A humidity-sensitive Arabidopsis copine mutant exhibits precocious cell death and increased disease resistance

SA-ABA antagonism in defense responses

Until recently, phytohormones were mostly studied separately. However, recent studies have suggested that the signaling pathways involved are highly interconnected. We recently reported the antagonistic effects of salicylic acid (SA) and abscisic acid (ABA) in the lesion mimic mutants, cpr22 and ssi4. After shifting these mutants from high humidity, where the lesion mimic phenotype is suppressed to permissive low humidity condition, both SA and ABA pathways were up-regulated. However, the increased levels of SA were able to block downstream ABA responses even though ABA signaling genes and endogenous ABA were elevated. Furthermore, these lesion mimic mutants displayed a partial ABA insensitivity with respect to germination, guard cell opening, and water loss. This increased water loss in detached mutant plants could also be mimicked by treating wild type plants with SA. An active SA analog, 5-chloro-salicylic acid also induced enhanced water loss, while an inactive analog, 4-hydroxy-benzoic acid, did not. Here, we report that the biological analogs of SA, the systemic acquired resistance (SAR) activators, BTH (benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester) and BIT (1,2-benzisothiazol-3(2H)-one1,1-dioxide), did not have the same effect as SA, suggesting that SA may have additional roles to defense, and that SAR activators may not mimic all SA effects.

Requirement of calcium binding, myristoylation, and protein-protein interaction for the Copine BON1 function in Arabidopsis

Copines are highly conserved proteins with lipid-binding activities found in animals, plants, and protists. They contain two calcium-dependent phospholipid binding C2 domains at the amino terminus and a VWA domain at the carboxyl terminus. The biological roles of most copines are not understood and the biochemical properties required for their functions are largely unknown. The Arabidopsis copine gene BON1/CPN1 is a negative regulator of cell death and defense responses. Here we probed the potential biochemical activities of BON1 through mutagenic studies. We found that mutations of aspartates in the C2 domains did not alter plasma membrane localization but compromised BON1 activity. Mutation at putative myristoylation residue glycine 2 altered plasma membrane localization of BON1 and rendered BON1 inactive. Mass spectrometry analysis of BON1 further suggests that the N-peptide of BON1 is modified. Furthermore, mutations that affect the interaction between BON1 and its functional partner BAP1 abolished BON1 function. This analysis reveals an unanticipated regulation of copine protein localization and function by calcium and lipid modification and suggests an important role in protein-protein interaction for the VWA domain of copines.

Copine A plays a role in the differentiation of stalk cells and the initiation of culmination in Dictyostelium development

Background

Copines are calcium-dependent phospholipid-binding proteins found in diverse eukaryotic organisms. We are studying the function of copines in Dictyostelium discoideum, a single-celled amoeba that undergoes cell differentiation and morphogenesis to form multicellular fruiting bodies when placed in starvation conditions. Previously, we showed that Dictyostelium cells lacking the copine A (cpnA) gene are not able to complete the developmental cycle, arresting at the slug stage. The aim of this study is to further characterize the developmental defect of the cpnA- cells.

Results

Time-lapse imaging revealed that cpnA- cells exhibited delayed aggregation and made large mounds that formed one large slug as compared to the smaller slugs of the wild-type cells. While the prespore cell patterning appeared to be normal within the cpnA- slugs, the prestalk cell patterning was different from wild-type. When cpnA- cells were mixed with a small percentage of wild-type cells, chimeric fruiting bodies with short stalks formed. When a small percentage of cpnA- cells was mixed with wild-type cells, the cpnA- cells labeled with GFP were found located throughout the chimeric slug and in both the stalk and sporehead of the fruiting bodies. However, there appeared to be a small bias towards cpnA- cells becoming spore cells. When cpnA- cells were developed in buffer containing EGTA, they were also able to differentiate into either stalk or spore cells to form fruiting bodies with short stalks.

Conclusions

Our results indicate that CpnA is involved in the regulation of aggregation, slug size, and culmination during Dictyostelium development. More specifically, CpnA appears to be involved in the function and differentiation of prestalk cells and plays a role in a calcium-regulated signaling pathway critical to triggering the initiation of culmination.

Novel functions of Stomatal Cytokinesis-Defective 1 (SCD1) in innate immune responses against bacteria

Eukaryotes employ complex immune mechanisms for protection against microbial pathogens. Here, we identified SCD1 (Stomatal Cytokinesis-Defective 1), previously implicated in growth and development through its role in cytokinesis and polarized cell expansion (Falbel, T. G., Koch, L. M., Nadeau, J. A., Segui-Simarro, J. M., Sack, F. D., and Bednarek, S. Y. (2003) Development 130, 4011-4024) as a novel component of innate immunity. In Arabidopsis, SCD1 is a unique gene encoding for the only protein containing a complete DENN (Differentially Expressed in Normal and Neoplastic cells) domain. The DENN domain is a largely uncharacterized tripartite protein motif conserved among eukaryotic proteins. We show that conditional scd1-1 plants containing a point mutation in a conserved DENN residue affected a subset of signaling responses to some bacterial pathogen-associated molecular patterns (PAMPs). Consistent with increased transcript accumulation of Pathogen-related (PR) genes, scd1-1 plants were more resistant to Pseudomonas syringae pathovar tomato (Pst) DC3000 infection implicating SCD1 as a negative regulator of basal resistance against bacteria. scd1-1 plants were different from known mutants exhibiting constitutive expressor of PR (cpr)-like phenotypes, in that growth impairment of scd1-1 plants was genetically independent of constitutive immune response activation. For scd1-1, shift to elevated temperature or introduction of a mutant allele in Salicylic acid Induction-Deficient 2 (SID2) suppressed constitutive defense response activation. sid2-2 also repressed the resistance phenotype of scd1-1. Temperature shift and sid2-2, however, did not rescue conditional growth and sterility defects of scd1-1. These results implicate SCD1 in multiple cellular pathways, possibly by affecting different proteins. Overall, our studies identified a novel role for eukaryotic DENN proteins in immunity against bacteria.

The lesion-mimic mutant cpr22 shows alterations in abscisic acid signaling and abscisic acid insensitivity in a salicylic acid-dependent manner

A number of Arabidopsis (Arabidopsis thaliana) lesion-mimic mutants exhibit alterations in both abiotic stress responses and pathogen resistance. One of these mutants, constitutive expresser of PR genes22 (cpr22), which has a mutation in two cyclic nucleotide-gated ion channels, is a typical lesion-mimic mutant exhibiting elevated levels of salicylic acid (SA), spontaneous cell death, constitutive expression of defense-related genes, and enhanced resistance to various pathogens; the majority of its phenotypes are SA dependent. These defense responses in cpr22 are suppressed under high-humidity conditions and enhanced by low humidity. After shifting plants from high to low humidity, the cpr22 mutant, but not the wild type, showed a rapid increase in SA levels followed by an increase in abscisic acid (ABA) levels. Concomitantly, genes for ABA metabolism were up-regulated in the mutant. The expression of a subset of ABA-inducible genes, such as RD29A and KIN1/2, was down-regulated, but that of other genes, like ABI1 and HAB1, was up-regulated in cpr22 after the humidity shift. cpr22 showed reduced responsiveness to ABA not only in abiotic stress responses but also in germination and stomatal closure. Double mutant analysis with nahG plants that degrade SA indicated that these alterations in ABA signaling were attributable to elevated SA levels. Furthermore, cpr22 displayed suppressed drought responses by long-term drought stress. Taken together, these results suggest an effect of SA on ABA signaling/abiotic stress responses during the activation of defense responses in cpr22.

The Arabidopsis thaliana Myo-inositol 1-phosphate synthase1 gene is required for Myo-inositol synthesis and suppression of cell death

l-myo-inositol 1-phosphate synthase (MIPS; EC 5.5.1.4) catalyzes the rate-limiting step in the synthesis of myo-inositol, a critical compound in the cell. Plants contain multiple MIPS genes, which encode highly similar enzymes. We characterized the expression patterns of the three MIPS genes in Arabidopsis thaliana and found that MIPS1 is expressed in most cell types and developmental stages, while MIPS2 and MIPS3 are mainly restricted to vascular or related tissues. MIPS1, but not MIPS2 or MIPS3, is required for seed development, for physiological responses to salt and abscisic acid, and to suppress cell death. Specifically, a loss in MIPS1 resulted in smaller plants with curly leaves and spontaneous production of lesions. The mips1 mutants have lower myo-inositol, ascorbic acid, and phosphatidylinositol levels, while basal levels of inositol (1,4,5)P(3) are not altered in mips1 mutants. Furthermore, mips1 mutants exhibited elevated levels of ceramides, sphingolipid precursors associated with cell death, and were complemented by a MIPS1-green fluorescent protein (GFP) fusion construct. MIPS1-, MIPS2-, and MIPS3-GFP each localized to the cytoplasm. Thus, MIPS1 has a significant impact on myo-inositol levels that is critical for maintaining levels of ascorbic acid, phosphatidylinositol, and ceramides that regulate growth, development, and cell death.

SPL28 encodes a clathrin-associated adaptor protein complex 1, medium subunit micro 1 (AP1M1) and is responsible for spotted leaf and early senescence in rice (Oryza sativa)

To expand our understanding of cell death in plant defense responses, we isolated a novel rice (Oryza sativa) spotted leaf mutant (spl28) that displays a lesion mimic phenotype in the absence of pathogen attack through treatment of Hwacheongbyeo (an elite Korean japonica cultivar) with N-methyl-N-nitrosourea (MNU). Early stage development of the spl28 mutant was normal. However, after flowering, spl28 mutants exhibited a significant decrease in chlorophyll content, soluble protein content, and photosystem II efficiency, and high concentrations of reactive oxygen species (ROS), phytoalexin, callose, and autofluorescent phenolic compounds that localized in or around the lesions. The spl28 mutant also exhibited significantly enhanced resistance to rice blast and bacterial blight. Using a map-based cloning approach, we determined that SPL28 encodes a clathrin-associated adaptor protein complex 1, medium subunit micro 1 (AP1M1), which is involved in the post-Golgi trafficking pathway. A green fluorescent protein (GFP) fusion protein of SPL28 (SPL28::GFP) localized to the Golgi apparatus, and expression of SPL28 complemented the membrane trafficking defect of apm1-1 Delta yeast mutants. SPL28 was ubiquitously expressed and contained a highly conserved adaptor complex medium subunit (ACMS) family domain. SPL28 appears to be involved in the regulation of vesicular trafficking, and SPL28 dysfunction causes the formation of hypersensitive response (HR)-like lesions, leading to the initiation of leaf senescence.

Characterization and mapping of complementary lesion-mimic genes lm1 and lm2 in common wheat

A lesion-mimic phenotype appeared in a segregating population of common wheat cross Yanzhan 1/Zaosui 30. The parents had non-lesion normal phenotypes. Shading treatment and histochemical analyses showed that the lesions were caused by light-dependent cell death and were not associated with pathogens. Studies over two cropping seasons showed that some lines with more highly expressed lesion-mimic phenotypes exhibited significantly lower grain yields than those with the normal phenotype, but there were no significant effects in the lines with weakly expressed lesion-mimic phenotypes. Among yield traits, one-thousand grain weight was the most affected by lesion-mimic phenotypes. Genetic analysis indicated that this was a novel type of lesion mimic, which was caused by interaction of recessive genes derived from each parent. The lm1 (lesion mimic 1) locus from Zaosui 30 was flanked by microsatellite markers Xwmc674 and Xbarc133/Xbarc147 on chromosome 3BS, at genetic distances of 1.2 and 3.8 cM, respectively, whereas lm2 from Yanzhan 1 was mapped between microsatellite markers Xgwm513 and Xksum154 on chromosome 4BL, at genetic distances of 1.5 and 3 cM, respectively. The linked microsatellite makers identified in this study might be useful for evaluating whether potential parents with normal phenotype are carriers of lesion-mimic alleles.

Bacterial growth restriction during host resistance to Pseudomonas syringae is associated with leaf water loss and localized cessation of vascular activity in Arabidopsis thaliana

The physiological mechanisms by which plants limit the growth of bacterial pathogens during gene-for-gene resistance are poorly understood. We characterized early events in the Arabidopsis thaliana-Pseudomonas syringae pathosystem to identify physiological changes for which the kinetics are consistent with bacterial growth restriction. Using a safranine-O dye solution to detect vascular activity, we demonstrated that A. thaliana Col-0 resistance to P. syringae pv. tomato DC3000 cells expressing avrRpm1 involved virtually complete cessation of vascular water movement into the infection site within only 3 h postinoculation (hpi), under the conditions tested. This vascular restriction preceded or was simultaneous with precipitous decreases in photosynthesis, stomatal conductance, and leaf transpiration, with the latter two remaining at detectable levels. Microscopic plant cell death was detected as early as 2 hpi. Interestingly, suppression of bacterial growth during AvrRpm1-mediated resistance was eliminated by physically blocking leaf water loss through the stomata without altering plant cell death and was nearly eliminated by incubating plants at high relative humidity. The majority of the population growth benefit from blocking leaf water loss occurred early after inoculation, i.e., between 4 and 8 hpi. Collectively, these results support a model in which A. thaliana suppresses P. syringae growth during gene-for-gene resistance, at least in part, by coupling restricted vascular flow to the infection site with water loss through partially open stomata; that is, the plants effectively starve the invading bacteria for water.

Primary Metabolism and Plant Defense—Fuel for the Fire

Plants have the ability to recognize and respond to a multitude of microorganisms. Recognition of pathogens results in a massive reprogramming of the plant cell to activate and deploy defense responses to halt pathogen growth. Such responses are associated with increased demands for energy, reducing equivalents, and carbon skeletons that are provided by primary metabolic pathways. Although pathogen recognition and downstream resistance responses have been the focus of major study, an intriguing and comparatively understudied phenomenon is how plants are able to recruit energy for the defense response. To that end, this review will summarize current research on energy-producing primary metabolism pathways and their role in fueling the resistance response.

Evidence that the BONZAI1/COPINE1 protein is a calcium- and pathogen-responsive defense suppressor

Copines are calcium-responsive, phospholipid-binding proteins involved in cellular signaling. The Arabidopsis BONZAI1/COPINE1 (BON1/CPN1) gene is a suppressor of defense responses controlled by the disease resistance (R) gene homolog SNC1. The BON1/CPN1 null mutant cpn1-1 has a recessive, temperature- and humidity-dependent, lesion mimic phenotype that includes activation of Pathogenesis-Related (PR) gene expression. Here, we demonstrated that the accumulation of BON1/CPN1 protein in wild-type plants was up-regulated by bacterial pathogen inoculation and by the activation of defense signaling responses controlled by two R genes, SNC1 and RPS2. Interestingly, however, over-accumulation of BON1/CPN1 in two BON1/CPN1 promoter T-DNA insertion mutants did not affect resistance to a bacterial pathogen. Promoter deletion analysis identified a 280 bp segment of the BON1/CPN1 promoter as being required for pathogen-induced gene expression; the same promoter region was also required for calcium ionophore-induced gene expression. Leaf infiltration with calcium ionophore triggered high-level PR gene expression specifically in cpn1-1 plants grown under permissive conditions, while co-infiltration of the calcium chelator EGTA attenuated this effect. These results explain the conditional nature of the cpn1-1 phenotype and are consistent with BON1/CPN1 being a calcium- and pathogen-responsive plant defense suppressor.

A cation/proton-exchanging protein is a candidate for the barley NecS1 gene controlling necrosis and enhanced defense response to stem rust

We characterized three lesion mimic necS1 (necrotic Steptoe) mutants, induced by fast neutron (FN) treatment of barley cultivar Steptoe. The three mutants are recessive and allelic. When infected with Puccinia graminis f. sp. tritici pathotypes MCC and QCC and P. graminis f. sp. secalis isolate 92-MN-90, all three mutants exhibited enhanced resistance compared to parent cultivar Steptoe. These results suggested that the lesion mimic mutants carry broad-spectrum resistance to stem rust. In order to identify the mutated gene responsible for the phenotype, transcript-based cloning was used. Two genes, represented by three Barley1 probesets (Contig4211_at and Contig4212_s_at, representing the same gene, and Contig10850_s_at), were deleted in all three mutants. Genetic analysis suggested that the lesion mimic phenotype was due to a mutation in one or both of these genes, named NecS1. Consistent with the increased disease resistance, all three mutants constitutively accumulated elevated transcript levels of pathogenesis-related (PR) genes. Barley stripe mosaic virus (BSMV) has been developed as a virus-induced gene-silencing (VIGS) vector for monocots. We utilized BSMV-VIGS to demonstrate that silencing of the gene represented by Contig4211_at, but not Contig10850_s_at caused the necrotic lesion mimic phenotype on barley seedling leaves. Therefore, Contig4211_at is a strong candidate for the NecS1 gene, which encodes a cation/proton exchanging protein (HvCAX1).

Promoter-sharing by different genes in human genome--CPNE1 and RBM12 gene pair as an example

Background

Regulation of gene expression plays important role in cellular functions. Co-regulation of different genes may indicate functional connection or even physical interaction between gene products. Thus analysis on genomic structures that may affect gene expression regulation could shed light on the functions of genes.

Results

In a whole genome analysis of alternative splicing events, we found that two distinct genes, copine I (CPNE1) and RNA binding motif protein 12 (RBM12), share the most 5' exons and therefore the promoter region in human. Further analysis identified many gene pairs in human genome that share the same promoters and 5' exons but have totally different coding sequences. Analysis of genomic and expressed sequences, either cDNAs or expressed sequence tags (ESTs) for CPNE1 and RBM12, confirmed the conservation of this phenomenon during evolutionary courses. The co-expression of the two genes initiated from the same promoter is confirmed by Reverse Transcription-Polymerase Chain Reaction (RT-PCR) in different tissues in both human and mouse. High degrees of sequence conservation among multiple species in the 5'UTR region common to CPNE1 and RBM12 were also identified.

Conclusion

Promoter and 5'UTR sharing between CPNE1 and RBM12 is observed in human, mouse and zebrafish. Conservation of this genomic structure in evolutionary courses indicates potential functional interaction between the two genes. More than 20 other gene pairs in human genome were found to have the similar genomic structure in a genome-wide analysis, and it may represent a unique pattern of genomic arrangement that may affect expression regulation of the corresponding genes.

Lesion mimic mutants: A classical, yet still fundamental approach to study programmed cell death

Over the last decade a substantial number of lesion mimic mutants (LMM) have been isolated and a growing number of the genes have been cloned. It is now becoming clear that these mutants are valuable tools to dissect various aspects of programmed cell death (PCD) and pathogen resistance pathways in plants. Together with other forward genetics approaches LMMs shed light on the PCD machinery in plant cells and revealed important roles for sphingolipids, Ca(2+) and chloroplast-derived porphyrin-metabolites during cell death development.

The evolution of spinnable cotton fiber entailed prolonged development and a novel metabolism

A central question in evolutionary biology concerns the developmental processes by which new phenotypes arise. An exceptional example of evolutionary innovation is the single-celled seed trichome in Gossypium (“cotton fiber”). We have used fiber development in Gossypium as a system to understand how morphology can rapidly evolve. Fiber has undergone considerable morphological changes between the short, tightly adherent fibers of G. longicalyx and the derived long, spinnable fibers of its closest relative, G. herbaceum, which facilitated cotton domestication. We conducted comparative gene expression profiling across a developmental time-course of fibers from G. longicalyx and G. herbaceum using microarrays with ∼22,000 genes. Expression changes between stages were temporally protracted in G. herbaceum relative to G. longicalyx, reflecting a prolongation of the ancestral developmental program. Gene expression and GO analyses showed that many genes involved with stress responses were upregulated early in G. longicalyx fiber development. Several candidate genes upregulated in G. herbaceum have been implicated in regulating redox levels and cell elongation processes. Three genes previously shown to modulate hydrogen peroxide levels were consistently expressed in domesticated and wild cotton species with long fibers, but expression was not detected by quantitative real time-PCR in wild species with short fibers. Hydrogen peroxide is important for cell elongation, but at high concentrations it becomes toxic, activating stress processes that may lead to early onset of secondary cell wall synthesis and the end of cell elongation. These observations suggest that the evolution of long spinnable fibers in cotton was accompanied by novel expression of genes assisting in the regulation of reactive oxygen species levels. Our data suggest a model for the evolutionary origin of a novel morphology through differential gene regulation causing prolongation of an ancestral developmental program.

Human domestication of plants has resulted in dramatic changes in mature structures, often over relatively short time frames. The availability of both wild and domesticated forms of domesticated species provides an opportunity to understand the genetic and developmental steps involved in domestication, thereby providing a model of how the evolutionary process shapes phenotypes. Here we use a comparative approach to explore the evolutionary innovations leading to modern cotton fiber, which represent some of the more remarkable single-celled hairs in the plant kingdom. We used microarrays assaying approximately 22,000 genes to elucidate expression differences across a developmental time-course of fibers from G. longicalyx, representing wild cotton, and G. herbaceum, a cultivated species. Expression changes between stages were temporally elongated in G. herbaceum relative to G. longicalyx, showing that domestication involved a prolongation of an ancestral developmental program. These data and quantitative real time-PCR experiments showed that long, spinnable fiber is associated with a number of genes implicated in regulating redox levels and cell elongation processes, suggesting that the evolution of spinnable cotton fiber entailed a novel metabolic regulatory program

Upregulation of Copine1 in trabecular meshwork cells of POAG patients: a membrane proteomics approach

PURPOSE

Primary open-angle glaucoma (POAG) is a leading cause of irreversible blindness worldwide, and its pathogenesis is still unknown. The purpose of this study was to determine molecular changes in membrane proteins in trabecular meshwork (TM) cells from POAG patients compared to those of age-matched normal controls.

METHODS

Two-dimensional (2-D) gel electrophoresis profiles of membrane extracts from normal and glaucomatous TM cells were compared. The desired spots were identified after trypsin digestion and mass spectrometric analysis. Based on the results, a calcium-dependant membrane-binding protein, copine1, was further approached for a possible role in glaucomatous TM cells. The intracellular calcium concentration ([Ca(2+)]i) of TM cells was increased by incubating with calcium ionophore, A23187. Relative quantification real-time polymerase chain reaction (PCR) and western blot analysis measured copine1 expression and localization both in untreated and A23187-treated TM cells.

RESULTS

Real-time PCR and western blot analysis confirmed that copine1 mRNA and protein expression were upregulated in glaucomatous TM cells when compared to normal ones. The cell distribution studies further showed that copine1 existed both in the membrane and cytoplasm fractions of glaucomatous TM cells but existed exclusively in cytoplasm fractions of their normal counterparts. More importantly, an influx of Ca(2+) markedly promoted the translocation of copine1 from the cytoplasm to membranes in glaucomatous TM cells.

CONCLUSIONS

Copine1 is upregulated in plasma membranes of TM cells in individuals with POAG, which may be partly explained by its Ca(2+)-dependent translocation from the cytoplasm to the membranes. Investigation of the role and functions of copine1 in TM cells should offer new insight into the abnormal intracellular Ca(2+)-signaling pathway in glaucomatous TM and help to clarify the molecular mechanism of POAG.

The Arabidopsis gain-of-function mutant ssi4 requires RAR1 and SGT1b differentially for defense activation and morphological alterations

A gain-of-function mutation in resistance (R) gene SSI4 causes constitutive activation of defense responses, spontaneous necrotic lesion formation, enhanced resistance against virulent pathogens, and a severe dwarf phenotype. Genetic analysis revealed that ssi4-induced H(2)O(2) accumulation and spontaneous cell death require RAR1, whereas ssi4-mediated stunting is dependent on SGT1b. By contrast, both RAR1 and SGT1b are required in a genetically additive manner for ssi4-induced disease resistance, SA accumulation, and lesion formation after pathogen infection. These data point to cooperative yet distinct functions of RAR1 and SGT1b in responses conditioned by a deregulated nucleotide-binding leucine-rich repeat protein. We also found that RAR1 and SGT1b together contribute to basal resistance because an ssi4 rar1 sgt1b triple mutant exhibited enhanced susceptibility to virulent pathogen infection compared with wild-type SSI4 plants. All ssi4-induced phenotypes were suppressed when plants were grown at 22 degrees C under high relative humidity. However, low temperature (16 degrees C) triggered ssi4-mediated cell death via an RAR1-dependent pathway even in the presence of high humidity. Thus, multiple environmental factors impact on ssi4 signaling, as has been observed for other constitutive defense mutants and R gene-triggered pathways.

The chimeric cyclic nucleotide-gated ion channel ATCNGC11/12 constitutively induces programmed cell death in a Ca2+ dependent manner

The hypersensitive response (HR) involves programmed cell death (PCD) in response to pathogen infection. To investigate the pathogen resistance signaling pathway, we previously identified the Arabidopsis mutant cpr22, which displays constitutive activation of multiple defense responses including HR like cell death. The cpr22 mutation has been identified as a 3 kb deletion that fuses two cyclic nucleotide-gated ion channel (CNGC)-encoding genes, ATCNGC11 and ATCNGC12, to generate a novel chimeric gene, ATCNGC11/12. In this study, we conducted a characterization of cell death induced by transient expression of ATCNGC11/12 in Nicotiana benthamiana. Electron microscopic analysis of this cell death showed similar characteristics to PCD, such as plasma membrane shrinkage and vesicle formation. The hallmark of animal PCD, fragmentation of nuclear DNA, was also observed in ATCNGC11/12-induced cell death. The development of cell death was significantly suppressed by caspase-1 inhibitors, suggesting the involvement of caspases in this process. Recently, vacuolar processing enzyme (VPE) was isolated as the first plant caspase-like protein, which is involved in HR development. In VPE-silenced plants development of cell death induced by ATCNGC11/12 was much slower and weaker compared to control plants, suggesting the involvement of VPE as a caspase in ATCNGC11/12-induced cell death. Complementation analysis using a Ca2+ uptake deficient yeast mutant demonstrated that the ATCNGC11/12 channel is permeable to Ca2+. Additionally, calcium channel blockers such as GdCl3 inhibited ATCNGC11/12-induced HR formation, whereas potassium channel blockers did not. Taken together, these results indicate that the cell death that develops in the cpr22 mutant is indeed PCD and that the chimeric channel, ATCNGC11/12, is at the point of, or up-stream of the calcium signal necessary for the development of HR.

The TIR-NB-LRR gene SNC1 is regulated at the transcript level by multiple factors

SNC1 (suppressor of NPR1, constitutive 1) is a haplotype-specific Toll and interleukin-1 receptor-like nucleotide-binding site leucine-rich repeat type of resistance (R)-like gene possibly mediating race-specific disease resistance. Inactivation of its negative regulator BON1 (BONZAI1)/CPN1 and BAP1 genes or upregulation of its expression epigenetically lead to constitutive defense responses and dwarf phenotype. Here, we report an autoactivation of SNC1 by introducing it into Arabidopsis as a transgene. The SNC1 genomic fragment confers a dwarf phenotype induced by defense response upregulation associated with a higher SNC1 transcript level. Analysis of the beta-glucuronidase reporter gene under the control of the SNC1 promoter suggests three modes of regulation on the SNC1 transcript level: a repression by the chromosomal structure, a feedback amplification from SNC1 on its promoter sequences, and a repression by BON1. These regulations appear to be independent of each other. The regulation of SNC1 possibly exemplifies a universally complex control of R genes to ensure a repression of R activation under nonstress conditions and a robust activation of defense responses once the R gene is induced.

The Arabidopsis BAP1 and BAP2 genes are general inhibitors of programmed cell death

Here we identify the BAP1 and BAP2 genes of Arabidopsis (Arabidopsis thaliana) as general inhibitors of programmed cell death (PCD) across the kingdoms. These two homologous genes encode small proteins containing a calcium-dependent phospholipid-binding C2 domain. BAP1 and its functional partner BON1 have been shown to negatively regulate defense responses and a disease resistance gene SNC1. Genetic studies here reveal an overlapping function of the BAP1 and BAP2 genes in cell death control. The loss of BAP2 function induces accelerated hypersensitive responses but does not compromise plant growth or confer enhanced resistance to virulent bacterial or oomycete pathogens. The loss of both BAP1 and BAP2 confers seedling lethality mediated by PAD4 and EDS1, two regulators of cell death and defense responses. Overexpression of BAP1 or BAP2 with their partner BON1 inhibits PCD induced by pathogens, the proapoptotic gene BAX, and superoxide-generating paraquat in Arabidopsis or Nicotiana benthamiana. Moreover, expressing BAP1 or BAP2 in yeast (Saccharomyces cerevisiae) alleviates cell death induced by hydrogen peroxide. Thus, the BAP genes function as general negative regulators of PCD induced by biotic and abiotic stimuli including reactive oxygen species. The dual roles of BAP and BON genes in repressing defense responses mediated by disease resistance genes and in inhibiting general PCD has implications in understanding the evolution of plant innate immunity.

Role of a novel pathogen-induced pepper C3-H-C4 type RING-finger protein gene, CaRFPI, in disease susceptibility and osmotic stress tolerance

Limited information is available about the roles of RING-finger proteins in plant defense. A pepper CaRFP1 encoding the C3-H-C4 type RING-finger protein that physically interacted with the basic PR-1 protein CABPR1 was isolated from pepper leaves infected by Xanthomonas campestris pv. vesicatoria. The CaRFP1 protein has VWFA domain, and N-terminal serine-rich and C-terminal cysteine-rich regions. The CaRFP1 transcripts accumulated earlier than did those of the basic PR-1 gene CABPR1 during the incompatible interaction of pepper leaves with X. campestris pv. vesicatoria, as well as in the systemic, uninoculated pepper leaf tissues. The CaRFP1 gene also was induced in pepper leaf tissues infected by Colletotrichum coccodes. The CaRFP1 gene was strongly induced much earlier by salicylic acid, ethylene and methyl jasmonate treatments, as well as environmental stresses including methyl viologen, mannitol and NaCl treatments. Overexpression of the CaRFP1 gene in the transgenic Arabidopsis plants conferred disease susceptibility to Pseudomonas syringae pv. tomato infection, accompanied by reduced PR-2 and PR-5 gene expression, suggesting that the CaRFP1 acts as an E3 ligase for polyubiquitination of target PR proteins. Exogenous salicylic acid treatment also abolished PR-2 and PR-5 gene expression in the transgenic plants. Differential osmotic stress tolerance was induced by high salt and drought in the CaRFPI-overexpressing plants during germination and seedling development, which was closely correlated with abscisic acid sensitivity of Arabidopsis plants. These results suggest that the CaRFP1 gene functions as an early defense regulator controlling bacterial disease susceptibility and osmotic stress tolerance.

MEKK1 is required for flg22-induced MPK4 activation in Arabidopsis plants

The Arabidopsis (Arabidopsis thaliana) gene MEKK1 encodes a mitogen-activated protein kinase kinase kinase that has been implicated in the activation of the map kinases MPK3 and MPK6 in response to the flagellin elicitor peptide flg22. In this study, analysis of plants carrying T-DNA knockout alleles indicated that MEKK1 is required for flg22-induced activation of MPK4 but not MPK3 or MPK6. Experiments performed using a kinase-impaired version of MEKK1 (K361M) showed that the kinase activity of MEKK1 may not be required for flg22-induced MPK4 activation or for other macroscopic FLS2-mediated responses. MEKK1 may play a structural role in signaling, independent of its protein kinase activity. mekk1 knockout mutants display a severe dwarf phenotype, constitutive callose deposition, and constitutive expression of pathogen response genes. This dwarf phenotype was largely rescued by introduction into mekk1 knockout plants of either the MEKK1 (K361M) construct or a nahG transgene that degrades salicylic acid. When treated with pathogenic bacteria, the K361M plants were slightly more susceptible to an avirulent strain of Pseudomonas syringae and showed a delayed hypersensitive response, suggesting a role for MEKK1 kinase activity in this aspect of plant disease resistance. Our results indicate that MEKK1 acts upstream of MPK4 as a negative regulator of pathogen response pathways, a function that may not require MEKK1's full kinase activity.

Copine A is required for cytokinesis, contractile vacuole function, and development in Dictyostelium

Copines make up a family of soluble, calcium-dependent, membrane binding proteins found in a variety of eukaryotic organisms. In an earlier study, we identified six copine genes in the Dictyostelium discoideum genome and focused our studies on cpnA. Our previous localization studies of green fluorescent protein-tagged CpnA in Dictyostelium suggested that CpnA may have roles in contractile vacuole function, endolysosomal trafficking, and development. To test these hypotheses, we created a cpnA- knockout strain, and here we report the initial characterization of the mutant phenotype. The cpnA- cells exhibited normal growth rates and a slight cytokinesis defect. When placed in starvation conditions, cpnA- cells appeared to aggregate into mounds and form fingers with normal timing; however, they were delayed or arrested in the finger stage. When placed in water, cpnA- cells formed unusually large contractile vacuoles, indicating a defect in contractile vacuole function, while endocytosis and phagocytosis rates for the cpnA- cells were similar to those seen for wild-type cells. These studies indicate that CpnA plays a role in cytokinesis and contractile vacuole function and is required for normal development, specifically in the later stages prior to culmination. We also used real-time reverse transcription-PCR to determine the expression patterns of all six copine genes during development. The six copine genes were expressed in vegetative cells, with each gene exhibiting a distinct pattern of expression throughout development. All of the copine genes except cpnF showed an upregulation of mRNA expression at one or two developmental transitions, suggesting that copines may be important regulators of Dictyostelium development.

Loss of Necrotic Spotted Lesions 1 associates with cell death and defense responses in Arabidopsis thaliana

We isolated a lesion mimic mutant, necrotic spotted lesions 1 (nsl1), from Ds-tagged Arabidopsis thaliana accession No-0. The nsl1 mutant exhibits a growth retardation phenotype and develops spotted necrotic lesions on its rosette and cauline leaves. These phenotypes occur in the absence of pathogens indicating that nsl1 mutants may constitutively express defense responses. Consistent with this idea, nsl1 accumulates high levels of callose and autofluorescent phenolic compounds localized to the necrotic lesions. Furthermore RNA gel blot analysis revealed that genes associated with disease resistance activation are upregulated in the nsl1 mutants and these plants contain elevated levels of salicylic acid (SA). Crossing nsl1 with an SA deficient mutant, eds16-1, revealed that the nsl1 lesions and growth retardation are dependent upon SA. The nsl1 phenotypes are not suppressed under either the rar1-10 or sgt1b-1 genetic background. NSL1 encodes a novel 612aa protein which contains a membrane-attack complex/perforin (MACPF) domain, which is conserved in bacteria, fungi, mammals and plants. The possible modes of action of NSL1 protein in negative regulation of cell death programs and defense responses are discussed.

Crosstalk between abiotic and biotic stress responses: a current view from the points of convergence in the stress signaling networks

Plants have evolved a wide range of mechanisms to cope with biotic and abiotic stresses. To date, the molecular mechanisms that are involved in each stress has been revealed comparatively independently, and so our understanding of convergence points between biotic and abiotic stress signaling pathways remain rudimentary. However, recent studies have revealed several molecules, including transcription factors and kinases, as promising candidates for common players that are involved in crosstalk between stress signaling pathways. Emerging evidence suggests that hormone signaling pathways regulated by abscisic acid, salicylic acid, jasmonic acid and ethylene, as well as ROS signaling pathways, play key roles in the crosstalk between biotic and abiotic stress signaling.

Systemic immunity

Systemic acquired resistance (SAR) provides enhanced, long-lasting systemic immunity to secondary infection by a range of biotrophic, hemibiotrophic and necrotrophic pathogens that have diverse modes of infection. Considerable effort has focussed on the conserved central positive regulator of SAR, NON-EXPRESSOR OF PATHOGENESIS-RELATED1 (NPR1), and its control by changes in cellular redox potential. Recently, genetic and genomic approaches have highlighted a critical role for nucleocytoplasmic communication and protein secretion in establishing effective systemic immunity. Identification of the mobile signals and the mechanisms by which they are perceived in distal tissues remains challenging, but emerging evidence suggests that signal translocation uses lipid-derived (possibly jasmonate-based) signals and lipid-binding chaperones. Furthermore, the demonstration that autophagy interdicts and inactivates a systemic cell death signal adds further complexity to elucidating how mobile signals are decoded and transduced for effective immunity.

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.

Analysis of Arabidopsis growth factor gene 1 (GFG1) encoding a nudix hydrolase during oxidative signaling

Maintenance of pyridine nucleotide homeostasis is vital for normal growth and development of plants and animals. We demonstrate that Arabidopsis Growth Factor Gene 1 (GFG1; At4g12720) encoding a nudix hydrolase, is an NADH pyrophosphatase and ADP-ribose pyrophosphatase. The affinity for NADH and ADP-ribose indicates that this enzyme could serve as a connection between sensing cellular redox changes and downstream signaling. GFG1 transcript levels were rapidly and transiently induced during both biotic stresses imposed by avirulent pathogens and abiotic stresses like ozone and osmoticum. T-DNA knock out plants of GFG1 gene, gfg1-1, exhibit pleiotropic phenotypes such as reduced size, increased levels of reactive oxygen species and NADH, microscopic cell death, constitutive expression of pathogenesis-related genes and enhanced resistance to bacterial pathogens. The recombinant protein failed to complement the mutator deficiency in SBMutT- strain of Escherichia coli, suggesting this protein may not play a role in sanitizing the nucleotide pool. Based on rapid transcriptional changes in response to various stresses, substrate specificity of the enzyme, and analysis of the knock out mutant, we propose that GFG1 is a key gene linking cellular metabolism and oxidative signaling.

The chimeric Arabidopsis CYCLIC NUCLEOTIDE-GATED ION CHANNEL11/12 activates multiple pathogen resistance responses

To investigate the resistance signaling pathways activated by pathogen infection, we previously identified the Arabidopsis thaliana mutant constitutive expresser of PR genes22 (cpr22), which displays constitutive activation of multiple defense responses. Here, we identify the cpr22 mutation as a 3-kb deletion that fuses two cyclic nucleotide-gated ion channel (ATCNGC)-encoding genes, ATCNGC11 and ATCNGC12, to generate a novel chimeric gene, ATCNGC11/12. Genetic, molecular, and complementation analyses suggest that ATCNGC11/12, as well as ATCNGC11 and ATCNGC12, form functional cAMP-activated ATCNGCs and that the phenotype conferred by cpr22 is attributable to the expression of ATCNGC11/12. However, because overexpression of ATCNGC12, but not ATCNGC11, suppressed the phenotype conferred by cpr22, the development of this phenotype appears to be regulated by the ratio between ATCNGC11/12 and ATCNGC12. Analysis of knockout lines revealed that both ATCNGC11 and ATCNGC12 are positive mediators of resistance against an avirulent biotype of Hyaloperonospora parasitica. Through epistatic analyses, cpr22-mediated enhanced resistance to pathogens was found to require NDR1-dependent and EDS1/PAD4-dependent pathways. In striking contrast, none of these pathways was required for cpr22-induced salicylic acid accumulation or PR-1 gene expression. These results demonstrate that NDR1, EDS1, and PAD4 mediate other resistance signaling function(s) in addition to salicylic acid and pathogenesis-related protein accumulation. Moreover, the requirement for both NDR1-dependent and EDS1/PAD4-dependent pathways for cpr22-mediated resistance suggests that these pathways are cross-regulated.

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.

Copine A, a calcium-dependent membrane-binding protein, transiently localizes to the plasma membrane and intracellular vacuoles in Dictyostelium

Background

Copines are soluble, calcium-dependent membrane binding proteins found in a variety of organisms. Copines are characterized as having two C2 domains at the N-terminal region followed by an "A domain" at the C-terminal region. The "A domain" is similar in sequence to the von Willebrand A (VWA) domain found in integrins. The presence of C2 domains suggests that copines may be involved in cell signaling and/or membrane trafficking pathways.

Results

We have identified six copines genes in the Dictyostelium discoideum genome, cpnA-cpnF, and have focused our studies on cpnA. CpnA is expressed throughout development and was shown to be capable of binding to membranes in a calcium-dependent manner in vitro. A GFP-tagged CpnA was also capable of binding to membranes in a calcium-dependent manner in vitro. In live wildtype Dictyostelium cells expressing GFP-CpnA, GFP-CpnA was typically found in the cytoplasm without any specific localization to membranes. However, in a small subset of starved cells, GFP-CpnA was observed to bind transiently (typically ~1–10 s) to the plasma membrane and intracellular vacuoles. In some cells, the transient membrane localization of GFP-CpnA was observed to occur multiple times in an oscillatory manner over several minutes. In plasma membrane disrupted cells, GFP-CpnA was observed to associate with the plasma membrane and intracellular vacuoles in a calcium-dependent manner. In fixed cells, GFP-CpnA labeled the plasma membrane and intracellular vacuoles, including contractile vacuoles, organelles of the endolysosomal pathway, and phagosomes.

Conclusion

Our results show that Dictyostelium has multiple copine homologs and provides an excellent system in which to study copine function. The localization of a GFP-tagged CpnA to the plasma membrane, contractile vacuoles, organelles of the endolysosomal pathway, and phagosomes suggests that CpnA may have a role in the function of these organelles or the trafficking to and from them. In addition, we hypothesize that the observed transient oscillatory membrane localization of GFP-CpnA in a small subset of starved cells is caused by fast calcium waves and speculate that CpnA may have a role in development, particularly in the differentiation of stalk cells.

High humidity represses Cf-4/Avr4- and Cf-9/Avr9-dependent hypersensitive cell death and defense gene expression

Gene-for-gene resistance is a well-known type of plant disease resistance. It is governed by plant resistance (R) genes and their matching pathogen avirulence (Avr) genes. This resistance is characterized by a hypersensitive response (HR) resulting from the interaction between products of a complementary R and Avr gene pair. The pathosystem of tomato (Lycopersicon esculentum Mill.) and its leaf mold fungal pathogen Cladosporium fulvum is a model system to study gene-for-gene resistance. HR occurs in tomato seedlings carrying a tomato Cf resistance gene and a matching C. fulvum Avr gene, including, for example, Cf-4/Avr4 and Cf-9/Avr9. Employing Cf/Avr tomato seedlings that both express a Cf gene and the matching Avr gene, here we report that both Cf-4/Avr4- and Cf-9/Avr9-dependent HR is delayed and reduced under high humidity (95%), and that Cf-9/Avr9-dependent HR is more sensitive to high humidity when compared to Cf-4/Avr4-dependent HR. Furthermore, high humidity acts synergistically with high temperature on HR suppression, resulting in complete blocking of the HR. The transcript profile of over 60 genes, related to HR, signaling and defense, in Cf/Avr seedlings grown under high humidity and thus showing no HR, significantly differed from that of the same seedlings grown under normal humidity and thus showing HR. These results demonstrate that high humidity probably acts at a very early point of the Cf downstream signaling pathway, or alternatively influences the interaction between the Cf and the Avr proteins.

Ascorbic acid deficiency activates cell death and disease resistance responses in Arabidopsis

Programmed cell death, developmental senescence, and responses to pathogens are linked through complex genetic controls that are influenced by redox regulation. Here we show that the Arabidopsis (Arabidopsis thaliana) low vitamin C mutants, vtc1 and vtc2, which have between 10% and 25% of wild-type ascorbic acid, exhibit microlesions, express pathogenesis-related (PR) proteins, and have enhanced basal resistance against infections caused by Pseudomonas syringae. The mutants have a delayed senescence phenotype with smaller leaf cells than the wild type at maturity. The vtc leaves have more glutathione than the wild type, with higher ratios of reduced glutathione to glutathione disulfide. Expression of green fluorescence protein (GFP) fused to the nonexpressor of PR protein 1 (GFP-NPR1) was used to detect the presence of NPR1 in the nuclei of transformed plants. Fluorescence was observed in the nuclei of 6- to 8-week-old GFP-NPR1 vtc1 plants, but not in the nuclei of transformed GFP-NPR1 wild-type plants at any developmental stage. The absence of senescence-associated gene 12 (SAG12) mRNA at the time when constitutive cell death and basal resistance were detected confirms that elaboration of innate immune responses in vtc plants does not result from activation of early senescence. Moreover, H2O2-sensitive genes are not induced at the time of systemic acquired resistance execution. These results demonstrate that ascorbic acid abundance modifies the threshold for activation of plant innate defense responses via redox mechanisms that are independent of the natural senescence program.

A plant-specific protein essential for blue-light-induced chloroplast movements

In Arabidopsis (Arabidopsis thaliana), light-dependent chloroplast movements are induced by blue light. When exposed to low fluence rates of light, chloroplasts accumulate in periclinal layers perpendicular to the direction of light, presumably to optimize light absorption by exposing more chloroplast area to the light. Under high light conditions, chloroplasts become positioned parallel to the incoming light in a response that can reduce exposure to light intensities that may damage the photosynthetic machinery. To identify components of the pathway downstream of the photoreceptors that mediate chloroplast movements (i.e. phototropins), we conducted a mutant screen that has led to the isolation of several Arabidopsis mutants displaying altered chloroplast movements. The plastid movement impaired1 (pmi1) mutant exhibits severely attenuated chloroplast movements under all tested fluence rates of light, suggesting that it is a necessary component for both the low- and high-light-dependant chloroplast movement responses. Analysis of pmi1 leaf cross sections revealed that regardless of the light condition, chloroplasts are more evenly distributed in leaf mesophyll cells than in the wild type. The pmi1-1 mutant was found to contain a single nonsense mutation within the open reading frame of At1g42550. This gene encodes a plant-specific protein of unknown function that appears to be conserved among angiosperms. Sequence analysis of the protein suggests that it may be involved in calcium-mediated signal transduction, possibly through protein-protein interactions.

Transgenic expression of the von Willebrand A domain of the BONZAI 1/COPINE 1 protein triggers a lesion-mimic phenotype in Arabidopsis

The copines are a newly identified, widely distributed class of Ca(2+)-dependent, phospholipid-binding proteins that may be involved in cellular signaling. The copines have a characteristic domain structure: two C2 domains in the N-terminal region and a von Willebrand A (VWA) domain in the C-terminal region. Studies suggest that copines interact with target protein(s) via their VWA domain and recruit the proteins to a membrane location through the activity of the C2 domains. Arabidopsis thaliana (L.) Heynh. plants with loss-of-function mutations in the BONZAI 1/COPINE 1 (BON1/CPN1) gene display aberrant regulation of defense responses, including development of a lesion-mimic phenotype, an accelerated hypersensitive response, and increased resistance to a bacterial and an oomycetous pathogen. The phenotype of mutants in BON1/CPN1 is both humidity- and temperature-sensitive. In this study, we generated transgenic plants expressing either the VWA or the C2 portions of BON1/CPN1 in the wild-type Columbia-0 (Col-0) genetic background. Transgenic plants expressing the BON1/CPN1 C2 domain portion appeared like wild-type plants. However, transgenic plants expressing the BON1/CPN1 VWA domain exhibited a lesion-mimic phenotype that partially phenocopied bon1/cpn1 mutant plants. Our data suggest that BON1/CPN1 VWA domain fragments may interfere with the function of the full-length endogenous BON1/CPN1 protein, possibly by competing with the full-length BON1/CPN1 protein for association with target proteins normally bound to the full-length BON1/CPN1 protein.

A major suppressor of cell death, slm1, modifies the expression of the maize (Zea mays L.) lesion mimic mutation les23

Disease lesion mimics provide an excellent biological system to study the genetic basis of cell death in plants. Many lesion mimics show variation in phenotype expression in different genetic backgrounds. Our goal was to identify quantitative trait loci (QTL) modifying lesion mimic expression thereby identifying genetic modifiers of cell death. A recessive lesion mimic, les23, in a severe-expressing line was crossed to the maize inbred line Mo20W, a lesion-suppressing line, and an F(2) population was developed for QTL analysis. In addition to locating les23 to the short arm of chromosome 2, this analysis detected significant loci for modification of lesion expression. One highly significant locus was found on the long arm of chromosome 2. The Mo20W allele at this QTL significantly delayed initiation of the lesion phenotype and decreased the final lesion severity. Other QTL with lesser effect affected severity of lesion expression without affecting lesion initiation date. Our results demonstrate that dramatic change in lesion phenotype can be controlled by a single major QTL. The presumed function of this QTL in normal plants is to regulate some aspect of the cell death pathway underlying the les23 phenotype.

Investigations into the role of the plastidial peptide methionine sulfoxide reductase in response to oxidative stress in Arabidopsis

Peptidyl Met residues are readily oxidized by reactive oxygen species to form Met sulfoxide. The enzyme peptide Met sulfoxide reductase (PMSR) catalyzes the reduction of Met sulfoxides back to Met. In doing so, PMSR is proposed to act as a last-chance antioxidant, repairing proteins damaged from oxidative stress. To assess the role of this enzyme in plants, we generated multiple transgenic lines with altered expression levels of the plastid form of PMSR (PMSR4). In transgenic plants, PMSR4 expression ranged from 95% to 40% (antisense) and more than 600% (overexpressing lines) of wild-type plants. Under optimal growing conditions, there is no effect of the transgene on the phenotype of the plants. When exposed to different oxidative stress conditions-methyl viologen, ozone, and high light-differences were observed in the rate of photosynthesis, the maximum quantum yield (Fv/Fm ratio), and the Met sulfoxide content of the isolated chloroplast. Plants that overexpressed PMSR4 were more resistant to oxidative damage localized in the chloroplast, and plants that underexpressed PMSR4 were more susceptible. The Met sulfoxide levels in proteins of the soluble fraction of chloroplasts were increased by methyl viologen and ozone, but not by high-light treatment. Under stress conditions, the overexpression of PMSR4 lowered the sulfoxide content and underexpression resulted in an overall increase in content.

The metabolic imbalance underlying lesion formation in Arabidopsis thaliana overexpressing farnesyl diphosphate synthase (isoform 1S) leads to oxidative stress and is triggered by the developmental decline of endogenous HMGR activity

Overexpression of Arabidopsis thaliana farnesyl diphosphate synthase isoform 1S (FPS1S) in transgenic A. thaliana (L.) Heynh. leads to necrotic lesion formation in leaves in planta and to premature senescence in detached leaves [A. Masferrer et al. (2002) Plant J 30:123-132]. Here we report that leaves of plants overexpressing FPS1S with symptoms of necrosis show increased H2O2 formation and induction of both the pathogenesis-related 1 (PR-1) and the alternative oxidase 1a (AOX1a) genes. These findings indicate that plants overexpressing FPS1S should be considered as lesion-mimic mutants and lead us to propose that H2O2 is the main inducing agent of necrosis in these plants. The onset of necrosis appears in a developmentally regulated manner that correlates with the developmental decline of endogenous 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity. Accordingly, constitutive overexpression of HMGR in plants overexpressing FPS1S prevents both necrosis and premature senescence. These observations demonstrate that both phenotypes are due to an insufficient supply of mevalonic acid and support the notion that the metabolic imbalance associated with FPS1S overexpression is, in fact, triggered by the developmental decline of HMGR activity. We also show that overexpression of FPS1S alleviates growth inhibition caused by overexpression of the catalytic domain of isoform HMGR1S. Overall, our results reinforce the view that the levels of specific intermediates of the mevalonic acid pathway must be strictly controlled, particularly those located at branch-point positions, in order to avoid deleterious effects on plant growth and development.

LESION SIMULATING DISEASE 1 is required for acclimation to conditions that promote excess excitation energy

The lsd1 mutant of Arabidopsis fails to limit the boundaries of hypersensitive cell death response during avirulent pathogen infection and initiates unchecked lesions in long day photoperiod giving rise to the runaway cell death (rcd) phenotype. We link here the initiation and propagation of rcd to the activity of photosystem II, stomatal conductance and ultimately to photorespiratory H(2)O(2). A cross of lsd1 with the chlorophyll a/b binding harvesting-organelle specific (designated cao) mutant, which has a reduced photosystem II antenna, led to reduced lesion formation in the lsd1/cao double mutant. This lsd1 mutant also had reduced stomatal conductance and catalase activity in short-day permissive conditions and induced H(2)O(2) accumulation followed by rcd when stomatal gas exchange was further impeded. All of these traits depended on the defense regulators EDS1 and PAD4. Furthermore, nonphotorespiratory conditions retarded propagation of lesions in lsd1. These data suggest that lsd1 failed to acclimate to light conditions that promote excess excitation energy (EEE) and that LSD1 function was required for optimal catalase activity. Through this regulation LSD1 can influence the effectiveness of photorespiration in dissipating EEE and consequently may be a key determinant of acclimatory processes. Salicylic acid, which induces stomatal closure, inhibits catalase activity and triggers the rcd phenotype in lsd1, also impaired acclimation of wild-type plants to conditions that promote EEE. We propose that the roles of LSD1 in light acclimation and in restricting pathogen-induced cell death are functionally linked.

The promotion of gonadal cell divisions by the Caenorhabditis elegans TRPM cation channel GON-2 is antagonized by GEM-4 copine

The initiation of postembryonic cell divisions by the gonadal precursors of C. elegans requires the activity of gon-2. gon-2 encodes a predicted cation channel (GON-2) of the TRPM subfamily of TRP proteins and is likely to mediate the influx of Ca(2+) and/or Mg(2+). We report here that mutations in gem-4 (gon-2 extragenic modifier) are capable of suppressing loss-of-function alleles of gon-2. gem-4 encodes a member of the copine family of Ca(2+)-dependent phosphatidylserine binding proteins. Overall, our data indicate that GEM-4 antagonizes GON-2. This antagonism could be mediated by a direct inhibition of GON-2 by GEM-4, since both proteins are predicted to be localized to the plasma membrane. Alternatively, GEM-4 could affect GON-2 activity levels by either promoting endocytosis or inhibiting exocytosis of vesicles that carry GON-2. It is also possible that GEM-4 and GON-2 act in parallel to each other. Mutation of gem-4 does not suppress the gonadal defects produced by inactivation of gon-4, suggesting that gon-4 either acts downstream of gem-4 and gon-2 or acts in a parallel regulatory pathway.

Calcium-dependent regulation of tumour necrosis factor-alpha receptor signalling by copine

The role of copines in regulating signalling from the TNF-alpha (tumour necrosis factor-alpha) receptor was probed by the expression of a copine dominant-negative construct in HEK293 (human embryonic kidney 293) cells. The construct was found to reduce activation of the transcription factor NF-kappaB (nuclear factor-kappaB) by TNF-alpha. The introduction of calcium into HEK293 cells either through the activation of muscarinic cholinergic receptors or through the application of the ionophore A23187 was found to enhance TNF-alpha-dependent activation of NF-kappaB. This effect of calcium was completely blocked by the copine dominant-negative construct. TNF-alpha was found to greatly enhance the expression of endogenous copine I, and the responsiveness of the TNF-alpha signalling pathway to muscarinic stimulation increased in parallel with the increased copine I expression. The copine dominant-negative construct also inhibited the TNF-alpha-dependent degradation of IkappaB, a regulator of NF-kappaB. All of the effects of the dominant-negative construct could be reversed by overexpression of full-length copine I, suggesting that the construct acts specifically through competitive inhibition of copine. One of the identified targets of copine I is the NEDD8-conjugating enzyme UBC12 (ubiquitin C12), that promotes the degradation of IkappaB through the ubiquitin ligase enzyme complex SCF(betaTrCP). Therefore the copine dominant-negative construct might inhibit TNF-alpha signalling by dysregulation or mislocalization of UBC12. Based on these results, a hypothesis is presented for possible roles of copines in regulating other signalling pathways in animals, plants and protozoa.

A haplotype-specific Resistance gene regulated by BONZAI1 mediates temperature-dependent growth control in Arabidopsis

Plant growth homeostasis and defense responses are regulated by BONZAI1 (BON1), an evolutionarily conserved gene. Here, we show that growth regulation by BON1 is mediated through defense responses. BON1 is a negative regulator of a haplotype-specific Resistance (R) gene SNC1. The bon1-1 loss-of-function mutation activates SNC1, leading to constitutive defense responses and, consequently, reduced cell growth. In addition, a feedback amplification of the SNC1 gene involving salicylic acid is subject to temperature control, accounting for the regulation of growth and defense by temperature in bon1-1 and many other mutants. Thus, plant growth homeostasis involves the regulation of an R gene by BON1 and the intricate interplay between defense responses and temperature responses.

Pseudomonas syringae pv. tomato cells encounter inhibitory levels of water stress during the hypersensitive response of Arabidopsis thaliana

During plant defense against bacterial pathogens, the hypersensitive response (HR) functions to restrict pathogen growth and spread. The mechanisms driving this growth restriction are poorly understood. We used a water stress-responsive transcriptional fusion to quantify the water potential sensed by individual Pseudomonas syringae pv. tomato DC3000 cells during infection of Arabidopsis thaliana leaves. A nonpathogenic DC3000 hrcC mutant defective in type III secretion, as well as the saprophyte Pseudomonas fluorescens A506, sensed water potentials of -0.3 to -0.4 MPa at 48 h postinfiltration (hpi). During pathogenesis, DC3000 sensed lower water potentials (-0.4 to -0.9 MPa), demonstrating that it can modify the intercellular environment, and these water potentials were associated with optimal DC3000 growth in culture. During the HR, DC3000 cells sensed water potentials (-1.6 to -2.2 MPa) that were low enough to prevent cell division in the majority of cells in culture. This water potential decrease occurred within only 4 hpi and was influenced by avirulence gene expression, with avrRpm1 expression associated with lower water potentials than avrRpt2 or avrB expression at 48 hpi. The population sizes of the DC3000 variants tested were significantly correlated with the apoplastic water potential at 48 hpi, with a decrease of -0.9 MPa associated with a 10-fold decrease in cells per gram of leaf. These results suggest that the apoplastic water potential is a determinant of endophytic bacterial population size, and water stress, resulting from high osmolarity or tissue desiccation, is at least one factor restricting bacterial growth during the HR.

Overexpression of (At)NPR1 in rice leads to a BTH- and environment-induced lesion-mimic/cell death phenotype

Systemic acquired resistance (SAR) is an inducible defense response that protects plants against a broad spectrum of pathogens. A central regulator of SAR in Arabidopsis is NPR1 (nonexpresser of pathogenesis-related genes). In rice, overexpression of Arabidopsis NPR1 enhances plant resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae. This report demonstrates that overexpression of (At)NPR1 in rice also triggers a lesion-mimic/cell death (LMD) phenotype. The LMD phenotype is environmentally regulated and heritable. In addition, the development of lesions and death correlates with the expression of rice defense genes and the accumulation of hydrogen peroxide. Application of the salicylic acid (SA) analog, benzo(1,2,3) thiadiazole-7-carbothioc acid S-methyl ester (BTH), potentiates this phenotype Endogenous SA levels are reduced in rice overexpressing (At)NPR1 when compared with wildtype plants, supporting the idea that (At)NPR1 may perceive and modulate the accumulation of SA. The association of (At)NPR1 expression in rice with the development of an LMD phenotype suggests that (At)NPR1 has multiple roles in plant stress responses that may affect its efficacy as a transgenic tool for engineering broad-spectrum resistance.

A gain-of-function mutation in a plant disease resistance gene leads to constitutive activation of downstream signal transduction pathways in suppressor of npr1-1, constitutive 1

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive 1 (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR-type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.

Mutation of a family 8 glycosyltransferase gene alters cell wall carbohydrate composition and causes a humidity-sensitive semi-sterile dwarf phenotype in Arabidopsis

The genome of Arabidopsis thaliana contains about 400 genes coding for glycosyltransferases, many of which are predicted to be involved in the synthesis and remodelling of cell wall components. We describe the isolation of a transposon-tagged mutant, parvus, which under low humidity conditions exhibits a severely dwarfed growth phenotype and failure of anther dehiscence resulting in semi-sterility. All aspects of the mutant phenotype were partially rescued by growth under high-humidity conditions, but not by the application of growth hormones or jasmonic acid. The mutation is caused by insertion of a maize Dissociation (Ds) element in a gene coding for a putative Golgi-localized glycosyltransferase belonging to family 8. Members of this family, originally identified on the basis of similarity to bacterial lipooligosaccharide glycosyltransferases, include enzymes known to be involved in the synthesis of bacterial and plant cell walls. Cell-wall carbohydrate analyses of the parvus mutant indicated reduced levels of rhamnogalacturonan I branching and alterations in the abundance of some xyloglucan linkages that may, however, be indirect consequences of the mutation.

Barley putative hypersensitive induced reaction genes: genetic mapping, sequence analyses and differential expression in disease lesion mimic mutants

The hypersensitive response (HR) is one of the most-efficient forms of plant defense against biotrophic pathogens, and results in localized cell death and the formation of necrotic lesions; however, the molecular components of pathways leading to HR remain largely unknown. Barley ( Hordeum vulgare ssp. vulgare L.) cDNAs for putative hypersensitive-induced reaction ( HIR) genes were isolated based on DNA and amino-acid homologies to maize HIR genes. Analyses of the cDNA and genomic sequences and genetic mapping found four distinct barley HIR genes, Hv-hir1, Hv-hir2, Hv-hir3 and Hv-hir4, on chromosomes 4(4H) bin10, 7(5H) bin04, 7(5H) bin07 and 1(7H) bin03, respectively. Hv-hir1, Hv-hir2 and Hv-hir3 genes were highly homologous at both DNA and the deduced amino-acid level, but the Hv-hir4 gene was similar to the other genes only at the amino-acid sequence level. Amino-acid sequence analyses of the barley HIR proteins indicated the presence of the SPFH protein-domain characteristic for the prohibitins and stomatins which are involved in control of the cell cycle and ion channels, as well as in other membrane-associated proteins from bacteria, plants and animals. HIR genes were expressed in all organs and developement stages analyzed, indicating a vital and non-redundant function. Barley fast-neutron mutants exhibiting spontaneous HR (disease lesion mimic mutants) showed up to a 35-fold increase in Hv-hir3 expression, implicating HIR genes in the induction of HR.

Regulation of Arabidopsis COPINE 1 gene expression in response to pathogens and abiotic stimuli

The copines are a widely distributed class of calcium-dependent, phospholipid-binding proteins of undetermined biological function. Mutation of the Arabidopsis CPN1 (COPINE 1) gene causes a humidity-sensitive lesion mimic phenotype with increased resistance to a bacterial and an oomyceteous pathogen, constitutive pathogenesis-related gene expression, and an accelerated hypersensitive cell death defense response. Here, we show that the disease resistance phenotype of the cpn1-1 mutant was also temperature sensitive, demonstrate increased CPN1 gene transcript accumulation in wild-type plants under low-humidity conditions, and present a detailed analysis of CPN1 gene transcript accumulation in response to bacterial pathogens. In wild-type plants, CPN1 transcript accumulation was rapidly, locally, and transiently induced by both avirulent and virulent strains of Pseudomonas syringae pv tomato bacteria. However, induction of CPN1 transcript accumulation by avirulent bacteria was much faster and stronger than that induced by virulent bacteria. Bacterial induction of CPN1 transcript accumulation was dependent on a functional type III bacterial protein secretion system. In planta expression of the avrRpt2 avirulence gene was sufficient to trigger rapid CPN1 transcript accumulation. CPN1 transcript accumulation was induced by salicylic acid treatment but was not observed during lesion formation in the lesion mimic mutants lsd1 and lsd5. These results are consistent with CPN1 playing a role in plant disease resistance responses, possibly as a suppressor of defense responses including the hypersensitive cell death defense response. The results also suggest that CPN1 may represent a link between plant disease resistance and plant acclimation to low-humidity and low-temperature conditions.

Identification of targets for calcium signaling through the copine family of proteins. Characterization of a coiled-coil copine-binding motif

We provide evidence that copines, members of a ubiquitous family of calcium-dependent, membrane-binding proteins, may represent a universal transduction pathway for calcium signaling because we find copines are capable of interacting with a wide variety of "target" proteins including MEK1, protein phosphatase 5, and the CDC42-regulated kinase, that are themselves components of intracellular signaling pathways. The copine target proteins were identified by yeast two-hybrid screening and the interactions were verified in vitro using purified proteins. In the majority of cases the copine binds to a domain of the target protein that is predicted to form a characteristic coiled-coil. A consensus sequence for the coiled-coil copine-binding site was derived and found to have predictive value for identifying new copine targets. We also show that interaction with copines may result in recruitment of target proteins to membrane surfaces and regulation of the enzymatic activities of target proteins.

HLM1, an essential signaling component in the hypersensitive response, is a member of the cyclic nucleotide-gated channel ion channel family

The hypersensitive response (HR) in plants is a programmed cell death that is commonly associated with disease resistance. A novel mutation in Arabidopsis, hlm1, which causes aberrant regulation of cell death, manifested by a lesion-mimic phenotype and an altered HR, segregated as a single recessive allele. Broad-spectrum defense mechanisms remained functional or were constitutive in the mutant plants, which also exhibited increased resistance to a virulent strain of Pseudomonas syringae pv tomato. In response to avirulent strains of the same pathogen, the hlm1 mutant showed differential abilities to restrict bacterial growth, depending on the avirulence gene expressed by the pathogen. The HLM1 gene encodes a cyclic nucleotide-gated channel, CNGC4. Preliminary study of the HLM1/CNGC4 gene pro-duct in Xenopus oocytes (inside-out patch-clamp technique) showed that CNGC4 is permeable to both K(+) and Na(+) and is activated by both cGMP and cAMP. HLM1 gene expression is induced in response to pathogen infection and some pathogen-related signals. Thus, HLM1 might constitute a common downstream component of the signaling pathways leading to HR/resistance.