An arabinogalactan protein(s) is a key component of a fraction that mediates local intercellular communication involved in tracheary element differentiation of zinnia mesophyll cells

Identification and expression profiles of xylogen-like arabinogalactan protein (XYLP) gene family in Phyllostachys edulis in different developmental tissues and under various abiotic stresses

Xylogen-like arabinogalactan protein (XYLP) is an atypical lipid transport protein. In this study, 23 Phyllostachys edulis XYLPs were identified, and their proteins contain characteristic structures of AGP and nsLTP domain. All PeXYLPs can be divided into four clades, and their genes were unevenly distributed on 11 chromosome scaffolds. Collinear analysis revealed that segmental duplication was the main driver for PeXYLP family expansion. The cis-acting elements presented in the promoter are involved in various regulations of PeXYLPs expression. G.O. annotation revealed that PeXYLPs are mainly interested in lipid transport and synthesis and primarily function at the plasma membrane. Transcriptome analysis revealed that PeXYLPs were spatiotemporally expressed and displayed significant variability during various tissue development. Besides that, some PeXYLPs also respond to multiple phytohormones and abiotic stresses. By semi-quantitative RT-PCR, the response of some PeXYLPs to MeJA was confirmed, and the proteins were shown to localize to the plasma membrane mainly. WGCNA in defined regions of fast-growing bamboo shoots revealed that 5 PeXYLPs in 4 gene co-expression modules showed a positive module-trait relationship with three fast-growing regions. This systematic analysis of the PeXYLP family will provide a foundation for further insight into the functions of individual PeXYLP in a specific tissue or organ development, phytohormone perception, and stress responses in the future.

An Arabidopsis thaliana arabinogalactan-protein (AGP31) and several cationic AGP fragments catalyse the boron bridging of rhamnogalacturonan-II

Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic ‘chaperones’ can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb²⁺ and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro. Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Biochemical characteristics of a diffusible factor that induces gametophyte to sporophyte switching in the brown alga Ectocarpus

The haploid-diploid life cycle of the filamentous brown alga Ectocarpus involves alternation between two independent and morphologically distinct multicellular generations, the sporophyte and the gametophyte. Deployment of the sporophyte developmental program requires two TALE homeodomain transcription factors OUROBOROS and SAMSARA. In addition, the sporophyte generation has been shown to secrete a diffusible factor that can induce uni-spores to switch from the gametophyte to the sporophyte developmental program. Here, we determine optimal conditions for production, storage, and detection of this diffusible factor and show that it is a heat-resistant, high molecular weight molecule. Based on a combined approach involving proteomic analysis of sporophyte-conditioned medium and the use of biochemical tools to characterize arabinogalactan proteins, we present evidence that sporophyte-conditioned medium contains AGP epitopes and suggest that the diffusible factor may belong to this family of glycoproteins.

Genome-wide identification, expression and functional analysis of Populus xylogen-like genes

As an extracellular arabinogalactan protein (AGP) containing a non-specific lipid transfer protein (nsLTP) domain, xylogen mediates the local intercellular communication required for tracheary element (TE) differentiation in Zinnia cell culture. Although XYLP (xylogen-like protein) gene families have been reported in Arabidopsis and rice, no comprehensive analysis has been performed in woody plants. In this work, 31 XYLP genes in five phylogenetic groups were identified from Populus trichocarpa genome and a comprehensive bioinformatic analysis including gene and protein structures, chromosomal locations and duplication events were conducted. In-silico data and qRT-PCR results indicated that PtXYLP1 is predominantly expressed in poplar apex, young leaves and roots, while PtXYLP2 is uniformly expressed across a variety of tissues with a low abundance. Analysis on PtXYLP1pro:GUS and PtXYLP2pro:GUS in Arabidopsis revealed their differential expression patterns during seed germination and specific inductions by exogenously applied phytohormones including auxin, cytokinin and GA. When overexpressed in Arabidopsis, PtXYLP1 but not PtXYLP2 resulted in cotyledons with defective venation patterns and interrupted secondary (2°) vein loops, which phenotype was underpinned by the down-regulation of genes indispensably required by embryonic venation development at procambium and/or vessel level.

Plant Phosphoglycerolipids: The Gatekeepers of Vascular Cell Differentiation

In higher plants, the plant vascular system has evolved as an inter-organ communication network essential to deliver a wide range of signaling factors among distantly separated organs. To become conductive elements, phloem and xylem cells undergo a drastic differentiation program that involves the degradation of the majority of their organelles. While the molecular mechanisms regulating such complex process remain poorly understood, it is nowadays clear that phosphoglycerolipids display a pivotal role in the regulation of vascular tissue formation. In animal cells, this class of lipids is known to mediate acute responses as signal transducers and also act as constitutive signals that help defining organelle identity. Their rapid turnover, asymmetrical distribution across subcellular compartments as well as their ability to rearrange cytoskeleton fibers make phosphoglycerolipids excellent candidates to regulate complex morphogenetic processes such as vascular differentiation. Therefore, in this review we aim to summarize, emphasize and connect our current understanding about the involvement of phosphoglycerolipids in phloem and xylem differentiation.

Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana

Vascular development is embedded into the developmental context of plant organ differentiation and can be divided into the consecutive phases of vascular patterning and differentiation of specific vascular cell types (phloem and xylem). To date, only very few genetic determinants of phloem development are known. Here, we identify OCTOPUS (OPS) as a potentiator of phloem differentiation. OPS is a polarly localised membrane-associated protein that is initially expressed in provascular cells, and upon vascular cell type specification becomes restricted to the phloem cell lineage. OPS mutants display a reduction of cotyledon vascular pattern complexity and discontinuous phloem differentiation, whereas OPS overexpressers show accelerated progress of cotyledon vascular patterning and phloem differentiation. We propose that OPS participates in vascular differentiation by interpreting longitudinal signals that lead to the transformation of vascular initials into differentiating protophloem cells.

Expression analyses of AtAGP17 and AtAGP19, two lysine-rich arabinogalactan proteins, in Arabidopsis

AtAGP17 and AtAGP19 are members of the lysine-rich arabinogalactan protein (AGP) subfamily in Arabidopsis. Detailed anatomical analysis of promoter activity of the AtAGP19 gene was carried out using transgenic Arabidopsis plants expressing a P(AtAGP19):GUS fusion. AtAGP19 promoter activity was tissue-specific and associated with vascular bundles, particularly differentiating xylem elements. Peptide-specific antibodies were raised against the Lys-rich regions of AtAGP17 and AtAGP19 and used to study the organ-specific expression patterns of these two AGPs. AtAGP17 and AtAGP19 were most abundant in roots and flowers, moderately abundant in stems, seedlings and siliques and virtually absent in leaves. Antibodies specific for AtAGP17 and AtAGP19, as reported here, represent valuable tools for understanding the biology of these two AGPs.

Changes to the proteome and targeted metabolites of xylem sap in Brassica oleracea in response to salt stress

Root-to-shoot signalling via xylem sap is an important mechanism by which plants respond to stress. This signalling could be mediated by alteration in the concentrations of inorganic and/or organic molecules. The effect of salt stress on the contents of xylem sap in Brassica olarecea has been analysed by mass spectrometry in order to quantify these changes. Subcellular location of arabinogalactan proteins (AGPs) by immunogold labelling and peroxidase isozymes was also analysed by isoelectrofocusing. The xylem sap metabolome analysis demonstrated the presence of many organic compounds such as sugars, organic acids and amino acids. Of these, amino acid concentrations, particularly that of glutamine, the major amino acid in the sap, were substantially reduced by salt stress. The xylem sap proteome analysis demonstrated the accumulation of enzymes involved in xylem differentiation and lignification, such as cystein proteinases, acid peroxidases, and a putative hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyl transferase under salt stress. The peroxidase isozyme pattern showed that salt stress induced a high accumulation of an acid isoform. These results suggest that xylem differentiation and lignification is induced by salt stress. The combination of different methods to analyse the xylem sap composition provides new insights into mechanisms in plant development and signalling under salt stress.

Establishment and maintenance of vascular cell communities through local signaling

During plant development, cell fates are often determined by cell-to-cell communication. The vascular system, in which procambial/cambial cells continue to provide cells to two conductive tissues, xylem and phloem, is an excellent model for understanding cell-to-cell communication as a developmental cue. Recent studies on vascular development have revealed several novel intercellular signaling molecules that regulate vascular cell fates by unique mechanisms. This review focuses on emerging novel concepts such that reciprocal signaling by a transcription factor and microRNAs between the stele and the endodermis determines xylem cell patterns, and that a small peptide secreted from phloem governs vascular stem-cell maintenance.

Genome-wide identification, classification, and expression analysis of the arabinogalactan protein gene family in rice (Oryza sativa L.)

Arabinogalactan proteins (AGPs) comprise a family of hydroxyproline-rich glycoproteins that are implicated in plant growth and development. In this study, 69 AGPs are identified from the rice genome, including 13 classical AGPs, 15 arabinogalactan (AG) peptides, three non-classical AGPs, three early nodulin-like AGPs (eNod-like AGPs), eight non-specific lipid transfer protein-like AGPs (nsLTP-like AGPs), and 27 fasciclin-like AGPs (FLAs). The results from expressed sequence tags, microarrays, and massively parallel signature sequencing tags are used to analyse the expression of AGP-encoding genes, which is confirmed by real-time PCR. The results reveal that several rice AGP-encoding genes are predominantly expressed in anthers and display differential expression patterns in response to abscisic acid, gibberellic acid, and abiotic stresses. Based on the results obtained from this analysis, an attempt has been made to link the protein structures and expression patterns of rice AGP-encoding genes to their functions. Taken together, the genome-wide identification and expression analysis of the rice AGP gene family might facilitate further functional studies of rice AGPs.

Involvement of phytosulfokine in the attenuation of stress response during the transdifferentiation of zinnia mesophyll cells into tracheary elements

Phytosulfokine (PSK) is a sulfated peptide hormone required for the proliferation and differentiation of plant cells. Here, we characterize the physiological roles of PSK in transdifferentiation of isolated mesophyll cells of zinnia (Zinnia elegans 'Canary Bird') into tracheary elements (TEs). Transcripts for a zinnia PSK precursor gene, ZePSK1, show two peaks of expression during TE differentiation; the first accumulation is transiently induced in response to wounding at 24 h of culture, and the second accumulation is induced in the final stage of TE differentiation and is dependent on endogenous brassinosteroids. Chlorate, a potent inhibitor of peptide sulfation, is successfully applied as an inhibitor of PSK action. Chlorate significantly suppresses TE differentiation. The chlorate-induced suppression of TE differentiation is overcome by exogenously applied PSK. In the presence of chlorate, expression of stress-related genes for proteinase inhibitors and a pathogenesis-related protein is enhanced and changed from a transient to a continuous pattern. On the contrary, administration of PSK significantly reduces the accumulation of transcripts for the stress-related genes. Even in the absence of auxin and cytokinin, addition of PSK suppresses stress-related gene expression. Microarray analysis reveals 66 genes down-regulated and 42 genes up-regulated in the presence of PSK. The large majority of down-regulated genes show significant similarity to various families of stress-related proteins, including chitinases, phenylpropanoid biosynthesis enzymes, 1-aminocyclopropane-1-carboxylic acid synthase, and receptor-like protein kinases. These results suggest the involvement of PSK in the attenuation of stress response and healing of wound-activated cells during the early stage of TE differentiation.

Hormone interactions during vascular development

Vascular tissue in plants is unique due to its diverse and dynamic cellular patterns. Signals controlling vascular development have only recently started to emerge through biochemical, genetic, and genomic approaches in several organisms, such as Arabidopsis, Populus, and Zinnia. These signals include hormones (auxin, brassinosteroids, and cytokinins, in particular), other small regulatory molecules, their transporters, receptors, and various transcriptional regulators. In recent years it has become apparent that plant growth regulators rarely act alone, but rather their signaling pathways are interlocked in complex networks; for example, polar auxin transport (PAT) regulates vascular development during various stages and an emerging theme is its modulation by other growth regulators, depending on the developmental context. Also, several synergistic or antagonistic interactions between various growth regulators have been described. Furthermore, shoot-root interactions appear to be important for this signal integration.

Non-cell-autonomous control of vascular stem cell fate by a CLE peptide/receptor system

Land plants evolved a long-distance transport system of water and nutrients composed of the xylem and phloem, both of which are generated from the procambium- and cambium-comprising vascular stem cells. However, little is known about the molecular mechanism of cell communication governing xylem-phloem patterning. Here, we show that a dodecapeptide (HEVHypSGHypNPISN; Hyp, 4-hydroxyproline), TDIF (tracheary element differentiation inhibitory factor), is secreted from the phloem and suppresses the differentiation of vascular stem cells into xylem cells through a leucine-rich repeat receptor-like kinase (LRR-RLK). TDIF binds in vitro specifically to the LRR-RLK, designated TDR (putative TDIF receptor), whose expression is restricted to procambial cells. However, the combined analysis of TDIF with a specific antibody and the expression profiles of the promoters of two genes encoding TDIF revealed that TDIF is synthesized mainly in, and secreted from, the phloem and its neighboring cells. The observation that TDIF is capable of promoting proliferation of procambial cells while suppressing xylem differentiation suggests that this small peptide functions as a phloem-derived, non-cell-autonomous signal that controls stem cell fate in the procambium. Our results indicate that we have discovered a cell communication system governing phloem-xylem cross-talk.

Characterization of a male sterile related gene BcMF15 from Brassica campestris ssp. chinensis

Data from cDNA-AFLP analysis based on the genome-wide transcriptional profiling on the flower buds of the male meiotic cytokinesis (mmc) mutant and its wild-type of Brassica campestris L. ssp. chinensis Makino, syn. B. rapa L. ssp. chinensis, indicated that mutation of the MMC gene resulted in changes in expression of a variety of genes. A transcript-derived fragment specifically accumulated in the wild-type flower buds was isolated, and the corresponding full-length cDNA and DNA was subsequently amplified. Bioinformatical analyses of this gene named BcMF15 (GenBank accession number EF600901) showed that it encoded a protein with 103 amino acids. The BcMF15 had a 88% nucleotide similarity to a lipid transfer protein-like gene. Moreover, sequence prediction indicated that BcMF15 might encode a membrane protein with a signal peptide at the N-terminus. Meanwhile, six domains were predicted in the deduced BcMF15 protein, such as the AAI domain existing in some crucial proteins of pollen development-preferential, signal peptide, transmembrane domain, vWF domain, ZnF_C4 domain, and Tryp_alpha_amyl domain. Spatial and temporal expression patterns analysis by RT-PCR indicated that BcMF15 was exclusively expressed in the fertile line, which indicated this gene is male sterile related. Phylogenetic analysis in Cruciferae revealed that the BcMF15 was relative conservative in evolution. We suppose BcMF15 may be a critical molecule in the transmembrane transportation and signal transduction during microspore development.

Peptide signaling in vascular development

In plants and animals, putative small peptide ligands have been suggested to play crucial roles in development as signal molecules of cell-cell communication. Recent studies of CLAVATA3/ENDOSPERM SURROUNDING REGION (CLE) genes and their products have revealed that distinctive dodeca-CLE peptide ligands function in various developmental processes. In particular, the finding and characterization of TDIF, a dodeca-CLE peptide suppressing tracheary element differentiation, indicates regulation of vascular organization by cell-cell communication through CLE peptides. In addition, other extracellular peptides such as phytosulfokine, proteins such as xylogen, and phytohormones all participate in the ordered formation of vascular tissues.

Microarray analysis of bast fibre producing tissues of Cannabis sativa identifies transcripts associated with conserved and specialised processes of secondary wall development

The mechanisms underlying bast fibre differentiation in hemp (Cannabis sativa L.) are largely unknown. We hybridised a cDNA microarray with RNA from fibre enriched tissues extracted at three different positions along the stem axis. Accordingly, we identified transcripts that were enriched in tissues in which phloem fibres were elongating or undergoing secondary wall thickening. These results were consistent with a dynamic pattern of cell wall deposition involving tissue specific expression of a large set of distinct glycosyltransferases and glycosylhydrolases apparently acting on polymers containing galactans, mannans, xylans, and glucans, as well as raffinose-series disaccharides. Putative arabinogalactan proteins and lipid transfer proteins were among the most highly enriched transcripts in various stem segments, with different complements of each expressed at each stage of development. We also detected stage-specific expression of brassinosteroid-related transcripts, various transporters, polyamine and phenylpropanoid related genes, and seven putative transcription factors. Finally, we observed enrichment of many transcripts with unknown biochemical function, some of which had been previously implicated in fibre development in poplar or cotton. Together these data complement and extend existing biochemical models of bast fibre development and secondary wall deposition and highlight uncharacterised, but conserved, components of these processes.

A lysine-rich arabinogalactan protein in Arabidopsis is essential for plant growth and development, including cell division and expansion

Arabinogalactan proteins (AGPs), a family of hydroxyproline-rich glycoproteins, occur throughout the plant kingdom. The lysine-rich classical AGP subfamily in Arabidopsis consists of three members, AtAGP17, 18 and 19. In this study, AtAGP19 was examined in terms of its gene expression pattern and function. AtAGP19 mRNA was abundant in stems, with moderate levels in flowers and roots and low levels in leaves. AtAGP19 promoter-controlled GUS activity was high in the vasculature of leaves, roots, stems and flowers, as well as styles and siliques. A null T-DNA knockout mutant of AtAGP19 was obtained and compared to wild-type (WT) plants. The atagp19 mutant had: (i) smaller, rounder and flatter rosette leaves, (ii) lighter-green leaves containing less chlorophyll, (iii) delayed growth, (iv) shorter hypocotyls and inflorescence stems, and (v) fewer siliques and less seed production. Several abnormalities in cell size, number, shape and packing were also observed in the mutant. Complementation of this pleiotropic mutant with the WT AtAGP19 gene restored the WT phenotypes and confirmed that AtAGP19 functions in various aspects of plant growth and development, including cell division and expansion, leaf development and reproduction.

The biology of arabinogalactan proteins

Arabinogalactan proteins is an umbrella term applied to a highly diverse class of cell surface glycoproteins, many of which contain glycosylphosphatidylinositol lipid anchors. The structures of protein and glycan moieties of arabinogalactan proteins are overwhelmingly diverse while the "hydroxproline contiguity hypothesis" predicts arabinogalactan modification of members of many families of extracellular proteins. Descriptive studies using monoclonal antibodies reacting with carbohydrate epitopes on arabinogalactan proteins and experimental work using beta-Yariv reagent implicate arabinogalactan proteins in many biological processes of cell proliferation and survival, pattern formation and growth, and in plant microbe interaction. Advanced structural understanding of arabinogalactan proteins and an emerging molecular genetic definition of biological roles of individual arabinogalactan protein species, in conjunction with potentially analogous extracellular matrix components of animals, stimulate hypotheses about their mode of action. Arabinogalactan proteins might be soluble signals, or might act as modulators and coreceptors of apoplastic morphogens; their amphiphilic molecular nature makes them prime candidates of mediators between the cell wall, the plasma membrane, and the cytoplasm.

Organic substances in xylem sap delivered to above-ground organs by the roots

Squash (Cucurbita maxima) xylem sap, an apoplastic fluid, contains t-zeatin riboside, glutamine, methylglycine, myo-inositol, fructose, oligosaccharides of arabinogalactan, glucan, galacturonan, and pectins (rhamnogalacturonan-I and rhamnogalacturonan-II), as well as various proteins, including arabinogalactan and pathogen-related proteins. These substances are mainly produced in stele (xylem) parenchyma and the pericycle in the root-hair zone where ion transporter genes are expressed. Glycine-rich protein genes (CRGRPs) cloned by antiserum raised against whole xylem sap of cucumber (Cucumis sativus) were abundantly expressed in the parenchyma cells surrounding xylem vessels in the root-hair zone. CRGRP proteins accumulated and immobilized in the lignified walls of metaxylem vessels and perivascular fibers in shoots, suggesting a systemic delivery mechanism of wall materials via xylem sap. A major 30-kDa protein (XSP30) found in cucumber xylem sap was homologous to the B chains of a lectin (ricin) and bound to a nonfucosylated core N-acetylglucosamine dimer of N-linked glycoproteins abundant in leaf parenchyma cells. XSP30 gene expression, abundant in root xylem parenchyma and pericycle, and the level of XSP30 protein fluctuated diurnally under the control of a circadian clock, and the amplitude was up-regulated by gibberellic acid produced in young leaves, suggesting a long-distance control system between organs.

Arabinogalactan proteins: involvement in plant growth and morphogenesis

Arabinogalactan proteins (AGPs) are highly glycosylated hydroxyproline-containing variously located proteoglycans dynamically regulated in the course of plant ontogenesis. Special functions of AGPs are still unclear, but their involvement in vegetative growth and reproduction of plants is well established. This review considers data on the structure, biosynthesis, and metabolism of AGPs. Special attention is given to involvement of AGPs in growth and morphogenesis, and possible mechanisms of their regulatory action are considered. AGPs are also compared with animal proteoglycans.

Vascular development: the long and winding road

Vascular development involves the specification of distinct meristematic cells that proliferate and then differentiate into two separate multicellular tissues: xylem and phloem. Organ-specific patterning, which requires the co-ordination of vascular development with organogenesis, introduces another layer of complexity to the development of vascular tissues. Because vascular tissues develop internally, analyses of their development are technically challenging. Nevertheless, the combined use of genetic and genomic approaches has provided significant insight into the mechanisms of vascular development. Notable highlights include the identification of class III HD-ZIP genes as regulators of both (pro)cambial activity and vascular tissue specification, the characterization of vesicle-trafficking components (SCARFACE [SFC]/VAN3) as being necessary for axial vein pattern, the genetic characterization of xylogen, and the identification of transcription factors and hormone signals that regulate vascular cell identity.

Beta-D-glucosyl and alpha-D-galactosyl Yariv reagents: syntheses from p-nitrophenyl-D-glycosides by transfer reduction using ammonium formate

Yariv beta-D-glucosyl (4a) and Yariv alpha-d-galactosyl (4b) reagents are multivalent phenylglycosides. The beta-D-glucosyl reagent is considered diagnostic for arabinogalactan proteins (AGPs) to which it can reversibly bind, stain, and precipitate. The alpha-D-galactosyl reagent does not bind AGPs and is used as a control. In a new strategy, we accomplished the large scale synthesis of the Yariv reagents in one continuous step by a transfer reduction method and without a need for any specialized apparatus. As the starting material, p-nitrophenyl-D-glycosides (1) were reduced to p-aminophenyl-D-glycosides (2) using ammonium formate as the hydrogen donor. The excess formate was converted to formic acid and ammonia, which then were removed from the reaction by simple distillation. Without isolation, p-aminophenyl-D-glycosides were diazotized (3) and coupled to phloroglucinol to give the Yariv reagents in approximately 40% yield. AGPs are a major component of gum arabic, an emulsifying agent widely used in the food and pharmaceutical industries. Increasing interest in AGPs prompted the development of a relatively easy and inexpensive method for the synthesis of these reagents.

Binding of arabinogalactan proteins by Yariv phenylglycoside triggers wound-like responses in Arabidopsis cell cultures

Arabinogalactan-proteins (AGPs) are cell wall proteoglycans and are widely distributed in the plant kingdom. Classical AGPs and some nonclassical AGPs are predicted to have a glycosylphosphatidylinositol lipid anchor and have been suggested to be involved in cell-cell signaling. Yariv phenylglycoside is a synthetic probe that specifically binds to plant AGPs and has been used to study AGP functions. We treated Arabidopsis suspension cell cultures with Yariv phenylglycoside and observed decreased cell viability, increased cell wall apposition and cytoplasmic vesiculation, and induction of callose deposition. The induction of cell wall apposition and callose synthesis led us to hypothesize that Yariv binding of plant surface AGPs triggers wound-like responses. To study the effect of Yariv binding to plant surface AGPs and to further understand AGP functions, an Arabidopsis whole genome array was used to monitor the transcriptional modifications after Yariv treatment. By comparing the genes that are induced by Yariv treatment with genes whose expressions have been previously shown to be induced by other conditions, we conclude that the gene expression profile induced by Yariv phenylglycoside treatment is most similar to that of wound induction. It remains uncertain whether the Yariv phenylglycoside cross-linking of cell surface AGPs induces these genes through a specific AGP-based signaling mechanism or through a general mechanical perturbation of the cell surface.

A proteoglycan mediates inductive interaction during plant vascular development

Inductive cell-cell interactions are essential for controlling cell fate determination in both plants and animals; however, the chemical basis of inductive signals in plants remains little understood. A proteoglycan-like factor named xylogen mediates local and inductive cell-cell interactions required for xylem differentiation in Zinnia cells cultured in vitro. Here we describe the purification of xylogen and cloning of its complementary DNA, and present evidence for its role in planta. The polypeptide backbone of xylogen is a hybrid-type molecule with properties of both arabinogalactan proteins and nonspecific lipid-transfer proteins. Xylogen predominantly accumulates in the meristem, procambium and xylem. In the xylem, xylogen has a polar localization in the cell walls of differentiating tracheary elements. Double knockouts of Arabidopsis lacking both genes that encode xylogen proteins show defects in vascular development: discontinuous veins, improperly interconnected vessel elements and simplified venation. Our results suggest that the polar secretion of xylogen draws neighbouring cells into the pathway of vascular differentiation to direct continuous vascular development, thereby identifying a molecule that mediates an inductive cell-cell interaction involved in plant tissue differentiation.

Analysis of sugars in squash xylem sap

Xylem sap contains organic and inorganic compounds that might be involved in root-to-shoot communication. To clarify the physiological functions of sugars in xylem sap, we characterized the sugar compounds of the xylem sap. The 80% ethanol-soluble fraction of xylem sap contained mainly myo-inositol and oligosaccharides. The 80% ethanol precipitate was solubilized with cyclohexanediamine tetraacetate and fractionated using anion exchange chromatography. The non-bound fraction from the anion-exchange column reacted with Yariv reagent and was rich in arabinogalactan, indicating the presence of arabinogalactan proteins (AGP). The bound fraction eluted with 50 mM ammonium formate buffer and separated using size exclusion chromatography producing the pectins rhamnogaracturonan (RG)-I and RG-II with apparent molecular masses of 15000 and 11000, respectively. These results indicate that the AGP, RG-I, borate cross-linked RG-II dimer and oligosaccharides produced by root tissues are transported to above-ground organs via xylem sap.

Visualization by comprehensive microarray analysis of gene expression programs during transdifferentiation of mesophyll cells into xylem cells

Plants have a unique transdifferentiation mechanism by which differentiated cells can initiate a new program of differentiation. We used a comprehensive analysis of gene expression in an in vitro zinnia (Zinnia elegans L.) culture model system to gather fundamental information about the gene regulation underlying the transdifferentiation of plant cells. In this model, photosynthetic mesophyll cells isolated from zinnia leaves transdifferentiate into xylem cells in a morphogenic process characterized by features such as secondary-wall formation and programmed cell death. More than 8,000 zinnia cDNA clones were isolated from an equalized cDNA library prepared from cultured cells transdifferentiating into xylem cells. Microarray analysis using these cDNAs revealed several types of unique gene regulation patterns, including: the transient expression of a set of genes during cell isolation, presumably induced by wounding; a rapid reduction in the expression of photosynthetic genes and the rapid induction of protein synthesis-associated genes during the first stage; the preferential induction of auxin-related genes during the subsequent stage; and the transient induction of genes closely associated with particular morphogenetic events, including cell-wall formation and degradation and programmed cell death during the final stage. This analysis also revealed a number of previously uncharacterized genes encoding proteins that function in signal transduction, such as protein kinases and transcription factors that are expressed in a stage-specific manner. These findings provide new clues to the molecular mechanisms of both plant transdifferentiation and wood formation.

Promotion of transcript accumulation of novel Zinnia immature xylem-specific HD-Zip III homeobox genes by brassinosteroids

We isolated three novel homeobox genes (ZeHB-10, -11 and -12) from Zinnia elegans to elucidate the molecular mechanism underlying vascular system formation. ZeHB-10, -11 and -12 encode for HD-Zip proteins of the class III to which Arabidopsis Athb-8, -9, -14, -15 and IFL1 belong. In situ hybridization analysis demonstrated that the ZeHB-10, -11 and -12 mRNAs accumulated preferentially in procambium and immature xylem cells in 14-day-old plants. Transcripts for the three genes also accumulated in cultured Zinnia cells in a xylogenesis-specific manner. The accumulation of transcripts for all of ZeHB-10, -11 and -12 in cultured Zinnia cells was suppressed strongly by uniconazole, an inhibitor of brassinosteroid synthesis, and such suppression was reversed by the addition of brassinolide, a biologically active brassinosteroid. Thus the expression of ZeHB-10, -11 and -12 may be regulated by endogenous levels of brassinosteroids. Taken together with the fact that ZeHB-10, -11 and -12 proteins can bind to each other in yeast, the roles of HD-Zip III genes in vascular development are discussed.

Analysis of early processes in wound-induced vascular regeneration using TED3 and ZeHB3 as molecular markers

Interruption of the vascular bundles of Zinnia internodes induced transdifferentiation of cells into tracheary elements (TEs) or sieve elements (SEs) within 4 d of wounding. The early stage of the regeneration processes was analyzed using two molecular marker genes, TED3 and ZeHB3, which are expressed specifically in TE precursor cells and immature phloem cells, respectively. An increase in the numbers of TED3 and ZeHB3 mRNA-expressing cells always preceded an increase in the numbers of TEs and SEs formed. The earliest sign of vascular differentiation was the appearance 24 h after wounding of a layer(s) of TED3 mRNA-expressing cells in the inter- and intrafascicular cambial-like regions along the severed vascular bundles. In contrast, the number of ZeHB3 mRNA-expressing cells decreased dramatically along the severed bundles 24 h after wounding, and increased again 36 h after wounding. These results clearly indicate that xylem and phloem differentiation are not synchronized during vascular regeneration. Treatment with 10(-3) M colchicine abolished the expression of ZeHB3 mRNA in pith parenchyma, but not TED3 mRNA; this suggests that cell division is a prerequisite for the transdifferentiation of pith parenchymal cells into immature phloem cells expressing ZeHB3. In contrast, transdifferentiation of pith parenchymal cells to TE precursor cells does not require preceding cell division. However, the inhibition of cell division prevented the formation of both radial files of TEs and the cambial-like layer(s) of TED3 mRNA-expressing cells, and, ultimately, vascular regeneration altogether. These results imply that wound-induced cambial-like activity in and between severed vascular bundles is essential for vascular regeneration.

Primary phloem-specific expression of a Zinnia elegans homeobox gene

Some plant homeobox genes are expressed specifically in vascular cells and are assumed to function in the differentiation of specific types of vascular cells. However, homeobox genes exhibiting primary phloem-specific expression have not been reported. To elucidate the molecular mechanisms of vascular development, we undertook to isolate from Zinnia elegans primary phloem-specific homeobox genes that may function in phloem development. An HD-Zip type homeobox gene, ZeHB3, was isolated. This gene encodes a class I HD-Zip protein, and constitutes a gene subfamily with the Daucus carota gene CHB6, and Arabidopsis thaliana genes Athb-5, Athb-6, and Athb-16. In situ hybridization of 1-, 14- and 50-day-old plants demonstrated that ZeHB3 mRNA accumulation is restricted to a few cells destined to differentiate into phloem cells and to the immature phloem cells surrounding the sieve elements and companion cells. ZeHB3 protein was also localized to immature phloem cells. These findings clearly indicate that ZeHB3 is a novel homeobox gene that marks, and may function in, the early stages of phloem differentiation.

Progress of lignification mediated by intercellular transportation of monolignols during tracheary element differentiation of isolated Zinnia mesophyll cells

Tracheary element (TE) differentiation is a typical example of programmed cell death (PCD) in higher plants, and maturation of TEs is completed by degradation of all cell contents. However, lignification of TEs progresses even after PCD. We investigated how and whence monolignols are supplied to TEs which have undergone PCD during differentiation of isolated Zinnia mesophyll cells into TEs. Higher densities of cell culture induced greater lignification of TEs. Whereas the continuous exchanging of culture medium suppressed lignification of TEs, further addition of coniferyl alcohol into the exchanging medium reduced the suppression of lignification. Analysis of the culture medium by HPLC and GC-MS showed that coniferyl alcohol, coniferaldehyde, and sinapyl alcohol accumulated in TE inductive culture. The concentration of coniferyl alcohol peaked at the beginning of secondary wall thickening, decreased rapidly during secondary wall thickening, then increased again. These results indicated that lignification on TEs progresses by supply of monolignols from not only TEs themselves but also surrounding xylem parenchyma-like cells through medium in vitro.