Gene expression patterns associated with in vitro tracheary element formation in isolated single mesophyll cells of Zinnia elegans

Expression and functions of proteases in vascular tissues

With the emergence of new models for wood formation and the increasing emphasis on improving the efficiency of cellulosic biofuel production, research on vascular tissue biology has intensified in recent years. Some of the most active areas of research focus on manipulating activity of enzymes in the cellulose, hemicellulose, pectin and lignin pathways. In addition, great strides have been made in the characterization of transcriptional networks controlling genes that affect differentiation, secondary cell wall synthesis and programmed cell death in xylem. Less attention has been devoted to the characterization of proteases that may be important regulators of post-translational events that affect vascular cell differentiation and function and cell wall composition. Several genes for proteases and components of the ubiquitin/26S proteasome pathway are upregulated in xylem and phloem and in cell culture systems for studying the differentiation of xylem tracheary elements (TEs). Although small molecule protease inhibitors have been used to explore the roles of proteases during the differentiation of cultured TEs, only a small number of vascular tissue-associated protease genes have been directly tested to determine whether they play roles in vascular tissue biology. In this report, we review roles for proteases in vascular cell differentiation and function as determined through the use of protease inhibitors and genetic analyses and conclude by identifying opportunities for future research in this area.

Signaling and gene regulatory programs in plant vascular stem cells

A key question about the development of multicellular organisms is how they precisely control the complex pattern formation during their growth. For plants to grow for many years, a tight balance between pluripotent dividing cells and cells undergoing differentiation should be maintained within stem cell populations. In this process, cell-cell communication plays a central role by creating positional information for proper cell type patterning. Cell-type specific gene regulatory networks govern differentiation of cells into particular cell types. In this review, we will provide a comprehensive overview of emerging key signaling and regulatory programs in the stem cell population that direct morphogenesis of plant vascular tissues.

Novel markers of xylogenesis in zinnia are differentially regulated by auxin and cytokinin

The characterization of in vitro xylogenic cultures of zinnia (Zinnia elegans) has led to major discoveries in the understanding of xylem formation in plants. We have constructed and characterized a subtractive library from zinnia cultures enriched in genes that are specifically expressed at the onset of secondary wall deposition and tracheary element (TE) programmed cell death. This Late Xylogenesis Library (LXL) consisted of 236 nonredundant cDNAs, 77% of which encoded novel sequences in comparison with the 17,622 expressed sequence tag sequences publicly available. cDNA arrays were constructed to examine dynamic global gene expression during the course of TE formation. As a first step in dissecting auxin and cytokinin signaling during TE differentiation, macroarrays were probed with cDNAs from cells cultured in different hormonal conditions. Fifty-one percent of the LXL genes were induced by either auxin or cytokinin individually, the large majority by auxin. To determine the potential involvement of these categories of genes in TE differentiation, multiplex in situ-reverse transcription-PCR was performed on cells for two genes encoding putative cell wall proteins: Gibberellin stimulated transcript-1, induced by auxin alone, and expansin 5, induced by cytokinin alone. All transcriptionally active TEs expressed both genes, indicating that, although these genes may not be considered as specific markers for TE differentiation per se, they are nevertheless an integral part of TE differentiation program. Among the non-TE population, four different gene expression-based cell types could be distinguished. Together, these results demonstrate the underlying complexity of hormonal perception and the existence of several different cell types in in vitro TE cell cultures.

Spatial and Temporal Tracing of Vessel Differentiation in Young Arabidopsis Seedlings by the Expression of an Immature Tracheary Element-specific Promoter

The vascular system is a complex tissue composed of several vascular cell types. However, little is known about the differentiation process of each vascular cell in situ. In this study, we found that the expression of the Zinnia cysteine protease 4 (ZCP4) promoter is restricted to only immature tracheary elements (TEs) in situ. Therefore, we monitored the early TE differentiation process in young Arabidopsis seedlings using a fusion gene of the ZCP4 promoter and the beta-glucuronidase gene as a molecular marker. This approach revealed unique processes of vessel differentiation during early seedling development, in which discontinuous initiation of vessel element differentiation occurs at distinct regions, followed by the simultaneous differentiation of protoxylem vessels and bidirectional differentiation of metaxylem vessels to form a vessel in the plant body.

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.

Subcellular localization of expansin mRNA in xylem cells

Terminal differentiation of many vascular cells involves cell wall changes. Cells first elongate their primary wall, then lay down a lignified secondary wall, which is often followed by digestion of the primary wall. Expansins are wall proteins that regulate wall changes, but little is known about the specific functions of the many individual expansin isoforms. An in vitro cell culture of synchronously differentiating tracheary elements was used to identify three new expansins and to compare their expression kinetics with the timing of wall changes. The genes encoding these expansins from zinnia (Zinnia elegans), designated ZeExp1, ZeExp2, ZeExp3, are expressed during cell elongation. ZeExp1 and ZeExp2 mRNA decrease at the early stage of secondary wall formation, whereas ZeExp3 does not. In planta, all three ZeExp mRNAs are found predominantly in a single flank of cells adjacent to protoxylem and metaxylem vessels and in cells roughly at the radial position of the fasicular and interfasicular cambium. Furthermore, within these cells, Exp mRNA is localized exclusively either to the apical or basipetal end of cells depending on the expansin gene and organ, providing the first evidence for polar localization of mRNA in plant cells. ZeExp1 and ZeExp3 mRNA are localized at the apical tip, whereas ZeExp2 mRNA is found in the basal tip. These observations indicate that these three expansins are xylem cell specific and possibly involved in the intrusive growth of the primary walls of differentiating xylem cells.

Purification and characterization of macrodontain I, a cysteine peptidase from unripe fruits of Pseudananas macrodontes (Morr.) harms (Bromeliaceae)

A new papain-like cysteine peptidase isolated from fruits of Pseudananas macrodontes (Morr.) Harms, a species closely related to pineapple (Ananas comosus L.), has been purified and characterized. The enzyme, named macrodontain I, is the main proteolytic component present in fruit extracts and was purified by acetone fractionation followed by anion-exchange chromatography. Separation was improved by selecting both an adequate pH value and a narrow saline gradient. Optimum pH range (more than 90% of maximum activity with casein) was achieved at pH 6.1-8.5. Homogeneity of the enzyme was confirmed by bidimensional electrophoresis and mass spectroscopy (MS). Molecular mass of the enzyme was 23,459 (MS) and its isoelectric point was 6.1. The alanine, glutamine, and tyrosine derivatives were strongly preferred when the enzyme was assayed on N-alpha-CBZ-l-amino acid p-nitrophenyl esters. The N-terminal sequence of macrodontain (by comparison with the N-terminus of 30 plant proteases with more than 50% homology) showed a great deal of sequence similarity to the other pineapple-stem-derived cysteine endopeptidases, being 85.7, 85. 2, and 77.8% identical to comosain, stem bromelain, and ananain, respectively. It seems clear that the Bromeliaceae endopeptidases are more closely related to each other than to other members of the papain family, suggesting relatively recent divergence.

Isolation and characterization of a cysteine protease from the latex of Araujia hortorum fruits

A new protease (araujiain h I) was purified to mass spectroscopy homogeneity from the latex of Araujia hortorum Fourn. (Asclepiadaceae) fruits by ultracentrifugation and ion exchange chromatography. The enzyme has a molecular mass of 24,031 (mass spectrometry) and an iso-electric point higher than 9.3. The optimum pH range for casein hydrolysis was 8.0-9.5. The enzyme showed remarkable caseinolytic activity at high temperatures, although its thermal stability decayed rapidly. The proteinase was activated by thiol compounds and inhibited by common thiol-blocking reagents, particularly E-64 and HgCl2, suggesting the enzyme belongs to the cysteine protease family. The concentration of active sites as determined by titration with E-64 was 3.3 microM. When assayed on N-alpha-CBZ-amino acid-p-nitrophenyl esters, the enzyme showed higher preference for the glutamine derivative, followed by those of alanine, asparagine, glycine, and leucine, in decreasing order. Partial homology (36-48%) with other plant cysteine proteinases was observed in an internal fragment obtained by Protease V8 treatment.

Identification of BFN1, a bifunctional nuclease induced during leaf and stem senescence in Arabidopsis

Nuclease I enzymes are responsible for the degradation of RNA and single-stranded DNA during several plant growth and developmental processes, including senescence. However, in the case of senescence the corresponding genes have not been reported. We describe the identification and characterization of BFN1 of Arabidopsis, and demonstrate that it is a senescence-associated nuclease I gene. BFN1 nuclease shows high similarity to the sequence of a barley nuclease induced during germination and a zinnia (Zinnia elegans) nuclease induced during xylogenesis. In transgenic plants overexpressing the BFN1 cDNA, a nuclease activity of about 38 kD was detected on both RNase and DNase activity gels. Levels of BFN1 mRNA were extremely low or undetectable in roots, leaves, and stems. In contrast, relatively high BFN1 mRNA levels were detected in flowers and during leaf and stem senescence. BFN1 nuclease activity was also induced during leaf and stem senescence. The strong response of the BFN1 gene to senescence indicated that it would be an excellent tool with which to study the mechanisms of senescence induction, as well as the role of the BFN1 enzyme in senescence using reverse genetic approaches in Arabidopsis.

IFL1, a gene regulating interfascicular fiber differentiation in Arabidopsis, encodes a homeodomain-leucine zipper protein

Arabidopsis inflorescence stems develop extraxylary fibers at specific sites in interfascicular regions. The spatial specification of interfascicular fiber differentiation is regulated by the INTERFASCICULAR FIBERLESS1 (IFL1) gene because mutation of that gene abolishes the formation of normal interfascicular fibers in Arabidopsis stems. To understand further the role of IFL1 in the specification of fiber differentiation, we cloned the IFL1 gene by using a positional cloning strategy. Sequence analysis showed that the IFL1 gene encodes a transcription factor that has the same features as a family of homeodomain-leucine zipper (HD-ZIP) proteins found only in plants. The predicted IFL1 protein is composed of three distinct domains, including a 60-amino acid HD at the N terminus followed by a 28-amino acid ZIP motif and a 724-amino acid C-terminal region. A nuclear targeting assay showed that IFL1 is able to direct a beta-glucuronidase fusion protein into the nucleus, which is consistent with IFL1's presumed function as a transcription factor. Gene expression analysis demonstrated that the IFL1 gene is expressed in the interfascicular regions in which fibers differentiate, which is consistent with its role in the control of interfascicular fiber differentiation. Furthermore, the IFL1 gene was shown to be expressed in the vascular regions, indicating its possible role in the regulation of vascular tissue formation. This possibility is supported by the observation that differentiation of both xylary fibers and vessel elements is altered in the vascular bundles of ifl1 mutants. Our results provide direct evidence that an HD-ZIP protein plays a role in the spatial control of fiber differentiation.

Transformation of the collateral vascular bundles into amphivasal vascular bundles in an Arabidopsis mutant

Arabidopsis inflorescence stems develop a vascular pattern similar to that found in most dicots. The arrangement of vascular tissues within the bundle is collateral, and vascular bundles in the stele are arranged in a ring. Although auxin has been shown to be an inducer of vascular differentiation, little is known about the molecular mechanisms controlling vascular pattern formation. By screening ethyl methanesufonate-mutagenized populations of Arabidopsis, we have isolated an avb1 (amphivasal vascular bundle) mutant with a novel vascular pattern. Unlike the collateral vascular bundles seen in the wild-type stems, the vascular bundles in the avb1 stems were similar to amphivasal bundles, i.e. the xylem completely surrounded the phloem. Furthermore, branching vascular bundles in the avb1 stems abnormally penetrated into the pith, which resulted in a disruption in the ring-like arrangement of vascular bundles in the stele. The avb1 mutation did not affect leaf venation pattern and root vascular organization. Auxin polar transport assay indicated that the avb1 mutation did not disrupt the auxin polar transport activity in inflorescence stems. The avb1 mutation also exhibited pleiotropic phenotypes, including curled stems and extra cauline branches. Genetic analysis indicated that the avb1 mutation was monogenic and partially dominant. The avb1 locus was mapped to a region between markers mi69 and ASB2, which is covered by a yeast artificial chromosome clone, CIC9E2, on chromosome 5. Isolation of the avb1 mutant provides a novel means to study the evolutionary mechanisms controlling the arrangement of vascular tissues within the bundle, as well as the mechanisms controlling the arrangement of vascular bundles in the stele.

Proteasome inhibitors prevent tracheary element differentiation in zinnia mesophyll cell cultures

To determine whether proteasome activity is required for tracheary element (TE) differentiation, the proteasome inhibitors clasto-lactacystin beta-lactone and carbobenzoxy-leucinyl-leucinyl-leucinal (LLL) were used in a zinnia (Zinnia elegans) mesophyll cell culture system. The addition of proteasome inhibitors at the time of culture initiation prevented differentiation otherwise detectable at 96 h. Inhibition of the proteasome at 48 h, after cellular commitment to differentiation, did not alter the final percentage of TEs compared with controls. However, proteasome inhibition at 48 h delayed the differentiation process by approximately 24 h, as indicated by examination of both morphological markers and the expression of putative autolytic proteases. These results indicate that proteasome function is required both for induction of TE differentiation and for progression of the TE program in committed cells. Treatment at 48 h with LLL but not clasto-lactacystin beta-lactone resulted in partial uncoupling of autolysis from differentiation. Results from gel analysis of protease activity suggested that the observed incomplete autolysis was due to the ability of LLL to inhibit TE cysteine proteases.

A pectate lyase from Zinnia elegans is auxin inducible

The Zinnia mesophyll cell system consists of isolated leaf mesophyll cells in culture that can be induced, by auxin and cytokinin, to reproducibly trans-differentiate into tracheary elements (TE) after 96 h, while in the presence of auxin alone the cells simply elongate. In a search for genes involved in modifications to cell-wall architecture before any overt signs of cell differentiation, a differential hybridization of a 72-h cDNA library with probes from mRNA at time-points of 24 h and 72 h was done revealing a number of transcripts up-regulated between these times. One of these cDNAs shows homology to pectate lyase, a pectin-degrading enzyme. The complete cDNA sequence (ZePel) corresponds to a translated protein of 44 kDa with an N-terminal signal peptide of about 2 kDa, and one potential N-glycosylation site. Northern analysis confirms that the strong expression of this gene during TE induction occurs at a very early stage of the process and is due solely to the presence of auxin in the induction medium. In situ hybridization studies in young Zinnia stems show that ZePel expression is associated with vascular bundles and shoot primordia. Recombinant protein made in Escherichia coli possesses calcium-dependent pectate lyase activity. Pectate lyase activity is detected in elongating and differentiating in vitro cell populations. The role of this enzyme in remodelling the cell wall during cell elongation and differentiation is discussed.

Association of caffeoyl coenzyme A 3-O-methyltransferase expression with lignifying tissues in several dicot plants

Caffeoyl coenzyme A 3-O-methyltransferase (CCoAOMT) was previously shown to be associated with lignification in both in vitro tracheary elements (TEs) and organs of zinnia (Zinnia elegans). However, it is not known whether this is a general pattern in dicot plants. To address this question, polyclonal antibodies against zinnia recombinant CCoAOMT fusion protein were raiseed and used for immunolocalization in several dicot plants. The antibodies predominantly recognized a protein band with a molecular mass of 28 kD on western analysis of tissue extracts from zinnia, forsythia (Forsythia suspensa), tobacco (Nicotiana tabacum), alfalfa (Medicago sativa), and soybean (Glycine max). Western analyses showed that the accumulation of CCoAOMT protein was closely correlated with lignification in in vitro TEs of zinnia. Immunolocalization results showed that CCoAOMT was localized in developing TEs of young zinnia stems and in TEs, xylem fibers, and phloem fibers of old stems. CCoAOMT was also found to be specifically associated with all lignifying tissues, including TEs, xylem fibers, and phloem fibers in stems of forsythia, tobacco, alfalfa, soybean, and tomato (Lycopersicon esculentum). The presence of CCoAOMT was evident in xylem ray parenchyma cells of forsythia, tobacco, and tomato. In forsythia and alfalfa, pith parenchyma cells next to the vascular cylinder were lignified. Accordingly, marked accumulation of CCoAOMT in these cells was observed. Taken together, these results showed a close association of CCoAOMT expression with lignification in dicot plants. This supports the hypothesis that the CCoAOMT-mediated methylation branch is a general one in lignin biosynthesis during normal growth and development in dicot plants.

Proteinase Activity during Tracheary Element Differentiation in Zinnia Mesophyll Cultures

The zinnia (Zinnia elegans) mesophyll cell culture tracheary element (TE) system was used to study proteinases active during developmentally programmed cell death. Substrate-impregnated gels and single-cell assays revealed high levels of proteinase activity in differentiating TEs compared with undifferentiated cultured cells and expanding leaves. Three proteinases (145, 28, and 24 kD) were exclusive to differentiating TEs. A fourth proteinase (59 kD), although detected in extracts from all tissues examined, was most active in differentiating TEs. The 28- and 24-kD proteinases were inhibited by thiol proteinase inhibitors, leupeptin, and N-[N-(L-3-trans-carboxirane-2-carbonyl)-L-leucyl]-agmatine (E-64). The 145- and 59-kD proteinases were inhibited by the serine proteinase inhibitor phenylmethylsulfonyl fluoride (PMSF). Extracts from the TE cultures contained sodium dodecyl sulfate-stimulated proteolytic activity not detected in control cultures. Sodium dodecyl sulfate-stimulated proteolysis was inhibited by leupeptin or E-64, but not by PMSF. Other tissues, sucrose-starved cells and cotyledons, that contain high levels of proteolytic activity did not contain TE-specific proteinases, but did contain higher levels of E-64-sensitive activities migrating as 36- to 31-kD enzymes and as a PMSF-sensitive 66-kD proteinase.

Proteolysis in plants: mechanisms and functions

Proteolysis is essential for many aspects of plant physiology and development. It is responsible for cellular housekeeping and the stress response by removing abnormal/misfolded proteins, for supplying amino acids needed to make new proteins, for assisting in the maturation of zymogens and peptide hormones by limited cleavages, for controlling metabolism, homeosis, and development by reducing the abundance of key enzymes and regulatory proteins, and for the programmed cell death of specific plant organs or cells. It also has potential biotechnological ramifications in attempts to improve crop plants by modifying protein levels. Accumulating evidence indicates that protein degradation in plants is a complex process involving a multitude of proteolytic pathways with each cellular compartment likely to have one or more. Many of these have homologous pathways in bacteria and animals. Examples include the chloroplast ClpAP protease, vacuolar cathepsins, the KEX2-like proteases of the secretory system, and the ubiquitin/26S proteasome system in the nucleus and cytoplasm. The ubiquitin-dependent pathway requires that proteins targeted for degradation become conjugated with chains of multiple ubiquitins; these chains then serve as recognition signals for selective degradation by the 26S proteasome, a 1.5 MDa multisubunit protease complex. The ubiquitin pathway is particularly important for developmental regulation by selectively removing various cell-cycle effectors, transcription factors, and cell receptors such as phytochrome A. From insights into this and other proteolytic pathways, the use of phosphorylation/dephosphorylation and/or the addition of amino acid tags to selectively mark proteins for degradation have become recurring themes.

XYLOGENESIS: INITIATION, PROGRESSION, AND CELL DEATH

Xylem cells develop from procambial or cambial initials in situ, and they can also be induced from parenchyma cells by wound stress and/or a combination of phytohormones in vitro. Recent molecular and biochemical studies have identified some of the genes and proteins involved in xylem differentiation, which have led to an understanding of xylem differentiation based on comparisons of events in situ and in vitro. As a result, differentiation into tracheary elements (TEs) has been divided into two processes. The "early" process involves the origination and development of procambial initials in situ. In vitro, the early process of transdifferentiation involves the dedifferentiation of cells and subsequent differentiation of dedifferentiated cells into TE precursor cells. The "late" process, observed both in situ and in vitro, involves a variety of events specific to TE formation, most of which have been observed in association with secondary wall thickenings and programmed cell death. In this review, I summarize these events, including coordinated expression of genes that are involved in secondary wall formation.

LIPID-TRANSFER PROTEINS IN PLANTS

Lipid-transfer proteins (LTP) are basic, 9-kDa proteins present in high amounts (as much as 4% of the total soluble proteinss) in higher plants. LTPs can enhance the in vitro transfer of phospholipids between membranes and can bind acyl chains. On the basis of these properties, LTPs were thought to participate in membrane biogenesis and regulation of the intracellular fatty acid pools. However, the isolation of several cDNAs and genes revealed the presence of a signal peptide indicating that LTPs could enter the secretory pathway. They were found to be secreted and located in the cell wall. Thus, novel roles were suggested for plant LTPs: participation in cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. The validity of these suggestions needs to be determined, in the hope that they will elucidate the role of this puzzling family of plant proteins.

Molecular genetics of cellular differentiation in leaves

Leaves of green plants vary widely in morphology. However, the underlying cell types and structures observed in leaves of different species are remarkably similar. Although we can adequately describe leaf development in morphological terms we cannot yet explain interactions at the cellular level. In recent years molecular genetics has been used extensively to address a variety of developmental questions. The isolation of a wide variety of mutants disrupted in numerous aspects of leaf ontogeny has led to the cloning of genes involved in various developmental processes. In this review we consider advances that have been made in understanding shoot apical meristem organization, leaf initiation and the development of leaf form. In particular we concentrate on progress, that has been made in understanding cellular differentiation in the epidermis, and within the interior of the leaf, namely the photosynthetic cells and the vasculature. CONTENTS Summary 533 I. Introduction 533 II. Shoot growth 533 III. Leaf initiation 534 IV. Development of leaf form 536 V. Cellular differentiation 537 VI. Perspectives 548 VII. Acknowledgements 549 VIII. References 549.

Induction of cysteine and serine proteases during xylogenesis in Zinnia elegans

The terminal process of xylogenesis, autolysis, is essential for the formulation of a tubular system for conduction of water and solutes throughout the whole plant. Several hydrolase types are implicated in autolysis responsible for the breakdown of cytoplasm. Here, we characterize p48h-17 cDNA from in vitro tracheary elements (TEs) of Zinnia elegans which encodes a preproprotein similar to papain. The putative mature protein, a cysteine protease, has a molecular mass of 22,699 Da with a pI of 5.7. DNA gel blot analysis indicated that p48h-17 is likely encoded by one or two genes. The p48h-17 mRNA accumulated markedly in in vitro differentiating TEs, whereas it appeared not to be induced in response to senescence and wounding in the leaves or H2O2 challenge in the cultured mesophyll cells. In stems, the expression of the p48h-17 gene was preferentially associated with differentiating xylem. Activity gel assays demonstrated that a cysteine and a serine protease, which had apparent molecular masses of 20 kDa and 60 kDa, respectively, were markedly induced during in vitro TE differentiation. The cysteine protease activity was also preferentially present in the xylem of Zinnia stems. Transient expression of the p48h-17 cDNA in tobacco protoplasts resulted in the production of a 20 kDa cysteine protease. Taken together, the results indicate that the p48h-17 gene appears to be preferentially associated with xylogenesis, and both the cysteine and serine proteases might be involved in autolysis during xylogenesis.

Isolation and characterization of cDNAs encoding xylogenesis-associated and wounding-induced ribonucleases in Zinnia elegans

The study of plant ribonuclease (RNase) functions is complicated by a complex profile of RNase activities detected in tissues. Thus, isolation of individual RNase genes will be desirable for the further understanding of function of each RNase. Here, we describe the isolation of cDNAs encoding two RNases, ZRNaseI and ZRNaseII, in differentiating tracheary elements (TEs) induced from isolated mesophyll cells of Zinnia elegans. Both the ZRNaseI and ZRNaseII exhibit putative secretion signal sequences at the amino-terminal ends with predicted molecular masses of 24 247 Da and 22 448 Da as mature proteins, respectively. DNA gel blot analysis showed that both RNases in Zinnia appear to be encoded by a small gene family. RNA gel blot analysis showed that the expression of the ZRNaseI gene was associated with the late stage of in vitro TE differentiation, whereas the ZRNaseII gene was mainly induced in response to stress. Neither RNase gene was induced in response to phosphate starvation, or to H2O2 challenge in the cultured mesophyll cells, or to senescence in the leaves. In young leaves, the ZRNaseI gene was not induced in response to wounding. But the ZRNaseII gene was markedly induced by 6 h after wounding. Tissue print hybridization showed that the expression of the ZRNaseI gene was preferentially associated with the differentiation TEs in Zinnia stems, while the ZRNaseII mRNA was not detected in unwounded Zinnia organs. Taken together, the results indicated that the ZRNaseI gene is expressed during the process of xylogenesis both in vitro and in the plant, whereas the ZRNaseII gene is predominantly induced in response to wounding. The identification of these RNase genes provides molecular tools for the dissection of the process of autolysis during xylogenesis, and for the dissection of the role of RNase in wounding response.

Secondary cell-wall-specific glycoprotein(s) from French bean hypocotyls

Specific labeling of secondary cell walls of tracheary elements and of xylary and phloem fibers has been observed when wheat germ agglutinin (WGA) and anti-WGA antibodies were used during ultrastructural studies of French bean (Phaseolus vulgaris L.) hypocotyls. In this report we demonstrate that at least part of this labeling is due to the presence of secondary cell-wall-specific glycoproteins. Three major novel glycoproteins with relative molecular weights of 55,000, 86,000, and 90,000, purified by means of WGA-Sepharose affinity chromatography, have been characterized. Their amino acid composition indicates that they are not the members of known classes of structural cell-wall proteins, since they contain no hydroxyproline, a lower level of glycine than seen in glycine-rich proteins, and very little proline. N-terminal sequences of all three proteins show no significant homology with other proteins. Antibodies were raised against electrophoretically pure 90-kD glycoprotein. These were used to localize this protein in secondary cell walls of xylem tracheary elements and in xylary and phloem fibers, i.e. in the same compartments where labeling with WGA has been observed. To our knowledge this is one of the first biochemical and ultrastructural demonstrations of secondary cell-wall-specific glycoproteins.

Differential expression of two O-methyltransferases in lignin biosynthesis in Zinnia elegans

Caffeic acid 3-O-methyltransferase (CAOMT) and caffeoyl-coenzyme A 3-O-methyltransferase (CCoAOMT) are involved in different methylation pathways in lignin biosynthesis. We previously showed that only the CCoAOMT was markedly induced during lignification in in vitro differentiating tracheary elements (TEs) of Zinnia elegans. To further examine the expression patterns of CAOMT in lignification, we isolated a cDNA clone for Zinnia CAOMT. RNA gel blot analysis showed that the expression of the CAOMT gene did not correlate well with lignification during in vitro TE differentiation from Zinnia-isolated mesophyll cells. Tissue-print hybridization showed that, in the young internodes, the CAOMT mRNA signal was much more evident in phloem fibers than in xylem, whereas the CCoAOMT mRNA signal was predominantly present in differentiating xylem regions. In the older internode, both the CAOMT and CCoAOMT mRNAs markedly accumulated in phloem fibers and differentiating xylem regions. Immunocytochemical localization showed that the CAOMT protein staining was much more evident in phloem fibers and xylem fibers than in xylem TEs. These results indicate that the expression of these two O-methyltransferases is differentially regulated during lignification in different cell types in Zinnia. We suggest that all the intermediates in the CAOMT-mediated methylation pathway might become substrates for the CCoAOMT-mediated methylation pathway after coenzyme A ligation when these two pathways occur in the same lignifying cell types.

Biochemistry and molecular biology of lignification

Lignins, which result from the dehydrogenative polymerization of cinnamyl alcohols, are complex heteropolymers deposited in the walls of specific cells of higher plants. Lignins have probably been associated to land colonization by plants but several aspects concerning their biosynthesis, structure and function are still only partially understood. This review focuses on the modern physicochemical methods of structural analysis of lignins, and on the new approaches of molecular biology and genetic engineering applied to lignification. The principles, advantages and limitations of three important analytical tools for studying lignin structure are presented. They include carbon 13 nuclear magnetic resonance, analytical pyrolysis and thioacidolysis. The use of these methods is illustrated by several examples concerning the characterization of grass lignins,'lignin-like'materials in protection barriers of plants and lignins produced by cell suspension cultures. Our present limited knowledge of the spatio temporal deposition of lignins during cell wall differentiation including the nature of the wall components associated to lignin deposition and of the cross-links between the different wall polymers is briefly reviewed. Emphasis is placed on the phenylpropanoid pathway enzymes and their corresponding genes which are described in relation to their potential roles in the quantitative and qualitative control of lignification. Recent findings concerning the promoter sequence elements responsible for the vascular expression of some of these genes are presented. A section is devoted to the enzymes specifically involved in the synthesis of monolignols: cinnamoyl CoA reductase and cinnamyl alcohol dehydrogenase. The recent characterization of the corresponding cDNAs/genes offers new possibilities for a better understanding of the regulation of lignification. Finally, at the level of the synthesis, the potential involvement of peroxidases and laccases in the polymerization of monolignols is critically discussed. In addition to previously characterized naturally occurring lignin mutants, induced lignin mutants have been obtained during the last years through genetic engineering. Some examples include plants transformed by O-methyltransferase and cinnamyl alcohol dehydrogenase antisense constructs which exhibit modified lignins. Such strategies offer promising perspectives in gaining a better understanding of lignin metabolism and functions and represent a realistic way to improve plant biomass. Contents Summary 203 I. Introduction 204 II. Main structural features of lignins 205 III. Lignification and cell wall differentiation: spatio-temporal deposition of lignins and inter-relations with other wall components 213 IV. Enzymes and genes involved in the biosynthesis and polymerization of monolignols 216 V. Lignin mutants as a way to improve plant biomass and to explore lignin biochemistry and metabolism 226 VI. Concluding remarks 229 Acknowledgements 230 References 230.

An alternative methylation pathway in lignin biosynthesis in Zinnia

S-Adenosyl-L-methionine:trans-caffeoyl-coenzyme A 3-O-methyltransferase (CCoAOMT) is implicated in disease resistant response, but whether it is involved in lignin biosynthesis is not known. We isolated a cDNA clone for CCoAOMT in differentiating tracheary elements (TEs) induced from Zinnia-isolated mesophyll cells. RNA gel blot analysis showed that the expression of the CCoAOMT gene was markedly induced during TE differentiation from the isolated mesophyll cells. Tissue print hybridization showed that the expression of the CCoAOMT gene is temporally and spatially regulated and that it is associated with lignification in xylem and in phloem fibers in Zinnia organs. Both CCoAOMT and caffeic acid O-methyltransferase (COMT) activities increased when the isolated Zinnia mesophyll cells were cultured, whereas only CCoAOMT activity was markedly enhanced during lignification in the in vitro-differentiating TEs. The induction pattern of the OMT activity using 5-hydroxyferuloyl CoA as substrate during lignification was the same as that using caffeoyl CoA. Taken together, the results indicate that CCoAOMT is associated with lignification during xylogenesis both in vitro and in the plant, whereas COMT is only involved in a stress response in vitro. We propose that CCoAOMT is involved in an alternative methylation pathway in lignin biosynthesis. In Zinnia in vitro-differentiating TEs, the CCoAOMT mediated methylation pathway is dominant.

Expression of an auxin- and cytokinin-regulated gene in cambial region in Zinnia

The expression patterns of a cDNA clone, p48h-10, of an auxin-induced gene were examined in isolated mesophyll cells of Zinnia and in the organs of Zinnia plants. In the isolated mesophyll cells, the mRNA accumulates in 48 hr of culture with 1-naphthaleneacetic acid alone. Because the first cell division occurs before 36 hr of culture, the gene probably is not involved in cell division. Benzyladenine does not induce expression of this gene, but the combination of 1-naphthaleneacetic acid and benzyladenine induces the mRNA accumulation about 24 hr earlier than does 1-naphthaleneacetic acid alone. Tissue print hybridization shows that the mRNA is present predominantly in the cambial region in stems, leaves, and roots and in the vascular bundles in flower buds but does not occur in the apical regions of shoot or root. The characteristics of the gene expression, including auxin- and cytokinin-regulated induction and cambial region localization, encourage us to suggest that the gene is involved in the early process of vascular differentiation.

Novel vascular cell-specific genes whose expression is regulated temporally and spatially during vascular system development

We have isolated three cDNA clones (TED2, TED3, and TED4) for genes expressed preferentially in cells that redifferentiate into tracheary elements from mesophyll cells isolated from leaves of Zinnia elegans. Sequence analyses of TED clones revealed that TED2 encodes a hydrophobic polypeptide with a significant similarity to the guinea pig lens-specific protein (zeta-crystallin) and that the deduced polypeptide of TED3 may be a novel cell wall protein. In situ hybridization of the TED probes with young Zinnia seedlings showed that expression of the three TED genes was restricted to vascular cells and regulated in a temporal and spatial manner during vascular development. TED3 transcripts were localized specifically to a few cells that are to differentiate or are differentiating into tracheary elements in all organs examined. TED4 transcripts were present mainly in the immature primary xylem both of cotyledons and of the boundary region between the root and hypocotyl and in the procambium of roots. In contrast, TED2 transcripts accumulated not only in immature primary xylem cells but also in immature phloem cells both in roots and in the boundary region between the root and hypocotyl. In addition, TED2 transcripts were expressed in the procambium cells of roots. In cotyledons, TED2 transcripts did not accumulate in xylem or phloem cells but only in two regions that might form a new vein just outside the phloem of the main leaf vein. Taken together, our findings indicate that TED2, TED3, and TED4 can be novel and efficient markers for development of the vascular system.