy. However, the yield often drops severely due to the frequently-abnormal fruitlet abscission, and few studies on the metabolism during its ripening process at the proteomic level have been executed so far. To get a better understanding regarding the sweet cherry abscission mechanism, proteomic analysis between the abscising carpopodium and non-abscising carpopodium of sweet cherry was accomplished using a newly developed Liquid chromatography-mass spectrometry/mass spectrometry with Tandem Mass Tag (TMT-LC-MS/MS) methodology. The embryo viability experiments showed that the vigor of the abscission embryos was significantly lower than that of retention embryo. The activity of cell wall degrading enzymes in abscising carpopodium was significantly higher than that in non-abscising carpopodium. The anatomy results suggested that cells in the abscission zone were small and separated. In total, 6280 proteins were identified, among which 5681 were quantified. It has been observed that differentially accumulated proteins (DAPs) influenced several biological functions and various subcellular localizations. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that plenty of metabolic pathways were notably enriched, particularly those involved in phytohormone biosynthesis, cell wall metabolism, and cytoskeletal metabolism, including 1-aminocyclopropane-1-carboxylate oxidase proteins which promote ethylene synthesis, and proteins promoting cell wall degradation, such as endoglucanases, pectinase, and polygalacturonase. Differential expression of proteins concerning phytohormone biosynthesis might activate the shedding regulation signals. Up-regulation of several cell wall degradation-related proteins possibly regulated the shedding of plant organs. Variations of the phytohormone biosynthesis and cell wall degradation-related proteins were explored during the abscission process. Furthermore, changes in cytoskeleton-associated proteins might contribute to the abscission of carpopodium. The current work represented the first study using comparative proteomics between abscising carpopodium and non-abscising carpopodium. These results indicated that embryo abortion might lead to phytohormone synthesis disorder, which effected signal transduction pathways, and hereby controlled genes involved in cell wall degradation and then caused the abscission of fruitlet. Overall, our data may give an intrinsic explanation of the variations in metabolism during the abscission of carpopodium.

Comparative gene expression profile analysis of ovules provides insights into Jatropha curcas L. ovule development

Jatropha curcas, an economically important biofuel feedstock with oil-rich seeds, has attracted considerable attention among researchers in recent years. Nevertheless, valuable information on the yield component of this plant, particularly regarding ovule development, remains scarce. In this study, transcriptome profiles of anther and ovule development were established to investigate the ovule development mechanism of J. curcas. In total, 64,325 unigenes with annotation were obtained, and 1723 differentially expressed genes (DEGs) were identified between different stages. The DEG analysis showed the participation of five transcription factor families (bHLH, WRKY, MYB, NAC and ERF), five hormone signaling pathways (auxin, gibberellic acid (GA), cytokinin, brassinosteroids (BR) and jasmonic acid (JA)), five MADS-box genes (AGAMOUS-2, AGAMOUS-1, AGL1, AGL11, and AGL14), SUP and SLK3 in ovule development. The role of GA and JA in ovule development was evident with increases in flower buds during ovule development: GA was increased approximately twofold, and JA was increased approximately sevenfold. In addition, the expression pattern analysis using qRT-PCR revealed that CRABS CLAW and AGAMOUS-2 were also involved in ovule development. The upregulation of BR signaling genes during ovule development might have been regulated by other phytohormone signaling pathways through crosstalk. This study provides a valuable framework for investigating the regulatory networks of ovule development in J. curcas.

Genome-Wide Identification and Expression Profiling Analysis of the Xyloglucan Endotransglucosylase/Hydrolase Gene Family in Tobacco (Nicotiana tabacum L.)

Xyloglucan endotransglucosylase/hydrolase genes (XTHs) encode enzymes required for the reconstruction and modification of xyloglucan backbones, which will result in changes of cell wall extensibility during growth. A total of 56 NtXTH genes were identified from common tobacco, and 50 cDNA fragments were verified by PCR amplification. The 56 NtXTH genes could be classified into two subfamilies: Group I/II and Group III according to their phylogenetic relationships. The gene structure, chromosomal localization, conserved protein domains prediction, sub-cellular localization of NtXTH proteins and evolutionary relationships among Nicotiana tabacum, Nicotiana sylvestrisis, Nicotiana tomentosiformis, Arabidopsis, and rice were also analyzed. The NtXTHs expression profiles analyzed by the TobEA database and qRT-PCR revealed that NtXTHs display different expression patterns in different tissues. Notably, the expression patterns of 12 NtXTHs responding to environment stresses, including salinity, alkali, heat, chilling, and plant hormones, including IAA and brassinolide, were characterized. All the results would be useful for the function study of NtXTHs during different growth cycles and stresses.

The Role of Auxin in Cell Wall Expansion

Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly regulated balance between rigidity and flexibility. Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth, which is triggered by a high intracellular turgor pressure. Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and shapes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Here, we review what is known on cell wall property regulation by auxin. We focus particularly on the auxin role during cell expansion linked directly to cell wall modifications. We also analyze downstream targets of transcriptional auxin signaling, which are related to the cell wall and could be linked to acid growth and the action of wall-loosening proteins. All together, this update elucidates the connection between hormonal signaling and cell wall synthesis and deposition.

Report on the Current Inventory of the Toolbox for Plant Cell Wall Analysis: Proteinaceous and Small Molecular Probes

Plant cell walls are highly complex structures composed of diverse classes of polysaccharides, proteoglycans, and polyphenolics, which have numerous roles throughout the life of a plant. Significant research efforts aim to understand the biology of this cellular organelle and to facilitate cell-wall-based industrial applications. To accomplish this, researchers need to be provided with a variety of sensitive and specific detection methods for separate cell wall components, and their various molecular characteristics in vitro as well as in situ. Cell wall component-directed molecular detection probes (in short: cell wall probes, CWPs) are an essential asset to the plant glycobiology toolbox. To date, a relatively large set of CWPs has been produced-mainly consisting of monoclonal antibodies, carbohydrate-binding modules, synthetic antibodies produced by phage display, and small molecular probes. In this review, we summarize the state-of-the-art knowledge about these CWPs; their classification and their advantages and disadvantages in different applications. In particular, we elaborate on the recent advances in non-conventional approaches to the generation of novel CWPs, and identify the remaining gaps in terms of target recognition. This report also highlights the addition of new "compartments" to the probing toolbox, which is filled with novel chemical biology tools, such as metabolic labeling reagents and oligosaccharide conjugates. In the end, we also forecast future developments in this dynamic field.

Distinct physiological and molecular responses in Arabidopsis thaliana exposed to aluminum oxide nanoparticles and ionic aluminum

Nano-aluminium oxide (nAl₂O₃) is one of the most widely used nanomaterials. However, nAl₂O₃ toxicity mechanisms and potential beneficial effects on terrestrial plant physiology remain poorly understood. Such knowledge is essential for the development of robust nAl₂O₃ risk assessment. In this study, we studied the influence of a 10-d exposure to a total selected concentration of 98 μM nAl₂O₃ or to the equivalent molar concentration of ionic Al (AlCl₃) (196 μM) on the model plant Arabidopsis thaliana on the physiology (e.g., growth and photosynthesis, membrane damage) and the transcriptome using a high throughput state-of-the-art technology, RNA-seq. We found no evidence of nAl₂O₃ toxicity on photosynthesis, growth and lipid peroxidation. Rather the nAl₂O₃ treatment stimulated root weight and length by 48% and 39%, respectively as well as photosynthesis opening up the door to the use of nAl₂O₃ in biotechnology and nano agriculture. Transcriptomic analyses indicate that the beneficial effect of nAl₂O₃ was related to an increase in the transcription of several genes involved in root growth as well as in root nutrient uptake (e.g., up-regulation of the root hair-specific gene family and root development genes, POLARIS protein). By contrast, the ionic Al treatment decreased shoot and root weight of Arabidopsis thaliana by 57.01% and 45.15%, respectively. This toxic effect was coupled to a range of response at the gene transcription level including increase transcription of antioxidant-related genes and transcription of genes involved in plant defense response to pathogens. This work provides an integrated understanding at the molecular and physiological level of the effects of nAl₂O₃ and ionic Al in Arabidopsis.

How the Depletion in Mineral Major Elements Affects Grapevine (Vitis vinifera L.) Primary Cell Wall

The noteworthy fine remodeling that plant cell walls (CWs) undergo to adapt to developmental, physiological and environmental cues and the observation that its composition and dynamics differ between species represents an opportunity to couple crop species agronomic studies with research on CW modifications. Vitis vinifera is one of the most important crops from an economic point-of-view due to the high value of the fruit, predominantly for winemaking. The availability of some information related to this species' CWs allows researching its responses to imposed conditions that affect the plant's development. Mineral deficiency, in particular nitrogen, phosphorus, potassium and sulfur, strongly affects plant metabolism, reducing both growth and crop yield. Despite the importance of mineral nutrition in development, its influence on CW synthesis and modifications is still insufficiently documented. Addressing this knowledge gap, V. vinifera experimental models were used to study CW responses to imposed mineral depletion in unorganized (callus) and organized (shoots) tissues. The discussion of the obtained results is the main focus of this review. Callus and shoots submitted to mineral restriction are impaired in specific CW components, predominantly cellulose. Reorganization on structure and deposition of several other polymers, in particular the degree and pattern of pectin methyl-esterification and the amount of xyloglucan (XyG), arabinan and extensin, is also observed. In view of recently proposed CW models that consider biomechanical hotspots and direct linkages between pectins and XyG/cellulose, the outcome of these modifications in explaining maintenance of CW integrity through compensatory stiffening can be debated. Nutrient stresses do not affect evenly all tissues with undifferentiated callus tissues showing more pronounced responses, followed by shoot mature internodes, and then newly formed internodes. The impact of nitrogen depletion leads to more noticeable responses, supporting this nutrient's primary role in plant development and metabolism. The consequential compensatory mechanisms highlight the pivotal role of CW in rearranging under environmental stresses.

Root morphogenic pathways in Eucalyptus grandis are modified by the activity of protein arginine methyltransferases

BACKGROUND

Methylation of proteins at arginine residues, catalysed by members of the protein arginine methyltransferase (PRMT) family, is crucial for the regulation of gene transcription and for protein function in eukaryotic organisms. Inhibition of the activity of PRMTs in annual model plants has demonstrated wide-ranging involvement of PRMTs in key plant developmental processes, however, PRMTs have not been characterised or studied in long-lived tree species.

RESULTS

Taking advantage of the recently available genome for Eucalyptus grandis, we demonstrate that most of the major plant PRMTs are conserved in E. grandis as compared to annual plants and that they are expressed in all major plant tissues. Proteomic and transcriptomic analysis in roots suggest that the PRMTs of E. grandis control a number of regulatory proteins and genes related to signalling during cellular/root growth and morphogenesis. We demonstrate here, using chemical inhibition of methylation and transgenic approaches, that plant type I PRMTs are necessary for normal root growth and branching in E. grandis. We further show that EgPRMT1 has a key role in root hair initiation and elongation and is involved in the methylation of β-tubulin, a key protein in cytoskeleton formation.

CONCLUSIONS

Together, our data demonstrate that PRMTs encoded by E. grandis methylate a number of key proteins and alter the transcription of a variety of genes involved in developmental processes. Appropriate levels of expression of type I PRMTs are necessary for the proper growth and development of E. grandis roots.

S-nitrosoglutathione promotes cell wall remodelling, alters the transcriptional profile and induces root hair formation in the hairless root hair defective 6 (rhd6) mutant of Arabidopsis thaliana

Nitric oxide (NO) exerts pleiotropic effects on plant development; however, its involvement in cell wall modification during root hair formation (RHF) has not yet been addressed. Here, mutants of Arabidopsis thaliana with altered root hair phenotypes were used to assess the involvement of S-nitrosoglutathione (GSNO), the primary NO source, in cell wall dynamics and gene expression in roots induced to form hairs. GSNO and auxin restored the root hair phenotype of the hairless root hair defective 6 (rhd6) mutant. A positive correlation was observed between increased NO production and RHF induced by auxin in rhd6 and transparent testa glabra (ttg) mutants. Deposition of an epitope within rhamnogalacturonan-I recognized by the CCRC-M2 antibody was delayed in root hair cells (trichoblasts) compared with nonhair cells (atrichoblasts). GSNO, but not auxin, restored the wild-type root glycome and transcriptome profiles in rhd6, modulating the expression of a large number of genes related to cell wall composition and metabolism, as well as those encoding ribosomal proteins, DNA and histone-modifying enzymes and proteins involved in post-translational modification. Our results demonstrate that NO plays a key role in cell wall remodelling in trichoblasts and suggest that it also participates in chromatin modification in root cells of A. thaliana.

Drought and Heat Differentially Affect XTH Expression and XET Activity and Action in 3-Day-Old Seedlings of Durum Wheat Cultivars with Different Stress Susceptibility

Heat and drought stress have emerged as major constraints for durum wheat production. In the Mediterranean area, their negative effect on crop productivity is expected to be exacerbated by the occurring climate change. Xyloglucan endotransglucosylase/hydrolases (XTHs) are chief enzymes in cell wall remodeling, whose relevance in cell expansion and morphogenesis suggests a central role in stress responses. In this work the potential role of XTHs in abiotic stress tolerance was investigated in durum wheat. The separate effects of dehydration and heat exposure on XTH expression and its endotransglucosylase (XET) in vitro activity and in vivo action have been monitored, up to 24 h, in the apical and sub-apical root regions and shoots excised from 3-day-old seedlings of durum wheat cultivars differing in stress susceptibility/tolerance. Dehydration and heat stress differentially influence the XTH expression profiles and the activity and action of XET in the wheat seedlings, depending on the degree of susceptibility/tolerance of the cultivars, the organ, the topological region of the root and, within the root, on the gradient of cell differentiation. The root apical region was the zone mainly affected by both treatments in all assayed cultivars, while no change in XET activity was observed at shoot level, irrespective of susceptibility/tolerance, confirming the pivotal role of the root in stress perception, signaling, and response. Conflicting effects were observed depending on stress type: dehydration evoked an overall increase, at least in the apical region of the root, of XET activity and action, while a significant inhibition was caused by heat treatment in most cultivars. The data suggest that differential changes in XET action in defined portions of the root of young durum wheat seedlings may have a role as a response to drought and heat stress, thus contributing to seedling survival and crop establishment. A thorough understanding of the mechanisms underlying these variations could represent the theoretical basis for implementing breeding strategies to develop new highly productive hybrids adapted to future climate scenarios.

The click-compatible sugar 6-deoxy-alkynyl glucose metabolically incorporates into Arabidopsis root hair tips and arrests their growth

Plant cell walls are dynamic structures whose polysaccharide components are rearranged and recycled during growth and morphogenesis. Covalent fluorescent tagging of these polysaccharides following a metabolic labeling approach can help elucidate these changes. Herein reported are the synthesis and seedling-incorporation of a plant polysaccharide chemical reporter, 6-deoxy-alkynyl glucose (6dAG), that is modeled on D-glucose. Whereas fucose-alkyne, a previously reported chemical reporter for pectin, incorporates diffusely throughout growing cell walls, 6dAG incorporated specifically into root hair tips. This incorporation occurs in a time- and concentration-dependent manner. 6dAG exposure both induces and colocalizes with callose deposition in this tissue, and arrests both root hair and root growth. These results show that plants can incorporate an additional alkynyl-modified sugar analog into their metabolism, and into a discrete subcellular location.

Regulation of cell wall remodeling in grapevine (Vitis vinifera L.) callus under individual mineral stress deficiency

Cell wall (CW) is a dynamic structure that determines the plant form, growth and response to environmental conditions. Vitis vinifera callus grown under nitrogen (-N), phosphorous (-P) and sulfur (-S) deficiency were used as a model system to address the influence of mineral stress in CW remodeling. Callus cells morphology was altered, mostly under -N, resulting in changes in cell length and width compared with the control. CW composition ascertained with specific staining and immuno-detection showed a decrease in cellulose and altered pattern of pectin methylesterification. Under mineral stress genes expression from candidate families disclosed mainly a downregulation of a glycosyl hydrolase family 9C (GH9C), xyloglucan transglycosylase/hydrolases (XTHs) with predicted hydrolytic activity and pectin methylesterases (PMEs). Conversely, upregulation of PMEs inhibitors (PMEIs) was observed. While methylesterification patterns can be associated to PME/PMEI gene expression, the lower cellulose content cannot be attributed to altered cellulose synthase (CesA) gene expression suggesting the involvement of other gene families. Salt extracts from -N and -P callus tissues increased plastic deformation in cucumber hypocotyls while no effect was observed with -S extracts. The lower endo-acting glycosyl hydrolase activity of -N callus extracts pinpoints a more expressive impact of -N on CW-remodeling.

Cell Fate Determination and the Switch from Diffuse Growth to Planar Polarity in Arabidopsis Root Epidermal Cells

Plant roots fulfill important functions as they serve in water and nutrient uptake, provide anchorage of the plant body in the soil and in some species form the site of symbiotic interactions with soil-living biota. Root hairs, tubular-shaped outgrowths of specific epidermal cells, significantly increase the root's surface area and aid in these processes. In this review we focus on the molecular mechanisms that determine the hair and non-hair cell fate of epidermal cells and that define the site on the epidermal cell where the root hair will be initiated (=planar polarity determination). In the model plant Arabidopsis, trichoblast and atrichoblast cell fate results from intra- and intercellular position-dependent signaling and from complex feedback loops that ultimately regulate GL2 expressing and non-expressing cells. When epidermal cells reach the end of the root expansion zone, root hair promoting transcription factors dictate the establishment of polarity within epidermal cells followed by the selection of the root hair initiation site at the more basal part of the trichoblast. Molecular players in the abovementioned processes as well as the role of phytohormones are discussed, and open areas for future experiments are identified.

A general method for assaying homo- and hetero-transglycanase activities that act on plant cell-wall polysaccharides

Transglycanases (endotransglycosylases) cleave a polysaccharide (donor-substrate) in mid-chain, and then transfer a portion onto another poly- or oligosaccharide (acceptor-substrate). Such enzymes contribute to plant cell-wall assembly and/or re-structuring. We sought a general method for revealing novel homo- and hetero-transglycanases, applicable to diverse polysaccharides and oligosaccharides, separating transglycanase-generated (3)H-polysaccharides from unreacted (3)H-oligosaccharides--the former immobilized (on filter-paper, silica-gel or glass-fiber), the latter eluted. On filter-paper, certain polysaccharides [e.g. (1→3, 1→4)-β-D-glucans] remained satisfactorily adsorbed when water-washed; others (e.g. pectins) were partially lost. Many oligosaccharides (e.g. arabinan-, galactan-, xyloglucan-based) were successfully eluted in appropriate solvents, but others (e.g. [(3)H]xylohexaitol, [(3)H]mannohexaitol [(3)H]cellohexaitol) remained immobile. On silica-gel, all (3)H-oligosaccharides left an immobile 'ghost' spot (contaminating any (3)H-polysaccharides), which was diminished but not prevented by additives e.g. sucrose or Triton X-100. The best stratum was glass-fiber (GF), onto which the reaction-mixture was dried then washed in 75% ethanol. Washing led to minimal loss or lateral migration of (3)H-polysaccharides if conducted by slow percolation of acidified ethanol. The effectiveness of GF-blotting was well demonstrated for Chara vulgaris trans-β-mannanase. In conclusion, our novel GF-blotting technique efficiently frees transglycanase-generated (3)H-polysaccharides from unreacted (3)H-oligosaccharides, enabling high-throughput screening of multiple postulated transglycanase activities utilising chemically diverse donor- and acceptor-substrates.

Comparative Morphophysiological Analyses and Molecular Profiling Reveal Pi-Efficient Strategies of a Traditional Rice Genotype

Phosphate (Pi) deficiency severely affects crop yield. Modern high yielding rice genotypes are sensitive to Pi deficiency whereas traditional rice genotypes are naturally compatible with low Pi ecosystems. However, the underlying molecular mechanisms for low Pi tolerance in traditional genotypes remain largely elusive. To delineate the molecular mechanisms for low Pi tolerance, two contrasting rice genotypes, Dular (low Pi tolerant), and PB1 (low Pi sensitive), have been selected. Comparative morphophysiological, global transcriptome and lipidome analyses of root and shoot tissues of both genotypes grown under Pi deficient and sufficient conditions revealed potential low Pi tolerance mechanisms of the traditional genotype. Most of the genes associated with enhanced internal Pi utilization (phospholipid remobilization) and modulation of root system architecture (RSA) were highly induced in the traditional rice genotype, Dular. Higher reserves of phospholipids and greater accumulation of galactolipids under low Pi in Dular indicated it has more efficient Pi utilization. Furthermore, Dular also maintained greater root growth than PB1 under low Pi, resulting in larger root surface area due to increased lateral root density and root hair length. Genes involved in enhanced low Pi tolerance of the traditional genotype can be exploited to improve the low Pi tolerance of modern high yielding rice cultivars.

Distribution of XTH, expansin, and secondary-wall-related CesA in floral and fruit abscission zones during fruit development in tomato (Solanum lycopersicum)

After fruit development is triggered by pollination, the abscission zone (AZ) in the fruit pedicel strengthens its adhesion to keep the fruit attached. We previously reported that xyloglucan and arabinan accumulation in the AZ accompanies the shedding of unpollinated flowers. After the fruit has developed and is fully ripened, shedding occurs easily in the AZ due to lignin accumulation. Regulation of cell wall metabolism may play an important role in these processes, but it is not well understood. In the present report, we used immunohistochemistry to visualize changes in the distributions of xyloglucan and arabinan metabolism-related enzymes in the AZs of pollinated and unpollinated flowers, and in ripened fruits. During floral abscission, we observed a gradual increase in polyclonal antibody labeling of expansin in the AZ. The intensities of LM6 and LM15 labeling of arabinan and xyloglucan, respectively, also increased. However, during floral abscission, we observed a large 1 day post anthesis (DPA) peak in the polyclonal antibody labeling of XTH in the AZ, which then decreased. These results suggest that expansin and XTH play important, but different roles in the floral abscission process. During fruit abscission, unlike during floral abscission, no AZ-specific expansin and XTH were observed. Although lignification was seen in the AZ of over-ripe fruit pedicels, secondary cell wall-specific cellulose synthase signals were not observed. This suggests that cellulose metabolism-related enzymes do not play important roles in the AZ prior to fruit abscission.

Trans-Golgi network localized small GTPase RabA1d is involved in cell plate formation and oscillatory root hair growth

BACKGROUND

Small Rab GTPases are important regulators of vesicular trafficking in plants. AtRabA1d, a member of the RabA1 subfamily of small GTPases, was previously found in the vesicle-rich apical dome of growing root hairs suggesting a role during tip growth; however, its specific intracellular localization and role in plants has not been well described.

RESULTS

The transient expression of 35S::GFP:RabA1d construct in Allium porrum and Nicotiana benthamiana revealed vesicular structures, which were further corroborated in stable transformed Arabidopsis thaliana plants. GFP-RabA1d colocalized with the trans-Golgi network marker mCherry-VTI12 and with early FM4-64-labeled endosomal compartments. Late endosomes and endoplasmic reticulum labeled with FYVE-DsRed and ER-DsRed, respectively, were devoid of GFP-RabA1d. The accumulation of GFP-RabA1d in the core of brefeldin A (BFA)-induced-compartments and the quantitative upregulation of RabA1d protein levels after BFA treatment confirmed the association of RabA1d with early endosomes/TGN and its role in vesicle trafficking. Light-sheet microscopy revealed involvement of RabA1d in root development. In root cells, GFP-RabA1d followed cell plate expansion consistently with cytokinesis-related vesicular trafficking and membrane recycling. GFP-RabA1d accumulated in disc-like structures of nascent cell plates, which progressively evolved to marginal ring-like structures of the growing cell plates. During root hair growth and development, GFP-RabA1d was enriched at root hair bulges and at the apical dome of vigorously elongating root hairs. Importantly, GFP-RabA1d signal intensity exhibited an oscillatory behavior in-phase with tip growth. Progressively, this tip localization dissapeared in mature root hairs suggesting a link between tip localization of RabA1d and root hair elongation. Our results support a RabA1d role in events that require vigorous membrane trafficking.

CONCLUSIONS

RabA1d is located in early endosomes/TGN and is involved in vesicle trafficking. RabA1d participates in both cell plate formation and root hair oscillatory tip growth. The specific GFP-RabA1d subcellular localization confirms a correlation between its specific spatio-temporal accumulation and local vesicle trafficking requirements during cell plate and root hair formation.

Review on shape formation in epidermal pavement cells of the Arabidopsis leaf

Epidermal pavement cells appear with a fascinating irregular wavy shape in the Arabidopsis thaliana leaf. This review addresses the questions of why this particular shape is produced during leaf development and how this is accomplished. To answer the first question most probably waviness offers some biomechanical benefits over other organisations. Different positions of lobe-formation are therefore explored and discussed. At the moment, however, no hard evidence that favours any one morphology is available. The latter question comprises the biomechanical accomplishment of shape and refers to the cell wall and cytoskeletal involvement herein. A current model for pavement cell development is discussed but remaining questions and pitfalls are put forward. Moreover, an overview of the genetic and biochemical regulatory pathways that are described up to date in the literature is presented.

Plant cell shape: modulators and measurements

Plant cell shape, seen as an integrative output, is of considerable interest in various fields, such as cell wall research, cytoskeleton dynamics and biomechanics. In this review we summarize the current state of knowledge on cell shape formation in plants focusing on shape of simple cylindrical cells, as well as in complex multipolar cells such as leaf pavement cells and trichomes. We summarize established concepts as well as recent additions to the understanding of how cells construct cell walls of a given shape and the underlying processes. These processes include cell wall synthesis, activity of the actin and microtubule cytoskeletons, in particular their regulation by microtubule associated proteins, actin-related proteins, GTP'ases and their effectors, as well as the recently-elucidated roles of plant hormone signaling and vesicular membrane trafficking. We discuss some of the challenges in cell shape research with a particular emphasis on quantitative imaging and statistical analysis of shape in 2D and 3D, as well as novel developments in this area. Finally, we review recent examples of the use of novel imaging techniques and how they have contributed to our understanding of cell shape formation.

Biochemistry and physiological roles of enzymes that 'cut and paste' plant cell-wall polysaccharides

The plant cell-wall matrix is equipped with more than 20 glycosylhydrolase activities, including both glycosidases and glycanases (exo- and endo-hydrolases, respectively), which between them are in principle capable of hydrolysing most of the major glycosidic bonds in wall polysaccharides. Some of these enzymes also participate in the 'cutting and pasting' (transglycosylation) of sugar residues-enzyme activities known as transglycosidases and transglycanases. Their action and biological functions differ from those of the UDP-dependent glycosyltransferases (polysaccharide synthases) that catalyse irreversible glycosyl transfer. Based on the nature of the substrates, two types of reaction can be distinguished: homo-transglycosylation (occurring between chemically similar polymers) and hetero-transglycosylation (between chemically different polymers). This review focuses on plant cell-wall-localized glycosylhydrolases and the transglycosylase activities exhibited by some of these enzymes and considers the physiological need for wall polysaccharide modification in vivo. It describes the mechanism of transglycosylase action and the classification and phylogenetic variation of the enzymes. It discusses the modulation of their expression in plants at the transcriptional and translational levels, and methods for their detection. It also critically evaluates the evidence that the enzyme proteins under consideration exhibit their predicted activity in vitro and their predicted action in vivo. Finally, this review suggests that wall-localized glycosylhydrolases with transglycosidase and transglycanase abilities are widespread in plants and play important roles in the mechanism and control of plant cell expansion, differentiation, maturation, and wall repair.

Invasive cells in animals and plants: searching for LECA machineries in later eukaryotic life

Invasive cell growth and migration is usually considered a specifically metazoan phenomenon. However, common features and mechanisms of cytoskeletal rearrangements, membrane trafficking and signalling processes contribute to cellular invasiveness in organisms as diverse as metazoans and plants – two eukaryotic realms genealogically connected only through the last common eukaryotic ancestor (LECA). By comparing current understanding of cell invasiveness in model cell types of both metazoan and plant origin (invadopodia of transformed metazoan cells, neurites, pollen tubes and root hairs), we document that invasive cell behavior in both lineages depends on similar mechanisms. While some superficially analogous processes may have arisen independently by convergent evolution (e.g. secretion of substrate- or tissue-macerating enzymes by both animal and plant cells), at the heart of cell invasion is an evolutionarily conserved machinery of cellular polarization and oriented cell mobilization, involving the actin cytoskeleton and the secretory pathway. Its central components - small GTPases (in particular RHO, but also ARF and Rab), their specialized effectors, actin and associated proteins, the exocyst complex essential for polarized secretion, or components of the phospholipid- and redox- based signalling circuits (inositol-phospholipid kinases/PIP2, NADPH oxidases) are aparently homologous among plants and metazoans, indicating that they were present already in LECA.

Reviewer: This article was reviewed by Arcady Mushegian, Valerian Dolja and Purificacion Lopez-Garcia.

Construction of a rice glycoside hydrolase phylogenomic database and identification of targets for biofuel research

Glycoside hydrolases (GH) catalyze the hydrolysis of glycosidic bonds in cell wall polymers and can have major effects on cell wall architecture. Taking advantage of the massive datasets available in public databases, we have constructed a rice phylogenomic database of GHs (http://ricephylogenomics.ucdavis.edu/cellwalls/gh/). This database integrates multiple data types including the structural features, orthologous relationships, mutant availability, and gene expression patterns for each GH family in a phylogenomic context. The rice genome encodes 437 GH genes classified into 34 families. Based on pairwise comparison with eight dicot and four monocot genomes, we identified 138 GH genes that are highly diverged between monocots and dicots, 57 of which have diverged further in rice as compared with four monocot genomes scanned in this study. Chromosomal localization and expression analysis suggest a role for both whole-genome and localized gene duplications in expansion and diversification of GH families in rice. We examined the meta-profiles of expression patterns of GH genes in twenty different anatomical tissues of rice. Transcripts of 51 genes exhibit tissue or developmental stage-preferential expression, whereas, seventeen other genes preferentially accumulate in actively growing tissues. When queried in RiceNet, a probabilistic functional gene network that facilitates functional gene predictions, nine out of seventeen genes form a regulatory network with the well-characterized genes involved in biosynthesis of cell wall polymers including cellulose synthase and cellulose synthase-like genes of rice. Two-thirds of the GH genes in rice are up regulated in response to biotic and abiotic stress treatments indicating a role in stress adaptation. Our analyses identify potential GH targets for cell wall modification.

Group III-A XTH genes of Arabidopsis encode predominant xyloglucan endohydrolases that are dispensable for normal growth

The molecular basis of primary wall extension endures as one of the central enigmas in plant cell morphogenesis. Classical cell wall models suggest that xyloglucan endo-transglycosylase activity is the primary catalyst (together with expansins) of controlled cell wall loosening through the transient cleavage and religation of xyloglucan-cellulose cross links. The genome of Arabidopsis (Arabidopsis thaliana) contains 33 phylogenetically diverse XYLOGLUCAN ENDO-TRANSGLYCOSYLASE/HYDROLASE (XTH) gene products, two of which were predicted to be predominant xyloglucan endohydrolases due to clustering into group III-A. Enzyme kinetic analysis of recombinant AtXTH31 confirmed this prediction and indicated that this enzyme had similar catalytic properties to the nasturtium (Tropaeolum majus) xyloglucanase1 responsible for storage xyloglucan hydrolysis during germination. Global analysis of Genevestigator data indicated that AtXTH31 and the paralogous AtXTH32 were abundantly expressed in expanding tissues. Microscopy analysis, utilizing the resorufin β-glycoside of the xyloglucan oligosaccharide XXXG as an in situ probe, indicated significant xyloglucan endohydrolase activity in specific regions of both roots and hypocotyls, in good correlation with transcriptomic data. Moreover, this hydrolytic activity was essentially completely eliminated in AtXTH31/AtXTH32 double knockout lines. However, single and double knockout lines, as well as individual overexpressing lines, of AtXTH31 and AtXTH32 did not demonstrate significant growth or developmental phenotypes. These results suggest that although xyloglucan polysaccharide hydrolysis occurs in parallel with primary wall expansion, morphological effects are subtle or may be compensated by other mechanisms. We hypothesize that there is likely to be an interplay between these xyloglucan endohydrolases and recently discovered apoplastic exo-glycosidases in the hydrolytic modification of matrix xyloglucans.

PFT1, a transcriptional Mediator complex subunit, controls root hair differentiation through reactive oxygen species (ROS) distribution in Arabidopsis

Root hair morphogenesis is driven by an amalgam of interacting processes controlled by complex signaling events. Redox processes and transcriptional control are critical for root hair development. However, the molecular mechanisms that integrate redox state and transcription are largely unknown. To elucidate a possible role of transcriptional Mediators in root hair formation, we analyzed the Arabidopsis root hair phenotype of T-DNA insertion lines that harbor homozygous mutations in genes encoding Mediator subunits. Genetic evidence indicates that the Mediator subunits PFT1/MED25 and MED8 are critical for root hair differentiation, but act via separate mechanisms. Transcriptional profiling of pft1 roots revealed that PFT1 activates a subset of hydrogen peroxide (H(2)O(2))-producing class III peroxidases. pft1 mutants showed perturbed H(2)O(2) and superoxide (O(2)(·-)) distribution, suggesting that PFT1 is essential to maintain redox homeostasis in the root. Chemical treatments rescued the pft1 mutant phenotype, indicating that correct reactive oxygen species (ROS) distribution is an essential prerequisite for root hair differentiation. In addition, PFT1 positively regulates cell wall remodeling genes that are essential for root hair formation. Our results demonstrate that PFT1 maintains ROS distribution which, in turn, controls root hair differentiation. Thus, our findings reveal a novel mechanism in which the Mediator controls ROS homeostasis by regulating the transcriptional machinery.

A tale of two tissues: AtGH9C1 is an endo-β-1,4-glucanase involved in root hair and endosperm development in Arabidopsis

Arabidopsis AtGH9C1 is an endo-β-1,4-glucanase possessing a carbohydrate-binding domain (CBM49). Analysis of AtGH9C1 expression by promoter-reporter GUS, RT-PCR, public transcriptome databases and GFP protein tagging demonstrated a high and selective expression of AtGH9C1 in root hairs and in the endosperm. Expression in root hair cells started prior to bulge formation and continued during hair elongation. AtGH9C1 expression increased with treatments that increase density (ACC) or length (sucrose) of root hairs. Expression in the endosperm extended sequentially to the micropylar, peripheral and chalazal compartments. A mutant with reduced AtGH9C1 expression had a delay in germination and a marked reduction in root hair presence. Complementation of the mutant partially improved both germination and root hair density. Experiments with ectopically expressed AtGH9C1-GFP with and without the CBM49, demonstrated that both forms of the protein are secreted and that CBM49 targets the protein to specific regions of the cell wall, but what makes these regions special is still unknown. The amino acid alignment of angiosperm GH9 genes with C-terminal extensions illustrate that AtGH9C1 belongs to a different clade than its tomato homolog, S1GH9C1. The latter has a CBM49 that was shown to bind crystalline cellulose. We suggest that AtGH9C1 is associated with the weakening of the cell wall during formation and growth of the root hair as well as with the sequential anterior-posterior breakdown of the endosperm cell wall that provides space for the growing embryo. Thus, is likely that the CBM49 of AtGH9C1 recognizes a form of cellulose or glucan polymer that is prevalent in the wall of these specialized tissues and that is different than the one recognized by S1GH9C1.

Characterization of resistance to pine wood nematode infection in Pinus thunbergii using suppression subtractive hybridization

BACKGROUND

Pine wilt disease is caused by the pine wood nematode, Bursaphelenchus xylophilus, which threatens pine forests and forest ecosystems worldwide and causes serious economic losses. In the 40 years since the pathogen was identified, the physiological changes occurring as the disease progresses have been characterized using anatomical and biochemical methods, and resistant trees have been selected via breeding programs. However, no studies have assessed the molecular genetics, e.g. transcriptional changes, associated with infection-induced physiological changes in resistant or susceptible trees.

RESULTS

We constructed seven subtractive suppression hybridization (SSH) cDNA libraries using time-course sampling of trees inoculated with pine wood nematode at 1, 3, or 7 days post-inoculation (dpi) in susceptible trees and at 1, 3, 7, or 14 dpi in resistant trees. A total of 3,299 sequences was obtained from these cDNA libraries, including from 138 to 315 non-redundant sequences in susceptible SSH libraries and from 351 to 435 in resistant SSH libraries. Using Gene Ontology hierarchy, those non-redundant sequences were classified into 15 subcategories of the biological process Gene Ontology category and 17 subcategories of the molecular function category. The transcriptional components revealed by the Gene Ontology classification clearly differed between resistant and susceptible libraries. Some transcripts were discriminative: expression of antimicrobial peptide and putative pathogenesis-related genes (e.g., PR-1b, 2, 3, 4, 5, 6) was much higher in susceptible trees than in resistant trees at every time point, whereas expression of PR-9, PR-10, and cell wall-related genes (e.g., for hydroxyproline-rich glycoprotein precursor and extensin) was higher in resistant trees than in susceptible trees at 7 and 14 dpi.

CONCLUSIONS

Following inoculation with pine wood nematode, there were marked differences between resistant and susceptible trees in transcript diversity and the timing and level of transcripts expressed in common; in particular, expression of stress response and defense genes differed. This study provided new insight into the differences in the physiological changes between resistant and susceptible trees that have been observed in anatomical and biochemical studies.

Emerging aspects of ER organization in root hair tip growth: lessons from RHD3 and Atlastin

Cell polarity is a fundamental aspect of eukaryotic cells. A central question for cell biologists is how the polarity of a cell is established and maintained. Root hairs are exceptionally polarized structures formed from specific root epidermal cells. The morphogenesis of root hairs is characterized by the localized cell growth in a small dome at the tip of the hair, a process called tip growth. Root hairs are thus an attractive model system to study the establishment and maintenance of cell polarity in eukaryotes. Research on Arabidopsis root hairs has identified a plethora of molecular and cellular components that are important for root hair tip growth. Recently, studies on RHD3 and Atlastin have revealed a surprising similarity with respect to the role of the tubular ER network in tip growth of root hairs in plants and the axonal outgrowth of corticospinal neurons in neurological disorders known as hereditary spastic paraplegia (HSP). In this mini-review, we highlight recent progress in understanding of the function and regulation of RHD3 in the generation of the tubular ER network and discussed ways in which RHD3 could be involved in the establishment and maintenance of root hair tip growth.

Conserved and unique features of the maize (Zea mays L.) root hair proteome

Root hairs are unicellular extensions of specialized epidermis cells. Under limiting conditions, they significantly increase the water and nutrient uptake capacity of plants by enlarging their root surface. Thus far, little is known about the initiation and growth of root hairs in the monocot model species maize. To gain a first insight into the protein composition of these specialized cells, the 2573 most abundant proteins of maize root hairs attached to four-day-old primary roots of the inbred line B73 were identified by combining 1DE with nanoLC-MS/MS in a shotgun proteomic experiment. Among the identified proteins, homologues of 252 proteins have been previously associated with root hair formation and development in other species. Comparison of the root hair reference proteome of the monocot species maize with the previously published root hair proteome of the dicot species soybean revealed conserved, but also unique, protein functions in root hairs of these two major groups of flowering plants.

Xyloglucan endotransglucosylase and cell wall extensibility

Transgenic tomato hypocotyls with altered levels of an XTH gene were used to study how XET activity could affect the hypocotyl growth and cell wall extensibility. Transgenic hypocotyls showed significant over-expression (line 13) or co-suppression (line 33) of the SlXTH1 in comparison with the wild type, with these results being correlated with the results on specific soluble XET activity, suggesting that SlXTH1 translates mainly for a soluble XET isoenzyme. A relationship between XET activity and cell wall extensibility was found, and the highest total extensibility was located in the apical hypocotyl segment of the over-expressing SlXTH1 line, where the XET-specific activity and hypocotyl growth were also highest compared with the wild line. Also, in the co-suppression SlXTH1 line, total extensibility values were lower than in the wild type line. The study of linkages between cell wall polysaccharides by FTIR showed that hypocotyls over-expressing SlXTH1 and having a higher XET-specific activity, were grouped away from the wild line, indicating that the linkages between pectins and between cellulose and xyloglucans might differ. These results suggested that the action of the increased XET activity in the transgenic line could be responsible for the cell wall structural changes, and therefore, alter the cell wall extensibility. On the other hand, results on xyloglucan oligosaccharides composition of the xyloglucan by MALDI TOF-MS showed no differences between lines, indicating that the xyloglucan structure was not affected by the XET action. These results provide evidences that XTHs from group I are involved mainly in the restructuring of the cell wall during growth and development, but they are not the limiting factor for plant growth.

Functions of xyloglucan in plant cells

While an increase in the number of xyloglucan tethers between the cellulose microfibrils in plant cell walls increases the walls' rigidity, the degradation of these tethers causes the walls to loosen. Degradation can occur either through the integration of xyloglucan oligosaccharides due to the action of xyloglucan endotransglucosylase or through direct hydrolysis due to the action of xyloglucanase. This is why the addition of xyloglucan and its fragment oligosaccharides causes plant tissue tension to increase and decrease so dramatically. Experiments involving the overexpression of xyloglucanase and cellulase have revealed the roles of xyloglucans in the walls. The degradation of wall xyloglucan in poplar by the transgenic expression of xyloglucanase, for example, not only accelerated stem elongation in the primary wall, but also blocked upright-stem gravitropism in the secondary wall. Overexpression of cellulase also reduced xyloglucan content in the walls as cellulose microfibrils were trimmed at their amorphous region, resulting in increased cell volume in Arabidopsis leaves and in sengon with disturbed leaf movements. The hemicellulose xyloglucan, in its function as a tether, plays a key role in the loosening and tightening of cellulose microfibrils: it enables the cell to change its shape in growth and differentiation zones and to retain its final shape after cell maturation.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

Differences in enzymic properties of five recombinant xyloglucan endotransglucosylase/hydrolase (XTH) proteins of Arabidopsis thaliana

Xyloglucan endotransglucosylase/hydrolases (XTHs) are cell wall enzymes that are able to graft xyloglucan chains to oligosaccharides or to other available xyloglucan chains and/or to hydrolyse xyloglucan chains. As they are involved in the modification of the load-bearing cell-wall components, they are believed to be very important in the regulation of growth and development. Given the large number (33) of XTH genes in Arabidopsis and the overlapping expression patterns, specific enzymic properties may be expected. Five predominantly root-expressed Arabidopsis thaliana XTHs belonging to subgroup I/II were analysed here. These represent two sets of closely related genes: AtXTH12 and 13 on the one hand (trichoblast-enriched) and AtXTH17, 18, and 19 on the other (expressed in nearly all cell types in the root). They were all recombinantly produced in the yeast Pichia pastoris and partially purified by ammonium sulphate precipitation before they were subsequently all subjected to a series of identical in vitro tests. The kinetic properties of purified AtXTH13 were investigated in greater detail to rule out interference with the assays by contaminating yeast proteins. All five proteins were found to exhibit only the endotransglucosylase (XET; EC 2.4.1.207) activity towards xyloglucan and non-detectable endohydrolytic (XEH; EC 3.2.1.151) activity. Their endotransglucosylase activity was preferentially directed towards xyloglucan and, in some cases, water-soluble cellulose acetate, rather than to mixed-linkage β-glucan. Isoforms differed in optimum pH (5.0-7.5), in temperature dependence and in acceptor substrate preferences.

Xyloglucan endotransglycosylase/hydrolase genes in cotton and their role in fiber elongation

Plant cell wall extensibility is mediated, in part, by xyloglucan endotransglycosylases/hydrolases (XTH) that are able to cleave and reattach xyloglucan polymers that make up the hemicelluloses matrix of type I cell walls. In Arabidopsis and other plants, XTHs are encoded by relatively large gene families that are regulated in specific spatial and temporal patterns. In silico screening of a cotton expressed sequence tag (EST) database identified 23 sequences with close sequence similarity to Arabidopsis XTH coding sequences. Analysis of full-length cotton cDNAs derived from these ESTs allow for the identification of three distinct GhXTH cDNAs (denoted GhXTH1, GhXTH2 and GhXTH3) based primarily on their 3' untranslated sequences. The three GhXTH genes were expressed differently with GhXTH1 predominantly expressed in elongating cotton fibers. The function of GhXTH1 in mediating cotton fiber elongation was analyzed in transgenic cotton plants that express a transgene consisting of the GhXTH1 coding sequence under transcriptional control of the CaMV 35S promoter. Plants that over-expressed GhXTH1 had increased XTH activity and produced mature cotton fibers that were between 15 and 20% longer than wild-type cotton plants under both greenhouse and field growth conditions. Segregation analysis showed that the 35S::GhXTH1 transgene acts as a dominant fiber length allele in transgenic cotton. These results confirm that GhXTH1 is the predominant XTH in elongating fibers and its expression limits cotton fiber elongation.

Mitochondrial beta-cyanoalanine synthase is essential for root hair formation in Arabidopsis thaliana

Cyanide is stoichiometrically produced as a coproduct of the ethylene biosynthesis pathway and is detoxified by β-cyanoalanine synthase enzymes. The molecular and phenotypical analysis of T-DNA insertion mutants of the mitochondrial β-cyanoalanine synthase CYS-C1 suggests that discrete accumulation of cyanide is not toxic for the plant and does not alter mitochondrial respiration rates but does act as a strong inhibitor of root hair development. The cys-c1 null allele is defective in root hair formation and accumulates cyanide in root tissues. The root hair defect is phenocopied in wild-type plants by the exogenous addition of cyanide to the growth medium and is reversed by the addition of hydroxocobalamin or by genetic complementation with the CYS-C1 gene. Hydroxocobalamin not only recovers the root phenotype of the mutant but also the formation of reactive oxygen species at the initial step of root hair tip growth. Transcriptional profiling of the cys-c1 mutant reveals that cyanide accumulation acts as a repressive signal for several genes encoding enzymes involved in cell wall rebuilding and the formation of the root hair tip as well as genes involved in ethylene signaling and metabolism. Our results demonstrate that mitochondrial β-cyanoalanine synthase activity is essential to maintain a low level of cyanide for proper root hair development.

Structural sterols are involved in both the initiation and tip growth of root hairs in Arabidopsis thaliana

Structural sterols are abundant in the plasma membrane of root apex cells in Arabidopsis thaliana. They specifically accumulate in trichoblasts during the prebulging and bulge stages and show a polar accumulation in the tip during root hair elongation but are distributed evenly in mature root hairs. Thus, structural sterols may serve as a marker for root hair initiation and growth. In addition, they may predict branching events in mutants with branching root hairs. Structural sterols were detected using the sterol complexing fluorochrome filipin. Application of filipin caused a rapid, concentration-dependent decrease in tip growth. Filipin-complexed sterols accumulated in globular structures that fused to larger FM4-64-positive aggregates in the tip, so-called filipin-induced apical compartments, which were closely associated with the plasma membrane. The plasma membrane appeared malformed and the cytoarchitecture of the tip zone was affected. Trans-Golgi network/early endosomal compartments containing molecular markers, such as small Rab GTPase RabA1d and SNARE Wave line 13 (VTI12), locally accumulated in these filipin-induced apical compartments, while late endosomes, endoplasmic reticulum, mitochondria, plastids, and cytosol were excluded from them. These data suggest that the local distribution and apical accumulation of structural sterols may regulate vesicular trafficking and plasma membrane properties during both initiation and tip growth of root hairs in Arabidopsis.

Global analysis of the root hair morphogenesis transcriptome reveals new candidate genes involved in root hair formation in barley

Root hairs are long tubular outgrowths of specialized root epidermal cells that play an important role in plant nutrition and water uptake. They are also an important model in studies of higher plant cell differentiation. In contrast to the model dicot Arabidopsis thaliana, currently very little is known about the genetic and molecular basis of root hair formation in monocots, including major cereals. To elucidate candidate genes controlling this developmental process in barley, we took advantage of the recently established Affymetrix GeneChip Barley1 Genome Array to carry out global transcriptome analyses of hairless and root hair primordia-forming roots of two barely mutant lines. Expression profiling of the root-hairless mutant rhl1.a and its wild type parent variety 'Karat' revealed 10 genes potentially involved in the early step of root hair formation in barley. Differential expression of all identified genes was confirmed by quantitative reverse transcription-polymerase chain reaction. The genes identified encode proteins associated with the cell wall and membranes, including one gene for xyloglucan endotransglycosylase, three for peroxidase enzymes and five for arabinogalactan protein, extensin, leucine-rich-repeat protein, phosphatidylinositol phosphatidylcholine transfer protein and a RhoGTPase GDP dissociation inhibitor, respectively. The molecular function of one gene is unknown at present. The expression levels of these genes were strongly reduced in roots of the root-hairless mutant rhl1.a compared to the parent variety, while expression of all 10 genes was similar in another mutant, i.e. rhp1.b, that has lost its ability to develop full root hairs but still forms hairs blocked at the primordium stage, and its wild type relative. This clearly indicates that the new genes identified are involved in the initiation of root hair morphogenesis in barley.