Signal transduction in the wound response of tomato plants

Influence of Coating Application Methods on the Postharvest Quality of Cassava

Various modes of edible coating application vary in their coat dispersion and film formation, hence the need to determine the most effective mode of application for cassava. Edible surface coatings have been found to be effective in preserving the quality of various food products. However, there are variations in effectiveness among the different coating solutions, hence the need for optimization of the concentrations of the gums used. This study aimed at determining the most efficient coating application method on the cassava postharvest quality. Physiologically mature cassava (variety KME 1) was harvested and divided into seven portions. The various portions were coated using 1.5% xanthan gum, 1.5% xanthan/guar gum, and 2% xanthan/guar gum by both dipping and spraying method. There was no significant difference on the colour, total cyanide, ethylene production, and total phenolic content between the two application methods. The 2% xanthan/guar gum coating showed a significant difference on the dry matter content while the 1.5% xanthan gum coating had a significant difference on the respiration rate and weight loss. The 1.5 xanthan treated roots had a final dry matter content of 72.5% for the sprayed samples and 75.98% for the dipped sample while the 2% xanthan/guar gum treated roots had a final dry matter content of 64.6% and 74.1% for the dipped and sprayed root samples, respectively. The 1.5% xanthan and 2% xanthan/guar gum treated roots showed no significant difference in their action on dry matter content. The 1.5% xanthan/guar dipped and sprayed samples differed significantly on their effect on flesh firmness with final values of 35.4N and 46.1N, respectively, at 20 days after harvest. This study suggested that based on the coating solution and the parameters being observed, there generally was no varying effect of dipping and spraying methods of coating application. The choice of the efficient mode of application to use will depend on other factors such as the easiness of application.

Long-distance electrical signals as a link between the local action of stressors and the systemic physiological responses in higher plants

Our review is devoted to the analysis of the role of long-distance electrical signals in the development of the fast systemic physiological responses in higher plants. The characteristics and mechanisms of basic electrical signals (variation potential, action potential and system potential) are analyzed, and a potential schema of the generation and propagation of the system potential is proposed. The review summarizes the physiological changes induced by the variation potential, action potential and system potential in higher plants, including changes in gene expressions, the production of phytohormones, photosynthesis, phloem mass-flow, respiration, ATP content, transpiration and plant growth. Potential mechanisms of the changes are analyzed. Finally, a hypothetical schema, which describes a hierarchy of the variation potential, action potential and system potential, in the development of the fast systemic non-specific adaptation of plants to stressors, is proposed.

Insights in the Fruit Flesh Browning Mechanisms in Solanum melongena Genetic Lines with Opposite Postcut Behavior

Color, taste, flavor, nutritional value, and shelf life are important factors determining quality and healthiness of food and vegetables. These factors are strongly affected by browning processes, occurring after fruit or vegetable cutting. Characterization of ten eggplant genotypes for chlorogenic acid (CGA) content, total phenols (TP), polyphenoloxidase (PPO) activity, and browning tendency corroborated a lack of significant correlations between biochemical factors and fruit flesh browning. Further in-depth molecular and biochemical analyses of two divergent eggplant genetic lines, AM199 (high browning) and AM086 (low browning), within 30 min from cutting, highlighted differences in the physiological mechanisms underlying the browning process. qRT-PCR analysis revealed distinct activation mechanisms of CGA biosynthetic and PPO genes in the two genetic lines. Metabolic data on CGA, sugars, and ascorbic acid contents confirmed that their different browning tendency matched with different metabolic responses to cutting. Our findings suggest that the complex mechanism of flesh browning in the two eggplant genetic lines might be mediated by multiple specific factors.

Plant vascular architecture determines the pattern of herbivore-induced systemic responses in Arabidopsis thaliana

The induction of systemic responses in plants is associated with the connectivity between damaged and undamaged leaves, as determined by vascular architecture. Despite the widespread appreciation for studying variation in induced plant defense, few studies have characterized spatial variability of induction in the model species, Arabidopsis thaliana. Here we show that plant architecture generates fine scale spatial variation in the systemic induction of invertase and phenolic compounds. We examined whether the arrangement of leaves along the stem (phyllotaxy) produces predictable spatial patterns of cell-wall bound and soluble invertase activities, and downstream phenolic accumulation following feeding by the dietary specialist herbivore, Pieris rapae and the generalist, Spodoptera exigua. Responses were measured in leaves within and outside of the damaged orthostichy (leaves sharing direct vascular connections), and compared to those from plants where source-sink transport was disrupted by source leaf removal and by an insertional mutation in a sucrose transporter gene (suc2-1). Following herbivore damage to a single, middle-aged leaf, induction of cell-wall and soluble invertase was most pronounced in young and old leaves within the damaged orthostichy. The pattern of accumulation of phenolics was also predicted by these vascular connections and was, in part, dependent on the presence of source leaves and intact sucrose transporter function. Induction also occurred in leaves outside of the damaged orthostichy, suggesting that mechanisms may exist to overcome vascular constraints in this system. Our results demonstrate that systemic responses vary widely according to orthostichy, are often herbivore-specific, and partially rely on transport between source and sink leaves. We also provide evidence that patterns of induction are more integrated in A. thaliana than previously described. This work highlights the importance of plant vascular architecture in determining patterns of systemic induction, which is likely to be ecologically important to insect herbivores and plant pathogens.

Recombinant monoclonal antibody yield in transgenic tobacco plants is affected by the wounding response via an ethylene dependent mechanism

Variability in recombinant IgG yield in transgenic tobacco plants has previously been observed in relation to leaf position, and is interpreted as a function of ageing and the senescence process, leading to increasing protein degradation. Here, similar findings are demonstrated in plants of different ages, expressing IgG but not IgG-HDEL, an antibody form that accumulates within the endoplasmic reticulum. Antibody yields declined following wounding in young transgenic plants expressing IgG but not in those expressing IgG-HDEL. However, in mature IgG plants, the opposite was demonstrated, with significant boosts in yield, while mature IgG-HDEL plants could not be boosted. The lack of response in IgG-HDEL plants suggests that the changes induced by wounding occur post-translationally, and the findings might be explained by wounding responses that differ in plants according to their developmental stages. Plant mechanisms involved in senescence and wounding overlap to a significant degree and compounds such as ethylene, jasmonic acid and salicylic acid are important for mediating downstream effects. Treatment of transgenic plants with ethylene also resulted in a decrease in recombinant IgG yield, which was consistent with the finding that wounded plants could induce lower IgG yields in neighbouring non-wounded plants. Treatment with 1-MCP, an ethylene antagonist, abrogated the IgG yield drop that resulted from wounding, but had no effect on the more gradual IgG yield loss associated with increasing plant age.

Systemin/Jasmonate-mediated systemic defense signaling in tomato

Wound-inducible proteinase inhibitors (PIs) in tomato plants provide a useful model system to elucidate the signal transduction pathways that regulate systemic defense response. Among the proposed intercellular signals for wound-induced PIs expression are the peptide systemin and the oxylipin-derived phytohormone jasmonic acid (JA). An increasing body of evidence indicates that systemin and JA work in the same signaling pathway to activate the expression of PIs and other defense-related genes. However, relatively less is known about how these signals interact to promote cell-to-cell communication over long distances. Genetic analysis of the systemin/JA signaling pathway in tomato plants provides a unique opportunity to study, in a single experimental system, the mechanism by which peptide and oxylipin signals interact to coordinate systemic expression of defense-related genes. Previously, it has been proposed that systemin is the long-distance mobile signal for defense gene expression. Recently, grafting experiments with tomato mutants defective in JA biosynthesis and signaling provide new evidence that JA, rather than systemin, functions as the systemic wound signal, and that the biosynthesis of JA is regulated by the peptide systemin. Further understanding of the systemin/JA signaling pathway promises to provide new insights into the basic mechanisms governing plant defense to biotic stress.

Increasing insight into induced plant defense mechanisms using elicitors and inhibitors

One of the strategies that plants employ to defend themselves against herbivore attack is the induced production of carnivore-attracting volatiles. Using elicitors and inhibitors of different steps of the signal-transduction pathways can improve our understanding of the mechanisms underlying induced plant defenses. For instance, we recently showed that application of jasmonic acid, a key hormone in the octadecanoid pathway involved in herbivore-induced defense, to Brassica oleracea affects gene expression, hormone levels, and volatile emission, as well as oviposition by herbivores and host location behavior by parasitoids. Such defense responses vary with the dose of the elicitor and with time since application. This addendum describes how the use of inhibitors, in addition to the use of elicitors like jasmonic acid, can be applied in bio-assays to investigate the role of signal-transduction pathways involved in induced plant defense. We show how inhibition of different steps of the octadecanoid pathway affects host location behavior by parasitoids.

Characterization of potato (Solanum tuberosum) and tomato (Solanum lycopersicum) protein phosphatases type 2A catalytic subunits and their involvement in stress responses

Protein phosphorylation/dephosphorylation plays critical roles in stress responses in plants. This report presents a comparative characterization of the serine/threonine PP2A catalytic subunit family in Solanum tuberosum (potato) and S. lycopersicum (tomato), two important food crops of the Solanaceae family, based on the sequence analysis and expression profiles in response to environmental stress. Sequence homology analysis revealed six isoforms in potato and five in tomato clustered into two subfamilies (I and II). The data presented in this work show that the expression of different PP2Ac genes is regulated in response to environmental stresses in potato and tomato plants and suggest that, in general, mainly members of the subfamily I are involved in stress responses in both species. However, the differences found in the expression profiles between potato and tomato suggest divergent roles of PP2A in the plant defense mechanisms against stress in these closely related species.

Agriculture and the new challenges for photosynthesis research

A rising human population and changing patterns of land use mean that world food production rates will need to be increased by at least 50% by 2050, a massive rise in harvestable yield per hectare of the major crops such as rice (Oryza sativa) and wheat (Triticum aestivum). Combinations of breeding for improved morphology-related traits such as harvest index and increased inputs of water and fertilizer, which have sustained yield increases since the 1960s, will be neither sufficient nor sustainable. An important limiting factor will be the capacity to produce sufficient biomass during favourable growing periods. Here we analyse this problem in the context of increasing the efficiency of conversion of solar energy into biomass, that is, leaf and canopy photosynthesis. Focussing on crops carrying out C3 photosynthesis, we analyse the evidence for 'losses' in the process of conversion of solar energy into crop biomass and we explore novel mechanisms of improving biomass production rates, which have arisen from recent research into the fundamental primary processes of photosynthesis and carbohydrate metabolism. We show that there are several lines of evidence that these processes are not fully optimized for maximum yield. We put forward the hypothesis that the chloroplast itself should be given greater prominence as a sensor, processor and integrator of highly variable environmental signals to allow a more efficient transduction of energy supply into biomass production.

Induction of plant defense gene expression by plant activators and Pseudomonas syringae pv. tomato in greenhouse-grown tomatoes

Plant activators provide an appealing management option for bacterial diseases of greenhouse-grown tomatoes. Two types of plant activators, one that induces systemic acquired resistance (SAR) and a second that activates induced systemic resistance (ISR), were evaluated for control of Pseudomonas syringae pv. tomato and effect on plant defense gene activation. Benzothiadiazole (BTH, SAR-inducing compound) effectively reduced bacterial speck incidence and severity, both alone and in combination with the ISR-inducing product. Application of BTH also led to elevated activation of salicylic acid and ethylene-mediated responses, based on real-time polymerase chain reaction analysis of marker gene expression levels. In contrast, the ISR-inducing product (made up of plant growth-promoting rhizobacteria) inconsistently modified defense gene expression and did not provide disease control to the same level as did BTH. No antagonism was observed by combining the two activators as control of bacterial speck was similar to or better than BTH alone.

Intercellular communication in plants: evidence for two rapidly transmitted systemic signals generated in response to electromagnetic field stimulation in tomato

Exposing all of a wild-type tomato plant to electromagnetic radiation evoked rapid and substantial accumulation of basic leucine-zipper transcription factor (bZIP) mRNA in the terminal leaf (#4) with kinetics very similar to that seen in response to wounding, while in the abscisic acid (ABA) mutant (Sitiens), the response was more rapid, but transient. Submitting just the oldest leaf (#1) of a wild-type plant to irradiation evoked bZIP mRNA accumulation both locally in the exposed leaf and systemically in the unexposed (distant) leaf #4, although systemic accumulation was delayed somewhat. Accumulation of Pin2 mRNA was less than bZIP in both the exposed and distant leaves in wild type, but there was no delay in the systemic response. In Sitiens, bZIP mRNA accumulation was far less than in wild type in both local and distant leaves, while Pin2 mRNA accumulation was stronger in the exposed leaf, but totally prevented in the systemic leaf. In the jasmonic acid (JA) mutant (JL-5) and in wild-type plants treated with the ABA biosynthesis inhibitor, naproxen, responses were similar to those in the ABA mutant, while treatment of the exposed leaf with calcium antagonists totally abolished both local and systemic increases in bZIP transcript accumulation.

Identification of Ethylene-Mediated Protein Changes during Nodulation in Medicago truncatula Using Proteome Analysis

Ethylene has been hypothesised to be a regulator of root nodule development in legumes, but its molecular mechanisms of action remain unclear. The skl mutant is an ethylene-insensitive legume mutant showing a hypernodulation phenotype when inoculated with its symbiont Sinorhizobium meliloti. We used the skl mutant to study the ethylene-mediated protein changes during nodule development in Medicago truncatula. We compared the root proteome of the skl mutant to its wild-type in response to the ethylene precursor aminocyclopropane carboxylic acid (ACC) to study ethylene-mediated protein expression in root tissues. We then compared the proteome of skl roots to its wild-type after Sinorhizobium inoculation to identify differentially displayed proteins during nodule development at 1 and 3 days post inoculation (dpi). Six proteins (pprg-2, Kunitz proteinase inhibitor, and ACC oxidase isoforms) were down-regulated in skl roots, while three protein spots were up-regulated (trypsin inhibitor, albumin 2, and CPRD49). ACC induced stress-related proteins in wild-type roots, such as pprg-2, ACC oxidase, proteinase inhibitor, ascorbate peroxidase, and heat-shock proteins. However, the expression of stress-related proteins such as pprg-2, Kunitz proteinase inhibitor, and ACC oxidase, was down-regulated in inoculated skl roots. We hypothesize that during early nodule development, the plant induces ethylene-mediated stress responses to limit nodule numbers. When a mutant defective in ethylene signaling, such as skl, is inoculated with rhizobia, the plant stress response is reduced, resulting in increased nodule numbers.

Bestatin, an inhibitor of aminopeptidases, provides a chemical genetics approach to dissect jasmonate signaling in Arabidopsis

Bestatin, a potent inhibitor of some aminopeptidases, was shown previously to be a powerful inducer of wound-response genes in tomato (Lycopersicon esculentum). Here, we present several lines of evidence showing that bestatin specifically activates jasmonic acid (JA) signaling in plants. First, bestatin specifically activates the expression of JA-inducible genes in tomato and Arabidopsis (Arabidopsis thaliana). Second, the induction of JA-responsive genes by bestatin requires the COI1-dependent JA-signaling pathway, but does not depend strictly on JA biosynthesis. Third, microarray analysis using Arabidopsis whole-genome chip demonstrates that the gene expression profile of bestatin-treated plants is similar to that of JA-treated plants. Fourth, bestatin promotes a series of JA-related developmental phenotypes. Taken together, the unique action mode of bestatin in regulating JA-signaled processes leads us to the hypothesis that bestatin exerts its effects through the modulation of some key regulators in JA signaling. We have employed bestatin as an experimental tool to dissect JA signaling through a chemical genetic screening, which yielded a collection of Arabidopsis bestatin-resistant (ber) mutants that are insensitive to the inhibitory effects of bestatin on root elongation. Further characterization efforts demonstrate that some ber mutants are defective in various JA-induced responses, which allowed us to classify the ber mutants into three phenotypic groups: JA-insensitive ber mutants, JA-hypersensitive ber mutants, and mutants insensitive to bestatin but showing normal response to JA. Genetic and phenotypic analyses of the ber mutants with altered JA responses indicate that we have identified several novel loci involved in JA signaling.

Cyanobacterial peptides — Nature's own combinatorial biosynthesis

Cyanobacterial secondary metabolites have attracted increasing scientific interest due to bioactivity of many compounds in various test systems. Among the known structures, oligopeptides are often found with many congeners sharing conserved substructures, while being highly variable in others. A major part of known oligopeptides are of non-ribosomal origin and can be grouped into classes with conserved structural properties. Thus, the overall structural diversity of cyanobacterial oligopeptides only seemingly suggests an equally high diversity of biosynthetic pathways and respective genes. For each class of peptides, some of which have been found in all major branches of the cyanobacterial evolutionary tree, homologous synthetases and genes can be inferred. This implies that non-ribosomal peptide synthetase genes are a very ancient part of the cyanobacterial genome and presumably have evolved by recombination and duplication events to reach the present structural diversity of cyanobacterial oligopeptides. In addition, peptide synthetases would appear to be an essential part of the cyanobacterial evolution and physiology. The present review presents an overview of the biosynthesis of cyanobacterial peptides and corresponding gene clusters, the structural diversity of structural types and structural variations within peptide classes, and implications for the evolution and plasticity of biosynthetic genes and the potential function of cyanobacterial peptides.

Identification of peptide metabolites of Microcystis (Cyanobacteria) that inhibit trypsin-like activity in planktonic herbivorous Daphnia (Cladocera)

Cyanobacteria are recognized as producers of a broad variety of bioactive metabolites. Among these, the peptides synthesized by the non-ribosomal peptide synthetase pathway occur in high structural variability. One class of cyanobacterial peptides, the cyanopeptolins or micropeptins, have been shown to be strong inhibitors of vertebrate serine proteases, like trypsin. In the present study we screened extracts of ten strains of the unicellular cyanobacterium Microcystis sp. for their potential to inhibit trypsin-like activity in the planktonic crustacea Daphnia, the main herbivores in freshwater ecosystem. Respective standardized IC(50)'s varied for nearly two orders of magnitude. In HPLC fractions we could identify mainly cyanopeptolins as active compounds by MALDI-TOF mass spectrometry. Cyanopeptolins were found in 22 structural variants with 13 variants produced by one strain alone. Peptides of the microviridin class were moderately active while no activity was evident for microginins and microcystins. Among the cyanopeptolins only those were active that had an arginine or lysine residue N-terminal to the modified amino acid 3-amino-6-hydroxy-piperidone. Structural variants that had a tyrosine residue at this particular position did not inhibit trypsin-like activity. The highly variable composition of the side chain of cyanopeptolins had no marked effect on the activity. Among the six cyanobacterial strains we tested intensively two did not produce any cyanopeptolins and were accordingly less active as crude extracts. The present study underlines the potential importance of the biochemistry of cyanobacteria for the feeding ecology of a planktonic herbivore.

The expression of the hydroxyproline-rich glycopeptide systemin precursor A in response to (a)biotic stress and elicitors is indicative of its role in the regulation of the wound response in tobacco (Nicotiana tabacum L.)

Two hydroxyproline-rich glycopeptide systemin (TobHS) precursor proteins known as preproTobHypSys-A and B were recently discovered in tobacco (Nicotiana tabacum L.) [Pearce et al. in Nature 411:817-820, 2001]. In this work, the effect of elicitors, insect damage, and abiotic stress on the expression of preproTobHypSys-A ppTobHS-A) in tobacco plants was evaluated. Foliar application of methyl jasmonate preferentially induced the systemic expression of ppTobHS-A in leaves phyllotactically one position above-treated leaves. Abscisic acid strongly induced ppTobHS-A, but water-stress did not. Mechanical wound-induction of ppTobHS-A in young plantlets was rapidly (1 h) and simultaneously detected in wounded and upper unwounded leaves, whereas in older plants induction was slow (12 h) and localized. ppTobHS-A was induced in plants infested with Bemisia tabaci or damaged by herbivory with Manduca sexta larvae. Compared to mechanical wounding, larval herbivory induced a stronger and more stable expression of ppTobHS-A. Moreover, exposure to Manduca-damaged plants induced its expression in neighboring intact plants. In most treatments, the expression patterns of ppTobHS-A coincided with those of selected wound-responsive (WR) genes (e.g., PIOX, NtPI-I, TPI). This correlation was tighter in the wounded and MeJA-treated leaves, whereas in distal, undamaged leaves, it appeared to depend on the type of WR gene examined and on the type of damage sustained by the plant. These results are consistent with the perceived role of the TobHS in defense signaling.

Protease activity in gut of Daphnia magna: evidence for trypsin and chymotrypsin enzymes

Two major protease activities were present in gut homogenates of the cladoceran crustacean Daphnia magna: (i) a trypsin activity that hydrolysed the synthetic substrate N-benzoyl-dl-arginine p-nitroanilide and was strongly inhibited by N-p-tosyl-lysine chloroketone (TLCK) and 4-(amidinophenyl)methanesulfonyl fluoride (APMSF) and not inhibited by chymostatin; and (ii) a chymotrypsin activity that hydrolysed synthetic chymotrypsin substrates containing more than one amino acid, did not hydrolyse N-benzoyl-l-tyrosine p-nitroanilide, and was strongly inhibited by chymostatin and not by TLCK and APMSF. Both activities had alkaline pH optima (pH 7-10), but were shown to be due to distinct types of proteases. These two enzyme activities accounted for 75-83% of the proteolytic activity of gut contents. Substrate SDS-polyacrylamide gel electrophoresis revealed nine different proteases ranging from 15 to 73 kDa.

Acclimation of photosynthesis to high irradiance in rice: gene expression and interactions with leaf development

Rice (Oryza sativa L.) has been used to study the long-term responses of photosynthesis to high irradiance focusing on the composition of the photosynthetic apparatus and leaf morphology. Typical sun/shade differences in chloroplast composition are seen in the fifth leaf following growth in high irradiance compared with low irradiance (1000 and 200 micromol m(-2) s(-1), respectively): higher light-saturated rates of photosynthesis (P(max)), higher amounts of Rubisco protein, and a lower chlorophyll a:b ratio. In addition, leaves were thicker under high light compared with low light. However, responses appear more complex when leaf developmental stage is considered. Using a system of transferring plants from low to high light in the laboratory responses that occur before and after full leaf extension have been studied. Acclimation of photosynthesis is limited by leaf age: the transfer to high light, post-leaf extension, is characterized by alterations in chlorophyll a:b but not in Rubisco protein, which may be limited by leaf morphology. Microarray analysis of gene expression was carried out on plants that were transferred to high light post-leaf extension. A down-regulation of light-harvesting genes was seen. No change in the expression level of Rubisco genes was observed. Up-regulation of genes involved in photoprotection was observed. It was also shown that high-light leaf morphology is established prior to formation of the zone of cellular elongation and division. The endogenous and environmental factors which establish the characteristics of high light acclimation may be important for attaining high rates of assimilation in leaves and crop canopies, and the fifth leaf in rice provides a convenient model system for the determination of the mechanisms involved.

Oxidative stress responses during cassava post-harvest physiological deterioration

A major constraint to the development of cassava (Manihot esculenta Crantz) as a crop to both farmers and processors is its starchy storage roots' rapid post-harvest deterioration, which can render it unpalatable and un-marketable within 24-72 h. An oxidative burst occurs within 15 min of the root being injured, that is followed by the altered regulation of genes, notably for catalase and peroxidase, related to the modulation of reactive oxygen species, and the accumulation of secondary metabolites, some of which show antioxidant properties. The interactions between these enzymes and compounds, in particular peroxidase and the coumarin, scopoletin, are largely confined to the vascular tissues where the visible symptoms of deterioration are observed. These, together with other data, are used to develop a tentative model of some of the principal events involved in the deterioration process.

Oxidative stress responses during cassava post-harvest physiological deterioration

A major constraint to the development of cassava (Manihot esculenta Crantz) as a crop to both farmers and processors is its starchy storage roots' rapid post-harvest deterioration, which can render it unpalatable and un-marketable within 24-72 h. An oxidative burst occurs within 15 min of the root being injured, that is followed by the altered regulation of genes, notably for catalase and peroxidase, related to the modulation of reactive oxygen species, and the accumulation of secondary metabolites, some of which show antioxidant properties. The interactions between these enzymes and compounds, in particular peroxidase and the coumarin, scopoletin, are largely confined to the vascular tissues where the visible symptoms of deterioration are observed. These, together with other data, are used to develop a tentative model of some of the principal events involved in the deterioration process.

Induction of ASCORBATE PEROXIDASE 2 expression in wounded Arabidopsis leaves does not involve known wound-signalling pathways but is associated with changes in photosynthesis

ASCORBATE PEROXIDASE 2 (APX2) encodes a key enzyme of the antioxidant network. In excess light-stressed Arabidopsis leaves, photosynthetic electron transport (PET), hydrogen peroxide (H(2)O(2)) and abscisic acid (ABA) regulate APX2 expression. Wounded leaves showed low induction of APX2 expression, and when exposed to excess light, APX2 expression was increased synergistically. Signalling pathways dependent upon jasmonic acid (JA), chitosan and ABA were not involved in the wound-induced expression of APX2, but were shown to require PET and were preceded by a depressed rate of CO(2) fixation. This led to an accumulation of H(2)O(2) in veinal tissue. Diphenyl iodonium (DPI), which has been shown previously to be a potent inhibitor of H(2)O(2) accumulation in the veins of wounded leaves, prevented induction of APX2 expression probably by inhibition of PET. Thus, the weak induction of APX2 expression in wounded leaves may require H(2)O(2) and PET only. As in other environmental stresses, wounding of leaves resulted in decreased photosynthesis leading to increased reactive oxygen species (ROS) production. This may signal the induction of many 'wound-responsive' genes not regulated by JA-dependent or other known JA-independent pathways.

Putative protease inhibitor gene discovery and transcript profiling during fruit development and leaf damage in grapefruit (Citrus paradisi Macf.)

Seven putative protease inhibitor (PPI) cDNAs, representing four protein families, were isolated from a grapefruit (Citrus paradisi Macf. Cv. Marsh) immature fruit flavedo cDNA library. Cloned open reading frames encoded proteins with similarity to, and protein signatures for: legume Kuntiz inhibitors (lkiL-1, lkiL-2, lkiL-3), potato trypsin inhibitor I (ptiIL-1), serpins (serpL-1), cystatins (cystL-1), and gamma thionins (gthL-1). Response of transcript abundance to fruit development and leaf wounding was determined for all but lkiL-1 using real-time RT-PCR. Immature leaves had the highest transcript levels for all PPIs. The gthL-1 transcript in immature leaves was the most abundant transcript but was absent from healthy mature leaves. In fruit flavedo, transcripts for all PPIs were most abundant in youngest fruit (<15 mm dia. fruit), and declined during development, but displayed different patterns of developmental change. Mechanical or Diaprepes root weevil (DRW) feeding damage to leaves caused a <10-fold reduction or had no effect on transcript level with the exception of gthL-1 which, as a result of damage, increased >50-fold in mature leaves and decreased >1400-fold in immature leaves. This developmental control of transcript response to wounding in a woody perennial is opposite of what has been observed for defensive proteinase inhibitors (PIs) in other plants (typically herbaceous and/or annual plants), where younger leaves typically invoke a higher defensive proteinase inhibitor transcript accumulation than older tissues. Except for gthL-1, the PPI transcripts were minimally responsive or unresponsive to wounding. Changes in PPI transcript levels suggest diverse roles for the products of these genes in citrus, with only gthL-1 responding in a defense-like manner.

Constraints to herbivore-induced systemic responses: bidirectional signaling along orthostichies in Nicotiana attenuata

We investigated the impact of leaf vascular connections on systemically transmitted herbivore-induced gene expression in Nicotiana attenuata. Although systemic signaling is clearly associated with the plant vascular system, few studies consider vascular architecture when measuring systemically induced defenses. N. attenuata is a plant with dispersed phyllotaxis approximating 3/8 in the rosette stage of growth. We mimicked Manduca sexta herbivory by introducing larval regurgitant to wounds produced with a standardized continuous mechanical wounding and investigated mRNA accumulation of genes. Herbivory in N. attenuata induces the expression of genes coding for a proteinase inhibitor protein (PI), threonine deaminase (TD, EC 4.3.1.19), a luminal-binding protein (BiP), and an alpha-dioxygenase (alpha-DOX). We measured the systemic response of sink leaves when orthostichous (growing at an angular distance of 0 degrees) source leaves were treated, and vice versa, and compared it to the systemic response of leaves growing at the maximum angular distance of 180 degrees. Vascular architecture clearly controlled the intensity of systemic mRNA accumulation within the 4-hr time frame of the experiment. In addition, we found signal translocation to be bidirectional, travelling from source to sink as well as from sink to source leaves, which argues against a phloem-based assimilate-linked signal identity.

Primary structure and reactive site of a novel wheat proteinase inhibitor of subtilisin and chymotrypsin

The proteinase inhibitor WSCI, active in inhibiting bacterial subtilisin and a number of animal chymotrypsins, was purified from endosperm of exaploid wheat (Triticum aestivum, c.v. San Pastore) by ion exchange chromatography and its complete amino acid sequence was established by automated Edman degradation. WSCI consists of a single polypeptide chain of 72 amino acid residues, has a molecular mass of 8126.3 Da and a pl of 5.8. The inhibition constants (Ki) for Bacillus licheniformis subtilisin and bovine pancreatic alpha-chymotrypsin are 3.92 x 10(-9) M and 7.24 x 10(-9) M, respectively. The inhibitor contains one methionine and of tryptophan residue and has a high content of essential amino acids (41 over a total of 72 residues), but no cysteines. The primary structure of WSCI shows high similarity with barley subtilisin-chymotrypsin isoinhibitors of the Cl-2 type and with maize subtilisinchymotrypsin inhibitor MPI. Significant degrees of similarity were also found between sequences of WSCI and of other members of the potato inhibitor I family of the serine proteinase inhibitors. The wheat inhibitor WSCI has a single reactive site (the peptide bond between methionyl-48 and glutamyl-49 residues) as identified by affinity chromatography and sequence analysis.

The tomato mutant spr1 is defective in systemin perception and the production of a systemic wound signal for defense gene expression

Wound-induced systemic expression of defensive proteinase inhibitor (PI) genes in tomato plants requires the action of systemin and its precursor protein prosystemin. Although it is well established that systemin induces PI expression through the octadecanoid pathway for jasmonic acid (JA) biosynthesis, relatively little is known about how systemin and JA interact to promote long-distance signaling between damaged and undamaged leaves. Here, this question was addressed by characterizing a systemin-insensitive mutant (spr1) that was previously identified as a suppressor of prosystemin-mediated responses. In contrast to JA biosynthetic or JA signaling mutants that lack both local and systemic PI expression in response to wounding, spr1 plants were deficient mainly in the systemic response. Consistent with this phenotype, spr1 plants exhibited normal PI induction in response to oligosaccharide signals that are thought to play a role in the local wound response. Moreover, spr1 abolished JA accumulation in response to exogenous systemin, and reduced JA accumulation in wounded leaves to approximately 57% of wild-type (WT) levels. Analysis of reciprocal grafts between spr1 and WT plants showed that spr1 impedes systemic PI expression by blocking the production of the long-distance wound signal in damaged leaves, rather than inhibiting the recognition of that signal in systemic undamaged leaves. These experiments suggest that Spr1 is involved in a signaling step that couples systemin perception to activation of the octadecanoid pathway, and that systemin acts at or near the site of wounding (i.e. in rootstock tissues) to increase JA synthesis to a level that is required for the systemic response. It was also demonstrated that spr1 plants are not affected in the local or systemic expression of a subset of rapidly induced wound-response genes, indicating the existence of a systemin-independent pathway for wound signaling.

Characterization and cDNA-microarray expression analysis of 12-oxophytodienoate reductases reveals differential roles for octadecanoid biosynthesis in the local versus the systemic wound response

12-Oxophytodienoate reductases (OPRs) belong to a family of flavin-dependent oxidoreductases. With two new tomato isoforms reported here, three OPRs have now been characterized in both tomato and Arabidopsis. Only one of these isoforms (OPR3) participates directly in the octadecanoid pathway for jasmonic acid biosynthesis, as only OPR3 reduces the 9S,13S-stereoisomer of 12-oxophytodienoic acid, the biological precursor of jasmonic acid. The subcellular localization of OPRs was analyzed in tomato and Arabidopsis. The OPR3 protein and activity were consistently found in peroxisomes where they co-localize with the enzymes of beta-oxidation which catalyze the final steps in the formation of jasmonic acid. The octadecanoid pathway is thus confined to plastids and peroxisomes and, in contrast to previous assumptions, does not involve the cytosolic compartment. The expression of tomato (Lycopersicon esculentum,Le) OPR3 was analyzed in the context of defense-related genes using a microarray comprising 233 cDNA probes. LeOPR3 was found to be up-regulated after wounding with induction kinetics resembling those of other octadecanoid pathway enzymes. In contrast to the induction of genes for wound response proteins (e.g. proteinase inhibitors), the accumulation of octadecanoid pathway transcripts was found to be more rapid and transient in wounded leaves, but hardly detectable in unwounded, systemic leaves. Consistent with the expression data, OPDA and JA were found to accumulate locally but not systemically in the leaves of wounded tomato plants. The transcriptional activation of the octadecanoid pathway and the accumulation of JA to high levels are, thus not required for the activation of defense gene expression in systemic tissues.

NHL25 and NHL3, two NDR1/HIN1-1ike genes in Arabidopsis thaliana with potential role(s) in plant defense

The Arabidopsis genome contains 28 genes with sequence homology to the Arabidopsis NDR1 gene and the tobacco HIN1 gene. Expression analysis of eight of these genes identified two (NHL25 and NHL3 for NDR1/HIN1-like) that show pathogen-dependent mRNA accumulation. Transcripts did not accumulate during infection with virulent Pseudomonas syringae pv. tomato DC3000 but did accumulate specifically when the bacteria carried any of the four avirulence genes avrRpm1, avrRpt2, avrB, or avrRps4. Furthermore, expression of avrRpt2 in plants containing the corresponding resistance gene, RPS2, was sufficient to induce transcript accumulation. However, during infection with an avirulent oomycete, Peronospora parasitica isolate Cala-2, only NHL25 expression was reproducibly induced. Salicylic acid (SA) treatment can induce expression of NHL25 and NHL3. Studies performed on nahG plants showed that, during interaction with avirulent bacteria, only the expression of NHL25 but not that of NHL3 was affected. This suggests involvement of separate SA-dependent and SA-independent pathways, respectively, in the transcriptional activation of these genes. Bacteria-induced gene expression was not abolished in ethylene- (etrl-3 and ein2-1) and jasmonate- (coil-1) insensitive mutants or in mutants impaired in disease resistance (ndr1-1 and pad4-1). Interestingly, NHL3 transcripts accumulated after infiltration with the avirulent hrcC mutant of Pseudomonas syringae pv. tomato DC3000 and nonhost bacteria but not with the virulent Pseudomonas syringae pv. tomato DC3000, suggesting that virulent bacteria may suppress NHL3 expression during pathogenesis. Hence, the expression patterns and sequence homology to NDR1 and HIN1 suggest one or more potential roles for these genes in plant resistance.

Long-distance CO(2) signalling in plants

Stomatal numbers are tightly controlled by environmental signals including light intensity and atmospheric CO(2) partial pressure. This requires control of epidermal cell development during the early phase of leaf growth and involves changes in both the density of cells on the leaf surface and the proportion of cells that adopt a stomatal fate. This paper reviews the current understanding of how stomata develop and describes recent advances that have given insights into the regulatory mechanisms involved using mutant Arabidopsis plants that implicates a role for long-chain fatty acids in cell-to-cell communication. Evidence is presented which indicates that long-distance signalling from mature to newly developing leaves forms part of the mechanism by which stomatal development responds to environmental cues. Analysis of mutant plants suggests that the plant hormones abscisic acid, ethylene and jasmonates are implicated in the long-distance signalling pathway and that the action may be mediated by reactive oxygen species.

A calcium-dependent protein kinase is systemically induced upon wounding in tomato plants

A full-length cDNA clone (LeCDPK1) from tomato (Lycopersicon esculentum) encoding a calcium-dependent protein kinase (CDPK) was isolated by screening a cDNA library from tomato cell cultures exposed to Cladosporium fulvum elicitor preparations. The predicted amino acid sequence of the cDNA reveals a high degree of similarity with other members of the CDPK family. LeCDPK1 has a putative N-terminal myristoylation sequence and presents a possible palmitoylation site. The in vitro translated protein conserves the biochemical properties of a member of the CDPK family. In addition, CDPK activity was detected in soluble and particulate extracts of tomato leaves. Basal levels of LeCDPK1 mRNA were detected by northern-blot analysis in roots, stems, leaves, and flowers of tomato plants. The expression of LeCDPK1 was rapidly and transiently enhanced in detached tomato leaves treated with pathogen elicitors and H2O2. Moreover, when tomato greenhouse plants were subjected to mechanical wounding, a transient increase of LeCDPK1 steady-state mRNA levels was detected locally at the site of the injury and systemically in distant non-wounded leaves. The increase observed in LeCDPK1 mRNA upon wounding correlates with an increase in the amount and in the activity of a soluble CDPK detected in extracts of tomato leaves, suggesting that this kinase is part of physiological plant defense mechanisms against biotic or abiotic attacks.

A calcium-dependent protein kinase is systemically induced upon wounding in tomato plants

A full-length cDNA clone (LeCDPK1) from tomato (Lycopersicon esculentum) encoding a calcium-dependent protein kinase (CDPK) was isolated by screening a cDNA library from tomato cell cultures exposed to Cladosporium fulvum elicitor preparations. The predicted amino acid sequence of the cDNA reveals a high degree of similarity with other members of the CDPK family. LeCDPK1 has a putative N-terminal myristoylation sequence and presents a possible palmitoylation site. The in vitro translated protein conserves the biochemical properties of a member of the CDPK family. In addition, CDPK activity was detected in soluble and particulate extracts of tomato leaves. Basal levels of LeCDPK1 mRNA were detected by northern-blot analysis in roots, stems, leaves, and flowers of tomato plants. The expression of LeCDPK1 was rapidly and transiently enhanced in detached tomato leaves treated with pathogen elicitors and H2O2. Moreover, when tomato greenhouse plants were subjected to mechanical wounding, a transient increase of LeCDPK1 steady-state mRNA levels was detected locally at the site of the injury and systemically in distant non-wounded leaves. The increase observed in LeCDPK1 mRNA upon wounding correlates with an increase in the amount and in the activity of a soluble CDPK detected in extracts of tomato leaves, suggesting that this kinase is part of physiological plant defense mechanisms against biotic or abiotic attacks.

Plant responses to insect herbivory: the emerging molecular analysis

Plants respond to herbivore attack with a bewildering array of responses, broadly categorized as direct and indirect defenses, and tolerance. Plant-herbivore interactions are played out on spatial scales that include the cellular responses, well-studied in plant-pathogen interactions, as well as responses that function at whole-plant and community levels. The plant's wound response plays a central role but is frequently altered by insect-specific elicitors, giving plants the potential to optimize their defenses. In this review, we emphasize studies that advance the molecular understanding of elicited direct and indirect defenses and include verifications with insect bioassays. Large-scale transcriptional changes accompany insect-induced resistance, which is organized into specific temporal and spatial patterns and points to the existence of herbivore-specific trans-activating elements orchestrating the responses. Such organizational elements could help elucidate the molecular control over the diversity of responses elicited by herbivore attack.

Proteins of circularly permuted sequence present within the same organism: the major serine proteinase inhibitor from Capsicum annuum seeds

The major serine proteinase inhibitor from bell pepper (Capsicum annuum, paprika) seeds was isolated, characterized, and sequenced, and its disulfide bond topology was determined. PSI-1.2 is a 52-amino-acid-long, cysteine-rich polypeptide that inhibits both trypsin (K(i) = 4.6 x 10(-9) M) and chymotrypsin (K(i) = 1.1 x 10(-8) M) and is a circularly permuted member of the potato type II inhibitor family. Mature proteins of this family are produced from precursor proteins containing two to eight repeat units that are proteolytically cleaved within, rather than between, the repeats. In contrast, PSI-1.2 corresponds to a complete repeat that was predicted as the putative ancestral protein of the potato type II family. To our knowledge, this is the first case in which two proteins related to each other by circular permutation are shown to exist in the same organism and are expressed within the same organ. PSI-1.2 is not derived from any of the known precursors, and it contains a unique amphiphilic segment in one of its loops. A systematic comparison of the related precursor repeat-sequences reveals common evolutionary patterns that are in agreement with the ancestral gene-duplication hypothesis.

Plant peptide hormones: The long and the short of it

A polypeptide from tobacco has been found to be processed into multiple functional peptides, each with independent hormone-like activities. This adds to a growing set of small peptides known to function as signal molecules in plants.

Alternative splicing of prosystemin pre-mRNA produces two isoforms that are active as signals in the wound response pathway

Systemin and its precursor protein, prosystemin, play an essential role in the systemic wound response pathway of tomato plants. We report here the isolation from tomato of a novel prosystemin cDNA (prosysB) that differs from the reported cDNA sequence (prosysA) by the addition of a CAG trinucleotide. Inspection of the prosystemin genomic sequence, which was mapped to the central region of chromosome 5, indicated that prosysA and prosysB transcripts are generated by an alternative splicing event that utilizes different 3' splice sites within intron 3. Quantitative RT-PCR analysis showed that prosysB transcripts accumulated to approximately twice the level of prosysA in all tissues that express the prosystemin gene. The relative abundance of the two mRNAs was unaffected by wounding or methyl jasmonate treatment, conditions that increase the level of total prosys mRNA. These findings indicate that alternative splicing of prosys pre-mRNA is a constitutive process. The amino acid sequence of prosysB is predicted to differ from that of prosysA by replacement of Arg-57 with Thr-Gly in the non-systemin portion of the protein. Over-expression of the prosysB cDNA in transgenic tomato plants conferred constitutive expression of defense genes that are regulated by wounding and systemin. We conclude that prosysB is the major prosystemin-encoding transcript in tomato, and that this isoform is active as a signal in the wound response pathway.

Calystegines in root cultures of Atropa belladonna respond to sucrose, not to elicitation

Calystegines are norpseudotropine alkaloids accumulating in root cultures of Atropa belladonna, together with tropine derivatives, e.g. hyoscyamine. Both alkaloid groups are derived from the tropane alkaloid pathway. For the investigation of the regulation and individual steps of tropane biosynthesis, methods for the induction of the pathway were tested. Elicitation by chitosan, or defence responses to ABA and methyl jasmonate did not enhance calystegine accumulation, but led to a more or less pronounced decrease. By blocking one arm of the diverged tropane pathway, calystegine accumulation can be increased, but total tropane alkaloid formation does not increase considerably. By elevation of sucrose supply, both, total alkaloids and calystegines in particular were increased approximately threefold. The mechanism of the induction of the biosynthesis by sucrose is not known and needs further experiments.

Localization and control of expression of Nt-Syr1, a tobacco SNARE protein

Syntaxins and other SNARE proteins are crucial for intracellular vesicle trafficking, fusion and secretion. Previously, we isolated the syntaxin-related protein Nt-Syr1 from Nicotiana in a screen for ABA-related signalling elements, and demonstrated its role in determining the ABA sensitivity of stomatal guard cells. Because the location and expression of SNAREs are often important clues to their functioning, we have examined the distribution and stimulus-dependent expression of Nt-Syr1 between tissues, as well as its location within the cell, using antisera raised against purified recombinant peptides corresponding to overlapping cytosolic domains of Nt-Syr1. The Nt-Syr1 epitope was strongly represented in roots and to lesser extents in stems, leaves and flowers of well-watered plants. Biochemical analysis and examination of immunogold labelling under the electron microscope indicated Nt-Syr1 to be located primarily at the plasma membrane. Expression of the protein in leaves and to a lesser extent in flowers and stems was transiently enhanced by ABA, but not by auxin, kinetin or gibberellic acid. Expression in leaves was promoted by salt stress and wounding, but not by cold. By contrast, Nt-Syr1 levels in the root were unaffected by ABA. In the leaves, enhanced expression of Nt-Syr1 by salt stress was not observed in aba1 mutant Nicotiana, which is deficient in ABA synthesis, and in plants carrying the Arabidopsis abi1 transgene that suppresses a number of ABA-evoked responses in these plants. However, an enhanced expression in response to wounding was observed, even in the mutant backgrounds. We conclude that Nt-Syr1 expression at the plasma membrane is important for its function and is subject to control by parallel, stress-related signalling pathways, both dependent on and independent of ABA. Nt-Syr1 may be associated with additional functions, especially in the roots, that are unrelated to ABA or stress responses in the plant.

THE GREAT ESCAPE: Phloem Transport and Unloading of Macromolecules1

The phloem of higher plants translocates a diverse range of macromolecules including proteins, RNAs, and pathogens. This review considers the origin and destination of such macromolecules. A survey of the literature reveals that the majority of phloem-mobile macromolecules are synthesized within companion cells and enter the sieve elements through the branched plasmodesmata that connect these cells. Examples of systemic macromolecules that originate outside the companion cell are rare and are restricted to viral and subviral pathogens and putative RNA gene-silencing signals, all of which involve a relay system in which the macromolecule is amplified in each successive cell along the pathway to companion cells. Evidence is presented that xenobiotic macromolecules may enter the sieve element by a default pathway as they do not possess the necessary signals for retention in the sieve element-companion cell complex. Several sink tissues possess plasmodesmata with a high-molecular-size exclusion limit, potentially allowing the nonspecific escape of a wide range of small (<50-kDa) macromolecules from the phloem. Larger macromolecules and systemic mRNAs appear to require facilitated transport through sink plasmodesmata. The fate of phloem-mobile macromolecules is considered in relation to current models of long-distance signaling in plants.

The systemin signaling pathway: differential activation of plant defensive genes

Systemin, an 18-amino-acid polypeptide released from wound sites on tomato leaves caused by insects or other mechanical damage, systemically regulates the activation of over 20 defensive genes in tomato plants in response to herbivore and pathogen attacks. Systemin is processed from a larger prohormone protein, called prosystemin, by proteolytic cleavages. However, prosystemin lacks a signal sequence and glycosylation sites and is apparently not synthesized through the secretory pathway, but in the cytoplasm. The polypeptide activates a lipid-based signal transduction pathway in which the 18:3 fatty acid, linolenic acid, is released from plant membranes and converted to the oxylipin signaling molecule jasmonic acid. A wound-inducible systemin cell surface receptor with an M(r) of 160,000 has recently been identified. The receptor regulates an intracellular cascade including, depolarization of the plasma membrane, the opening of ion channels, an increase in intracellular Ca(2+), activation of a MAP kinase activity and a phospholipase A(2) activity. These rapid changes appear to play important roles leading to the intracellular release of linolenic acid from membranes and its subsequent conversion to jasmonic acid, a potent activator of defense gene transcription. Although the mechanisms for systemin processing, release, and transport are still unclear, studies of the timing of the synthesis and of the intracellular localization of wound- and systemin-inducible mRNAs and proteins indicates that differential syntheses of signal pathway genes and defensive genes are occurring in different cell types. This signaling cascade in plants exhibits extraordinary analogies with the signaling cascade for the inflammatory response in animals.

Proteinase inhibitor-inducing activity of the prohormone prosystemin resides exclusively in the C-terminal systemin domain

Prosystemin is the 200-amino acid precursor of the 18-amino acid polypeptide defense hormone, systemin. Herein, we report that prosystemin was found to be as biologically active as systemin when assayed for proteinase inhibitor induction in young tomato plants and nearly as active in the alkalinization response in Lycopersicon esculentum suspension-cultured cells. Similar to many animal prohormones that harbor multiple signals, the systemin precursor contains five imperfect repetitive domains N-terminal to a single systemin domain. Whether the five repetitive domains contain defense signals has not been established. N-terminal deletions of prosystemin had little effect on its activity in tomato plants or suspension-cultured cells. Deletion of the C-terminal region of prosystemin containing the 18-amino acid systemin domain completely abolished its proteinase inhibitor induction and alkalinization activities. The apoplastic fluid from tomato leaves and the medium of cultured cells were analyzed for proteolytic activity that could process prosystemin to systemin. These experiments showed that proteolytic enzymes present in the apoplasm and medium could cleave prosystemin into large fragments, but the enzymes did not produce detectable levels of systemin. Additionally, inhibitors of these proteolytic enzymes did not affect the biological activity of prosystemin. The cumulative data indicated that prosystemin and/or large fragments of prosystemin can be active inducers of defense responses in both tomato leaves and suspension-cultured cells and that the only region of prosystemin that is responsible for activating the defense response resides in the systemin domain.