Effects of Ca2+ on phytoalexin induction by fungal elicitor in soybean cells

Mitogen-Activated Protein Kinase Kinase OsMEK2 Positively Regulates Ca2+ Influx and Ferroptotic Cell Death during Rice Immune Responses

Mitogen-activated protein (MAP) kinase (MAPK) signaling pathway is important in plant immune responses, involved in iron- and reactive oxygen species (ROS)-dependent ferroptotic cell death mediated by Ca2+. High Ca2+ influx triggered iron-dependent ROS accumulation, lipid peroxidation, and subsequent hypersensitive response (HR) cell death in rice (Oryza sativa). Apoplastic Ca2+ chelation by EGTA during avirulent Magnaporthe oryzae infection altered Ca2+, ROS, and Fe2+ accumulation, increasing rice susceptibility to infection. By contrast, acibenzolar-S-methyl (ASM), a plant defense activator, significantly enhanced Ca2+ influx, and H2O2 accumulation, triggering rice ferroptotic cell death during virulent Magnaporthe oryzae infection. Here, we report a novel role of the MAPK signaling pathway in regulating cytoplasmic Ca2+ increase during ferroptotic cell death in rice immunity, using the ΔOsmek2 knockout mutant rice. The knockout of rice OsMEK2 impaired the ROS accumulation, lipid peroxidation, and iron accumulation during avirulent M. oryzae infection. This study has shown that OsMEK2 could positively regulate iron- and ROS-dependent ferroptotic cell death in rice by modulating the expression of OsNADP-ME, OsRBOHB, OsPLC, and OsCNGC. This modulation indicates a possible mechanism for how OsMEK2 participates in Ca2+ regulation in rice ferroptotic cell death, suggesting its broader role in plant immune responses in response to M. oryzae infection.

Factors Involved on Tiger-Stripe Foliar Symptom Expression of Esca of Grapevine

Esca of grapevine causes yield losses correlated with incidence and severity symptom expression. Factors associated with leaf symptom mechanisms are yet to be fully clarified. Therefore, in 2019 and 2020, macro and microelement analyses and leaf reflectance measurements were carried out on leaves at different growth stages in a vineyard located in Abruzzo, central Italy. Surveys were carried out on leaves of both never leaf-symptomatic vines and different categories of diseased vine shoots. Never leaf-symptomatic and diseased vines were also treated with a fertilizer mixture that proved to be able to limit the symptom expression. Results showed that untreated asymptomatic diseased vines had high calcium contents for most of the vegetative season. On the contrary, treated asymptomatic diseased vines showed higher contents of calcium, magnesium, and sodium, at berries pea-sized, before the onset of symptoms. These vines had better physiological efficiency showing higher water index (WI), normalized difference vegetation index (NDVI), and green normalized difference vegetation index (GNDVI) values, compared to untreated asymptomatic vines, at fruit set. Results confirmed the strong response of the plant to symptom expression development and the possibility of limiting this response with calcium and magnesium applications carried out before the symptom onset.

A comparative transcriptome and proteomics analysis reveals the positive effect of supplementary Ca(2+) on soybean sprout yield and nutritional qualities

Effects of Ca(2+) on yield and nutritional qualities of soybean sprout were investigated. Ca-treated sprouts had higher yield than water-treated ones. Metabolism of selected storage materials and bioactive substances in soybean sprouts was strengthened by Ca(2+). The phytic acid and saponin content of Ca-treated soybean sprouts were lower than those of control. Supplemental Ca(2+) increased content of gamma-aminobutyric acid, isoflavones, phenolics, and vitamins, respectively. These findings indicate that supplemental Ca(2+) can increase soybean sprout yield and improve its nutritional qualities. The comparative transcriptome and proteomics between water-treated and Ca-treated soybean sprouts were studied. As consequence 1912 genes and 460 proteins were up- or down-regulated after 4days of Ca(2+) treatment. The functional classification of these differentially expressed genes and proteins indicated their connection with primary/secondary metabolic pathways, ion transport, signal transduction, and transcriptional regulation. The results obtained here will enable to understand how changes in yield and nutritional quality are regulated by extra Ca(2+) in soybean sprouts.

BIOLOGICAL SIGNIFICANCE

In this study, a total of 1912 genes and 460 proteins involved in the growth, storage material decomposition, and bioactive substance synthesis in soybean sprouts after treated with Ca(2+) were identified. This is the first report of a comprehensive transcriptomic and proteomic analysis of soybean sprout in response to supplemental Ca(2+).

ABC transporter PEN3/PDR8/ABCG36 interacts with calmodulin that, like PEN3, is required for Arabidopsis nonhost resistance

Nonhost resistance (NHR) is the most prevalent form of plant immunity. In Arabidopsis, NHR requires membrane-localized ATP-binding cassette (ABC) transporter PENETRATION (PEN) 3. Upon perception of pathogen-associated molecular patterns, PEN3 becomes phosphorylated, suggestive of PEN3 regulation by post-translational modification. Here, we investigated the PEN3 protein interaction network. We probed the Arabidopsis protein microarray AtPMA-5000 with the N-terminal cytoplasmic domain of PEN3. Several of the proteins identified to interact with PEN3 in vitro represent cellular Ca(2+) sensors, including calmodulin (CaM) 3, CaM7 and several CaM-like proteins, pointing to the importance of Ca(2+) sensing to PEN3-mediated NHR. We demonstrated co-localization of PEN3 and CaM7, and we confirmed PEN3-CaM interaction in vitro and in vivo by PEN3 pull-down with CaM Sepharose, CaM overlay assay and bimolecular fluorescence complementation. We also show that just like in pen3, NHR to the nonadapted fungal pathogens Phakopsora pachyrhizi and Blumeria graminis f.sp. hordei is compromised in the Arabidopsis cam7 and pen3 cam7 mutants. Our study discloses CaM7 as a PEN3-interacting protein crucial to Arabidopsis NHR and emphasizes the importance of Ca(2+) sensing to plant immunity.

Glyceollin, a soybean phytoalexin with medicinal properties

This review covers the biosynthesis of glyceollin and its biological activities including antiproliferative/antitumor action (toward B16 melanoma cells, LNCaP prostate cancer cells, and BG-1 ovarian cancer cells), anti-estrogenic action (through estrogen receptors α- and β-), antibacterial action (toward Erwinia carotovora, Escherichia coli, Bradyrhizobium japonicum, Sinorhizobium fredii ), antinematode activity, and antifungal activity (toward Fusarium solani, Phakospora pachyrhizi, Diaporthe phaseolorum, Macrophomina phaseolina, Sclerotina sclerotiorum, Phytophthora sojae, Cercospora sojina, Phialophora gregata, and Rhizoctonia solani). Other activities include insulinotropic action and attenuation of vascular contractions in rat aorta.

Molecular cloning and characterisation of two calmodulin isoforms of the Madagascar periwinkle Catharanthus roseus

Involvement of Ca(2+) signalling in regulation of the biosynthesis of monoterpene indole alkaloids (MIA) in Catharanthus roseus has been extensively studied in recent years, albeit no protein of this signalling pathway has been isolated. Using a PCR strategy, two C. roseus cDNAs encoding distinct calmodulin (CAM) isoforms were cloned and named CAM1 and CAM2. The deduced 149 amino acid sequences possess four Ca(2+) binding domains and exhibit a close identity with Arabidopsis CAM isoforms (>91%). The ability of CAM1 and CAM2 to bind Ca(2+) was demonstrated following expression of the corresponding recombinant proteins. Furthermore, transient expression of CAM1-GFP and CAM2-GFP in C. roseus cells showed a typical nucleo-cytoplasm localisation of both CAMs, in agreement with the wide distribution of CAM target proteins. Using RNA blot analysis, we showed that CAM1 and CAM2 genes had a broad pattern of expression in C. roseus organs and are constitutively expressed during a C. roseus cell culture cycle, with a slight inhibitory effect of auxin for CAM1. Using RNA in situ hybridisation, we also detected CAM1 and CAM2 mRNA in the vascular bundle region of young seedling cotyledons. Finally, using specific inhibitors, we also showed that CAMs are required for MIA biosynthesis in C. roseus cells by acting on regulation of expression of genes encoding enzymes that catalyse early steps of MIA biosynthesis, such as 1-deoxy-d-xylulose 5-phosphate reductoisomerase and geraniol 10-hydroxylase.

Implication of signaling pathways involving calcium, phosphorylation and active oxygen species in methyl jasmonate-induced defense responses in grapevine cell cultures

Perception of elicitors triggers plant defense responses via various early signal transduction pathways. Methyl jasmonate (MeJA) stimulates defense responses in grapevine (Vitis vinifera). We investigated the involvement of various partners (calcium, ROS, reversible phosphorylation) in MeJA-induced responses by using a pharmacological approach. We used specific calcium channel effectors and inhibitors of serine/threonine phosphatases, superoxide dismutase and NAD(P)H oxidase and investigated production of stilbenes (resveratrol and its glucoside, piceid, the major form), which are the grapevine phytoalexins. RNA accumulation of two genes encoding enzymes involved in stilbene synthesis (PAL and STS), three genes encoding pathogenesis-related proteins (CHIT4C, PIN and GLU) and one gene encoding an enzyme producing jasmonates (LOX) were also assessed. Calcium and its origin seemed to play a major role in MeJA-induced grapevine defense responses. Phytoalexin production was strongly affected if calcium from the influx plasma membrane was inhibited, whereas calcium from the intracellular compartments did not seem to be involved. ROS production seemed to interfere with MeJA-stimulated defense responses, and protein phosphorylation/dephosphorylation events also played a direct role.

Two distinct intracellular Ca2+-release components act in opposite ways in the regulation of the auxin-dependent MIA biosynthesis in Catharanthus roseus cells

Calcium-mediated signalling is ubiquitous in both animals and plants. Changes in cytoplasmic free Ca(2+) concentration couple diverse arrays of stimuli to their specific responses, the specificity of the stimulus being determined by integrated actions between multiple Ca(2+) mobilization pathways. In this work, a pharmacological approach is reported, aimed at deciphering the role of calcium as a second messenger in the transduction pathway leading to the inhibitory effect of 2,4-dichlorophenoxyacetic acid (2,4-D), in regulating monoterpene indole alkaloid (MIA) biosynthesis in Catharanthus roseus cells. It is demonstrated here that auxin-dependent MIA biosynthesis is differentially regulated by two distinct calcium release components from internal stores in C. roseus showing pharmacological profiles similar to those displayed by animal RyR and IP3 channels. MIA biosynthesis is stimulated by caffeine (Ca(2+)-release activator through RyR channels) and by heparin and TMB8 (Ca(2+)-release inhibitors of IP3 channels) whereas MIA biosynthesis is inhibited by mastoparan (Ca(2+)-release activator of IP3 channels) and by ruthenium red and DHBP (Ca(2+)-release inhibitors of RyR channels). Furthermore, calcium, as 2,4-D, acts on MIA biosynthesis by regulating the monoterpene moiety of the MIA biosynthesis pathway since calcium channel modulators preferentially modulate g10h expression, the gene encoding the enzyme of the secoiridoid monoterpene pathway, that is the major target of 2,4-D action. In addition, the simultaneous use of caffeine (an activator of RyR channel in animals) and TMB8 (an inhibitor of the IP3 channel) in 2,4-D treated cells triggers a synergistic effect on MIA accumulation. This finding suggests an opposite and co-ordinated action of multiple Ca(2+)-release pathways in 2,4-D signal transduction, adding a new level of complexity to calcium signalling in plants and questioning the existence of RyR and IP3 channels in plants.

Some common signal transduction events are not necessary for the elicitor-induced accumulation of silymarin in cell cultures of Silybum marianum

A variety of pharmacological effectors of signal transduction pathways were used to investigate the elicitor-activated sequence of cellular responses by which yeast extract (YE) or methyljasmonate (MeJA) enhanced production of silymarin in cell cultures of Silybum marianum. As we recently showed that inhibition of external and internal calcium fluxes significantly increased flavonolignan production in S. marianum cultures, we examined whether calcium mediates signaling events leading to enhancement of silymarin production upon YE or MeJA elicitation. Pre-treatment of cultures with calcium chelators, calcium blockers or intracellular antagonists enhanced the elicitor effect of YE or MeJA. The increase of intracellular-free Ca(2+) level also promoted the elicitor effect, suggesting that an external source of calcium or alterations in internal calcium fluxes were not required for the elicitation to occur. Activation of phosphorylation/dephosphorylation cascades did not appear to mediate the elicitation mechanism; the increase in silymarin induced by elicitation was not suppressed by inhibitors of protein phosphatases or by protein kinase inhibitors. No H(2)O(2) generation was detected at any time after elicitation. Also, diphenyleneiodonium, a potent inhibitor of NAD(P)H-oxidase, did not block silymarin production in elicited cultures. From these results, we conclude that S. marianum cell cultures do not appear to employ conserved signaling components in the transduction of the elicitor signal to downstream responses such as silymarin production.

Activation of members of a MAPK module in beta-glucan elicitor-mediated non-host resistance of soybean

Plants recognize microbial pathogens by discriminating pathogen-associated molecular patterns from self-structures. We study the non-host disease resistance of soybean (Glycine max L.) to the oomycete, Phytophthora sojae. Soybean senses a specific molecular pattern consisting of a branched heptaglucoside that is present in the oomycetal cell walls. Recognition of this elicitor may be achieved through a beta-glucan-binding protein, which forms part of a proposed receptor complex. Subsequently, soybean mounts a complex defense response, which includes the increase of the cytosolic calcium concentration, the production of reactive oxygen species, and the activation of genes responsible for the synthesis of phytoalexins. We now report the identification of two mitogen-activated protein kinases (MAPKs) and one MAPK kinase (MAPKK) that may function as signaling elements in triggering the resistance response. The use of specific antisera enabled the identification of GmMPKs 3 and 6 whose activity is enhanced within the signaling pathway leading to defense reactions. Elicitor specificity of MAPK activation as well as the sensitivity against inhibitors suggested these kinases as part of the beta-glucan signal transduction pathway. An upstream GmMKK1 was identified based on sequence similarity to other plant MAPKKs and its interaction with the MAPKs was analyzed. Recombinant GmMKK1 interacted predominantly with GmMPK6, with concomitant phosphorylation of the MAPK protein. Moreover, a preferential physical interaction between GmMKK1 and GmMPK6 was demonstrated in yeast. These results suggest a role of a MAPK cascade in mediating beta-glucan signal transduction in soybean, similar to other triggers that activate MAPKs during innate immune responses in plants.

Early signaling events induced by elicitors of plant defenses

Plant pathogen attacks are perceived through pathogen-issued compounds or plant-derived molecules that elicit defense reactions. Despite the large variety of elicitors, general schemes for cellular elicitor signaling leading to plant resistance can be drawn. In this article, we review early signaling events that happen after elicitor perception, including reversible protein phosphorylations, changes in the activities of plasma membrane proteins, variations in free calcium concentrations in cytosol and nucleus, and production of nitric oxide and active oxygen species. These events occur within the first minutes to a few hours after elicitor perception. One specific elicitor transduction pathway can use a combination or a partial combination of such events which can differ in kinetics and intensity depending on the stimulus. The links between the signaling events allow amplification of the signal transduction and ensure specificity to get appropriate plant defense reactions. This review first describes the early events induced by cryptogein, an elicitor of tobacco defense reactions, in order to give a general scheme for signal transduction that will be use as a thread to review signaling events monitored in different elicitor or plant models.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Ajmalicine production in methyl jasmonate-induced Catharanthus roseus cell cultures depends on Ca2+ level

Cytosolic Ca(2+) and jasmonate mediate signals that induce defense responses in plants. In this study, the interaction between Ca(2+) and methyl jasmonate (MJ) in modulating defense responses was investigated by monitoring ajmalicine production in Catharanthus roseus suspension cultures. C. roseus suspensions were treated with nine combinations of CaCl(2) (3, 23, and 43 mM) and MJ (0, 10, and 100 microM) on day 6 of growth. Increased Ca(2+) influx through the addition of extracellular CaCl(2) suppressed ajmalicine production in MJ-induced cultures. The highest ajmalicine production (4.75 mg/l) was observed when cells were treated with a low level of calcium (3 mM) combined with a high level of MJ (100 microM). In the presence of 3 mM CaCl(2) in the medium, the addition of Ca(2+) chelator EGTA (1, 2.5, and 5 mM) or Ca(2+) channel blocker verapamil (1, 10, and 50 muM) to MJ-induced (100 microM) cultures on day 6 also inhibited ajmalicine production at higher levels of the Ca(2+) inhibitors. Hence, ajmalicine production in MJ-induced C. roseus cultures depended on the intracellular Ca(2+) concentration and a low extracellular Ca(2+) concentration (3 mM) enhanced MJ-induced ajmalicine production.

Enhanced silymarin accumulation is related to calcium deprivation in cell suspension cultures of Silybum marianum (L.) Gaertn

Elimination of calcium ions from the medium of cell cultures of Silybum marianum (L.) Gaertn increased flavonolignan production. Silymarin accumulation was not altered by treatment of cultures with the calcium ionophore A23187. The specific Ca2+ chelator, EGTA, enhanced the silymarin content in cells by 200%, and its secretion by 3-4 times. The inorganic ion La3+, as well as the calcium channel inhibitor verapamil, also stimulated production. Several reagents known to block intracellular calcium movement, such as ruthenium red, thapsigargin and TMB-8 appreciably increased silymarin accumulation. These results suggest that inhibition of external and internal calcium fluxes plays a significant role in flavonolignan metabolism of S. marianum cell cultures.

Proteomic analysis of amphiphilic proteins of hexaploid wheat kernels

Wheat proteins and specially gluten proteins have been well studied and are closely associated with baking products. Amphiphilic proteins (proteins that are soluble using nonionic detergent Triton X-114 ) also play an important role in wheat quality. Some of them, like puroindolines, are lipid binding proteins, and are strongly linked to dough foaming properties and to fine crumb texture. However many amphiphilic proteins are still unknown and both their physiological and technological functions remain to be analysed. In order to explore these proteins, proteomic analysis was carried out using 81 F9 lines, progeny obtained from an interspecific cross "W7984"x"Opata", and already used to built a map of more than 2000 molecular markers (International Triticeae Mapping Initiative, ITMImap). Two-dimensional electrophoresis (immobilized pH gradient (pH 6-11)x sodium dodecyl sulfate-polyacrylamide gel electrophoresis) was performed on amphiphilic proteins with three to five replicates for each line. Silver stained gels were analysed using Melanie 3 software. Genetic determinism was carried out on 170 spots segregating between the two parental hexaploïd wheats. Many of these spots were mapped on different chromosomes of the ITMImap. Spots of interest were identified using matrix-assisted laser desorption/ionization-time of flight and some of them were partly sequenced using electrospray ionization-tandem mass spectrometry. This proteomic approach provided some very useful information about some proteic components linked to bread wheat quality and particularly to kernel hardness.

Mlo, a modulator of plant defense and cell death, is a novel calmodulin-binding protein. Isolation and characterization of a rice Mlo homologue

Transient influx of Ca(2+) constitutes an early event in the signaling cascades that trigger plant defense responses. However, the downstream components of defense-associated Ca(2+) signaling are largely unknown. Because Ca(2+) signals are mediated by Ca(2+)-binding proteins, including calmodulin (CaM), identification and characterization of CaM-binding proteins elicited by pathogens should provide insights into the mechanism by which Ca(2+) regulates defense responses. In this study, we isolated a gene encoding rice Mlo (Oryza sativa Mlo; OsMlo) using a protein-protein interaction-based screening of a cDNA expression library constructed from pathogen-elicited rice suspension cells. OsMlo has a molecular mass of 62 kDa and shares 65% sequence identity and scaffold topology with barley Mlo, a heptahelical transmembrane protein known to function as a negative regulator of broad spectrum disease resistance and leaf cell death. By using gel overlay assays, we showed that OsMlo produced in Escherichia coli binds to soybean CaM isoform-1 (SCaM-1) in a Ca(2+)-dependent manner. We located a 20-amino acid CaM-binding domain (CaMBD) in the OsMlo C-terminal cytoplasmic tail that is necessary and sufficient for Ca(2+)-dependent CaM complex formation. Specific binding of the conserved CaMBD to CaM was corroborated by site-directed mutagenesis, a gel mobility shift assay, and a competition assay with a Ca(2+)/CaM-dependent enzyme. Expression of OsMlo was strongly induced by a fungal pathogen and by plant defense signaling molecules. We propose that binding of Ca(2+)-loaded CaM to the C-terminal tail may be a common feature of Mlo proteins.

The matrix metalloproteinase gene GmMMP2 is activated in response to pathogenic infections in soybean

Matrix metalloproteinases (MMPs) play an important role in host defense responses against pathogens in mammals where their activities lead to the production of antimicrobial peptides. We have identified a novel soybean (Glycine max) metalloproteinase gene, GmMMP2, that is transcriptionally up-regulated in infected tissues. The deduced amino acid sequence indicates that this gene belongs to the MMP family. It is a preproprotein containing an N-terminal signal peptide, a cysteine switch, a zinc-binding catalytic motif, and a C-terminal transmembrane domain. The GmMMP2 expressed in and purified from Escherichia coli exhibited an in vitro enzymatic activity in digesting myelin basic protein. All plant metalloproteinases reported so far have no known functions. However, they have been suggested to be involved in extracellular cell matrix degradation during development or senescence. Our investigations demonstrate that the GmMMP2 transcript levels were rapidly increased in compatible and incompatible interactions of soybean tissues with the oomycete pathogen Phytophthora sojae or the bacterial pathogen Pseudomonas syringae pv. glycinea. In agreement with the GmMMP2 activation, a metalloproteinase activity was gradually increased in suspension-cultured cells following the bacterial infection. GmMMP2 was also activated in response to wounding and dehydration. However, GmMMP2 activation did not correlate with the oxidative burst leading to the hypersensitive response cell death or the tissue senescence progress that involves programmed cell death. Our investigations suggest that GmMMP2 may be involved in a novel defense response of soybean against pathogenic infections.

Unravelling response-specificity in Ca2+ signalling pathways in plant cells

Considerable advances have been made, both in the technologies available to study changes in intracellular cytosolic free Ca²⁺ ([Ca²⁺ ]_i ), and in our understanding of Ca²⁺ signalling cascades in plant cells, but how specificity can be generated from such a ubiquitous component as Ca²⁺ is questionable. Recently the concept of 'Ca²⁺ signatures' has been formulated; tight control of the temporal and spatial characteristics of alterations in [Ca²⁺ ]_i signals is thought to be responsible, at least in part, for the specificity of the response. However, the way in which Ca²⁺ signatures are decoded, which depends on the nature and location of the targets of the Ca²⁺ signals, has received little attention. In a few key systems, progress is being made on how diverse Ca²⁺ signatures might be transduced within cells in response to specific signals. Valuable pieces of the signal-specificity puzzle are being put together and this is illustrated here using some key examples; these emphasize the global importance of Ca²⁺ -mediated signal-transduction cascades in the responses of plants to a wide diversity of extracellular signals. However, the way in which signal specificity is encoded and transduced is still far from clear. Contents Summary 7 I. Introduction: Ca²⁺ as a signal transducer 8 II. Alterations in intracellular [Ca²⁺ ] 8 1. Measuring alterations in [Ca²⁺ ] 8 Imaging [Ca²⁺ ]_i using Ca²⁺ -sensitive dyes 8 Measuring [Ca²⁺ ]_i using aequorin 9 Imaging [Ca²⁺ ]_i using cameleon 10 2. The concept of the 'Ca²⁺ signature 10 3. How might specific Ca²⁺ signatures be generated? 11 Control of intracellular Ca²⁺ release 11 Control of influx of extracellular Ca²⁺ 12 4. Examples of Ca²⁺ signatures and cellular responses to increases in [Ca²⁺ ] 13 Ca²⁺ signatures in stomatal guard cells in response to abscisic acid signals 14 Ca²⁺ signals in response to abiotic stimuli1 8 Ca²⁺ signatures involved in plant-pathogen responses 19 Ca²⁺ signatures in control of plant reproduction 20 Ca²⁺ signatures in root hairs in response to nodulation signals 23 III. Decoding the [Ca²⁺ ]_i signatures 24 1. Coupling Ca²⁺ signals to responses through CaM 26 2. Coupling Ca²⁺ signals to responses through CDPK 27 3. Novel Ca²⁺ binding proteins as primary Ca²⁺ sensors 28 Conclusions and Perspective 28 References 29.

Early events in the signal pathway for the oxidative burst in soybean cells exposed to avirulent pseudomonas syringae pv glycinea

Soybean (Glycine max) cv Williams 82 suspension cultures exhibit an oxidative burst approximately 3 h after challenge with Pseudomonas syringae pv glycinea (Psg) harboring the avrA (avirulence) gene. Pretreatment with the tyrosine (Tyr) kinase inhibitor herbimycin A or the serine/threonine kinase inhibitor K252a abolished the burst and subsequent induction of glutathione S-transferase. However, imposition of a 45-min rest period between pathogen challenge and subsequent addition of the kinase inhibitors resulted in escape from inhibition by herbimycin A, whereas inhibition by K252a persisted. Suramin, a G-protein inhibitor, inhibited the burst if added up to 90 min after pathogen challenge. The burst was also induced by the ion channel generator amphotericin B, and this induction was sensitive to suramin and K252a. Conversely, the ion channel blocker anthracene-9-carboxylate inhibited the Psg:avrA-induced burst. Psg:avrA rapidly induced Tyr phosphorylation of several proteins, and this was inhibited by herbimycin A or anthracene 9-carboxylic acid. These data suggest that the activation of ion channels is followed by an upstream Tyr kinase before the serine/threonine kinase-dependent steps in the signal pathway leading to the oxidative burst. Psg:avrA-dependent induction of phenylalanine ammonia-lyase was not inhibited by herbimycin or suramin, suggesting the operation of different signal pathways for the oxidative burst and phenylpropanoid-derived defense responses.

Recent advances in the study of nod factor perception and signal transduction

Rhizobial lipochitooligosaccharidic Nod factors mediate the specific recognition between leguminous plants and their prokaryotic symbionts. This review summarizes recent findings on the way plants could perceive and transduce these bacterial signals. It starts by summarizing knowledge about Nod factor binding sites, before moving to the potential implications in Nod factor signal transduction of G proteins, root-hair plasma membrane depolarisation, cytoplasmic and extracellular alkalinisation and finally variations in cytoplasmic calcium concentration.

Elicitor-stimulated ion fluxes and O2- from the oxidative burst are essential components in triggering defense gene activation and phytoalexin synthesis in parsley

Fungal elicitor stimulates a multicomponent defense response in cultured parsley cells (Petroselinum crispum). Early elements of this receptor-mediated response are ion fluxes across the plasma membrane and the production of reactive oxygen species (ROS), sequentially followed by defense gene activation and phytoalexin accumulation. Omission of Ca2+ from the culture medium or inhibition of elicitor-stimulated ion fluxes by ion channel blockers prevented the latter three reactions, all of which were triggered in the absence of elicitor by amphotericin B-induced ion fluxes. Inhibition of elicitor-stimulated ROS production using diphenylene iodonium blocked defense gene activation and phytoalexin accumulation. O2- but not H2O2 stimulated phytoalexin accumulation, without inducing proton fluxes. These results demonstrate a causal relationship between early and late reactions of parsley cells to the elicitor and indicate a sequence of signaling events from receptor-mediated activation of ion channels via ROS production and defense gene activation to phytoalexin synthesis. Within this sequence, O2- rather than H2O2 appears to trigger the subsequent reactions.

Transgenic tobacco expressing a foreign calmodulin gene shows an enhanced production of active oxygen species

A strategy for elucidating specific molecular targets of calcium and calmodulin in plant defense responses has been developed. We have used a dominant-acting calmodulin mutant (VU-3, Lys to Arg115) to investigate the oxidative burst and nicotinamide co-enzyme fluxes after various stimuli (cellulase, harpin, incompatible bacteria, osmotic and mechanical) that elicit plant defense responses in transgenic tobacco cell cultures. VU-3 calmodulin differs from endogenous plant calmodulin in that it cannot be methylated post-translationally, and as a result it hyperactivates calmodulin-dependent NAD kinase. Cells expressing VU-3 calmodulin exhibited a stronger active oxygen burst that occurred more rapidly than in normal control cells challenged with the same stimuli. Increases in NADPH level were also greater in VU-3 cells and coincided both in timing and magnitude with development of the active oxygen species (AOS) burst. These data show that calmodulin is a target of calcium fluxes in response to elicitor or environmental stress, and provide the first evidence that plant NAD kinase may be a downstream target which potentiates AOS production by altering NAD(H)/NADP(H) homeostasis.

Tansley Review No. 86 Accumulation of phytoalexins: defence mechanism and stimulus response system

Phytoalexin synthesis is a defence-response- that is characterized by a requirement for a number of distinct elements, all of which must be present for the response to be expressed fully. These same elements: a signal, a cellular receptor, a signal transduction system and a responsive metabolic system, are also used to describe a stimulus-response system. A number of molecular species can function as signal molecules or elicitors of phytoalexin synthesis, including poly- and oligosaccharides, proteins and polypeptides, and fatty acids. Few receptors for elicitors have been identified but those that have been are proteins located on the plasma membrane of the plant. Induction of phytoalexin synthesis involves selective and co-ordinated activation of specific defence response genes, including those encoding the enzymes of phytoalexin synthesis, and these genes constitute the responsive metabolic system. The separate, and distant, locations of the receptor and the responsive genes means that the event in which the signal is perceived by the receptor must be relayed to the genes by means of a second messenger system. Several second messengers are candidates for such a coupling- or signal transduction-system, including udenosine-3',5'-cyclic monophosphate, Ca²⁺ , diacylglycerol and inositol 1,4,5-trisphosphate, active oxygen species and jasmonic acid. Each has been examined as a possible component of the signal transduction system mediating between the elicitor receptor interaction and the phytoalexin synthesis it induces. Analysis of the signalling events is made complex by the simultaneous solicitation by the invading micro-organism of several defence responses, each of which might involve elements of a different signal system. The same complexity is evident which the role of phytoalexin accumulation in resistance is analysed. Evaluation of the contribution made by phytoalexin accumulation towards resistance has been attempted by the use of various inhibitors and enhancers of the process. Transgenic and mutant plants with specific alterations in one or more ot those elements necessary for the plant to respond to the signals for phytoalexin synthesis and other defence responses, are beginning to aid resolution of the complex pattern ot signalling events and the respective roles of the inducible defence mechanisms in resistance. CONTENTS Summary 1 I. Introduction 2 II. Chemistry of phytoalexins 3 III. Phytoalexin accumulation as a determinant of resistance 6 IV. Elicitation of phytoalexin accumulation 11 References 34.

Infection-induced rapid cell death in plants: a means of efficient pathogen defense

The hypersensitive reaction represents one of the major means by which plants actively defend themselves against infection by pathogenic bacteria, fungi, viruses, and nematodes. This complex defense reaction, often associated with the synthesis of phytoalexins (antimicrobial secondary metabolites), involves at the cellular level highly dynamic cytoplasmic rearrangements, rapid metabolic changes, and finally cell death. It also correlates with the rapid and transient activation of various defense-related genes in a region of tissue surrounding infection sites and later, with the systemic increase in expression of a number of other genes. Examination of the reactions of individual living cells of potato leaves infected with Phytophthora infestans enabled the comprehensive description of the dynamic aspects of all stages of the defense response. Cytochemical investigations, employing cultured cells of parsley infected with P. infestans as a versatile model system, have contributed to a better understanding of cytoplasmic and metabolic processes occurring during the defense response, and suggest that hypersensitive cell death requires the preceding activation of respiration and specific metabolic pathways. Key words: defense responses, defense-related genes, hypersensitive reaction, programmed cell death.

Rapid changes in oxidative metabolism as a consequence of elicitor treatment of suspension-cultured cells of French bean (Phaseolus vulgaris L.)

Stressed plant cells often show increased oxygen uptake which can manifest itself in the transient production of active oxygen species, the oxidative burst. There is a lack of information on the redox status of cells during the early stages of biotic stress. In this paper we measure oxygen uptake and the levels of redox intermediates NAD/NADH and ATP and show the transient induction of the marker enzyme for redox stress, alcohol dehydrogenase. Rapid changes in the redox potential of elicitor-treated suspension cultures of French bean cells indicate that, paradoxically, during the period of maximum oxygen uptake the levels of ATP and the NADH/NAD ratio fall in a way that indicates the occurrence of stress in oxidative metabolism. This period coincides with the maximum production of active oxygen species particularly H2O2. The cells recover and start producing ATP immediately of H2O2 production. This indicates that the increased O2 uptake is primarily incorporated into active O2 species. A second consequence of these changes is probably a transient compromising of the respiratory status of the cells as indicated in expression of alcohol dehydrogenase. Elicitor-induced bean ADH was purified to homogeneity and the M(r) 40,000 polypeptide was subjected to amino acid sequencing. 15% of the whole protein was sequenced from three peptides and was found to have nearly 100% sequence similarity to the amino acid sequence for pea ADH1 (PSADH1). The cDNA coding for the pea enzyme was used to demonstrate the transient induction of ADH mRNA in elicitor-treated bean cells. Enzyme activity levels also increased transiently subsequently. Increased oxygen uptake has previously been thought to be associated with provision of energy for the changes in biosynthesis that occur rapidly after perception of the stress signal. However the present work shows that this rapid increase in oxygen uptake as a consequence of elicitor action is not wholly associated with respiration.

Microbial recognition and activation of plant defense systems

Molecules released or generated during microbial entry (elicitors) are recognized by components of plant cells, ultimately resulting in the induction of a battery of plant defense responses. The molecular mechanisms underlying these signaling systems, as well as the plant defense responses they control, are becoming increasingly well characterized.

Evidence that generation of inositol 1,4,5-trisphosphate and hydrolysis of phosphatidylinositol 4,5-bisphosphate are rapid responses following addition of fungal elicitor which induces phytoalexin synthesis in lucerne (Medicago sativa) suspension culture cells

Treatment of lucerne suspension culture cells with glycoprotein elicitor from the phytopathogenic fungus Verticillium albo-atrum R & B triggers Ca(2+)-mediated induction of antimicrobial secondary metabolites termed phytoalexins. The present study investigated the possible role of polyphosphoinositide signal transduction in phytoalexin elicitation. Within 1 min of addition of elicitor to lucerne suspension culture cells we found a 100-160% (15-25 pmol/g fresh wt) increase in the level of compound with chromatographic and electrophoretic properties expected for an inositol trisphosphate (InsP3) and which was strongly bound by an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-specific binding protein; after 3 min the level of this compound had fallen below that observed prior to elicitor challenge. In 32P-prelabelled cells, the relative proportion of radioactivity which cochromatographed with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to have decreased by 48% 1 min after elicitor addition and that rapid depletion of membrane lipid radioactivity was specific to this lipid fraction. The rapid, transient increase in level of Ins(1,4,5)P3 and concomitant fall in PtdIns(4,5)P2 suggests that Ins(1,4,5)P3 generated by hydrolysis of PtdIns(4,5)P2 may provide a Ca(2+)-mobilizing signal in phytoalexin elicitation in lucerne.

Cell wall oligogalacturonides increase cytosolic free calcium in carrot protoplasts

The mode of action of cell wall elicitors in the induction of various plant cell responses, such as the activation of host defence mechanisms, is unknown, but signal transduction through cytosolic free calcium ([Ca2+]i) has been suggested. This paper shows that polygalacturonic acid or oligogalacturonides cause a prolonged increase in [Ca2+]i of carrot protoplasts within 20 min of induction. Our data support the view that a special conformation of the oligogalacturonides possessing >9 residues is necessary to induce an elevation in [Ca2+]i. The localization of [Ca2+]i elevation around the periphery of protoplast cytoplasm and the inhibition of the response with verapamil suggest that exogenous Ca2+ is the major source for the rise in [Ca2+]i.

Emerging strategies for enhancing crop resistance to microbial pathogens

There are marked differences in the pattern of host gene expression in incompatible plant:microbial pathogen interactions compared with compatible interactions, associated with the elaboration of inducible defenses. Constitutive expression of genes encoding a chitinase or a ribosome-inactivating protein in transgenic plants confers partial protection against fungal attack, and a large repertoire of such antimicrobial genes has been identified for further manipulation. In addition, strategies are emerging for the manipulation of multigenic defenses such as lignin deposition and synthesis of phytoalexin antibiotics by overexpression of genes encoding rate determining steps, modification of transcription factors or other regulatory genes, and engineering production of novel phytoalexins by interspecies transfer of biosynthetic genes. The imminent cloning of disease resistance genes, further molecular dissection of stress signal perception and transduction mechanisms, and identification of genes that affect symptom development will provide attractive new opportunities for enhancing crop protection. Combinatorial integration of these novel strategies into ongoing breeding programs should make an important contribution to effective, durable field resistance.

The protein kinase inhibitor, K-252a, decreases elicitor-induced Ca2+ uptake and K+ release, and increases coumarin synthesis in parsley cells

An elicitor preparation from fungal cell walls known to induce coumarin synthesis in suspension-cultured parsley cells also elicits a rapid and transient Ca2+ uptake, K+ release and external alkalinization, and increases uptake of 45Ca2+ into the cells. The latter three responses were inhibited by the protein kinase inhibitor K-252a at 0.2 microM. Elicitor-induced coumarin synthesis, a process which requires gene activation, was greatly enhanced by K-252a. These results suggest that protein phosphorylation might be involved in the initial steps of signal transduction as well as in the long-term induction of coumarin synthesis.

An endogenous factor from soybean (Glycine max L.) cell cultures activates phosphorylation of a protein which is dephosphorylated in vivo in elicitor-challenged cells

The existence of specific binding sites for a β-glucan elicitor of phytoalexin synthesis derived from the fungus Phytophthora megasperma f.sp. glycinea at the plasma membrane of soybean (Glycine max L.) tissues (W.E. Schmidt, J. Ebel (1987) Proc. Natl. Acad. Sci. USA 84, 4117-4121) might imply that stimulation of phytoalexin formation by the elicitor is a membrane-mediated process. Addition of the β-glucan elicitor to soybean cellsuspension cultures, which has previously been shown to induce phytoalexin accumulation, also results in rapid changes in the phosphate turnover of several phosphoproteins. The effect of the elicitor on protein phosphorylation was tested after labeling of the cells with [(32)P]orthophosphate. As shown by analysis using one-and two-dimensional gel electrophoresis, decreases as well as increases in the labeling of several phosphoroteins occurred rapidly, being detectable within 5 min after elicitor application, and persisted for at least 15 min. As judged by their relative molecular masses (Mr) and isoelectric points (pI), a number of proteins which were radioactively labeled in vivo were also phosphorylated in vitro by endogenous protein-kinase activity in the presence of Ca(2+). The most pronounced effect was observed with a protein substrate with Mr=69000 and pI=5.7 (pp69) whose phosphate labeling markedly decreased in response to elicitor treatment in vivo. Phosphorylation of pp69 in vitro in the presence of γ-[(32)P]ATP was strongly enhanced by a phosphorylation-stimulating factor (effector) derived from soybean cell cultures and occurred predominantly at serine residues. The effector possessed a low apparent Mr (≤1000), was negatively charged at pH 7.3, and was relatively heat stable. The effector was inactivated by treatment with alkaline phosphatase from calf intestine. Phosphorylation of pp69 was only slightly stimulated by Ca(2+), and was insensitive to cAMP, cGMP, calmodulin, a lipid mixture, a ganglioside mixture, or spermine under the assay conditions used. A 10 mM concentration of 3-phosphoglycerate increased pp69 phosphorylation to the extent of about 50% of that induced by the soybean effector. There was no evidence, however, that such concentrations of 3-phosphoglycerate occurred in effector preparations. The results are discussed in relation to hypothetical signal transduction during elicitor action on soybean cells.