Isolation of a vascular cell wall-specific monoclonal antibody recognizing a cell polarity by using a phage display subtraction method

Monoclonal antibody-based analysis of cell wall remodeling during xylogenesis

Xylogenesis, a process by which woody tissues are formed, entails qualitative and quantitative changes in the cell wall. However, the molecular events that underlie these changes are not completely understood. Previously, we have isolated two monoclonal antibodies, referred to as XD3 and XD27, by subtractive screening of a phage-display library of antibodies raised against a wall fraction of Zinnia elegans xylogenic culture cells. Here we report the biochemical and immunohistochemical characterization of those antibodies. The antibody XD3 recognized (1→4)-β-D-galactan in pectin fraction. During xylogenesis, the XD3 epitope was localized to the primary wall of tracheary-element precursor cells, which undergo substantial cell elongation, and was absent from mature tracheary elements. XD27 recognized an arabinogalactan protein that was bound strongly to a germin-like protein. The XD27 epitope was localized to pre-lignified secondary walls of tracheary elements. Thus these cell-wall-directed monoclonal antibodies revealed two molecular events during xylogenesis. The biological significance of these events is discussed in relation to current views of the plant cell wall.

Single-chain Fv antibody with specificity for Listeria monocytogenes

Single chain antibodies (scFv) exhibiting specific binding to Listeria monocytogenes strains were isolated from a pool of random scFvs expressed on the surface of filamentous bacteriophages. Positive selection (panning) using L. monocytogenes was used to enrich for phage clones with the desired binding affinity, and negative selection using L. innocua and L. ivanovii was used to remove phages expressing cross-reactive antibody fragments. A single phage clone, P4:A8, was selected using two independent panning schemes. A rapid assay was devised to determine phage antibody binding specificity and was used to develop a selectivity profile for individual phage clones. The P4:A8 clone was screened against a panel of bacteria consisting of eight strains of L. monocytogenes, one each of the other six species of Listeria and nine other relevant bacterial species. A collection of individual clones from the penultimate panning was also screened against a subset of the panel of bacteria. The selectivity profiles indicate that multiple clones, including P4:A8, exhibit binding to one or more strains of L. monocytogenes without cross-reactivity toward any other species in the panel. This is the first report of a species-specific antibody for viable cells of L. monocytogenes (i.e., the ability to bind to L. monocytogenes without cross-reactivity toward any other species of Listeria).

Higher extracellular pH suppresses tracheary element differentiation by affecting auxin uptake

In an optimized liquid medium containing auxin and cytokinin, mesophyll cells isolated from Zinnia elegans L. seedlings can be induced to differentiate into tracheary elements (TEs) at high frequency. However, it is known that buffering the medium at neutral pH severely suppresses TE differentiation. In the process of modifying the medium, we found that excessive administration of auxin restored the suppression. Based on this finding, we physiologically characterized auxin actions involved in TE differentiation by focusing on the influence of extracellular pH. First, dose/response relationships between auxin [1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D)] concentrations and differentiated cell ratios were determined under various extracellular pH conditions. Secondly, intracellular concentrations of free forms and metabolites of auxin species were determined by analyzing extracts from cells cultured with radiolabeled NAA and 2,4-D under different extracellular pH conditions with liquid scintillation counting and thin-layer chromatography autoradiograms. Higher extracellular pH was found to reduce both the auxin potency for inducing TE differentiation and intracellular auxin accumulation. Reduction levels correlatively varied depending on the auxin species. These results suggest that the weakening in auxin potency at higher extracellular pH is ascribed to lower auxin uptake, which leads to decreased intracellular perception of the auxin signal. A model to predict auxin action that considers membrane transport, metabolism, and the perception of auxin is also presented.

Screening of carbohydrate-specific phage antibodies against recombinant human erythropoietin (rhuEPO) using a phage display antibody library: preliminary study

This paper is a preliminary report on development of a screening method for carbohydrate-specific phage antibodies against recombinant human erythropoietin (rHuEPO), using a phage display antibody library. rHuEPO was oxidized with sodium periodate or treated with 1,4-dithiothreitol and guanidine hydrochloride for detecting the specificity of obtained phage antibodies. Of 100 phage clones, three initially showed higher carbohydrate-related specificity. One of them (No. 62) bound specifically to the carbohydrate chains of rHuEPO, while the other two (Nos. 63 and 83) might recognize the steric conformation related to both the carbohydrate and the polypeptide chain of rHuEPO. These phage antibodies may serve as useful capture ligands for future development of efficient analytical methods for rHuEPO.

Class III homeodomain leucine-zipper proteins regulate xylem cell differentiation

Although it has been suggested that class III homeodomain leucine-zipper proteins (HD-Zip III) are involved in vascular development, details of the function of individual HD-Zip III proteins in vascular differentiation have not been resolved. To understand the function of each HD-Zip III protein in vascular differentiation precisely, we analyzed the in vitro transcriptional activity and in vivo function of Zinnia HD-Zip III genes, ZeHB-10, ZeHB-11 and ZeHB-12, which show xylem-related expression. Transgenic Arabidopsis plants harboring cauliflower mosaic virus 35S-driven ZeHB-10 and ZeHB-12 with a mutation in the START domain (mtZeHB-10, mtZeHB-12) showed a higher production of tracheary elements (TEs) and xylem precursor cells, respectively. A systematic analysis with Genechip arrays revealed that overexpression of mtZeHB-12 rapidly induced various genes, including brassinosteroid-signaling pathway-related genes and genes for transcription factors that are expressed specifically in vascular tissues in situ. Furthermore, mtZeHB-12 overexpression did not induce TE-specific genes, including genes related to programmed cell death and lignin polymerization, but did induce lignin monomer synthesis-related genes, which are expressed in xylem parenchyma cells. These results suggest that ZeHB-12 is involved in the differentiation of xylem parenchyma cells, but not of TEs.

Development of a quantitative cell-based intracellular ELISA for the screening of B cell hybridoma supernatants: a novel rapid assay to detect positive clones

The primary screening of hybridoma clones secreting monoclonal antibodies (MAbs) requires the testing of a large number of hybridoma culture supernatants within a short time and is very labor-intensive. In addition, the type of antigen and its location in the cell have to be considered when selecting the appropriate screening procedure, but relatively few reagents are available for analyzing these molecules. We have developed an intracellular and cell surface ELISA technique for screening hybridoma supernatants that hastens the screening procedure of a large number of clones in a short period of time, as the supernatants of fused cells grown in 96-well plates are used directly in the assay. This novel screening technique is rapid, sensitive, specific, and applicable to MAbs specific for a wide variety of intracellular and/or cell surface proteins.

The ATE genes are responsible for repression of transdifferentiation into xylem cells in Arabidopsis

We isolated three recessive mutants of Arabidopsis (Arabidopsis thaliana) showing ectopic expression of the xylem-specific marker, pAtxyn3::YFP. Genetic analysis indicated that the phenotypes were caused by mutations in three different genes, designated Abnormal Tracheary Element formation-related gene expression (ate1-3). The ate1 mutants showed a normal DR5::GUS gene expression pattern, and the ate1 mutation did not affect the abnormal vascular pattern formation in the van3 and pin1 mutants, indicating that the ate1 mutation does not affect the vascular pattern organization governed by auxin. The ate mutants showed ectopic lignin deposition, patterned secondary wall thickenings, and cell death, which are characteristic of mature tracheary elements (TEs) in cells ectopically expressing the pAtxyn3::YFP gene. Ectopic TE formation was rapidly induced in parenchymal tissue of the ate mutants in a TE-inducible system with excised hypocotyl. Furthermore, reverse transcription-polymerase chain reaction experiments showed that the expression of TE formation-related genes is up-regulated in the ate mutants. The ate1 mutation also caused ectopic expression of another xylem-specific marker gene, pAt3g62160::YFP. Overall, our results suggest that the ATE genes are responsible for the in situ repression of transdifferentiation into TEs in Arabidopsis and could be participants in the transdifferentiation-masking system.

Two azuki bean XTH genes, VaXTH1 and VaXTH2, with similar tissue-specific expression profiles, are differently regulated by auxin

To gain insight into the functional diversity of the XTH (xyloglucan endotransglucosylase/hydrolase) gene family, we analyzed the expression profiles of two azuki bean genes, VaXTH1 and VaXTH2, which share a striking resemblance in their amino acid sequences. The two XTH genes exhibit essentially similar tissue-specific expression profiles, in that both mRNAs are found predominantly in the phloem fibers of growing internodes. However, their expression profiles are not identical. Whereas VaXTH1 is expressed in xylem cells in the basal part of the internode, little or no expression of VaXTH2 is found in the xylem. Furthermore, they exhibit spatially divergent RNA distribution profiles along the internode, VaXTH1 being expressed nearer to the top of the internode than VaXTH2. This indicates their temporally divergent expression profiles during development of the phloem fiber. Indole-3-acetic acid (IAA) up-regulates both of the mRNA levels. However, this effect of IAA on the VaXTH1 gene is nullified in 0.25 M mannitol, which prevents cell expansion without affecting auxin action per se. In contrast, the IAA-induced up-regulation of the VaXTH2 gene is not affected by mannitol. Furthermore, fusicoccin, which promotes acidification and growth, up-regulates VaXTH1 expression, but not VaXTH2 expression. Thus, the two XTH genes are committed to different steps of the cell wall dynamics in the same cell type at different stages of phloem fiber development, and are regulated by IAA in different ways.

Phage display: practicalities and prospects

Phage display is a molecular technique by which foreign proteins are expressed at the surface of phage particles. Such phage thereby become vehicles for expression that not only carry within them the nucleotide sequence encoding expressed proteins, but also have the capacity to replicate. Using phage display vast numbers of variant nucleotide sequences may be converted into populations of variant peptides and proteins which may be screened for desired properties. It is now some seventeen years since the first demonstration of the feasibility of this technology and the intervening years have seen an explosion in its applications. This review discusses the major uses of phage display including its use for elucidating protein interactions, molecular evolution and for the production of recombinant antibodies.

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.

Calcium signaling through protein kinases. The Arabidopsis calcium-dependent protein kinase gene family

In plants, numerous Ca(2+)-stimulated protein kinase activities occur through calcium-dependent protein kinases (CDPKs). These novel calcium sensors are likely to be crucial mediators of responses to diverse endogenous and environmental cues. However, the precise biological function(s) of most CDPKs remains elusive. The Arabidopsis genome is predicted to encode 34 different CDPKs. In this Update, we analyze the Arabidopsis CDPK gene family and review the expression, regulation, and possible functions of plant CDPKs. By combining emerging cellular and genomic technologies with genetic and biochemical approaches, the characterization of Arabidopsis CDPKs provides a valuable opportunity to understand the plant calcium-signaling network.

Differential expression of cell-wall-related genes during the formation of tracheary elements in the Zinnia mesophyll cell system

Plants, animals and some fungi undergo processes of cell specialization such that specific groups of cells are adapted to carry out particular functions. One of the more remarkable examples of cellular development in higher plants is the formation of water-conducting cells that are capable of supporting a column of water from the roots to tens of metres in the air for some trees. The Zinnia mesophyll cell system is a remarkable tool with which to study this entire developmental pathway in vitro. We have recently applied an RNA fingerprinting technology, to allow the detection of DNA fragments derived from RNA using cDNA synthesis and subsequent PCR-amplified fragment length polymorphisms (cDNA-AFLP), to systematically characterize hundreds of the genes involved in the process of tracheary element formation. Building hoops of secondary wall material is the key structural event in forming functional tracheary elements and we have identified over 50 partial sequences related to cell walls out of 600 differentially expressed cDNA fragments. The Zinnia system is an engine of gene discovery which is allowing us to identify and characterize candidate genes involved in cell wall biosynthesis and assembly.

Inhibition of proteasome activity by the TED4 protein in extracellular space: a novel mechanism for protection of living cells from injury caused by dying cells

In maturation process of tracheary element (TE) differentiation, many hydrolases are activated to execute programmed cell death of TEs. Such hydrolases are released from maturing TEs into extracellular space. The release of hydrolases should be harmful to surrounding cells. The TED4 protein, a tentative plant non-specific lipid transfer protein that is expressed preferentially in TE-induced culture of zinnia (Zinnia elegans L.), is secreted into the apoplastic space prior to and associated with morphological changes of TEs. Our studies on the interrelationship between the TED4 protein and proteolytic activities using an in vitro TE differentiation system of zinnia revealed the following facts. (1) Active proteasome is released into medium at maturation stage of TE differentiation. (2) The TED4 protein forms a complex with proteasome in culture medium. (3) The TED4 protein inhibits proteasome activity in the medium and crude extracts of zinnia cells. (4) The depletion of the TED4 protein from culture medium results in an increase in mortality of other living cells. These results strongly suggest that the secreted TED4 protein acts as an inhibitor of proteasome to protect other cells from undesirable injury due to proteolytic activities exudated from dying TEs.

Xylogenesis: the birth of a corpse

Xylogenesis is a complex developmental process culminating in programmed cell death as a truly terminal differentiation event. In Arabidopsis, the availability of vascular-patterning mutants, and the identification of genes and their products that are involved in cell specification, secondary-wall deposition and lignification, are providing clues to the functions of some of the sequences in the large expressed sequence tag databases derived from the xylem-rich tissues of trees. An in vitro system, the Zinnia mesophyll cell system, provides an alternative system for those cell-biological experiments that are difficult to tackle in intact plants. In particular, a combination of molecular-genetic and cell-biological approaches has made possible the elucidation of some of the features of plant programmed cell death.

Autolysis during in vitro tracheary element differentiation: formation and location of the perforation

Tracheary elements differentiated from isolated Zinnia: mesophyll cells were observed at various times of culture under a scanning electron microscope. Perforation occurred on the primary wall at one of the longitudinal ends in single tracheary elements. In double tracheary elements, which both of two cells derived from a single cell differentiated into, the pore opened on the primary walls both at the junction of the two tracheary elements and at a longitudinal end of one of the two tracheary elements. These results suggest not only that a single tracheary element has its own program to form a perforation at one end without being affected by neighboring cells, but also that isolated cells indeed hold some traces of polarity and cell-cell communication.