Fumonisin B1-induced cell death in arabidopsis protoplasts requires jasmonate-, ethylene-, and salicylate-dependent signaling pathways

Pseudomonas syringae pv. tomato type III effectors AvrPto and AvrPtoB promote ethylene-dependent cell death in tomato

The type III secretion system (TTSS) of Pseudomonas syringae pv. tomato (Pst) injects into the plant cell effector proteins that play an essential role in the formation of bacterial speck disease. To investigate the molecular roles of TTSS effectors in disease formation, we used a cDNA microarray to analyze the expression of approximately 8600 random tomato genes in response to wild-type Pst strain DC3000 and a mutant lacking a functional TTSS. Many of the differentially expressed genes identified encode proteins associated with hormone response or hormone biosynthesis pathways. Using isogenic mutant strains of DC3000, we monitored host transcriptional changes in response to the TTSS effector proteins AvrPto and AvrPtoB, both of which are important virulence factors on susceptible tomato lines. We found that AvrPto and AvrPtoB induce a set of host genes involved in ethylene biosynthesis and signaling, and in particular they regulate the expression of two genes, LeACO1 and LeACO2, encoding the ethylene-forming enzyme ACC oxidase. Analysis of transgenic tomato lines with diminished ACC oxidase activity revealed that ethylene production by the host is required for the full virulence activity of both AvrPto and AvrPtoB. AvrPto and AvrPtoB therefore appear to promote enhanced disease in tomato leaves, in part, by upregulating genes involved in ethylene production.

Zinc induces mitogen-activated protein kinase activation mediated by reactive oxygen species in rice roots

It is well known that zinc (Zn) is one of the micronutrients essential for normal growth and development of plants. However, the molecular mechanisms responsible for the regulation of plant growth by Zn are still not completely understood. The aim of this study was to investigate the signalling transduction pathways activated by Zn. We show that Zn elicited a remarkable increase in myelin basic protein (MBP) kinase activities. By immunoblot analysis, we suggest that Zn-activated 40- and 42-kDa MBP kinases are mitogen-activated protein kinases (MAPK). Pre-treatment of rice roots with reactive oxygen species (ROS) scavenger, sodium benzoate, was able to effectively prevent Zn-induced MAPK activation. However, phosphoinositide 3-kinase (PI-3K) inhibitor, LY294002, was unable to inhibit Zn-induced MAPK activation. These results suggest that the ROS may function in the Zn-triggered MAPK signalling pathway in rice roots.

AtGLB1 enhances the tolerance of Arabidopsis to hydrogen peroxide stress

Hydrogen peroxide (H2O2) as a widespread molecule plays an important role in plant stress responses. Here, we showed that an Arabidopsis line overexpressing hemoglobin 1 (AtGLB1) can enhance its tolerance to severe hypoxic stress. In our research, Arabidopsis lines with different hemoglobin levels were employed to study the relationship between H2O2 level and the tolerance to hypoxic stress. The relatively low endogenous H2O2 level of AtGLB1-overexpressing plants could be one of the main factors for the increased tolerance of plants to hypoxic stress. Further investigation indicated that the activity of the antioxidant system involved in scavenging H2O2 increased in all three lines examined during hypoxic treatment, while only the line overexpressing AtGLB1 could retain these relatively high levels up to 48 h of the treatment, suggesting that the antioxidant system might play a role in the low H2O2 level of Arabidopsis overexpressing AtGLB1.

Two Pseudomonas syringae Type III Effectors Inhibit RIN4-Regulated Basal Defense in Arabidopsis

Plant cells have two defense systems that detect bacterial pathogens. One is a basal defense system that recognizes complex pathogen-associated molecular patterns (PAMPs). A second system uses disease-resistance (R) proteins to recognize type lll effector proteins that are delivered into the plant cell by the pathogen's type III secretion system. Here we show that these two pathways are linked. We find that two Pseudomonas syringae type III effectors, AvrRpt2 and AvrRpm1, inhibit PAMP-induced signaling and thus compromise the host's basal defense system. RIN4 is an Arabidopsis protein targeted by AvrRpt2 and AvrRpm1 for degradation and phosphorylation, respectively. We find that RIN4 is itself a regulator of PAMP signaling. The R proteins, RPS2 and RPM1, sense type III effector-induced perturbations of RIN4. Thus, R proteins guard the plant against type III effectors that inhibit PAMP signaling and provide a mechanistic link between the two plant defense systems.

The role of salicylic acid in the induction of cell death in Arabidopsis acd11

Salicylic acid (SA) is implicated in the induction of programmed cell death (PCD) associated with pathogen defense responses because SA levels increase in response to PCD-inducing infections, and PCD development can be inhibited by expression of salicylate hydroxylase encoded by the bacterial nahG gene. The acd11 mutant of Arabidopsis (Arabidopsis thaliana L. Heynh.) activates PCD and defense responses that are fully suppressed by nahG. To further study the role of SA in PCD induction, we compared phenotypes of acd11/nahG with those of acd11/eds5-1 and acd11/sid2-2 mutants deficient in a putative transporter and isochorismate synthase required for SA biosynthesis. We show that sid2-2 fully suppresses SA accumulation and cell death in acd11, although growth inhibition and premature leaf chlorosis still occur. In addition, application of exogenous SA to acd11/sid2-2 is insufficient to restore cell death. This indicates that isochorismate-derived compounds other than SA are required for induction of PCD in acd11 and that some acd11 phenotypes require NahG-degradable compounds not synthesized via isochorismate.

Advances on plant products with potential to control toxigenic fungi: a review

In recent years, public pressure to reduce the use of synthetic fungicides in agriculture has increased. Concerns have been raised about both the environmental impact and the potential health risk related to the use of these compounds. Therefore, considerable efforts have been made towards the development of alternative crop protectants. The European Commission has been actively encouraging the development and commercial implementation of new compounds known as 'green chemicals'. In this context, an increase in the knowledge of plant defence responses to toxigenic fungi, which is covered in this review, will help to discover new plant products with antifungal activity and to design new strategies to improve plant resistance to these pathogens.

Polyamines inhibit NADPH oxidase-mediated superoxide generation and putrescine prevents programmed cell death induced by polyamine oxidase-generated hydrogen peroxide

Our previous results indicate that during protoplast isolation an oxidative burst occurs [A.K. Papadakis and KA Roubelakis-Angelakis (1999) Plant Physiol 127:197-205] and that suppression of totipotency is correlated with reduced antioxidant activity and low redox state [A.K. Papadakis et al. (2001b) Plant Physiol 126:434-444]. Polyamines are known to affect cell development and to act as antioxidants. Polyamines applied during isolation of tobacco (Nicotiana tabacum L.) protoplasts reduced the accumulation of O2*- but not that of H2O2. This antioxidant effect is probably due to the inhibition of microsomal membrane NADPH oxidase, which occurred in a concentration-dependent manner, with spermine exerting the highest inhibitory effect. However, during protoplast culture, polyamine oxidase activity increased severalfold in spermidine- and spermine-treated protoplasts, concomitant with H2O2 titers. A cell death program was executed in untreated protoplasts, as documented by membrane malfunction, induced DNase activity, DNA fragmentation and a positive TUNEL reaction. Protoplast cell death was prevented in protoplasts treated with putrescine, but not by treatment with spermidine or spermine, which rather had the opposite effect. The data presented suggest that PAs may be implicated in the expression of plant protoplast totipotency.

Arabidopsis AtSPL14, a plant-specific SBP-domain transcription factor, participates in plant development and sensitivity to fumonisin B1

The recessive Arabidopsis thalianafumonisin B1-resistant (fbr6) mutant was identified by its ability to survive in the presence of a programmed cell death (PCD)-inducing fungal toxin FB1. The fbr6 mutant also displays altered plant architecture in the absence of FB1, most notably elongated petioles and enhanced leaf margin serration. These phenotypes are a result of a T-DNA insertion in the SQUAMOSA promoter binding protein (SBP) domain gene, AtSPL14. AtSPL14 encodes a plant-specific protein with features characteristic of a transcriptional regulator, including a nuclear localization signal sequence, a plant-specific DNA binding domain (the SBP box), and a protein interaction motif (ankyrin repeats). A transiently expressed fusion of the AtSPL14 protein to green fluorescent protein is directed to the plant nucleus. DNA sequences immediately upstream of the translation start site direct expression of the beta-glucuronidase reporter gene primarily in the vascular tissues, consistent with the phenotypes of the fbr6 mutant. AtSPL14 activates transcription in yeast, with a transactivation domain residing within the N-terminal region of the protein. Recombinant AtSPL14 protein binds A. thaliana genomic DNA in vitro in the absence of other proteins. These results indicate that FBR6/SPL14 functions as a transcriptional regulator that plays a role not only in sensitivity to FB1, but also in the development of normal plant architecture.

Signal crosstalk and induced resistance: straddling the line between cost and benefit

This review discusses recent progress in our understanding of signaling in induced plant resistance and susceptibility to pathogens and insect herbivores, with a focus on the connections and crosstalk among phytohormone signaling networks that regulate responses to these and other stresses. Multiple stresses, often simultaneous, reduce growth and yield in plants. However, prior challenge by a pathogen or insect herbivore also can induce resistance to subsequent challenge. This resistance, or failure of susceptibility, must be orchestrated within a larger physiological context that is strongly influenced by other biotic agents and by abiotic stresses such as inadequate light, temperature extremes, drought, nutrient limitation, and soil salinity. Continued research in this area is predicated on the notion that effective utilization of induced resistance in crop protection will require a functional understanding of the physiological consequences of the "induced" state of the plant, coupled with the knowledge of the specificity and compatibility of the signaling systems leading to this state. This information may guide related strategies to improve crop performance in suboptimal environments, and define the limits of induced resistance in certain agricultural contexts.

Ethylene Biosynthesis and Signaling: An Overview

Hormones are key regulators of plant growth and development. Genetic and biochemical studies have identified major factors that mediate ethylene biosynthesis and signal transduction. Substantial progress in the elucidation of the ethylene signal transduction pathway has been made, mainly by research on Arabidopsis thaliana. Research on ethylene biosynthesis and its regulation provided new insights, particularly on the posttranslational regulation of ethylene synthesis and the feedback from ethylene signal transduction. The identification of new components in the ethylene-response pathway and the elucidation of their mode of action provide a framework for understanding not only how plants sense and respond to this hormone but also how the signal is integrated with other inputs, ultimately determining the plant phenotype.

Concurrent activation of cell death-regulating signaling pathways by singlet oxygen in Arabidopsis thaliana

Upon a dark/light shift the conditional flu mutant of Arabidopsis starts to generate singlet oxygen ((1)O(2)), a non-radical reactive oxygen species that is restricted to the plastid compartment. Immediately after the shift, plants stop growing and develop necrotic lesions. We have established a protoplast system, which allows detection and characterization of the death response in flu induced by the release of (1)O(2). Vitamin B6 that quenches (1)O(2) in fungi was able to protect flu protoplasts from cell death. Blocking ethylene production was sufficient to partially inhibit the death reaction. Similarly, flu mutant seedlings expressing transgenic NahG were partially protected from the death provoked by the release of (1)O(2), indicating a requirement for salicylic acid (SA) in this process, whereas in cells depleted of both, ethylene and SA, the extent of cell death was reduced to the wild-type level. The flu mutant was also crossed with the jasmonic acid (JA)-depleted mutant opr3, and with the JA, OPDA and dinor OPDA (dnOPDA)-depleted dde2-2 mutant. Analysis of the resulting double mutants revealed that in contrast to the JA-induced suppression of H(2)O(2)/superoxide-dependent cell death reported earlier, JA promotes singlet oxygen-mediated cell death in flu, whereas other oxylipins such as OPDA and dnOPDA antagonize this death-inducing activity of JA.

Induction of programmed cell death in lily by the fungal pathogen Botrytis elliptica

SUMMARY The genus Botrytis contains necrotrophic plant pathogens that have a wide host range (B. cinerea) or are specialized on a single host species, e.g. B. elliptica on lily. In this study, it was found that B. elliptica-induced cell death of lily displays hallmark features of animal programmed cell death or apoptosis including cytoplasmic shrinkage, nuclear DNA fragmentation and the accumulation of NO as well as H(2)O(2). A pharmacological approach showed that B. elliptica-induced cell death could be modulated by serine and cysteine protease inhibitors including one caspase inhibitor. Blocking phosphatase activity stimulated cell death and concomitant lesion formation, suggesting that B. elliptica-induced cell death is mediated by kinase/phosphatase pathways. Blocking Ca(2+) influx restricted cell death. Blocking steps of sphingolipid biosynthesis delayed lily cell death for several days. B. elliptica culture filtrate (CF) was able to induce lily cell death by means of secreted proteins. Induction of cell death is necessary and sufficient for pathogenicity and host specialization because prior infiltration of B. elliptica CF enabled subsequent infection of lily by the otherwise incompatible pathogens B. cinerea and B. tulipae. The secreted B. elliptica proteins also induced cell death in some but not all Arabidopsis accessions and mutants. Arabidopsis accessions that respond to infiltration of B. elliptica CF also display cell death symptoms upon inoculation with B. elliptica conidia.

The mitochondrion--an organelle commonly involved in programmed cell death in Arabidopsis thaliana

Plant cells undergoing programmed cell death (PCD) at late stages typically show chromatin condensation and endonucleolytic cleavage prior to obvious membrane or organelle ultrastructural changes. To investigate possible early PCD-associated events, we used microscopic observations and flow cytometry to quantitate mitochondrial membrane potential (DeltaPsim) changes during PCD at the single cell and population levels using Arabidopsis protoplasts. A DeltaPsim loss was commonly induced early during plant PCD and was important for PCD execution, as evidenced by the concomitant reduction of the change in DeltaPsim and PCD by cyclosporin A, which inhibits mitochondrial permeability transition pores in animal cells. DeltaPsim loss occurred prior to nuclear morphological changes and was only associated with mitochondrial cytochrome c release (an apoptotic trigger in animals) in response to one of three PCD elicitors. Three different stimuli in wild type implicated DeltaPsim changes in PCD: ceramide, protoporphyrin IX, and the hypersensitive response elicitor AvrRpt2. Additionally, the behavior of the conditional ectopic cell death mutant accelerated cell death2 and ACD2-overproducing plants also implicated DeltaPsim alteration as key for PCD execution. Because ACD2 is largely a chloroplast component in mature plants, the observation that the cell death in acd2 mutants requires changes in mitochondrial functions implicates communication between chloroplasts and mitochondria in mediating PCD activation. We suggest that DeltaPsim loss is a common early marker in plant PCD, similar to what has been documented in animals. However, unlike in animal cells, in plant cells, mitochondrial cytochrome c release is not an obligatory step in PCD control.

Light conditions influence specific defence responses in incompatible plant-pathogen interactions: uncoupling systemic resistance from salicylic acid and PR-1 accumulation

In incompatible plant-pathogen interactions, disease resistance is generated by rapid activation of a multitude of plant defence reactions. Little is known about the dependency of these resistance responses on external factors. The plasticity of plant defence mechanisms in terms of light conditions is studied here. Interaction of Arabidopsis thaliana (L.) Heynh. with an avirulent strain of Pseudomonas syringae pv. maculicola in the dark resulted in increased apoplastic bacterial growth and therefore reduced local resistance as compared to an infection process in the presence of light. Several characteristic defence reactions, including activation of phenylalanine ammonia-lyase, accumulation of salicylic acid (SA), expression of the pathogenesis-related protein PR-1 and the development of a microscopically defined hypersensitive response, proved to be light dependent. In contrast, the extent of the oxidative burst, as estimated by induction of the protectant gene glutathione- S-transferase, was not weakened by the absence of light. Moreover, pathogen-induced accumulation of jasmonic acid, production of the phytoalexin camalexin and transcriptional induction of a pathogen-inducible myrosinase were even more pronounced in the dark. Apart from affecting local defence responses, light also influenced the establishment of systemic acquired resistance (SAR). SAR development in response to infection by avirulent bacteria was completely lost when the primary infection process occurred in the absence of light. SAR developed both under medium (70 micromol photons m(-2) s(-1)) and strong (500 micromol photons m(-2) s(-1)) light conditions but was in the latter case not associated with an accumulation of SA and PR-1 in systemic leaves, demonstrating that SAR can be executed independently from these molecular SAR markers. Our results are consistent with the notion that SA accumulation in infected primary leaves is necessary for induction of systemic resistance and indicate that defence mechanisms different from SA signalling and PR-protein action exist in systemic tissue to confer resistance during SAR.

Transcriptional profiling by cDNA-AFLP and microarray analysis reveals novel insights into the early response to ethylene in Arabidopsis

A comprehensive transcriptome analysis by means of cDNA-amplified fragment length polymorphism (AFLP) and cDNA-microarray technology was performed in order to gain further understanding of the molecular mechanisms of immediate transcriptional response to ethylene. Col-0 plants were treated with exogenous ethylene and sampled at six different time-points ranging from 10 min until 6 h. In order to isolate truly ethylene-responsive genes, both the ethylene-insensitive mutant ein2-1 and the constitutive mutant (ctr1-1) were analysed in parallel by cDNA-AFLP while ein2-1 was included for the microarray experiment. Out of the cDNA-transcript profiling covering about 5% of the Arabidopsis transcriptome, 46 ethylene-responsive genes were isolated, falling in different classes of expression pattern and including a number of novel genes. Out of the 6008 genes present on the chip, 214 genes were significantly (alpha = 0.001) differentially expressed between Col-0 and ein2-1 over time. Cluster analysis and functional grouping of co-regulated genes allowed to determine the major ethylene-regulated classes of genes. In particular, a large number of genes involved in cell rescue, disease and defence mechanisms were identified as early ethylene-regulated genes. Furthermore, the data provide insight into the role of protein degradation in ethylene signalling and ethylene-regulated transcription and protein fate. Novel interactions between ethylene response and responses to several other signals have been identified by this study. Of particular interest is the overlap between ethylene response and responses to abscisic acid, sugar and auxin. In conclusion, the data provide unique insight into early regulatory steps of ethylene response.

Ethylene response factor 1 mediates Arabidopsis resistance to the soilborne fungus Fusarium oxysporum

Ethylene response factor 1 (ERF1) is a transcriptional factor from Arabidopsis thaliana that regulates plant resistance to the necrotrophic fungi Botrytis cinerea and Plectosphaerella cucumerina and whose overexpression enhances resistance to these fungi. Here, we show that ERF1 also mediates Arabidopsis resistance to the soilborne fungi Fusarium oxysporum sp. conglutinans and F. oxysporum f. sp. lycopersici, because its constitutive expression in Arabidopsis confers enhanced resistance to these pathogens. Expression of ERF1 was upregulated after inoculation with F. oxysporum f. sp. conglutinans, and this response was blocked in ein2-5 and coi1-1 mutants, impaired in the ethylene (ET) and jasmonic acid (JA) signal pathways, respectively, which further indicates that ERF1 is a downstream component of ET and JA defense responses. The signal transduction network controlling resistance to F. oxysporum fungi was explored using signaling-defective mutants in ET (ein2-5), JA (jar1-1), and salicylic acid (SA) (NahG, sid2-1, eds5-1, npr1-1, pad4-1, eds1-1, and pad2-1) transduction pathways. This analysis revealed that Arabidopsis resistance to F. oxysporum requires the ET, JA, and SA signaling pathways and the NPR1 gene, although it is independent of the PAD4 and EDS1 functions.

Sensing and signalling during plant flooding

Flooding is a major issue for plant survival in many regions of the world. Soil inundation induces multiple plant physiological dysfunctions, leading to a decline in plant growth and survival capacity. Some of the most important effects of flooding include a reduction in water and nutrient uptake and a decrease in metabolism. Prolonged soil flooding will also ultimately lead to anoxia conditions with profound effects on plant respiratory metabolism. However, it is still unclear which signals and which sensory mechanisms are responsible for triggering the plant response. In contrast, it is now established that flooding responses are typified by enhanced ethylene production, accompanied by a signalling cascade which includes a network of hormones and other common secondary signalling molecules. In recent years, there has been significant progress in the understanding of some of the signalling pathways involved during plant stress responses. Here, we present an overview of recent hypothesises on sensing and signalling during plant flooding.

An introduction to plant sphingolipids and a review of recent advances in understanding their metabolism and function

Sphingolipids are ubiquitous constituents of eukaryotic cells, and have been intensively investigated in mammals and yeast for decades. Aspects of sphingolipid biochemistry in plants have been explored only recently. To date, progress has been made in determining the structure and occurrence of sphingolipids in plant tissues; in characterizing the enzymatic steps involved in production and turnover of sphingolipids (and, in some cases, the genes encoding the relevant enzymes); and in identifying a variety of biological functions for sphingolipids in plants. Given that these efforts are far from complete and much remains to be learned, this review represents a status report on the burgeoning field of plant sphingolipid biochemistry. Contents Summary 677 I. Introduction 678 II. Plant sphingolipid structure 678 III. Sphingolipid metabolism in plants 683 IV. Sphingolipid functions in plants 693 V. Conclusions 696 Acknowledgements 696 References 696.

Divergent roles in Arabidopsis thaliana development and defense of two homologous genes, aberrant growth and death2 and AGD2-LIKE DEFENSE RESPONSE PROTEIN1, encoding novel aminotransferases

The disease-resistant Arabidopsis thaliana aberrant growth and death2 (agd2-1) mutant has elevated levels of the defense signal salicylic acid (SA), altered leaf morphology, and mild dwarfism. AGD2 and its close homolog ALD1 (for AGD2-LIKE DEFENSE RESPONSE PROTEIN1) encode aminotransferases that act on an overlapping set of amino acids in vitro. However, kinetic parameters indicate that AGD2 and ALD1 may drive the aminotransferase reaction in opposite directions. ALD1-deficient mutants have the opposite phenotypes from agd2-1, showing reduced SA production and increased disease susceptibility. Furthermore, ALD1 transcript levels are elevated in agd2-1 and are induced in the wild type by bacterial pathogen infection. ALD1 is responsible for some of the elevated SA content and a majority of the disease resistance and dwarfism of agd2-1. A complete knockout of AGD2 renders embryos inviable. We suggest that AGD2 synthesizes an important amino acid-derived molecule that promotes development and suppresses defenses, whereas ALD1 generates a related amino acid-derived molecule important for activating defense signaling.

Ceramides modulate programmed cell death in plants

The balance between the bioactive sphingolipid ceramide and its phosphorylated derivative has been proposed to modulate the amount of programmed cell death (PCD) in eukaryotes. We characterized the first ceramide kinase (CERK) mutant in any organism. The Arabidopsis CERK mutant, called accelerated cell death 5, accumulates CERK substrates and shows enhanced disease symptoms during pathogen attack and apoptotic-like cell death dependent on defense signaling late in development. ACD5 protein shows high specificity for ceramides in vitro. Strikingly, C2 ceramide induces, whereas its phosphorylated derivative partially blocks, plant PCD, supporting a role for ceramide phosphorylation in modulating cell death in plants.

Increased tryptophan decarboxylase and monoamine oxidase activities induce Sekiguchi lesion formation in rice infected with Magnaporthe grisea

Sekiguchi lesion (sl)-mutant rice infected with Magnaporthe grisea showed increased light-dependent tryptophan decarboxylase (TDC) and monoamine oxidase (MAO) activities. TDC and MAO activities were observed before the penetration of M. grisea to rice cells and maintained high levels even after Sekiguchi lesion formation. Light-dependent expression of TDC gene was observed in leaves inoculated with M. grisea before Sekiguchi lesion formation. Spore germination fluid (SGF) of M. grisea also induced Sekiguchi lesion formation accompanied by increased enzymes activities and tryptamine accumulation. Sekiguchi lesion was also induced by treatments with tryptamine and beta-phenylethylamine, which are substrates for MAO, but was not induced by non-substrates such as indole-3-propionic acid, (+/-)-phenylethylamine and tryptophan under light. Light-dependent induction of Sekiguchi lesion by tryptamine was significantly inhibited in the presence of MAO inhibitors, metalaxyl and semicarbazide, and H2O2-scavengers, ascorbic acid and catalase. H2O2 in M. grisea-infected leaves with and without Sekiguchi lesions was demonstrated directly in situ by strong 3,3'-diaminobenzidine (DAB) staining. On the other hand, H2O2 induced Sekiguchi lesions on leaves of cv. Sekiguchi-asahi under light, but not in darkness. This difference was associated with the decrease of catalase activity in infected leaves under light and the absence of decrease in darkness. We hypothesize that the H2O2-induced breakdown of cellular organelles such as chloroplasts and mitochondria in mesophyll cells may cause high TDC and MAO activities and the development of Sekiguchi lesion, and that the sl gene products in wild-type rice may function as a suppressor of organelle breakdown caused by chemical or environmental stress.

Overproduced ethylene causes programmed cell death leading to temperature-sensitive lethality in hybrid seedlings from the cross Nicotiana suaveolens x N. tabacum

Reproductive isolation mechanisms (RIMs) often become obstacles in crossbreeding. Hybrid lethality is a subtype of RIM but its physiological mechanism remains poorly elucidated. Interspecific hybrids of Nicotiana suaveolens Lehm. x N. tabacum L. cv. Hicks-2 expressed temperature-sensitive lethality. This lethality was induced by programmed cell death (PCD) that was accompanied by the characteristic changes of animal apoptosis in hybrid seedlings at 28 degrees C but not at 36 degrees C. When hybrid seedlings were cultured at 28 degrees C, DNA fragmentation started in the cotyledon, and nuclear fragmentation subsequently progressed with lethal symptoms spreading throughout the seedlings. At 28 degrees C, ethylene production in hybrid seedlings was detectable at a high level compared with the level in parental seedlings. In contrast, the ethylene production rate in hybrid seedlings cultured at 36 degrees C was equal to that in parental seedlings. Treatment with ethylene biosynthetic inhibitors, amino-oxyacetic acid and amino-ethoxyvinyl glycine, suppressed lethal symptoms and apoptotic changes, and also prolonged survival of hybrid seedlings. Thus, the increase in the ethylene production rate correlated closely with expression of lethal symptoms and apoptotic changes in hybrid seedlings. From these observations, we conclude that overproduced ethylene acts as an essential factor mediating PCD and subsequent lethality in hybrid seedlings. Furthermore, the present study has provided the first evidence that ethylene is involved in the phenomenon of hybrid lethality.

Synthesis of 4-hydroxysphinganine and characterization of sphinganine hydroxylase activity in corn

Sphinganine and 4-hydroxysphinganine (phytosphingosine) are the predominant free long-chain bases in lipid extracts of plant tissues. While the synthesis of sphinganine in plants has been investigated, the metabolic origin of 4-hydroxysphinganine is not known. Three different approaches utilizing fumonisin B(1), an inhibitor of sphinganine acylation, alone or in combination with beta-chloroalanine, an inhibitor of sphinganine synthesis, were used to establish that free 4-hydroxysphinganine is produced in excised corn shoots by the direct hydroxylation of sphinganine and not from the breakdown of complex sphingolipids. Sphinganine hydroxylase activity was characterized in microsomes isolated from corn. The enzyme was found to utilize D-erythro-sphinganine (with half-maximal activity observed at a substrate concentration of approximately 60 microM) and either NADPH (K(m)=33 microM) or NADH (K(m)=58 microM) as substrates. Ceramide hydroxylation was also demonstrated in corn microsomes, and the lack of competition between ceramide and sphinganine suggests the presence of distinct enzymes responsible for hydroxylating these two substrates. Using marker assays, sphinganine hydroxylase activity was localized to the endoplasmic reticulum. Sphinganine hydroxylase activity in microsomes isolated from corn shoots treated with fumonisin B(1) increased more than 3-fold compared to controls. The results of this study shed light on sphingolipid long-chain base synthesis and modification in plant tissues and suggest a possible contribution of sphinganine hydroxylase in manifesting the effects of fumonisin in plants.

The unusual Arabidopsis extensin gene atExt1 is expressed throughout plant development and is induced by a variety of biotic and abiotic stresses

We detail the expression of the Arabidopsis thaliana (L.) Heynh. atExt1 extensin gene. atExt1 is normally expressed in roots and inflorescences, and is induced by wounding, exogenously supplied salicylic acid, methyl jasmonate, auxins and brassinosteroids. Northern assays and histochemical analysis of transgenics expressing an atExt1:: gus fusion show that this gene is also induced by the brassica pathogen Xanthomonas campestris pv. campestris and that this induction is restricted to tissues close to the site of infection. Expression at regions of abscission and senescence also implicates atExt1 in these important developmental processes.

Reactive oxygen species and hormonal control of cell death

The accumulation of reactive oxygen species (ROS) is involved in regulating cell death. Pathogen- and ozone-induced processes have become important models for the study of cell death regulation by ROS. Hydrogen peroxide and superoxide have emerged as the two key ROS and recent studies have addressed their sources and control of their production. ROS signals interact directly or indirectly with several other signaling pathways, such as nitric oxide, and the stress hormones salicylic acid, jasmonic acid and ethylene. The interaction and balance of these pathways determines whether the cell lives or dies.

Plant disease resistance genes: recent insights and potential applications

Plant disease resistance genes (R genes) encode proteins that detect pathogens. R genes have been used in resistance breeding programs for decades, with varying degrees of success. Recent molecular research on R proteins and downstream signal transduction networks has provided exciting insights, which will enhance the use of R genes for disease control. Definition of conserved structural motifs in R proteins has facilitated the cloning of useful R genes, including several that are functional in multiple crop species and/or provide resistance to a relatively wide range of pathogens. Numerous signal transduction components in the defense network have been defined, and several are being exploited as switches by which resistance can be activated against diverse pathogens.

Selective desensitization of jasmonate- and pH-dependent signaling in the induction of benzophenanthridine biosynthesis in cells of Eschscholzia californica

The biosynthesis of benzophenanthridine alkaloids, phytoalexins of Eschscholzia californica, in cultured cells can be induced by a glycoprotein preparation from yeast, methyljasmonate, artificial acidification with permeant acids, or mild osmotic stress. Each of these stimuli strongly attenuated the subsequent response to the same stimulus (homologous desensitization). Elicitor contact and artificial acidification mutually desensitized the cells for either signal. In contrast, elicitor-treated cells maintained their responsiveness to methyljasmonate or hyperosmolarity (sorbitol). Elicitor concentrations that nearly saturated the alkaloid response did not cause a detectable increase of jasmonate content. Transient acidification of the cytoplasm is a necessary step of signaling by low elicitor concentrations but was not detectable after jasmonate treatment. Seen together, the data indicate the existence of a jasmonate-dependent and jasmonate-independent (Delta pH controlled) signal pathway towards the expression of benzophenanthridine biosynthesis. Selective desensitization allows either stimulus to activate a distinct share of the biosynthetic capacity of the cell and limits the accumulation of toxic defense metabolites.

Multiple mediators of plant programmed cell death: interplay of conserved cell death mechanisms and plant-specific regulators

Programmed cell death (PCD) is a process aimed at the removal of redundant, misplaced, or damaged cells and it is essential to the development and maintenance of multicellular organisms. In contrast to the relatively well-described cell death pathway in animals, often referred to as apoptosis, mechanisms and regulation of plant PCD are still ill-defined. Several morphological and biochemical similarities between apoptosis and plant PCD have been described, including DNA laddering, caspase-like proteolytic activity, and cytochrome c release from mitochondria. Reactive oxygen species (ROS) have emerged as important signals in the activation of plant PCD. In addition, several plant hormones may exert their respective effects on plant PCD through the regulation of ROS accumulation. The possible plant PCD regulators discussed in this review are integrated in a model that combines plant-specific regulators with mechanisms functionally conserved between animals and plants.

Ozone-induced ethylene production is dependent on salicylic acid, and both salicylic acid and ethylene act in concert to regulate ozone-induced cell death

Ethylene is known to influence plant defense responses including cell death in response to both biotic and abiotic stress factors. However, whether ethylene acts alone or in conjunction with other signaling pathways is not clearly understood. Ethylene overproducer mutants, eto1 and eto3, produced high levels of ethylene and developed necrotic lesions in response to an acute O3 exposure that does not induce lesions in O3-tolerant wild-type Col-0 plants. Treatment of plants with ethylene inhibitors completely blocked O3-induced ethylene production and partially attenuated O3-induced cell death. Analyses of the responses of molecular markers of specific signaling pathways indicated a relationship between salicylic acid (SA)- and ethylene-signaling pathways and O3 sensitivity. Both eto1 and eto3 plants constitutively accumulated threefold higher levels of total SA and exhibited a rapid increase in free SA and ethylene levels prior to lesion formation in response to O3 exposure. SA pre-treatments increased O3 sensitivity of Col-0, suggesting that constitutive high SA levels prime leaf tissue to exhibit increased magnitude of O3-induced cell death. NahG and npr1 plants compromised in SA signaling failed to produce ethylene in response to O3 and other stress factors suggesting that SA is required for stress-induced ethylene production. Furthermore, NahG expression in the dominant eto3 mutant attenuated ethylene-dependent PR4 expression and rescued the O3-induced HR (hypersensitive response) cell death phenotype exhibited by eto3 plants. Our results suggest that both SA and ethylene act in concert to influence cell death in O3-sensitive genotypes, and that O3-induced ethylene production is dependent on SA.

MAP kinase signalling cascade in Arabidopsis innate immunity

There is remarkable conservation in the recognition of pathogen-associated molecular patterns (PAMPs) by innate immune responses of plants, insects and mammals. We developed an Arabidopsis thaliana leaf cell system based on the induction of early-defence gene transcription by flagellin, a highly conserved component of bacterial flagella that functions as a PAMP in plants and mammals. Here we identify a complete plant MAP kinase cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) and WRKY22/WRKY29 transcription factors that function downstream of the flagellin receptor FLS2, a leucine-rich-repeat (LRR) receptor kinase. Activation of this MAPK cascade confers resistance to both bacterial and fungal pathogens, suggesting that signalling events initiated by diverse pathogens converge into a conserved MAPK cascade.

Knockout of Arabidopsis accelerated-cell-death11 encoding a sphingosine transfer protein causes activation of programmed cell death and defense

We describe the lethal, recessive accelerated-cell-death11 Arabidopsis mutant (acd11). Cell death in acd11 exhibits characteristics of animal apoptosis monitored by flow cytometry, and acd11 constitutively expresses defense-related genes that accompany the hypersensitive response normally triggered by avirulent pathogens. Global transcriptional changes during programmed cell death (PCD) and defense activation in acd11 were monitored by cDNA microarray hybridization. The PCD and defense pathways activated in acd11 are salicylic acid (SA) dependent, but do not require intact jasmonic acid or ethylene signaling pathways. Light is required for PCD execution in acd11, as application of an SA-analog to SA-deficient acd11 induced death in the light, but not in the dark. Epistatic analysis showed that the SA-dependent pathways require two regulators of SA-mediated resistance responses, PAD4 and EDS1. Furthermore, acd11 PR1 gene expression, but not cell death, depends on the SA signal tranducer NPR1, suggesting that the npr1-1 mutation uncouples resistance responses and cell death in acd11. The acd11 phenotype is caused by deletion of the ACD11 gene encoding a protein homologous to a mammalian glycolipid transfer protein (GLTP). In contrast to GLTP, ACD11 accelerates the transfer of sphingosine, but not of glycosphingolipids, between membranes in vitro.

Host-selective toxins and avirulence determinants: what's in a name?

Host-selective toxins, a group of structurally complex and chemically diverse metabolites produced by plant pathogenic strains of certain fungal species, function as essential determinants of pathogenicity or virulence. Investigations into the molecular and biochemical responses to these disease determinants reveal responses typically associated with host defense and incompatibility induced by avirulence determinants. The characteristic responses that unify these disparate disease phenotypes are numerous, yet the evidence implicating a causal relationship of these responses, whether induced by host-selective toxins or avirulence factors, in determining the consequences of the host-pathogen interaction is equivocal. This review summarizes some examples of the action of host-selective toxins to illustrate the similarity in responses with those to avirulence determinants.

The oxylipin pathway in Arabidopsis

Oxylipins are acyclic or cyclic oxidation products derived from the catabolism of fatty acids which regulate many defense and developmental pathways in plants. The dramatic increase in the volume of publications and reviews on these compounds since 1997 documents the increasing interest in this compound and its role in plants. Research on this topic has solidified our understanding of the chemistry and biosynthetic pathways for oxylipin production. However, more information is still needed on how free fatty acids are produced and the role of beta-oxidation in the biosynthetic pathway for oxylipins. It is also becoming apparent that oxylipin content and composition changes during growth and development and during pathogen or insect attack. Oxylipins such as jasmonic acid (JA) or 12-oxo-phytodienoic acid modulate the expression of numerous genes and influence specific aspects of plant growth, development and responses to abiotic and biotic stresses. Although oxylipins are believed to act alone, several examples were presented to illustrate that JA-induced responses are modulated by the type and the nature of crosstalk with other signaling molecules such as ethylene and salicylic acid. How oxylipins cause changes in gene expression and instigate a physiological response is becoming understood with the isolation of mutations in both positive and negative regulators in the jasmonate signaling pathway and the use of cDNA microarrays.

Regulation and execution of programmed cell death in response to pathogens, stress and developmental cues

Recent studies have expanded our view of the interactions between small molecule signals that regulate the hypersensitive response and other forms of cell suicide in plants. The mitochondrion has received increasing support as a mediator of at least some forms of programmed cell death in plants. In addition, new information provides a glimpse of how plant hormone signaling may be integrated with extensive autolysis, sensitivity to reactive oxygen intermediates and cell death.

Characterization of an Arabidopsis cDNA encoding a subunit of serine palmitoyltransferase, the initial enzyme in sphingolipid biosynthesis

Serine palmitoyltransferase (SPT; EC 2.3.1.50) catalyzes the condensation of serine with palmitoyl-CoA to form 3-ketosphinganine in the first step of de novo sphingolipid biosynthesis. In this study, we describe the cloning and functional characterization of a cDNA from Arabidopsis thaliana encoding the LCB2 subunit of SPT. The Arabidopsis LCB2 (AtLCB2) cDNA contains an open reading frame of 1,467 nucleotides, encoding 489 amino acids. The predicted polypeptide contains three transmembrane helices and a highly conserved motif involved in pyridoxal phosphate binding. Expression of this open reading frame in the Saccharomyces cerevisiae mutant strains defective in SPT activity resulted in the expression of a significant level of sphinganine, suggesting that AtLCB2 cDNA encodes SPT. Southern blot analysis and inspection of the complete Arabidopsis genome sequence database suggest that there is a second LCB2-like gene in Arabidopsis. Expression of a green fluorescent protein (GFP) fusion product in suspension-cultured tobacco BY-2 cells showed that AtLCB2 is localized to the endoplasmic reticulum. AtLCB2 cDNA may be used to study how sphingolipid synthesis is regulated in higher plants.

Three Tnt1 subfamilies show different stress-associated patterns of expression in tobacco. Consequences for retrotransposon control and evolution in plants

The genomes of most Nicotiana species contain three different subfamilies of the Tnt1 retrotransposon, which differ completely in their U3 sequence, whereas the rest of the sequence is relatively constant. The results presented here show that all three Tnt1 subfamilies are expressed in tobacco (Nicotiana tabacum) and that the U3 sequence variability correlates with differences in the pattern of expression of the Tnt1 elements. Each of the three Tnt1 subfamilies is induced by stress, but their promoters have a different response to different stress-associated signaling molecules. The Tnt1A subfamily is particularly strongly induced by elicitors and methyl jasmonate, whereas expression of the Tnt1C subfamily is more sensitive to salicylic acid and auxins. The direct relationship between U3 sequence variability and differences in the stress-associated expression of the Tnt1 elements present in a single host species gives support to our model that postulates that retrotransposons have adapted to their host genomes through the evolution of highly regulated promoters that mimic those of the stress-induced plant genes. Moreover, here we show that the analysis of the transcriptional control of a retrotransposon population such as Tnt1 provides new insights into the study of the complex and still poorly understood network of defense- and stress-induced plant signal transduction pathways.

Genetic dissection of systemic acquired resistance

Significant progress has been made in the past year in understanding the mechanism of systemic acquired resistance. Mitogen-activated protein kinase cascades have been implicated as negative regulators of salicyclic acid accumulation and the induction of resistance. The salicylic acid signal is transduced through NPR1, a nuclear-localized protein that interacts with transcription factors that are involved in regulating salicylic-acid-mediated gene expression. Both promoter analyses and genetic studies have shown that gene expression in systemic acquired resistance requires not only the activation of a transcriptional activator(s) but also inhibition of a transcriptional repressor(s). Microarray experiments have been performed to search for those genes whose expression is transcriptionally regulated during systemic acquired resistance and to identify common promoter elements that control these genes.

Abrogation of disease development in plants expressing animal antiapoptotic genes

An emerging topic in plant biology is whether plants display analogous elements of mammalian programmed cell death during development and defense against pathogen attack. In many plant-pathogen interactions, plant cell death occurs in both susceptible and resistant host responses. For example, specific recognition responses in plants trigger formation of the hypersensitive response and activation of host defense mechanisms, resulting in restriction of pathogen growth and disease development. Several studies indicate that cell death during hypersensitive response involves activation of a plant-encoded pathway for cell death. Many susceptible interactions also result in host cell death, although it is not clear how or if the host participates in this response. We have generated transgenic tobacco plants to express animal genes that negatively regulate apoptosis. Plants expressing human Bcl-2 and Bcl-xl, nematode CED-9, or baculovirus Op-IAP transgenes conferred heritable resistance to several necrotrophic fungal pathogens, suggesting that disease development required host-cell death pathways. In addition, the transgenic tobacco plants displayed resistance to a necrogenic virus. Transgenic tobacco harboring Bcl-xl with a loss-of-function mutation did not protect against pathogen challenge. We also show that discrete DNA fragmentation (laddering) occurred in susceptible tobacco during fungal infection, but does not occur in transgenic-resistant plants. Our data indicate that in compatible plant-pathogen interactions apoptosis-like programmed cell death occurs. Further, these animal antiapoptotic genes function in plants and should be useful to delineate resistance pathways. These genes also have the potential to generate effective disease resistance in economically important crops.

The complexity of disease signaling in Arabidopsis

Not more than 10 years ago it was generally accepted that pathogen-inducible defense mechanisms in plants are triggered through a central signaling cascade that regulates a multicomponent defense response. Now we know that the plant defense system is regulated through a complex network of various signaling cascades.

The Arabidopsis-accelerated cell death gene ACD2 encodes red chlorophyll catabolite reductase and suppresses the spread of disease symptoms

accelerated cell death 2 (acd2) mutants of Arabidopsis have spontaneous spreading cell death lesions and constitutive activation of defenses in the absence of pathogen infection. Lesion formation in acd2 plants can be triggered by the bacterial toxin coronatine through a light-dependent process. Coronatine-triggered and spontaneous lesion spreading in acd2 plants also requires protein translation, indicating that cell death occurs by an active process. We have cloned the ACD2 gene; its predicted product shows significant and extensive similarity to red chlorophyll catabolite reductase, which catalyzes one step in the breakdown of the porphyrin component of chlorophyll [Wüthrich, K. L., Bovet, L., Hunziger, P. E., Donnison, I. S. & Hörtensteiner, S. (2000) Plant J. 21, 189-198]. Consistent with this, ACD2 protein contains a predicted chloroplast transit peptide, is processed in vivo, and purifies with the chloroplast fraction in subcellular fractionation experiments. At some stages of development, ACD2 protein also purifies with the mitochondrial fraction. We hypothesize that cell death in acd2 plants is caused by the accumulation of chlorophyll breakdown products. Such catabolites might be specific triggers for cell death or they might induce cellular damage through their ability to absorb light and emit electrons that generate free radicals. In response to infection by Pseudomonas syringae, transgenic plants expressing excess ACD2 protein show reduced disease symptoms but not reduced growth of bacteria. Thus, breakdown products of chlorophyll may act to amplify the symptoms of disease, including cell death and yellowing. We suggest that economically important plants overexpressing ACD2 might also show increased tolerance to pathogens and might be useful for increasing crop yields.

The Arabidopsis-accelerated cell death gene ACD2 encodes red chlorophyll catabolite reductase and suppresses the spread of disease symptoms

accelerated cell death 2 (acd2) mutants of Arabidopsis have spontaneous spreading cell death lesions and constitutive activation of defenses in the absence of pathogen infection. Lesion formation in acd2 plants can be triggered by the bacterial toxin coronatine through a light-dependent process. Coronatine-triggered and spontaneous lesion spreading in acd2 plants also requires protein translation, indicating that cell death occurs by an active process. We have cloned the ACD2 gene; its predicted product shows significant and extensive similarity to red chlorophyll catabolite reductase, which catalyzes one step in the breakdown of the porphyrin component of chlorophyll [Wüthrich, K. L., Bovet, L., Hunziger, P. E., Donnison, I. S. & Hörtensteiner, S. (2000) Plant J. 21, 189-198]. Consistent with this, ACD2 protein contains a predicted chloroplast transit peptide, is processed in vivo, and purifies with the chloroplast fraction in subcellular fractionation experiments. At some stages of development, ACD2 protein also purifies with the mitochondrial fraction. We hypothesize that cell death in acd2 plants is caused by the accumulation of chlorophyll breakdown products. Such catabolites might be specific triggers for cell death or they might induce cellular damage through their ability to absorb light and emit electrons that generate free radicals. In response to infection by Pseudomonas syringae, transgenic plants expressing excess ACD2 protein show reduced disease symptoms but not reduced growth of bacteria. Thus, breakdown products of chlorophyll may act to amplify the symptoms of disease, including cell death and yellowing. We suggest that economically important plants overexpressing ACD2 might also show increased tolerance to pathogens and might be useful for increasing crop yields.

Male fertility: a case of enzyme identity

The plant hormone jasmonate has been implicated in male fertility in Arabidopsis. Recent studies have identified the enzyme required for a critical step of jasmonate synthesis in anthers and shown that this enzyme really is required for male fertility.

Three unique mutants of Arabidopsis identify eds loci required for limiting growth of a biotrophic fungal pathogen

To identify components of the defense response that limit growth of a biotrophic fungal pathogen, we isolated Arabidopsis mutants with enhanced disease susceptibility to Erysiphe orontii. Our initial characterization focused on three mutants, eds14, eds15, and eds16. None of these is considerably more susceptible to a virulent strain of the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm). All three mutants develop a hypersensitive response when infiltrated with Psm expressing the avirulence gene avrRpt2, which activates resistance via the LZ-NBS/LRR resistance protein encoded by RPS2. The growth of Psm(avrRpt2), while somewhat greater in the mutants than in the wild type, is less than growth of the isogenic virulent strain. These results indicate that resistance mediated via LZ-NBS/LRR R genes is functional. Analysis of the growth of avirulent Peronospora parasitica strains showed that the resistance pathway utilized by TIR-NBS/LRR R genes is also operative in all three mutants. Surprisingly, only eds14 and eds16 were more susceptible to Erysiphe cichoracearum. Analysis of the expression profiles of PR-1, BGL2, PR-5 and PDF1.2 in eds14, eds15, and eds16 revealed differences from the wild type for all the lines. In contrast, these mutants were not significantly different from wild type in the deposition of callose at sites of E. orontii penetration. All three mutants have reduced levels of salicylic acid after infection. eds16 was mapped to the lower arm of chromosome I and found by complementation tests to be allelic to the salicylic acid-deficient mutant sid2.

Simulation of fungal-mediated cell death by fumonisin B1 and selection of fumonisin B1-resistant (fbr) Arabidopsis mutants

Fumonisin B1 (FB1), a programmed cell death-eliciting toxin produced by the necrotrophic fungal plant pathogen Fusarium moniliforme, was used to simulate pathogen infection in Arabidopsis. Plants infiltrated with 10 microM FB1 and seedlings transferred to agar media containing 1 microM FB1 develop lesions reminiscent of the hypersensitive response, including generation of reactive oxygen intermediates, deposition of phenolic compounds and callose, accumulation of phytoalexin, and expression of pathogenesis-related (PR) genes. Arabidopsis FB1-resistant (fbr) mutants were selected directly by sowing seeds on agar containing 1 microM FB1, on which wild-type seedlings fail to develop. Two mutants chosen for further analyses, fbr1 and fbr2, had altered PR gene expression in response to FB1. fbr1 and fbr2 do not exhibit differential resistance to the avirulent bacterial pathogen Pseudomonas syringae pv maculicola (ES4326) expressing the avirulence gene avrRpt2 but do display enhanced resistance to a virulent isogenic strain that lacks the avirulence gene. Our results demonstrate the utility of FB1 for high-throughput isolation of Arabidopsis defense-related mutants and suggest that pathogen-elicited programmed cell death of host cells may be an important feature of compatible plant-pathogen interactions.