Molecular cloning, nucleotide sequence, and abscisic acid induction of a suberization-associated highly anionic peroxidase

A new versatile peroxidase with extremophilic traits over-produced in MicroTom cell cultures

Peroxidases are widespread key antioxidant enzymes that catalyse the oxidation of electron donor substrates in parallel with the decomposition of H2O2. In this work, a novel tomato peroxidase, named SAAP2, was isolated from MicroTom cell cultures, purified, and characterised. The enzyme was identified with 64% sequence coverage as the leprx21 gene product (suberization-associated anionic peroxidase 2-like) from Solanum lycopersicum, 334 amino acids long. Compared to other plant peroxidases, SAAP2 was more active at elevated temperatures, with the optimal temperature and pH at 90 °C and 5.0, respectively. Furthermore, the enzyme retained more than 80% of its maximal activity over the range of 70-80 °C and the presence of NaCl (1.0-4.5 M). It also exhibited broad pH versatility (65% relative activity over the pH range 2.0-7.0), acid-tolerance (80% residual activity after 22 h at pH 2.0-7.0), high thermostability (50% residual activity after 2 h at 80 °C) and proteolytic resistance. SAAP2 exhibited exceptional resistance under thermo-acidic conditions compared to the horseradish peroxidase benchmark, suggesting that it may find potential applications as a supplement or anti-pollution agent in the food industry.

Nitric oxide is involved in hydrogen sulfide-induced adventitious rooting in tomato (Solanum lycopersicum)

Nitric oxide (NO) and hydrogen sulfide (H2 S) are signalling molecules that regulate adventitious rooting in plants. However, little is known about the cross-talk between NO and H2 S during adventitious rooting. Tomato (Solanum lycopersicum L.) explants were used to investigate the roles of and relationships between NO and H2 S during rooting. Effects of the NO donor sodium nitroprusside (SNP) and the H2 S donor sodium hydrosulfide (NaHS) on adventitious rooting were dose-dependent, and the greatest biological responses were observed under 25μM SNP and 50μM NaHS. The positive effect of NaHS was reversed by the NO scavenger 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), indicating that the H2 S-induced response was partially NO-dependent. Peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD) activities significantly increased by SNP and NaHS treatment, and indoleacetic acid oxidase (IAAO) activity and the O2 - and H2 O2 content significantly decreased by SNP and NaHS treatment. SNP and NaHS treatment also increased the content of soluble sugar and protein and indole-3-acetic acid (IAA). cPTIO significantly mitigated the increases in POD, PPO and SOD activity and soluble sugar, protein and IAA content induced by NaHS. SNP and NaHS upregulated the expression of auxin-related genes (ARF4 and ARF16 ), cell cycle-related genes (CYCD3 , CYCA3 and CDKA1 ), and antioxidant-related genes (TPX2 , SOD and POD ); whereas cPTIO significantly inhibited the increase in the expression of these genes induced by NaHS. Overall, these results show that NO may be involved in H2 S-induced adventitious rooting by regulating the activity of rooting-related enzymes, the expression of related genes, and the content of various nutrients.

Host susceptibility factors render ripe tomato fruit vulnerable to fungal disease despite active immune responses

The increased susceptibility of ripe fruit to fungal pathogens poses a substantial threat to crop production and marketability. Here, we coupled transcriptomic analyses with mutant studies to uncover critical processes associated with defense and susceptibility in tomato (Solanum lycopersicum) fruit. Using unripe and ripe fruit inoculated with three fungal pathogens, we identified common pathogen responses reliant on chitinases, WRKY transcription factors, and reactive oxygen species detoxification. We established that the magnitude and diversity of defense responses do not significantly impact the interaction outcome, as susceptible ripe fruit mounted a strong immune response to pathogen infection. Then, to distinguish features of ripening that may be responsible for susceptibility, we utilized non-ripening tomato mutants that displayed different susceptibility patterns to fungal infection. Based on transcriptional and hormone profiling, susceptible tomato genotypes had losses in the maintenance of cellular redox homeostasis, while jasmonic acid accumulation and signaling coincided with defense activation in resistant fruit. We identified and validated a susceptibility factor, pectate lyase (PL). CRISPR-based knockouts of PL, but not polygalacturonase (PG2a), reduced susceptibility of ripe fruit by >50%. This study suggests that targeting specific genes that promote susceptibility is a viable strategy to improve the resistance of tomato fruit against fungal disease.

A member of wheat class III peroxidase gene family, TaPRX-2A, enhanced the tolerance of salt stress

BACKGROUND

Salt and drought are the main abiotic stresses that restrict the yield of crops. Peroxidases (PRXs) are involved in various abiotic stress responses. Furthermore, only few wheat PRXs have been characterized in the mechanism of the abiotic stress response.

RESULTS

In this study, a novel wheat peroxidase (PRX) gene named TaPRX-2A, a member of wheat class III PRX gene family, was cloned and its response to salt stress was characterized. Based on the identification and evolutionary analysis of class III PRXs in 12 plants, we proposed an evolutionary model for TaPRX-2A, suggesting that occurrence of some exon fusion events during evolution. We also detected the positive selection of PRX domain in 13 PRXs involving our evolutionary model, and found 2 or 6 positively selected sites during TaPRX-2A evolution. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) results showed that TaPRX-2A exhibited relatively higher expression levels in root tissue than those exhibited in leaf and stem tissues. TaPRX-2A expression was also induced by abiotic stresses and hormone treatments such as polyethylene glycol 6000, NaCl, hydrogen peroxide (H2O2), salicylic acid (SA), methyljasmonic acid (MeJA) and abscisic acid (ABA). Transgenic wheat plants with overexpression of TaPRX-2A showed higher tolerance to salt stress than wild-type (WT) plants. Confocal microscopy revealed that TaPRX-2A-eGFP was mainly localized in cell nuclei. Survival rate, relative water content, and shoot length were higher in TaPRX-2A-overexpressing wheat than in the WT wheat, whereas root length was not significantly different. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were enhanced in TaPRX-2A-overexpressing wheat compared with those in the WT wheat, resulting in the reduction of reactive oxygen species (ROS) accumulation and malondialdehyde (MDA) content. The expression levels of downstream stress-related genes showed that RD22, TLP4, ABAI, GST22, FeSOD, and CAT exhibited higher expressions in TaPRX-2A-overexpressing wheat than in WT under salt stress.

CONCLUSIONS

The results show that TaPRX-2A plays a positive role in the response to salt stress by scavenging ROS and regulating stress-related genes.

Silencing against the conserved NAC domain of the potato StNAC103 reveals new NAC candidates to repress the suberin associated waxes in phellem

Both suberin and its associated waxes contribute to the formation of apoplastic barriers that protect plants from the environment. Some transcription factors have emerged as regulators of the suberization process. The potato StNAC103 gene was reported as a repressor of suberin polyester and suberin-associated waxes deposition because its RNAi-mediated downregulation (StNAC103-RNAi) over-accumulated suberin and associated waxes in the tuber phellem concomitantly with the induction of representative biosynthetic genes. Here, to explore if other genes of the large NAC gene family participate to this repressive function, we extended the silencing to other NAC members by targeting the conserved NAC domain of StNAC103 (StNAC103-RNAi-c). Transcript profile of the StNAC103-RNAi-c phellem indicated that StNAC101 gene was an additional potential target. In comparison with StNAC103-RNAi, the silencing with StNAC103-RNAi-c construct resulted in a similar effect in suberin but yielded an increased load of associated waxes in tuber phellem, mainly alkanes and feruloyl esters. Globally, the chemical effects in both silenced lines are supported by the transcript accumulation profile of genes involved in the biosynthesis, transport and regulation of apoplastic lipids. In contrast, the genes of polyamine biosynthesis were downregulated. Altogether these results point out to StNAC101 as a candidate to repress the suberin-associated waxes.

Roles of apoplastic peroxidases in plant response to wounding

Apoplastic class III peroxidases (EC 1.11.1.7) play key roles in the response of plants to pathogen infection and abiotic stresses, including wounding. Wounding is a common stress for plants that can be caused by insect or animal grazing or trampling, or result from agricultural practices. Typically, mechanical damage to a plant immediately induces a rapid release and activation of apoplastic peroxidases, and an oxidative burst of reactive oxygen species (ROS), followed by the upregulation of peroxidase genes. We discuss how plants control the expression of peroxidases genes upon wounding, and also the sparse information on peroxidase-mediated signal transduction pathways. Evidence reviewed here suggests that in many plants production of the ROS that comprise the initial oxidative burst results from a complex interplay of peroxidases with other apoplastic enzymes. Later responses following wounding include various forms of tissue healing, for example through peroxidase-dependent suberinization, or cell death. Limited data suggest that ROS-mediated death signalling during the wound response may involve the peroxidase network, together with other redox molecules. In conclusion, the ability of peroxidases to both generate and scavenge ROS plays a key role in the involvement of these enigmatic enzymes in plant stress tolerance.

Molecular characterization of fruit-specific class III peroxidase genes in tomato (Solanum lycopersicum)

In this study, expression of four peroxidase genes, LePrx09, LePrx17, LePrx35 and LePrxA, was identified in immature tomato fruits, and the function in the regulation of fruit growth was characterized. Analysis of amino acid sequences revealed that these genes code for class III peroxidases, containing B, D and F conserved domains, which bind heme groups, and a buried salt bridge motif. LePrx35 and LePrxA were identified as novel peroxidase genes in Solanum lycopersicum (L.). The temporal expression patterns at various fruit growth stages revealed that LePrx35 and LePrxA were expressed only in immature green (IMG) fruits, whereas LePrx17 and LePrx09 were expressed in both immature and mature green fruits. Tissue-specific expression profiles indicated that only LePrx09 was expressed in the mesocarp but not the inner tissue of immature fruits. The effects of hormone treatments and stresses on the four genes were examined; only the expression levels of LePrx17 and LePrx09 were altered. Transcription of LePrx17 was up-regulated by jasmonic acid (JA) and pathogen infection and expression of LePrx09 was induced by ethephon, salicylic acid (SA) and JA, in particular, as well as wounding, pathogen infection and H2O2 stress. Tomato plants over-expressing LePrx09 displayed enhanced resistance to H2O2 stress, suggesting that LePrx09 may participate in the H2O2 signaling pathway to regulate fruit growth and disease resistance in tomato fruits.

In planta anthocyanin degradation by a vacuolar class III peroxidase in Brunfelsia calycina flowers

In contrast to detailed knowledge regarding the biosynthesis of anthocyanins, the largest group of plant pigments, little is known about their in planta degradation. It has been suggested that anthocyanin degradation is enzymatically controlled and induced when beneficial to the plant. Here we investigated the enzymatic process in Brunfelsia calycina flowers, as they changed color from purple to white. We characterized the enzymatic process by which B. calycina protein extracts degrade anthocyanins. A candidate peroxidase was partially purified and characterized and its intracellular localization was determined. The transcript sequence of this peroxidase was fully identified. A basic peroxidase, BcPrx01, is responsible for the in planta degradation of anthocyanins in B. calycina flowers. BcPrx01 has the ability to degrade complex anthocyanins, it co-localizes with these pigments in the vacuoles of petals, and both the mRNA and protein levels of BcPrx01 are greatly induced parallel to the degradation of anthocyanins. Both isoelectric focusing (IEF) gel analysis and 3D structure prediction indicated that BcPrx01 is cationic. Identification of BcPrx01 is a significant breakthrough both in the understanding of anthocyanin catabolism in plants and in the field of peroxidases, where such a consistent relationship between expression levels, in planta subcellular localization and activity has seldom been demonstrated.

The potato suberin feruloyl transferase FHT which accumulates in the phellogen is induced by wounding and regulated by abscisic and salicylic acids

The present study provides new insights on the role of the potato (Solanum tuberosum) suberin feruloyl transferase FHT in native and wound tissues, leading to conclusions about hitherto unknown properties of the phellogen. In agreement with the enzymatic role of FHT, it is shown that its transcriptional activation and protein accumulation are specific to tissues that undergo suberization such as the root boundary layers of the exodermis and the endodermis, along with the tuber periderm. Remarkably, FHT expression and protein accumulation within the periderm is restricted to the phellogen derivative cells with phellem identity. FHT levels in the periderm are at their peak near harvest during periderm maturation, with the phellogen becoming meristematically inactive and declining thereafter. However, periderm FHT levels remain high for several months after harvest, suggesting that the inactive phellogen retains the capacity to synthesize ferulate esters. Tissue wounding induces FHT expression and the protein accumulates from the first stages of the healing process onwards. FHT is up-regulated by abscisic acid and down-regulated by salicylic acid, emphasizing the complex regulation of suberin synthesis and wound healing. These findings open up new prospects important for the clarification of the suberization process and yield important information with regard to the skin quality of potatoes.

Suspension cell culture as a tool for the characterization of class III peroxidases in sugarcane

Secreted class III peroxidases (EC 1.11.1.7) are implicated in a broad range of physiological processes throughout the plant life cycle. However, the unambiguous determination of the precise biological role of an individual class III peroxidase isoenzyme is still a difficult task due to genetic redundancy and broad substrate specificity in vitro. In addition, many difficulties are encountered during extraction and analysis of cell wall proteins. Since class III peroxidases are also secreted into the apoplast, the use of suspension cell cultures can facilitate isolation and functional characterization of individual isoforms. Here, we report on the characterization of class III peroxidases secreted in the spent medium of sugarcane suspension cell cultures. After treatment with specific inducers of cell wall lignification, peroxidases were isolated and activities assayed with guaiacol, syringaldazine and coniferyl alcohol. Enzymatic activity was not significantly different after treatments, regardless of the substrate, with the exception of methyl-jasmonate treatment, which led to a decreased guaiacol peroxidase activity. Remarkably, peroxidases isolated from the medium were capable of oxidizing syringaldazine, an analog to sinapyl alcohol, suggesting that sugarcane cultures can produce peroxidases putatively correlated to lignification. A proteomic approach using activity staining of 2-DE gels revealed a complex isoperoxidase profile, composed predominantly of cationic isoforms. Individual spots were excised and analyzed by LC-ESI-Q-TOF and homology-based search against the Sugarcane EST Database resulted in the identification of several proteins. Spatio-temporal expression pattern of selected genes was determined for validation of identified class III peroxidases that were preferentially expressed during sugarcane stem development.

Fruit-specific overexpression of wound-induced tap1 under E8 promoter in tomato confers resistance to fungal pathogens at ripening stage

Based on high economic importance and nutritious value of tomato fruits and as previous studies employed E8 promoter in fruit ripening-specific gene expression, we have developed transgenic tomato plants overexpressing tomato anionic peroxidase cDNA (tap1) under E8 promoter. Stable transgene integration was confirmed by polymerase chain reaction (PCR) and Southern analysis for nptII. Northern blotting confirmed elevated tap1 levels in the breaker- and red-ripe stages of T(1) transgenic fruits, whereas wild-type (WT) plants did not show tap1 expression in these developmental stages. Further, tap1 expression levels were significantly enhanced in response to wounding in breaker- and red-ripe stages of transgenic fruits, whereas wound-induced expression of tap1 was not detected in WT fruits. Confocal microscopy revealed high accumulation of phenolic compounds at the wound site in transgenic fruits suggesting a role of tap1 in wound-induced phenolic polymerization. Total peroxidase activity has increased remarkably in transgenic pericarp tissues in response to wounding, while very less or minimal levels were recorded in WT pericarp tissues. Transgenic fruits also displayed reduced post-harvest decay and increased resistance toward Alternaria alternata and Fusarium solani infection with noticeable inhibition in lesion formation. Conidiospore germination and mycelial growth of F. solani were severely inhibited when treated with E8-tap1 fruit extracts compared to WT fruits. 3-(4,5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay showed reduced spore viability when incubated in E8-tap1 fruit extracts. Thus, fruit-specific expression of tap1 using E8 promoter is associated with enhanced total peroxidase activity and high phenolic accumulation in fruits with minimized post-harvest deterioration caused by wounding and fungal attack in tomato fruits.

A potato skin SSH library yields new candidate genes for suberin biosynthesis and periderm formation

Potato (Solanum tuberosum) tubers are underground storage organs covered by the skin or periderm, a suberized layer that protects inner flesh from dehydration and pathogens. Understanding the molecular processes associated with periderm formation is of great importance for a better knowledge of this protective tissue and for improving the storage life of tubers. Here, to isolate new candidate genes for potato periderm, a suppression subtractive hybridization library from potato skin was performed. This library yielded a comprehensive list of 108 candidate genes that were manually sorted in functional categories according to the main cellular and metabolic processes in periderm. As expected, the list contains Suberin and wax genes, including some genes with a demonstrated role in the biosynthesis of these cell wall aliphatic compounds. Moreover, Regulation and Stress and defence genes are highly abundant in the library in general agreement with previous potato skin proteomic studies. The putative function of the genes in periderm is discussed.

Molecular characterization of the sweet potato peroxidase SWPA4 promoter which responds to abiotic stresses and pathogen infection

Previously, the swpa4 peroxidase gene has been shown to be inducible by a variety of abiotic stresses and pathogenic infections in sweet potato (Ipomoea batatas). To elucidate its regulatory mechanism at the transcriptional level under various stress conditions, we isolated and characterized the promoter region (2374 bp) of swpa4 (referred to as SWPA4). We performed a transient expression assay in tobacco protoplasts with deletions from the 5'-end of SWPA4 promoter fused to the beta-glucuronidase (GUS) reporter gene. The -1408 and -374 bp deletions relative to the transcription start site (+1) showed 8 and 4.5 times higher GUS expression than the cauliflower mosaic virus 35S promoter, respectively. In addition, transgenic tobacco plants expressing GUS under the control of -2374, -1408 or -374 bp region of SWPA4 promoter were generated and studied in various tissues under abiotic stresses and pathogen infection. Gel mobility shift assays revealed that nuclear proteins from sweet potato cultured cells specifically interacted with 60-bp fragment (-178/-118) in -374 bp promoter region. In silico analysis indicated that four kinds of cis-acting regulatory sequences, reactive oxygen species-related element activator protein 1 (AP1), CCAAT/enhancer-binding protein alpha element, ethylene-responsive element (ERE) and heat-shock element, are present in the -60 bp region (-178/-118), suggesting that the -60 bp region might be associated with stress inducibility of the SWPA4 promoter.

Potato skin proteome is enriched with plant defence components

Periderm is a tissue of secondary origin that replaces damaged epidermis. It can be found in underground plant organs, as an above-ground tissue of woody species (cork), and as a wound-healing tissue. Its outer layers are composed of phellem cells with suberized walls that constitute a protective barrier, preventing pathogen invasion and fluid loss. In potato, a model for periderm studies, periderm tissue replaces the epidermis early in tuber development and the suberized phellems constitute the tuber's skin. To identify factors involved in phellem/skin development and that play a role in its defensive characteristics, two-dimensional gel electrophoresis was used to compare the skin and parenchymatic flesh proteomes of young developing tubers. Proteins exhibiting differentially high signal intensity in the skin were sorted by functional categories. As expected, the differential skin proteome was enriched in proteins whose activity is characteristic of actively dividing tissues such as cell proliferation, C(1) metabolism, and the oxidative respiratory chain. Interestingly, the major functional category consisted of proteins (63%) involved in plant defence responses to biotic and abiotic stresses. This group included three isozymes of caffeoyl-CoA O-methyltransferase and five isozymes of peroxidase that may play a role in suberization processes. The differential expression of these proteins in the skin was further verified by RT-PCR of their corresponding transcripts in skin and tuber flesh samples. The results presented here shed light on the early events in skin development and further expand the concept of the periderm as a protective tissue containing an array of plant defence components.

A genomic approach to suberin biosynthesis and cork differentiation

Cork (phellem) is a multilayered dead tissue protecting plant mature stems and roots and plant healing tissues from water loss and injuries. Cork cells are made impervious by the deposition of suberin onto cell walls. Although suberin deposition and cork formation are essential for survival of land plants, molecular studies have rarely been conducted on this tissue. Here, we address this question by combining suppression subtractive hybridization together with cDNA microarrays, using as a model the external bark of the cork tree (Quercus suber), from which bottle cork is obtained. A suppression subtractive hybridization library from cork tree bark was prepared containing 236 independent sequences; 69% showed significant homology to database sequences and they corresponded to 135 unique genes. Out of these genes, 43.5% were classified as the main pathways needed for cork biosynthesis. Furthermore, 19% could be related to regulatory functions. To identify genes more specifically required for suberin biosynthesis, cork expressed sequence tags were printed on a microarray and subsequently used to compare cork (phellem) to a non-suberin-producing tissue such as wood (xylem). Based on the results, a list of candidate genes relevant for cork was obtained. This list includes genes for the synthesis, transport, and polymerization of suberin monomers such as components of the fatty acid elongase complexes, ATP-binding cassette transporters, and acyltransferases, among others. Moreover, a number of regulatory genes induced in cork have been identified, including MYB, No-Apical-Meristem, and WRKY transcription factors with putative functions in meristem identity and cork differentiation.

Structural motifs of syringyl peroxidases predate not only the gymnosperm-angiosperm divergence but also the radiation of tracheophytes

* The most distinctive variation in the monomer composition of lignins in vascular land plants is that found between the two main groups of seed plants. Thus, while gymnosperm lignins are typically composed of guaiacyl (G) units, angiosperm lignins are largely composed of similar levels of G and syringyl (S) units. * However, and contrary to what might be expected, peroxidases isolated from basal (Cycadales and Ginkgoales) and differentially evolved (Coniferales and Gnetales) gymnosperms are also able to oxidize S moieties, and this ability is independent of the presence or absence of S-type units in their lignins. * The results obtained led us to look at the protein database to search for homologies between gymnosperm peroxidases and true eudicot S-peroxidases, such as the Zinnia elegans peroxidase. * The findings showed that certain structural motifs characteristic of eudicot S-peroxidases (certain amino acid sequences and beta-sheet secondary structures) predate the gymnosperm-angiosperm divergence and the radiation of tracheophytes, since they are found not only in peroxidases from basal gymnosperms, ferns and lycopods, but also in peroxidases from the moss Physcomitrella patens (Bryopsida) and the liverwort Marchantia polymorpha (Marchantiopsida), which, as typical of bryophytes, do not have xylem tissue nor lignins.

Analysis of sugar chain-binding specificity of tomato lectin using lectin blot: recognition of high mannose-type N-glycans produced by plants and yeast

The sugar chain-binding specificity of tomato lectin (LEA) against glycoproteins was investigated qualitatively using lectin blot analysis. Glycoproteins containing tri- and tetra-antennary complex-type N-glycans were stained with LEA. Unexpectedly, glycoproteins containing high mannose-type N-glycans and a horseradish peroxidase were stained with LEA. LEA blot analysis of the glycoproteins accompanied by treatment with exoglycosidase revealed that the binding site of LEA for the complex-type N-glycans was the N-acetyllactosaminyl side chains, whereas the proximal chitobiose core appeared to be the binding site of LEA for high mannose-type N-glycans. Despite these results, the glycoproteins did not inhibit the hemagglutinating activity of LEA. Among the chitin-binding lectins compared, potato tuber lectin showed specificity similar to LEA on lectin blot analysis, while Datura stramonium lectin and wheat germ agglutinin (WGA) did not interact with glycoproteins containing high mannose-type N-glycans, except that RNase B was stained by WGA. Based on these observations, LEA blot analysis was applied to sugar chain analysis of tomato glycoproteins. The most abundant LEA-reactive glycoprotein was purified from the exocarp of ripe tomato fruits, and was identified as the tomato anionic peroxidase1 (TAP1). These results suggest that LEA interacts with glycoproteins produced by tomatoes, which participate in biological activities in tomato plants.

Molecular cloning and tissue-specific transcriptional regulation of the first peroxidase family member, Udp1, in stinging nettle (Urtica dioica)

A full-length cDNA clone, designated Udp1, was isolated from Urtica dioica (stinging nettle), using a polymerase chain reaction based strategy. The putative Udp1 protein is characterized by a cleavable N-terminal signal sequence, likely responsible for the rough endoplasmic reticulum entry and a 310 amino acids mature protein, containing all the important residues, which are evolutionary conserved among different members of the plant peroxidase family. A unique structural feature of the Udp1 peroxidase is defined into the short carboxyl-terminal extension, which could be associated with the vacuolar targeting process. Udp1 peroxidase is differentially regulated at the transcriptional level and is specifically expressed in the roots. Interestingly, wounding and ultraviolet radiation stress cause an ectopic induction of the Udp1 gene expression in the aerial parts of the plant. A genomic DNA fragment encoding the Udp1 peroxidase was also cloned and fully sequenced, revealing a structural organization of three exons and two introns. The phylogenetic relationships of the Udp1 protein to the Arabidopsis thaliana peroxidase family members were also examined and, in combination with the homology modelling approach, dictated the presence of distinct structural elements, which could be specifically involved in the determination of substrate recognition and subcellular localization of the Udp1 peroxidase.

Profiling of wheat class III peroxidase genes derived from powdery mildew-attacked epidermis reveals distinct sequence-associated expression patterns

A cDNA library was constructed from leaf epidermis of diploid wheat (Triticum monococcum) infected with the powdery mildew fungus (Blumeria graminis f. sp. tritici) and was screened for genes encoding peroxidases. From 2,500 expressed sequence tags (ESTs), 36 cDNAs representing 10 peroxidase genes (designated TmPRX1 to TmPRX10) were isolated and further characterized. Alignment of the deduced amino acid sequences and phylogenetic clustering with peroxidases from other plant species demonstrated that these peroxidases fall into four distinct groups. Differential expression and tissue-specific localization among the members were observed during the B. graminis f. sp. tritici attack using Northern blots and reverse-transcriptase polymerase chain reaction analyses. Consistent with its abundance in the EST collection, TmPRX1 expression showed the highest induction during pathogen attack and fluctuated in response to the fungal parasitic stages. TmPRX1 to TmPRX6 were expressed predominantly in mesophyll cells, whereas TmPRX7 to TmPRX10, which feature a putative C-terminal propeptide, were detectable mainly in epidermal cells. Using TmPRX8 as a representative, we demonstrated that its C-terminal propeptide was sufficient to target a green fluorescent protein fusion protein to the vacuoles in onion cells. Finally, differential expression profiles of the TmPRXs after abiotic stresses and signal molecule treatments were used to dissect the potential role of these peroxidases in multiple stress and defense pathways.

Peroxidase activity associated with suberization processes of the muskmelon (Cucumis melo) rind

The rind of fruits of muskmelon (Cucumis melo L. var. reticulatus) contains a network of suberized tissue referred to as the 'netting', and peroxidase (EC 1.11.1.7) activity is necessary to the polymerization of the aromatic domain of suberin. Peroxidase activity increased dramatically during the early stages of melon fruit netting, and in fruits exhibiting incomplete netting, peroxidase activity was significantly higher in netted than in non-netted regions of the same fruit. Moreover, analysis of peroxidase activity in three varieties of smooth-rind melons (Cucumis melo var. inodorous) indicated lower levels of the activity in rind samples, taken throughout fruit development, than in rinds of netted varieties. Netting-associated anionic peroxidase (NAPOD) was isolated from the melon rind at an early stage of netting development, partially purified, microsequenced and its cDNA was cloned. It was found to be a single-copy gene within the genome of netted and smooth melon varieties, and highly homologous to other Cucurbitaceous anionic peroxidases. A high transcript level was only detected in the rind of the netted variety. Monitoring the gene expression of netting-associated anionic peroxidase, together with other enzymes involved in the netting will shed light on the molecular control of the suberization processes in the melon rind and in plants in general.

Identification of suberization-associated anionic peroxidase as a possible allergenic protein from tomato

A 45 kDa protein, which is recognized by IgE antibodies in sera of food-allergic patients, was purified and characterized as an allergenic protein from the tomato. The IgE-binding protein purified from tomato extract was found to be a glycoprotein with a molecular weight of approximately 45,000, an isoelectric point of 4.2, and no free N-terminal amino group. Furthermore, it was shown that the purified protein had peroxidase activity. From the amino acid sequence of a peptide fragment prepared by lysylendopeptidase digestion, the allergenic protein was identified to be the tomato suberization-associated anionic peroxidase 1 known as one of the pathogenesis-related proteins widely distributed in plants. These properties suggested the protein isolated from tomato to be a new allergenic protein in plant foodstuffs.

Plant defense genes associated with quantitative resistance to potato late blight in Solanum phureja x dihaploid S. tuberosum hybrids

Markers corresponding to 27 plant defense genes were tested for linkage disequilibrium with quantitative resistance to late blight in a diploid potato population that had been used for mapping quantitative trait loci (QTLs) for late blight resistance. Markers were detected by using (i) hybridization probes for plant defense genes, (ii) primer pairs amplifying conserved domains of resistance (R) genes, (iii) primers for defense genes and genes encoding transcriptional regulatory factors, and (iv) primers allowing amplification of sequences flanking plant defense genes by the ligation-mediated polymerase chain reaction. Markers were initially screened by using the most resistant and susceptible individuals of the population, and those markers showing different allele frequencies between the two groups were mapped. Among the 308 segregating bands detected, 24 loci (8%) corresponding to six defense gene families were associated with resistance at chi2 > or = 13, the threshold established using the permutation test at P = 0.05. Loci corresponding to genes related to the phenylpropanoid pathway (phenylalanine ammonium lyase [PAL], chalcone isomerase [CHI], and chalcone synthase [CHS]), loci related to WRKY regulatory genes, and other -defense genes (osmotin and a Phytophthora infestans-induced cytochrome P450) were significantly associated with quantitative disease resistance. A subset of markers was tested on the mapping population of 94 individuals. Ten defense-related markers were clustered at a QTL on chromosome III, and three defense-related markers were located at a broad QTL on chromosome XII. The association of candidate genes with QTLs is a step toward understanding the molecular basis of quantitative resistance to an important plant disease.

A wound-inducible tobacco peroxidase gene expresses preferentially in the vascular system

A tobacco peroxidase gene tpoxN1 was reported to be expressed within 1 h after wounding in leaves [Hiraga et al. (2000a) Plant Cell Physiol. 41: 165]. We describe here further results on the wound-induced tpoxN1 expression. The quick tpoxN1 induction occurred preferentially in stems and petioles, but was negligible in leaf blades even 8 h after wounding. Induced GUS activity was also detected rapidly after wounding in the stem of transgenic tobacco plants carrying the tpoxN1 promoter::GUS fusion gene, localized mainly in the vascular systems where it was maintained this level for 14 d or more. Strong GUS activity was also found in the petiole and veinlet as well as the epidermal tissue in the stem. Treatment of known inducers for wound-responsive genes such as jasmonate, 1-aminocyclopropane-1-carboxylate, spermine, phytohormones and other stress treatments did not enhance wound-induced tpoxN1 gene expression in stems at all, but rather repressed it in some cases. Studies using metabolic inhibitors suggested that phosphorylation and dephosphorylation of proteins together with de novo protein synthesis are likely to be involved in the wound-induced tpoxN1 expression as well as some other wound-responsive genes. Thus, tpoxN1 is a unique wound-inducible and possible wound-healing gene which is rapidly expressed being maintained for a long time in veins via an unknown wound-signaling pathway(s).

A basic peroxidase from wheat kernel with antifungal activity

A basic heme-peroxidase (WP1) was purified to homogeneity from wheat (Triticum aestivum) kernels. The protein was not glycosylated and exhibited a molecular mass of 36 kDa and a pI of 8.0. The N-terminal amino acid sequence revealed a very high similarity with a wheat flour peroxidase allergen associated with baker's asthma. WPI showed indole-3-acetic acid oxidase activity in the presence of Mn2+ and phenolic cofactors. Antifungal assays performed in vitro towards phytopathogenic fungi indicated that WP1 was active in inhibiting germ tube elongation. This first report on antifungal properties of a heme-peroxidase gives experimental support to the idea that peroxidases play a defensive role against invading pathogens.

A large family of class III plant peroxidases

Class III plant peroxidase (POX), a plant-specific oxidoreductase, is one of the many types of peroxidases that are widely distributed in animals, plants and microorganisms. POXs exist as isoenzymes in individual plant species, and each isoenzyme has variable amino acid sequences and shows diverse expression profiles, suggesting their involvement in various physiological processes. Indeed, studies have provided evidence that POXs participate in lignification, suberization, auxin catabolism, wound healing and defense against pathogen infection. Little, however, is known about the signal transduction for inducing expression of the pox genes. Recent studies have provided information on the regulatory mechanisms of wound- and pathogen-induced expression of some pox genes. These studies suggest that pox genes are induced via different signal transduction pathways from those of other known defense-related genes.

Structure of soybean seed coat peroxidase: a plant peroxidase with unusual stability and haem-apoprotein interactions

Soybean seed coat peroxidase (SBP) is a peroxidase with extraordinary stability and catalytic properties. It belongs to the family of class III plant peroxidases that can oxidize a wide variety of organic and inorganic substrates using hydrogen peroxide. Because the plant enzyme is a heterogeneous glycoprotein, SBP was produced recombinant in Escherichia coli for the present crystallographic study. The three-dimensional structure of SBP shows a bound tris(hydroxymethyl)aminomethane molecule (TRIS). This TRIS molecule has hydrogen bonds to active site residues corresponding to the residues that interact with the small phenolic substrate ferulic acid in the horseradish peroxidase C (HRPC):ferulic acid complex. TRIS is positioned in what has been described as a secondary substrate-binding site in HRPC, and the structure of the SBP:TRIS complex indicates that this secondary substrate-binding site could be of functional importance. SBP has one of the most solvent accessible delta-meso haem edge (the site of electron transfer from reducing substrates to the enzymatic intermediates compound I and II) so far described for a plant peroxidase and structural alignment suggests that the volume of Ile74 is a factor that influences the solvent accessibility of this important site. A contact between haem C8 vinyl and the sulphur atom of Met37 is observed in the SBP structure. This interaction might affect the stability of the haem group by stabilisation/delocalisation of the porphyrin pi-cation of compound I.

Xylem-specific expression of wound-inducible rice peroxidase genes in transgenic plants

A peroxidase gene, poxA, was isolated from a rice (Oryza sativa L.) genomic library. The gene consists of four exons whose combined sequences were identical to that of the prxRPA mRNA whose levels were dramatically stimulated by wounding as well as by treatment of rice shoots with ethephon or UV irradiation [H. Ito, F. Kimizuka, A. Ohbayashi, H. Matsui, M. Honma, A. Shinmyo, Y. Ohashi, A.B. Caplan, R.L. Rodriguez, Molecular cloning and characterization of two complementary DNAs encoding putative peroxidases from rice (Oryza sativa L.) shoots, Plant Cell Rep. 13 (1994) 361-366]. The temporal and spatial expression properties of the poxA gene promoter as well as that from a second related peroxidase gene, poxN, were analyzed in transgenic tobacco and rice plants using the uidA gene as a reporter. In transgenic tobacco, UV- and wound-responsive cis-elements were located within 144 bp from the translational start codon of the poxA gene. The poxN promoter, however, was inactive in the heterologous host as no significant GUS activity was evident. On the other hand, chimeric uidA genes containing 2.2 kb of the poxA promoter or 1.4 kb of poxN promoter were active in transgenic rice plants. Both peroxidase promoters directed GUS activities in a spatial and tissue specific manner coincident with the expression patterns exhibited by their mRNAs. Histochemical analysis of transgenic rice plants showed that both peroxidase genes are expressed in the vascular bundles of the shoot apex and lamina joint, and in xylem-parenchyma cells of the leaf blade and sheath.

An HR-induced tobacco peroxidase gene is responsive to spermine, but not to salicylate, methyl jasmonate, and ethephon

In Tobacco mosaic virus (TMV)-infected tobacco plants carrying the N resistance gene, a hypersensitive reaction or response (HR) occurs to enclose the virus in the infected tissue. Although a contribution of peroxidases to the resistance has been proposed, no evidence has been presented that tobacco peroxidase genes respond to HR. Here, we describe the HR-induced expression of a tobacco peroxidase gene (tpoxC1) whose induction kinetics were slightly different from those of acidic and basic tobacco pathogenesis-related (PR) protein genes. Interestingly, tpoxC1 was insensitive to the inducers of PR genes such as salicylic acid, methyl jasmonate, and ethephon. Spermine activated tpoxC1 gene expression at a low level and both acidic and basic PR gene expression at a considerably higher level. These results indicate that the induced expression of tpoxC1 is regulated differently from that of classical tobacco PR genes in the N gene-mediated self-defense system in tobacco plants.

Novel bacterial peroxidase without catalase activity from Flavobacterium meningosepticum: purification and characterization

A novel bacterial peroxidase co-produced intracellularly with H(2)O(2)-forming nucleoside oxidase, was purified from the cell-free extract of Flavobacterium meningosepticum to homogeneity with 10.3% overall recovery through simple purification procedures including successive DEAE-Sephacel, phenyl-Sepharose CL-4B and Sephacryl S-300 chromatography. The relative molecular mass of the native enzyme was 220¿ omitted¿000 Da, and that of its subunit was 54¿ omitted¿000 Da. In contrast to other major intercellular peroxidases of bacterial origin, the enzyme did not show any catalase activity. The amino acid sequences of the 92 NH(2)-terminal amino acids and three internal peptides showed no significant homology with known peroxidases. The enzyme was not sensitive to the typical peroxidase inhibitors NaCN, NaF and NaN(3), while mercuric ion strongly inhibited the enzyme activity, and some carbonyl reagents were also found to have inhibitory effects. The enzyme showed a small K(m) value for H(2)O(2) (9.5 microM) compared to other peroxidases. On the basis of its visible absorption spectrum, the enzyme contained about 1.3 mol of heme per molecule.

Biochemical characterization of the suberization-associated anionic peroxidase of potato

The anionic peroxidase associated with the suberization response in potato (Solanum tuberosum L.) tubers during wound healing has been purified and partially characterized at the biochemical level. It is a 45-kD, class III (plant secretory) peroxidase that is localized to suberizing tissues and shows a preference for feruloyl (o-methoxyphenol)-substituted substrates (order of substrate preference: feruloyl > caffeoyl > p-coumaryl approximately syringyl) such as those that accumulate in tubers during wound healing. There was little influence on oxidation by side chain derivatization, although hydroxycinnamates were preferred over the corresponding hydroxycinnamyl alcohols. The substrate specificity pattern is consistent with the natural substrate incorporation into potato wound suberin. In contrast, the cationic peroxidase(s) induced in response to wound healing in potato tubers is present in both suberizing and nonsuberizing tissues and does not discriminate between hydroxycinnamates and hydroxycinnamyl alcohols. A synthetic polymer prepared using E-[8-(13)C]ferulic acid, H(2)O(2), and the purified anionic enzyme contained a significant amount of cross-linking through C-8, albeit with retention of unsaturation.

Computational analyses and annotations of the Arabidopsis peroxidase gene family

Classical heme-containing plant peroxidases have been ascribed a wide variety of functional roles related to development, defense, lignification, and hormonal signaling. More than 40 peroxidase genes are now known in Arabidopsis thaliana for which functional association is complicated by a general lack of peroxidase substrate specificity. Computational analysis was performed on 30 near full-length Arabidopsis peroxidase cDNAs for annotation of start codons and signal peptide cleavage sites. A compositional analysis revealed that 23 of the 30 peroxidase cDNAs have 5' untranslated regions containing 40-71% adenine, a rare feature observed also in cDNAs which predominantly encode stress-induced proteins, and which may indicate translational regulation.

The macromolecular aromatic domain in suberized tissue: a changing paradigm

As a structural feature of specialized cell walls, suberization remains an enigma, despite its obvious importance both during normal growth and development and as a stress response in plants. While it is clear that suberized tissues contain both polyaromatic and polyaliphatic domains, and that each of these has its own unique characteristics, whether there is a contiguous macromolecule that can be called suberin is an open question. From a structural perspective, the aromatic domain is unique and distinct from lignin, and is apparently comprised primarily of (poly)hydroxycinnamates, such as amides (e.g., feruloyltyramine). The aliphatic domain is also unique, being quite distinct from cutin in terms of both its chemical composition and cellular location. In the present paper, histochemical, structural and biochemical data, particularly, regarding the polyaromatic domain of suberized tissues, are critically reviewed. A revised description of the polyaromatic domain of suberized tissues, based on the consensus that is emerging from the current data, is presented and especially includes a spatially distinct (poly)hydroxycinnamoyl-containing macromolecule.

Cloning and biochemical characterization of an anionic peroxidase from Zea mays

We have isolated, cloned and characterized a cDNA from Zea mays L., denoted ZmAP1, coding for an anionic peroxidase. The open reading frame of ZmAP1 starting 72 residues from the 5' end of the cDNA predicts a 37,778 dalton protein of 356 amino acid residues. The protein has high similarity to other peroxidases and contains two peroxidase motifs that carry two highly conserved histidines in the active center. We expressed recombinant ZmAP1 protein in E. coli as a fusion with maltose-binding protein. The fusion protein was biochemically active after addition of hemin to the apoprotein. The maize peroxidase ZmAP1 has a pH optimum at pH 4.0 and a Km of 0.2 mM for the substrate 2,2'-azino-bis-(3-ethyl-benzothiazolin-6-sulfonic acid) at this pH. In maize seedlings the ZmAP1 gene is expressed predominantly in roots, the mesocotyl, the coleoptile and to a lower extent in the node, whereas no expression in the primary leaf was found. In situ hybridization shows that the expression of ZmAP1 in the young maize root is confined to the epidermis, hypodermis and the pericycle.

Characterization of acquired resistance in lesion-mimic transgenic potato expressing bacterio-opsin

The lesion-mimic mutants of certain plants display necrotic lesions resembling those of the hypersensitive response and activate local and systemic defense responses in the absence of pathogens. We have engineered a lesion-mimic phenotype in transgenic Russet Burbank potato plants through constitutive expression of a bacterio-opsin (bO) proton pump derived from Halobacterium halobium. Transgenic potato plants exhibiting a lesion-mimic phenotype had increased levels of salicylic acid and overexpressed several pathogenesis-related messenger RNAs, all hallmarks of systemic acquired resistance (SAR). The lesion-mimic plants also displayed enhanced resistance to the US1 isolate (A1 mating type) of a fungal pathogen, Phytophthora infestans, a causal agent of late blight disease. In contrast, little resistance was observed against the US8 isolate (A2 mating type) of this pathogen. Furthermore, a majority of the transgenic plants displaying the lesion-mimic phenotype had increased susceptibility to potato virus X. The tubers of these plants were not resistant to the bacterial pathogen Erwinia carotovora. These results indicate that expression of bO can result in the activation of defense responses in transgenic potato plants and show for the first time that bO expression can confer resistance to a pathogenic fungus. However, our results also demonstrate that like SAR, this "engineered" resistance is likely to be limited to certain pathogens and particular cultivars.

Expression of a highly basic peroxidase gene in NaCl-adapted tomato cell suspensions

A tomato peroxidase gene, TPX2, that is only weakly expressed in the roots of young tomato seedlings is highly expressed in tomato suspension cells adapted to high external NaCl concentration. The protein encoded by this gene, with an isolectric point value of approximately 9.6, is found in the culture medium of the growing cells. Our data suggest that the expression of TPX2 in the salt-adapted cells is not the result of the elicitation imposed by the in vitro culture or the presence of high NaCl concentration in the medium.

Molecular cloning and expression analysis of peroxidase genes from wheat

A PCR-based screening approach was used to isolate genomic clones from wheat encoding peroxidase isozymes. Three complete genes (pox1, pox2 and pox4) and one truncated gene (pox3) were characterized. The nucleotide sequences predicted mature proteins of 31 kDa, in which all the highly conserved motifs of secreted plant peroxidases were preserved. The coding regions showed 73-83% DNA sequence identity, with the highest level of similarity noted for the tandemly oriented pox2 and pox3. Expression of respective pox genes in various tissues of wheat was assessed by the RT-PCR technique, which showed that all four genes are active. The primary pox1 mRNA was spliced to remove three introns, whereas processing of the other pox transcripts involved only two intervening sequences. Splicing occurred at consensus GU/AG splice sites except for the first introns of pox1, pox2 and pox4 transcripts, where processing took place at unusual GC donor sites. The RNA analysis suggested that the pox1, pox2 and pox4 genes are predominantly expressed in roots. Lower levels of expression were found for pox4 and pox3 in leaves. Infection of wheat by the powdery mildew fungus selectively induced expression of pox2 in leaves.

Molecular cloning of two tandemly arranged peroxidase genes from Populus kitakamiensis and their differential regulation in the stem

A genomic library was prepared from Populus kitakamiensis and screened with the cDNA for an anionic peroxidase from P. kitakamiensis. One genomic clone was isolated that contained two tandemly oriented genes for anionic peroxidases, prxA3a and prxA4a. Both genes consisted of four exons and three introns; the introns had consensus nucleotides, namely, GT and AG, at their 5' and 3' ends, respectively. The prxA3a and prxA4a genes encoded 347 and 343 amino acid residues, respectively, including putative signal sequences at the amino-termini. Putative promoters and polyadenylation signals were found in the flanking regions of both genes. The sequence of the coding region of prxA3a was completely identical to that of the cDNA clone pA3, whereas the sequence of the coding region of prxA4a was only 73% identical to that of the cDNA clone pA3. Northern blot analysis showed that the patterns of expression of the mRNAs that corresponded to prxA3a and prxA4a differed in stems of P. kitakamiensis.

Two pathogen-responsive genes in parsley encode a tyrosine-rich hydroxyproline-rich glycoprotein (hrgp) and an anionic peroxidase

Two recently isolated cDNAs representing genes that are transcriptionally activated in fungus-infected parsley leaves or elicitor-treated, cultured parsley cells are shown to encode a hydroxyproline-rich glycoprotein (HRGP) and an anionic peroxidase. The deduced HRGP protein is rich in tyrosine residues, a feature also found in other pathogen- and wound-induced plant HRGPs. Expression of the peroxidase gene(s) is induced rapidly upon elicitation and precedes that of the HRGP gene. In situ hybridization experiments demonstrate the presence of HRGP and peroxidase mRNAs in parsley tissue around fungal infection sites. Peroxidase mRNA accumulation is particularly sharply restricted to plant cells directly adjacent to fungal hyphae. These results provide further evidence for an important role of specific cell wall modifications in plant defense.

Signals involved in wound-induced proteinase inhibitor II gene expression in tomato and potato plants

Chemical and physical signals have been reported to mediate wound-induced proteinase inhibitor II (Pin2) gene expression in tomato and potato plants. Among the chemical signals, phytohormones such as abscisic acid (ABA) and jasmonic acid (JA) and the peptide systemin represent the best characterized systems. Furthermore, electrical and hydraulic mechanisms have also been postulated as putative Pin2-inducing systemic signals. Most of the chemical agents are able to induce Pin2 gene expression without any mechanical wounding. Thus, ABA, JA, and systemin initiate Pin2 mRNA accumulation in the directly treated leaves and in the nontreated leaves (systemic) that are located distal to the treated ones. ABA-deficient tomato and potato plants do not respond to wounding by accumulation of Pin2 mRNA, therefore providing a suitable model system for analysis of the signal transduction pathway involved in wound-induced gene activation. It was demonstrated that the site of action of JA is located downstream to the site of action of ABA. Moreover, systemin represents one of the initial steps in the signal transduction pathway regulating the wound response. Recently, it was reported that heat treatment and mechanical injury generate electrical signals, which propagate throughout the plant. These signals are capable of inducing Pin2 gene expression in the nontreated leaves of wounded plants. Furthermore, electrical current application to tomato leaves leads to an accumulation of Pin2 mRNA in local and systemic tissues. Examination of photosynthetic parameters (assimilation and transpiration rate) on several types of stimuli suggests that heat-induced Pin2 gene expression is regulated by an alternative pathway from that mediating the electrical current and mechanical wound response.

Induction of chilling resistance by water stress, and cDNA sequence analysis and expression of water stress-regulated genes in rice

Exposure of seedlings of a chilling-sensitive variety of rice (Oryza sativa L. cv. Wasetoittu) to water stress (0.5 M mannitol, 30 min) at room temperature induced a degree of chilling resistance. No such resistance was induced by exogenous abscisic acid (ABA) application (10 microM, 60 min). Upon short-term water stress, new transcripts were expressed in both seedlings and suspension-cultured cells. We suggest that the genes induced by short-term water stress, and not those induced by ABA, are related to acquired chilling resistance in this chilling-sensitive rice variety. A total of nine different cDNA clones, specifically induced by short-term water stress, were isolated by differential hybridization and partial sequencing. Northern hybridization analysis using RNAs from the seedlings subjected to chilling after water stress treatment reveal three distinct groups of above mentioned nine cDNA clones: wsi (water stress-induced) 18, 76, and 724, representative of genes whose expression increases, decreases, and remains almost fixed during chilling, respectively. The nucleotide and deduced amino acid sequences of the three representative clones were determined. Characteristic features of wsi18 are the presence of one set of amino acid sequence repeats, a conserved amino acid sequence common to LEA-group genes in the N-terminal region, and an alanine- and lysine-rich tract in the C-terminal region.

Induction of a tomato peroxidase gene in vascular tissue

Expression of a tomato peroxidase gene that is constitutively expressed only in roots was induced in stems and leaves as a result of mechanical wounding. However, wound-induction of TPX1 transcript accumulation in leaves was limited to the mid-rib. No TPX1 transcript was detected in the lamina of the leaf after wounding. Peroxidase isozyme studies indicated the presence of a unique basic isoform in stems after wounding.