AtBI-1, a plant homologue of Bax inhibitor-1, suppresses Bax-induced cell death in yeast and is rapidly upregulated during wounding and pathogen challenge

Soybean Mosaic Virus 6K1 Interactors Screening and GmPR4 and GmBI1 Function Characterization

Host proteins are essential during virus infection, and viral factors must target numerous host factors to complete their infectious cycle. The mature 6K1 protein of potyviruses is required for viral replication in plants. However, the interaction between 6K1 and host factors is poorly understood. The present study aims to identify the host interacting proteins of 6K1. Here, the 6K1 of Soybean mosaic virus (SMV) was used as the bait to screen a soybean cDNA library to gain insights about the interaction between 6K1 and host proteins. One hundred and twenty-seven 6K1 interactors were preliminarily identified, and they were classified into six groups, including defense-related, transport-related, metabolism-related, DNA binding, unknown, and membrane-related proteins. Then, thirty-nine proteins were cloned and merged into a prey vector to verify the interaction with 6K1, and thirty-three of these proteins were confirmed to interact with 6K1 by yeast two-hybrid (Y2H) assay. Of the thirty-three proteins, soybean pathogenesis-related protein 4 (GmPR4) and Bax inhibitor 1 (GmBI1) were chosen for further study. Their interactions with 6K1 were also confirmed by bimolecular fluorescence complementation (BiFC) assay. Subcellular localization showed that GmPR4 was localized to the cytoplasm and endoplasmic reticulum (ER), and GmBI1 was located in the ER. Moreover, both GmPR4 and GmBI1 were induced by SMV infection, ethylene and ER stress. The transient overexpression of GmPR4 and GmBI1 reduced SMV accumulation in tobacco, suggesting their involvement in the resistance to SMV. These results would contribute to exploring the mode of action of 6K1 in viral replication and improve our knowledge of the role of PR4 and BI1 in SMV response.

Cell Death Signaling From Endoplasmic Reticulum Stress: Plant-Specific and Conserved Features

The endoplasmic reticulum (ER) stress response is triggered by any condition that disrupts protein folding and promotes the accumulation of unfolded proteins in the lumen of the organelle. In eukaryotic cells, the evolutionarily conserved unfolded protein response is activated to clear unfolded proteins and restore ER homeostasis. The recovery from ER stress is accomplished by decreasing protein translation and loading into the organelle, increasing the ER protein processing capacity and ER-associated protein degradation activity. However, if the ER stress persists and cannot be reversed, the chronically prolonged stress leads to cellular dysfunction that activates cell death signaling as an ultimate attempt to survive. Accumulating evidence implicates ER stress-induced cell death signaling pathways as significant contributors for stress adaptation in plants, making modulators of ER stress pathways potentially attractive targets for stress tolerance engineering. Here, we summarize recent advances in understanding plant-specific molecular mechanisms that elicit cell death signaling from ER stress. We also highlight the conserved features of ER stress-induced cell death signaling in plants shared by eukaryotic cells.

Calmodulin7: recent insights into emerging roles in plant development and stress

Analyses of the function of Arabidopsis Calmodulin7 (CAM7) in concert with multiple regulatory proteins involved in various signal transduction processes. Calmodulin (CaM) plays various regulatory roles in multiple signaling pathways in eukaryotes. Arabidopsis CALMODULIN 7 (CAM7) is a unique member of the CAM family that works as a transcription factor in light signaling pathways. CAM7 works in concert with CONSTITUTIVE PHOTOMORPHOGENIC 1 and ELONGATED HYPOCOTYL 5, and plays an important role in seedling development. Further, it is involved in the regulation of the activity of various Ca²⁺-gated channels such as cyclic nucleotide gated channel 6 (CNGC6), CNGC14 and auto-inhibited Ca²⁺ ATPase 8. Recent studies further indicate that CAM7 is also an integral part of multiple signaling pathways including hormone, immunity and stress. Here, we review the recent advances in understanding the multifaceted role of CAM7. We highlight the open-ended questions, and also discuss the diverse aspects of CAM7 characterization that need to be addressed for comprehensive understanding of its cellular functions.

The role of alternative oxidase in plant hypersensitive response

The innate immune system of plants is crucial in defining the fate of a plant cell during plant-pathogen interactions. This response is often accompanied by a hypersensitive reaction leading to the death of a plant cell and restricted pathogen growth. Plant mitochondria, in this case, play a key role by maintaining a balance between cell respiration and reactive oxygen species formation. One of the key features of the hypersensitive response is the shift of the normal plant respiratory pathway to a special 'alternative' pathway. Plants contain an enzyme, alternative oxidase, for maintaining metabolic homeostasis of the cell. This energy dissipating respiration provides a branch in normal respiration by using ubiquinone to form water and heat, thus maintaining the energy status of the cell. Alternative oxidase is thought to minimize production of reactive oxygen species and can also function in 'anti-apoptotic' machinery in plant cells. In this mini review, we briefly describe the alternative respiratory pathway and explain the role of alternative oxidase in important cellular processes, such as programmed cell death and the hypersensitive response.

EDS1-Dependent Cell Death and the Antioxidant System in Arabidopsis Leaves is Deregulated by the Mammalian Bax

Cell death is the ultimate end of a cell cycle that occurs in all living organisms during development or responses to biotic and abiotic stresses. In the course of evolution, plants and animals evolve various molecular mechanisms to regulate cell death; however, some of them are conserved among both these kingdoms. It was found that mammalian proapoptotic BCL-2 associated X (Bax) protein, when expressed in plants, induces cell death, similar to hypersensitive response (HR). It was also shown that changes in the expression level of genes encoding proteins involved in stress response or oxidative status regulation mitigate Bax-induced plant cell death. In our study, we focused on the evolutional compatibility of animal and plant cell death molecular mechanisms. Therefore, we studied the deregulation of reactive oxygen species burst and HR-like propagation in Arabidopsis thaliana expressing mammalian Bax. We were able to diminish Bax-induced oxidative stress and HR progression through the genetic cross with plants mutated in ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), which is a plant-positive HR regulator. Plants expressing the mouse Bax gene in eds1-1 null mutant background demonstrated less pronounced cell death and exhibited higher antioxidant system efficiency compared to Bax-expressing plants. Moreover, eds1/Bax plants did not show HR marker genes induction, as in the case of the Bax-expressing line. The present study indicates some common molecular features between animal and plant cell death regulation and can be useful to better understand the evolution of cell death mechanisms in plants and animals.

Uncovering Bax inhibitor-1 dual role in the legume-rhizobia symbiosis in common bean roots

Bax-inhibitor 1 (BI-1) is a cell death suppressor conserved in all eukaryotes that modulates cell death in response to abiotic stress and pathogen attack in plants. However, little is known about its role in the establishment of symbiotic interactions. Here, we demonstrate the functional relevance of an Arabidopsis thaliana BI-1 homolog (PvBI-1a) to symbiosis between the common bean (Phaseolus vulgaris) and Rhizobium tropici. We show that the changes in expression of PvBI-1a observed during early symbiosis resemble those of some defence response-related proteins. By using gain- and loss-of-function approaches, we demonstrate that the overexpression of PvBI-1a in the roots of common bean increases the number of rhizobial infection events (and therefore the final number of nodules per root), but induces the premature death of nodule cells, affecting their nitrogen fixation efficiency. Nodule morphological alterations are known to be associated with changes in the expression of genes tied to defence, autophagy, and vesicular trafficking. Results obtained in the present work suggest that BI-1 has a dual role in the regulation of programmed cell death during symbiosis, extending our understanding of its critical function in the modulation of host immunity while responding to beneficial microbes.

ShORR-1, a Novel Tomato Gene, Confers Enhanced Host Resistance to Oidium neolycopersici

A previous complementary cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis examined responses to the powdery mildew pathogen Oidium neolycopersici (On) of the resistant cultivar Solanum habrochiates G1.1560, carrying the Ol-1 resistance gene, and susceptible cultivar S. lycopersicum Moneymaker (MM). Among other findings, a differentially expressed transcript-derived fragment (DE-TDF) (M14E72-213) was upregulated in near isogenic line (NIL)-Ol-1, but absent in MM. This DE-TDF showed high homology to a gene of unknown function, which we named ShORR-1 (Solanum habrochaites Oidium Resistance Required-1). However, MM homolog of ShORR-1 (named ShORR-1-M) was still found with 95.26% nucleic acid sequence similarity to ShORR-1 from G1.1560 (named ShORR-1-G); this was because the cut sites of restriction enzymes in the previous complementary cDNA-AFLP analysis was absent in ShORR-1-M and differs at 13 amino acids from ShORR-1-G. Transient expression in onion epidermal cells showed that ShORR-1 is a membrane-localized protein. Virus-induced gene silencing (VIGS) of ShORR-1-G in G1.1560 plants increased susceptibility to On. Furthermore, overexpressing of ShORR-1-G conferred MM with resistance to On, involving extensive hydrogen peroxide accumulation and formation of abnormal haustoria. Knockdown of ShORR-1-M in MM did not affect its susceptibility to On, while overexpressing of ShORR-1-M enhanced MM's susceptibility to On. We also found that changes in transcript levels of six well-known hormone signaling and defense-related genes are involved in ShORR-1-G-mediated resistance to On. The results indicate that ShORR-1-M and ShORR-1-G have antagonistic effects in tomato responses to On, and that ShORR-1 is essential for Ol-1-mediated resistance in tomato.

The Wheat Bax Inhibitor-1 Protein Interacts with an Aquaporin TaPIP1 and Enhances Disease Resistance in Arabidopsis

Bax inhibitor-1 (BI-1) is an endoplasmic reticulum (ER)-resident cell death suppressor evolutionarily conserved in eukaryotes. The ability of BI-1 to inhibit the biotic and abiotic stresses have been well-studied in Arabidopsis, while the functions of wheat BI-1 are largely unknown. In this study, the wheat BI-1 gene TaBI-1.1 was isolated by an RNA-seq analysis of Fusarium graminearum (Fg)-treated wheat. TaBI-1.1 expression was induced by a salicylic acid (SA) treatment and down-regulated by an abscisic acid (ABA) treatment. Based on β-glucuronidase (GUS) staining, TaBI-1.1 was expressed in mature leaves and roots but not in the hypocotyl or young leaves. Constitutive expression of TaBI-1.1 in Arabidopsis enhanced its resistance to Pseudomonas syringae pv. Tomato (Pst) DC3000 infection and induced SA-related gene expression. Additionally, TaBI-1.1 transgenic Arabidopsis exhibited an alleviation of damage caused by high concentrations of SA and decreased the sensitivity to ABA. Consistent with the phenotype, the RNA-seq analysis of 35S::TaBI-1.1 and Col-0 plants showed that TaBI-1.1 was involved in biotic stresses. These results suggested that TaBI-1.1 positively regulates SA signals and plays important roles in the response to biotic stresses. In addition, TaBI-1.1 interacted with the aquaporin TaPIP1, and both them were localized to ER membrane. Furthermore, we demonstrated that TaPIP1 was up-regulated by SA treatment and TaPIP1 transgenic Arabidopsis enhanced the resistance to Pst DC3000 infection. Thus, the interaction between TaBI-1.1 and TaPIP1 on the ER membrane probably occurs in response to SA signals and defense response.

Plant Bax Inhibitor-1 interacts with ATG6 to regulate autophagy and programmed cell death

Autophagy is an evolutionarily conserved catabolic process and is involved in the regulation of programmed cell death during the plant immune response. However, mechanisms regulating autophagy and cell death are incompletely understood. Here, we demonstrate that plant Bax inhibitor-1 (BI-1), a highly conserved cell death regulator, interacts with ATG6, a core autophagy-related protein. Silencing of BI-1 reduced the autophagic activity induced by both N gene-mediated resistance to Tobacco mosaic virus (TMV) and methyl viologen (MV), and enhanced N gene-mediated cell death. In contrast, overexpression of plant BI-1 increased autophagic activity and surprisingly caused autophagy-dependent cell death. These results suggest that plant BI-1 has both prosurvival and prodeath effects in different physiological contexts and both depend on autophagic activity.

TMBIM-mediated Ca2+ homeostasis and cell death

Ca²⁺ is a ubiquitous intracellular messenger that regulates numerous physiological activities in humans, animals, plants, and bacteria. Cytosolic Ca²⁺ is kept at a low level, but subcellular organelles such as the endoplasmic reticulum (ER) and Golgi apparatus maintain high-concentration Ca²⁺ stores. Under resting conditions, store Ca²⁺ homeostasis is dynamically regulated to equilibrate between active Ca²⁺ uptake and passive Ca²⁺ leak processes. The evolutionarily conserved Transmembrane BAX Inhibitor-1 Motif-containing (TMBIM) proteins mediate Ca²⁺ homeostasis and cell death. This review focuses on recent advances in functional and structural analysis of TMBIM proteins in regulation of the two related functions. The roles of TMBIM proteins in pathogen infection and cancer are also discussed with prospects for treatment. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Roles of plant hormones and anti-apoptosis genes during drought stress in rice (Oryza sativa L.)

We previously identified the rice (Oryza sativa) senescence-associated gene OsSAP which encodes a highly conserved protein involved in anti-apoptotic activity. This novel Bax suppressor-related gene regulates tolerance to multiple stresses in yeast. Here, we show the effects of drought stress on leaf and root tissues of plants over-expressing OsSAP in relation to the levels of phytohormones, abscisic acid (ABA), jasmonic acid (JA), indole-3-carboxylic acid (ICA), gibberellic acid (GA3), and zeatin. Results showed that rice plants over-expressing SAP were tolerant to drought stress compared to wild type and the plants over-expressing AtBI-1, which is a homolog of the human Bax inhibitor-1 in Arabidopsis. ABA and JA levels in OsSAP and AtBI-1 transgenic plants consistently increased up to at least 3 days after drought treatment, whereas lower GA3 levels were recorded during early drought period. Comparison between control and transgenic plants overexpressing anti-apoptosis genes OsSAP and AtBI-1 resulted in different patterns of hormone levels, indicating that these genes are involved in the plant responses to drought stress and present an opportunity for further study on drought stress tolerance in rice and other plant species.

Transcriptomics of tomato plants infected with TYLCSV or expressing the central TYLCSV Rep protein domain uncover changes impacting pathogen response and senescence

To establish a successful infection viruses need to overcome plant innate immune responses and redirect host gene expression for their multiplication and diffusion. Tomato yellow leaf curl Sardinia virus (TYLCSV) is a geminivirus, which causes significant economic losses in tomato. The multifunctional replication associated geminivirus protein (Rep) has an important role during viral infection. In particular, the Rep central domain spanning from aa 120 to 180 is known to interact with viral and host factors. In this study, we used long serial analysis of gene expression to analyse the transcriptional profiles of transgenic tomato plants expressing the first 210 amino acids of TYLCSV Rep (Rep210) and TYLCSV-infected wild-type tomato plants (Wt-Ty). Also, we compared these profiles with those of transgenic Rep130 tomatoes. Comparison of Wt-Ty and Rep210 libraries with the wild-type one identified 118 and 203 differentially expressed genes (DEGs), respectively. Importantly, 55% of Wt-Ty DEGs were in common with Rep210, and no ones showed opposite expression. Conversely, a negligible overlap was found between Rep130 DEGs and Wt-Ty and Rep210 ones. TYLCSV- and Rep210-repressed genes, but not induced ones, overlapped with the leaf senescence process. Interestingly, TYLCSV upregulates expression of genes involved in the negative regulation of programmed cell death (PCD), several of which were also regulated by the abscisic acid. Rep210 upregulated genes related to defence response, immune system processes and negative regulation of PCD. Collectively, our results support a model in which the Rep central domain has a pivotal role in redirecting host plant gene expression.

Overexpression of BAX INHIBITOR-1 Links Plasma Membrane Microdomain Proteins to Stress

BAX INHIBITOR-1 (BI-1) is a cell death suppressor widely conserved in plants and animals. Overexpression of BI-1 enhances tolerance to stress-induced cell death in plant cells, although the molecular mechanism behind this enhancement is unclear. We recently found that Arabidopsis (Arabidopsis thaliana) BI-1 is involved in the metabolism of sphingolipids, such as the synthesis of 2-hydroxy fatty acids, suggesting the involvement of sphingolipids in the cell death regulatory mechanism downstream of BI-1. Here, we show that BI-1 affects cell death-associated components localized in sphingolipid-enriched microdomains of the plasma membrane in rice (Oryza sativa) cells. The amount of 2-hydroxy fatty acid-containing glucosylceramide increased in the detergent-resistant membrane (DRM; a biochemical counterpart of plasma membrane microdomains) fraction obtained from BI-1-overexpressing rice cells. Comparative proteomics analysis showed quantitative changes of DRM proteins in BI-1-overexpressing cells. In particular, the protein abundance of FLOTILLIN HOMOLOG (FLOT) and HYPERSENSITIVE-INDUCED REACTION PROTEIN3 (HIR3) markedly decreased in DRM of BI-1-overexpressing cells. Loss-of-function analysis demonstrated that FLOT and HIR3 are required for cell death by oxidative stress and salicylic acid, suggesting that the decreased levels of these proteins directly contribute to the stress-tolerant phenotypes in BI-1-overexpressing rice cells. These findings provide a novel biological implication of plant membrane microdomains in stress-induced cell death, which is negatively modulated by BI-1 overexpression via decreasing the abundance of a set of key proteins involved in cell death.

Arabidopsis Bax Inhibitor-1 inhibits cell death induced by pokeweed antiviral protein in Saccharomyces cerevisiae

Apoptosis is an active form of programmed cell death (PCD) that plays critical roles in the development, differentiation and resistance to pathogens in multicellular organisms. Ribosome inactivating proteins (RIPs) are able to induce apoptotic cell death in mammalian cells. In this study, using yeast as a model system, we showed that yeast cells expressing pokeweed antiviral protein (PAP), a single-chain ribosome-inactivating protein, exhibit apoptotic-like features, such as nuclear fragmentation and ROS production. We studied the interaction between PAP and AtBI-1 (Arabidopsis thaliana Bax Inhibitor-1), a plant anti-apoptotic protein, which inhibits Bax induced cell death. Cells expressing PAP and AtBI-1 were able to survive on galactose media compared to PAP alone, indicating a reduction in the cytotoxicity of PAP in yeast. However, PAP was able to depurinate the ribosomes and to inhibit total translation in the presence of AtBI-1. A C-terminally deleted AtBI-1 was able to reduce the cytotoxicity of PAP. Since anti-apoptotic proteins form heterodimers to inhibit the biological activity of their partners, we used a co-immunoprecipitation assay to examine the binding of AtBI-1 to PAP. Both full length and C-terminal deleted AtBI-1 were capable of binding to PAP. These findings indicate that PAP induces cell death in yeast and AtBI-1 inhibits cell death induced by PAP without affecting ribosome depurination and translation inhibition.

Bovine serum albumin in saliva mediates grazing response in Leymus chinensis revealed by RNA sequencing

Background

Sheepgrass (Leymus chinensis) is an important perennial forage grass across the Eurasian Steppe and is adaptable to various environmental conditions, but little is known about its molecular mechanism responding to grazing and BSA deposition. Because it has a large genome, RNA sequencing is expensive and impractical except for the next-generation sequencing (NGS) technology.

Results

In this study, NGS technology was employed to characterize de novo the transcriptome of sheepgrass after defoliation and grazing treatments and to identify differentially expressed genes (DEGs) responding to grazing and BSA deposition. We assembled more than 47 M high-quality reads into 120,426 contigs from seven sequenced libraries. Based on the assembled transcriptome, we detected 2,002 DEGs responding to BSA deposition during grazing. Enrichment analysis of Gene ontology (GO), EuKaryotic Orthologous Groups (KOG) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways revealed that the effects of grazing and BSA deposition involved more apoptosis and cell oxidative changes compared to defoliation. Analysis of DNA fragments, cell oxidative factors and the lengths of leaf scars after grazing provided physiological and morphological evidence that BSA deposition during grazing alters the oxidative and apoptotic status of cells.

Conclusions

This research greatly enriches sheepgrass transcriptome resources and grazing-stress-related genes, helping us to better understand the molecular mechanism of grazing in sheepgrass. The grazing-stress-related genes and pathways will be a valuable resource for further gene-phenotype studies.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1126) contains supplementary material, which is available to authorized users.

The tomato cell death suppressor Adi3 is restricted to the endosomal system in response to the Pseudomonas syringae effector protein AvrPto

The tomato (Solanum lycopersicum) AGC protein kinase Adi3 functions as a suppressor of cell death and was first identified as an interactor with the tomato resistance protein Pto and the Pseudomonas syringae effector protein AvrPto. Models predict that loss of Adi3 cell death suppression (CDS) activity during Pto/AvrPto interaction leads to the cell death associated with the resistance response initiated from this interaction. Nuclear localization is required for Adi3 CDS. Prevention of nuclear accumulation eliminates Adi3 CDS and induces cell death by localizing Adi3 to intracellular punctate membrane structures. Here we use several markers of the endomembrane system to show that the punctate membrane structures to which non-nuclear Adi3 is localized are endosomal in nature. Wild-type Adi3 also localizes in these punctate endosomal structures. This was confirmed by the use of endosomal trafficking inhibitors, which were capable of trapping wild-type Adi3 in endosomal-like structures similar to the non-nuclear Adi3. This suggests Adi3 may traffic through the cell using the endomembrane system. Additionally, Adi3 was no longer found in the nucleus but was visualized in these punctate endosomal-like membranes during the cell death induced by the Pto/AvrPto interaction. Therefore we propose that inhibiting nuclear import and constraining Adi3 to the endosomal system in response to AvrPto is a mechanism to initiate the cell death associated with resistance.

Arabidopsis Bax inhibitor-1 promotes sphingolipid synthesis during cold stress by interacting with ceramide-modifying enzymes

Bax inhibitor-1 (BI-1) is a widely conserved cell death suppressor localized in the endoplasmic reticulum membrane. Our previous results revealed that Arabidopsis BI-1 (AtBI-1) interacts with not only Arabidopsis cytochrome b 5 (Cb5), an electron transfer protein, but also a Cb5-like domain (Cb5LD)-containing protein, Saccharomyces cerevisiae fatty acid 2-hydroxylase 1, which 2-hydroxylates sphingolipid fatty acids. We have now found that AtBI-1 binds Arabidopsis sphingolipid Δ8 long-chain base (LCB) desaturases AtSLD1 and AtSLD2, which are Cb5LD-containing proteins. The expression of both AtBI-1 and AtSLD1 was increased by cold exposure. However, different phenotypes were observed in response to cold treatment between an atbi-1 mutant and a sld1sld2 double mutant. To elucidate the reasons behind the difference, we analyzed sphingolipids and found that unsaturated LCBs in atbi-1 were not altered compared to wild type, whereas almost all LCBs in sld1sld2 were saturated, suggesting that AtBI-1 may not be necessary for the desaturation of LCBs. On the other hand, the sphingolipid content in wild type increased in response to low temperature, whereas total sphingolipid levels in atbi-1 were unaltered. In addition, the ceramide-modifying enzymes AtFAH1, sphingolipid base hydroxylase 2 (AtSBH2), acyl lipid desaturase 2 (AtADS2) and AtSLD1 were highly expressed under cold stress, and all are likely to be related to AtBI-1 function. These findings suggest that AtBI-1 contributes to synthesis of sphingolipids during cold stress by interacting with AtSLD1, AtFAH1, AtSBH2 and AtADS2.

The membrane-associated transcription factor NAC089 controls ER-stress-induced programmed cell death in plants

The unfolded protein response (UPR) is activated to sustain cell survival by reducing misfolded protein accumulation in the endoplasmic reticulum (ER). The UPR also promotes programmed cell death (PCD) when the ER stress is severe; however, the underlying molecular mechanisms are less understood, especially in plants. Previously, two membrane-associated transcriptions factors (MTFs), bZIP28 and bZIP60, were identified as the key regulators for cell survival in the plant ER stress response. Here, we report the identification of another MTF, NAC089, as an important PCD regulator in Arabidopsis (Arabidopsis thaliana) plants. NAC089 relocates from the ER membrane to the nucleus under ER stress conditions. Inducible expression of a truncated form of NAC089, in which the transmembrane domain is deleted, induces PCD with increased caspase 3/7-like activity and DNA fragmentation. Knock-down NAC089 in Arabidopsis confers ER stress tolerance and impairs ER-stress-induced caspase-like activity. Transcriptional regulation analysis and ChIP-qPCR reveal that NAC089 plays important role in regulating downstream genes involved in PCD, such as NAC094, MC5 and BAG6. Furthermore, NAC089 is up-regulated by ER stress, which is directly controlled by bZIP28 and bZIP60. These results show that nuclear relocation of NAC089 promotes ER-stress-induced PCD, and both pro-survival and pro-death signals are elicited by bZIP28 and bZIP60 during plant ER stress response.

Protein folding is fundamentally important for development and responses to environmental stresses in eukaryotes. When excess misfolded proteins are accumulated in the endoplasmic reticulum (ER), the unfolded protein response (UPR) is triggered to promote cell survival through optimizing protein folding, and also promote programmed cell death (PCD) when the stress is severe. However, the link from ER-stress-sensing to PCD is largely unknown. Here, we report the identification of one membrane-associated transcription factor NAC089 as an important regulator of ER stress-induced PCD in plants. We have established a previously unrecognized molecular connection between ER stress sensors and PCD regulators. We have shown that organelle-to-organelle translocation of a transcription factor is important for its function in transcriptional regulation. Our results have provided novel insights into the molecular mechanisms of PCD in plants, especially under ER stress conditions.

Protective effects of ETC complex III and cytochrome c against hydrogen peroxide-induced apoptosis in yeast

In mammals, the mitochondrial electron transfer components (ETC) complex III and cytochrome c (cyt c) play essential roles in reactive oxygen species (ROS)-induced apoptosis. However, in yeast, the functions of cyt c and other ETC components remain unclear. In this study, three ETC-defective yeast mutants qcr7Δ, cyc1Δcyc7Δ, and cox12Δ, lacking cyt c oxidoreductase (complex III), cyt c, and cyt c oxidase (complex IV), respectively, were used to test the roles of these proteins in the response of cells to hydrogen peroxide (H₂O₂). Mutants qcr7Δ and cyc1Δcyc7Δ displayed greater H₂O₂ sensitivity than the wild-type or cox12Δ mutant. Consistent with this, qcr7Δ and cyc1Δcyc7Δ produced higher ROS levels, displayed derepressed expression of the proapoptotic genes AIF1, NUC1, and NMA111, but not YCA1, at the mRNA level, and were more vulnerable to H₂O₂-induced apoptosis. Interestingly, mutants lacking these proapoptotic genes displayed enhanced H₂O₂ tolerance, but unaffected ROS accumulation. Furthermore, the overexpression of antiapoptotic genes (Bcl-2, Ced-9, AtBI-1, and PpBI-1) reduced the levels of AIF1, NUC1, and NMA111 mRNAs, and reduced H₂O₂-induced cell death. Our findings identify two ETC components as early-inhibitory members of the ROS-mediated apoptotic pathway, suggesting their essential roles in metabolizing H₂O₂, probably by providing reduced cyt c, allowing cyt c peroxidase to remove H₂O₂ from the cells.

Transcriptome-wide mapping of pea seed ageing reveals a pivotal role for genes related to oxidative stress and programmed cell death

Understanding of seed ageing, which leads to viability loss during storage, is vital for ex situ plant conservation and agriculture alike. Yet the potential for regulation at the transcriptional level has not been fully investigated. Here, we studied the relationship between seed viability, gene expression and glutathione redox status during artificial ageing of pea (Pisum sativum) seeds. Transcriptome-wide analysis using microarrays was complemented with qRT-PCR analysis of selected genes and a multilevel analysis of the antioxidant glutathione. Partial degradation of DNA and RNA occurred from the onset of artificial ageing at 60% RH and 50°C, and transcriptome profiling showed that the expression of genes associated with programmed cell death, oxidative stress and protein ubiquitination were altered prior to any sign of viability loss. After 25 days of ageing viability started to decline in conjunction with progressively oxidising cellular conditions, as indicated by a shift of the glutathione redox state towards more positive values (>-190 mV). The unravelling of the molecular basis of seed ageing revealed that transcriptome reprogramming is a key component of the ageing process, which influences the progression of programmed cell death and decline in antioxidant capacity that ultimately lead to seed viability loss.

Co-immunoprecipitation-based identification of putative BAX INHIBITOR-1-interacting proteins involved in cell death regulation and plant-powdery mildew interactions

The endoplasmic reticulum (ER)-resident BAX INHIBITOR-1 (BI-1) protein is one of a few cell death suppressors known to be conserved in animals and plants. The function of BI-1 proteins in response to various biotic and abiotic stress factors is well established. However, little is known about the underlying mechanisms. We conducted co-immunoprecipitation (co-IP) experiments to identify Arabidopsis thaliana BI-1-interacting proteins to obtain a potentially better understanding of how BI-1 functions during plant-pathogen interactions and as a suppressor of cell death. Liquid chromatography and tandem mass spectrometry (LC-MS/MS) identified 95 proteins co-immunoprecipitated with green fluorescing protein (GFP)-tagged BI-1. Five selected candidate proteins, a RIBOPHORIN II (RPN2) family protein, VACUOLAR ATP SYNTHASE SUBUNIT A (VHA-A), cytochrome P450 83A1 (CYP83A1), H(+) -ATPASE 1 (AHA1) and PROHIBITIN 2 (PHB2), were further investigated with regard to their role in BI-1-associated processes. To this end, we analysed a set of Arabidopsis mutants in the interaction with the adapted powdery mildew fungus Erysiphe cruciferarum and on cell death-inducing treatments. Two independent rpn2 knock-down mutants tended to better support powdery mildew, and a phb2 mutant showed altered responses to cell death-inducing Alternaria alternata f.sp. lycopersici (AAL) toxin treatment. Two independent cyp83a1 mutants showed a strong powdery mildew resistance phenotype and enhanced sensitivity to AAL toxin. Moreover, co-localization studies and fluorescence resonance energy transfer (FRET) experiments suggested a direct interaction of BI-1 with CYP83A1 at the ER.

Aluminum-induced programmed cell death promoted by AhSAG, a senescence-associated gene in Arachis hypoganea L

Programmed cell death (PCD) is a foundational cellular process in plant development and elimination of damaged cells under environmental stresses. In this study, Al induced PCD in two peanut (Arachis hypoganea L.) cultivars Zhonghua 2 (Al-sensitive) and 99-1507 (Al-tolerant) using DNA ladder, TUNEL detection and electron microscopy. The concentration of Al-induced PCD was lower in Zhonghua 2 than in 99-1507. AhSAG, a senescence-associated gene was isolated from cDNA library of Al-stressed peanut with PCD. Open reading frame (ORF) of AhSAG was 474bp, encoding a SAG protein composed of 157 amino acids. Compared to the control and the antisense transgenic tobacco plants, the fast development and blossom of the sense transgenic plants happened to promote senescence. The ability of Al tolerance in sense transgenic tobacco was lower than in antisense transgenic tobacco according to root elongation and Al content analysis. The expression of AhSAG-GFP was higher in sense transgenic tobacco than in antisense transgenic tobacco. Altogether, these results indicated that there was a negative relationship between Al-induced PCD and Al-resistance in peanut, and the AhSAG could induce or promote the occurrence of PCD in plants.

BAX inhibitor-1 silencing suppresses white spot syndrome virus replication in red swamp crayfish, Procambarus clarkii

BAX inhibitor-1 (BI-1) was originally described as an anti-apoptotic protein in both animal and plant cells. BI-1 overexpression suppresses ER stress-induced apoptosis in animal cells. Inhibition of BI-1 activity could induce the cell death in mammals and plants. However, the function of BI-1 in crustacean immunity was unclear. In this paper, the full-length cDNA of a BI-1 protein in red swamp crayfish, Procambarus clarkii (PcBI-1) was cloned and its expression profiles in normal and infected crayfish were analyzed. The results showed that PcBI-1 was expressed in hemocytes, heart, hepatopancreas, gills, stomach, and intestines of the crayfish and was upregulated after challenged with Vibrio anguillarum and with white spot syndrome virus (WSSV). To determine the function of PcBI-1 in the innate immunity of the crayfish, the RNA interference against PcBI-1 was performed and the results indicated the hemocyte programmed cell death rate was increased significantly and WSSV replication was declined after PcBI-1 knocked down. Altogether, PcBI-1 plays an anti-apoptotic role, wherein high PcBI-1 expression suppresses programmed cell death, which is beneficial for WSSW replication in crayfish.

Endoplasmic reticulum (ER) stress response and its physiological roles in plants

The endoplasmic reticulum (ER) stress response is a highly conserved mechanism that results from the accumulation of unfolded or misfolded proteins in the ER. The response plays an important role in allowing plants to sense and respond to adverse environmental conditions, such as heat stress, salt stress and pathogen infection. Since the ER is a well-controlled microenvironment for proper protein synthesis and folding, it is highly susceptible to stress conditions. Accumulation of unfolded or misfolded proteins activates a signaling pathway, called the unfolded protein response (UPR), which acts to relieve ER stress and, if unsuccessful, leads to cell death. Plants have two arms of the UPR signaling pathway, an arm involving the proteolytic processing of membrane-associated basic leucine zipper domain (bZIP) transcription factors and an arm involving RNA splicing factor, IRE1, and its mRNA target. These signaling pathways play an important role in determining the cell's fate in response to stress conditions.

Recombinant antibody production in Arabidopsis seeds triggers an unfolded protein response

Among the many plant-based production systems that are being tested for molecular farming, seeds are very attractive, as they provide a stable environment in which the accumulating recombinant proteins can be stored. However, it is not known exactly how high production levels of recombinant antibodies influence the endogenous transcriptome and proteome of the developing seed. To address this question, we studied the transcriptomic status in developing Arabidopsis (Arabidopsis thaliana) seeds 13 d post anthesis of three transgenic lines, producing varying levels of recombinant VHH or single-chain Fv antibody fragments fused to the human immunoglobulin G1-derived Fc fragment under the control of the β-PHASEOLIN seed-specific promoter. Using genome-wide Tiling arrays, we demonstrated that only a small proportion of the transcriptome was significantly changed in each of the lines compared with the wild type. Strikingly, in all three lines, we found a large overlap of up-regulated genes corresponding to protein folding, glycosylation/modification, translocation, vesicle transport, and protein degradation, suggestive of a state of cellular stress called the unfolded protein response. Moreover, the gene up-regulation amplitude was similar in all three lines. We hypothesize that the production of recombinant antibodies in the endoplasmic reticulum triggers endoplasmic reticulum stress, causing a disturbance of the normal cellular homeostasis.

Centrality of host cell death in plant-microbe interactions

Programmed cell death (PCD) is essential for proper growth, development, and cellular homeostasis in all eukaryotes. The regulation of PCD is of central importance in plant-microbe interactions; notably, PCD and features associated with PCD are observed in many host resistance responses. Conversely, pathogen induction of inappropriate cell death in the host results in a susceptible phenotype and disease. Thus, the party in control of PCD has a distinct advantage in these battles. PCD processes appear to be of ancient origin, as indicated by the fact that many features of cell death strategy are conserved between animals and plants; however, some of the details of death execution differ. Mammalian core PCD genes, such as caspases, are not present in plant genomes. Similarly, pro- and antiapoptotic mammalian regulatory elements are absent in plants, but, remarkably, when expressed in plants, successfully impact plant PCD. Thus, subtle structural similarities independent of sequence homology appear to sustain operational equivalence. The vacuole is emerging as a key organelle in the modulation of plant PCD. Under different signals for cell death, the vacuole either fuses with the plasmalemma membrane or disintegrates. Moreover, the vacuole appears to play a key role in autophagy; evidence suggests a prosurvival function for autophagy, but other studies propose a prodeath phenotype. Here, we describe and discuss what we know and what we do not know about various PCD pathways and how the host integrates signals to activate salicylic acid and reactive oxygen pathways that orchestrate cell death. We suggest that it is not cell death as such but rather the processes leading to cell death that contribute to the outcome of a given plant-pathogen interaction.

Endoplasmic reticulum stress responses in plants

Endoplasmic reticulum (ER) stress is of considerable interest to plant biologists because it occurs in plants subjected to adverse environmental conditions. ER stress responses mitigate the damage caused by stress and confer levels of stress tolerance to plants. ER stress is activated by misfolded proteins that accumulate in the ER under adverse environmental conditions. Under these conditions, the demand for protein folding exceeds the capacity of the system, which sets off the unfolded protein response (UPR). Two arms of the UPR signaling pathway have been described in plants: one that involves two ER membrane-associated transcription factors (bZIP17 and bZIP28) and another that involves a dual protein kinase (RNA-splicing factor IRE1) and its target RNA (bZIP60). Under mild or short-term stress conditions, signaling from IRE1 activates autophagy, a cell survival response. But under severe or chronic stress conditions, ER stress can lead to cell death.

Over-expression of Arabidopsis Bax inhibitor-1 delays methyl jasmonate-induced leaf senescence by suppressing the activation of MAP kinase 6

Methyl jasmonate (MeJA) is an important signalling molecule that has been reported to be able to promote plant senescence. The cell death suppressor Bax inhibitor-1 (BI1) has been found to suppress stress factor-mediated cell death in yeast and Arabidopsis. However, the effect and the genetic mechanism of Arabidopsis thaliana BI1 (AtBI1) on leaf senescence remain unclear. It was found here that the AtBI1 mutant, atbi1-2 (a gene knock-out), showed accelerated progression of MeJA-induced leaf senescence, while the AtBI1 complementation lines displayed similar symptoms as the WT during the senescence process. In addition, over-expression of the AtBI1 gene delayed the onset of MeJA-induced leaf senescence. Further analyses showed that during the process of MeJA-induced senescence, the activity of MPK6, a mitogen-activated protein kinase (MAPK), increased in WT plants, whereas it was significantly suppressed in AtBI1-overexpressing plants. Under the MeJA treatment, cytosolic calcium ([Ca(2+)](cyt)) functioned upstream of MPK6 activation and the elevation of [Ca(2+)](cyt) was reduced in AtBI1-overexpressing leaves. These results suggested a role of AtBI1 over-expression in delaying MeJA-induced leaf senescence by suppressing the [Ca(2+)](cyt)-dependent activation of MPK6, thus providing a new insight into the function and mechanism of AtBI1 in plant senescence.

Wheat BAX inhibitor-1 contributes to wheat resistance to Puccinia striiformis

BAX inihibitor-1 (BI-1) is proposed to be a cell death suppressor conserved in both animals and plants. The ability of BI-1 genes to inhibit programmed cell death (PCD) has been well studied in animals, but the physiological importance of BI-1 in plant-microbe interactions remains unclear. This study characterized BI-1 from wheat infected by Puccinia striiformis f. sp. tritici (Pst). The deduced TaBI-1 protein contained a Bax inhibitor domain and seven transmembrane regions conserved among members of the BI-1 family. Transcription of TaBI-1 was detected in all wheat tissues tested (culms, roots, leaves, anthers, and spikelets). Furthermore, TaBI-1 exhibited positive transcriptional responses to Pst infection and abiotic stresses. Overexpression of TaBI-1 in tobacco blocked Bax-induced cell death. Silencing TaBI-1 in plants of a resistant wheat genotype converted a resistant reaction to a relatively susceptible reaction when inoculated with an avirulent pathotype of the pathogen, and increased the area per infection site, but the percentage of necrotic cells did not change significantly, indicating that TaBI-1, a negative cell death regulator, contributes to wheat resistance to stripe rust. These results provide a better understanding of the molecular mechanism of wheat resistance to stripe rust.

Fortunella margarita transcriptional reprogramming triggered by Xanthomonas citri subsp. citri

BACKGROUND

Citrus canker disease caused by the bacterial pathogen Xanthomonas citri subsp. citri (Xcc) has become endemic in areas where high temperature, rain, humidity, and windy conditions provide a favourable environment for the dissemination of the bacterium. Xcc is pathogenic on many commercial citrus varieties but appears to elicit an incompatible reaction on the citrus relative Fortunella margarita Swing (kumquat), in the form of a very distinct delayed necrotic response. We have developed subtractive libraries enriched in sequences expressed in kumquat leaves during both early and late stages of the disease. The isolated differentially expressed transcripts were subsequently sequenced. Our results demonstrate how the use of microarray expression profiling can help assign roles to previously uncharacterized genes and elucidate plant pathogenesis-response related mechanisms. This can be considered to be a case study in a citrus relative where high throughput technologies were utilized to understand defence mechanisms in Fortunella and citrus at the molecular level.

RESULTS

cDNAs from sequenced kumquat libraries (ESTs) made from subtracted RNA populations, healthy vs. infected, were used to make this microarray. Of 2054 selected genes on a customized array, 317 were differentially expressed (P < 0.05) in Xcc challenged kumquat plants compared to mock-inoculated ones. This study identified components of the incompatible interaction such as reactive oxygen species (ROS) and programmed cell death (PCD). Common defence mechanisms and a number of resistance genes were also identified. In addition, there were a considerable number of differentially regulated genes that had no homologues in the databases. This could be an indication of either a specialized set of genes employed by kumquat in response to canker disease or new defence mechanisms in citrus.

CONCLUSION

Functional categorization of kumquat Xcc-responsive genes revealed an enhanced defence-related metabolism as well as a number of resistant response-specific genes in the kumquat transcriptome in response to Xcc inoculation. Gene expression profile(s) were analyzed to assemble a comprehensive and inclusive image of the molecular interaction in the kumquat/Xcc system. This was done in order to elucidate molecular mechanisms associated with the development of the hypersensitive response phenotype in kumquat leaves. These data will be used to perform comparisons among citrus species to evaluate means to enhance the host immune responses against bacterial diseases.

Yeast Bax inhibitor, Bxi1p, is an ER-localized protein that links the unfolded protein response and programmed cell death in Saccharomyces cerevisiae

Bax inhibitor-1 (BI-1) is an anti-apoptotic gene whose expression is upregulated in a wide range of human cancers. Studies in both mammalian and plant cells suggest that the BI-1 protein resides in the endoplasmic reticulum and is involved in the unfolded protein response (UPR) that is triggered by ER stress. It is thought to act via a mechanism involving altered calcium dynamics. In this paper, we provide evidence that the Saccharomyces cerevisiae protein encoded by the open reading frame, YNL305C, is a bona fide homolog for BI-1. First, we confirm that yeast cells from two different strain backgrounds lacking YNL305C, which we have renamed BXI1, are more sensitive to heat-shock induced cell death than wildtype controls even though they have indistinguishable growth rates at 30°C. They are also more susceptible both to ethanol-induced and to glucose-induced programmed cell death. Significantly, we show that Bxi1p-GFP colocalizes with the ER localized protein Sec63p-RFP. We have also discovered that Δbxi1 cells are not only more sensitive to drugs that induce ER stress, but also have a decreased unfolded protein response as measured with a UPRE-lacZ reporter. Finally, we have discovered that deleting BXI1 diminishes the calcium signaling response in response to the accumulation of unfolded proteins in the ER as measured by a calcineurin-dependent CDRE-lacZ reporter. In toto, our data suggests that the Bxi1p, like its metazoan homologs, is an ER-localized protein that links the unfolded protein response and programmed cell death.

Bax inhibitor-1: a highly conserved endoplasmic reticulum-resident cell death suppressor

In spite of fundamental differences between plant and animal cells, it is remarkable that some cell death regulators that were identified to control cell death in metazoans can also function in plants. The fact that most of these proteins do not have structural homologs in plant genomes suggests that they may be targeting a highly conserved 'core' mechanism with conserved functions that is present in all eukaryotes. The ubiquitous Bax inhibitor-1 (BI-1) is a common cell death suppressor in eukaryotes that has provided a potential portal to this cell death core. In this review, we will update the current status of our understanding on the function and activities of this intriguing protein. Genetic, molecular and biochemical studies have so far suggested a consistent view that BI-1 is an endoplasmic reticulum (ER)-resident transmembrane protein that can interact with multiple partners to alter intracellular Ca(2+) flux control and lipid dynamics. Functionally, the level of BI-1 protein has been hypothesized to have the role of a rheostat to regulate the threshold of ER-stress inducible cell death. Further, delineation of the cell death suppression mechanism by BI-1 should shed light on an ancient cell death core-control pathway in eukaryotes, as well as novel ways to improve stress tolerance.

Arabidopsis DAL1 and DAL2, two RING finger proteins homologous to Drosophila DIAP1, are involved in regulation of programmed cell death

Programmed cell death (PCD) is a precise, genetically controlled cellular process with important roles in plant growth, development, and response to biotic and abiotic stress. However, the genetic mechanisms that control PCD in plants are unclear. Two Arabidopsis genes, DAL1 and DAL2 (for Drosophila DIAP1 like 1 and 2), encoding RING finger proteins with homology to DIAP1 were identified, and a series of experiments were performed to elucidate their roles in the regulation of PCD and disease resistance. Expression of DAL1 and DAL2 genes was induced in Arabidopsis plants after inoculation with virulent and avirulent strains of Pseudomonas syrinage pv. tomato (Pst) DC3000 or after infiltration with fumonisin B1 (FB1). Plants with mutations in the DAL1 and DAL2 genes displayed more severe disease after inoculation with an avirulent strain of Pst DC3000, but they showed similar disease severity as the wild-type plant after inoculation with a virulent strain of Pst DC3000. Significant accumulations of reactive oxygen species (ROS) and increased cell death were observed in the dal1 and dal2 mutant plants after inoculation with the avirulent strain of Pst DC3000. The dal mutant plants underwent extensive PCD upon infiltration of FB1 and displayed higher levels of ROS accumulation, callose deposition, and autofluorescence than the wild-type plants. Our data suggest that DAL1 and DAL2 may act as negative regulators of PCD in Arabidopsis.

Identification of the essential Brucella melitensis porin Omp2b as a suppressor of Bax-induced cell death in yeast in a genome-wide screening

Background

Inhibition of apoptosis is one of the mechanisms selected by numerous intracellular pathogenic bacteria to control their host cell. Brucellae, which are the causative agent of a worldwide zoonosis, prevent apoptosis of infected cells, probably to support survival of their replication niche.

Methodology/Principal Findings

In order to identify Brucella melitensis anti-apoptotic effector candidates, we performed a genome-wide functional screening in yeast. The B. melitensis ORFeome was screened to identify inhibitors of Bax-induced cell death in S. cerevisiae. B. melitensis porin Omp2b, here shown to be essential, prevents Bax lethal effect in yeast, unlike its close paralog Omp2a. Our results based on Omp2b size variants characterization suggest that signal peptide processing is required for Omp2b effect in yeast.

Conclusion/Significance

We report here the first application to a bacterial genome-wide library of coding sequences of this “yeast-rescue” screening strategy, previously used to highlight several new apoptosis regulators. Our work provides B. melitensis proteins that are candidates for an anti-apoptotic function, and can be tested in mammalian cells in the future. Hypotheses on possible molecular mechanisms of Bax inhibition by the B. melitensis porin Omp2b are discussed.

Identification of expressed genes during compatible interaction between stripe rust (Puccinia striiformis) and wheat using a cDNA library

BACKGROUND

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat worldwide. To establish compatibility with the host, Pst forms special infection structures to invade the plant with minimal damage to host cells. Although compatible interaction between wheat and Pst has been studied using various approaches, research on molecular mechanisms of the interaction is limited. The aim of this study was to develop an EST database of wheat infected by Pst in order to determine transcription profiles of genes involved in compatible wheat-Pst interaction.

RESULTS

Total RNA, extracted from susceptible infected wheat leaves harvested at 3, 5 and 8 days post inoculation (dpi), was used to create a cDNA library, from which 5,793 ESTs with high quality were obtained and clustered into 583 contigs and 2,160 singletons to give a set of 2,743 unisequences (GenBank accessions: GR302385 to GR305127). The BLASTx program was used to search for homologous genes of the unisequences in the GenBank non-redundant protein database. Of the 2,743 unisequences, 52.8% (the largest category) were highly homologous to plant genes; 16.3% to fungal genes and 30% of no-hit. The functional classification of all ESTs was established based on the database entry giving the best E-value using the Bevan's classification categories. About 50% of the ESTs were significantly homologous to genes encoding proteins with known functions; 20% were similar to genes encoding proteins with unknown functions and 30% did not have significant homology to any sequence in the database. The quantitative real-time PCR (qRT-PCR) analysis determined the transcription profiles and their involvement in the wheat-Pst interaction for seven of the gene.

CONCLUSION

The cDNA library is useful for identifying the functional genes involved in the wheat-Pst compatible interaction, and established a new database for studying Pst pathogenesis genes and wheat defense genes. The transcription patterns of seven genes were confirmed by the qRT-PCR assay to be differentially expressed in wheat-Pst compatible and incompatible interaction.

A BAX inhibitor-1 gene in Capsicum annuum is induced under various abiotic stresses and endows multi-tolerance in transgenic tobacco

Programmed cell death (PCD) is a highly conserved cellular suicide process important in developmental processes and elimination of damaged cells upon environmental stresses. Among the important regulators of PCD, much interest has been centered on BCL2-associated x protein (BAX) as the pro-PCD factor. On the other hand, BAX inhibitor-1 (BI-1) has been implicated as an anti-PCD factor that balances out the activity of BAX in the developmental processes and responses to environment. A cDNA clone coding a BI-1 gene was isolated from a cDNA library of heat-stressed hot pepper (Capsicum annuum) and named as CaBI-1. This gene contains an open reading frame (ORF) of 248 amino acids encoding a BI-1 protein. Genomic DNA-blot analysis for CaBI-1 suggested one or two loci in the C. annuum genome. Transcription of CaBI-1 was induced in response to high or low temperatures, drought, high salinity, flooding and heavy metal stresses, and ABA. We introduced the ORF of CaBI-1 under the control of the CaMV 35S promoter (P(35S)) into tobacco (Nicotiana tabacum cv. Wisconsin 38) genome by Agrobacterium-mediated transformation. The P(35S):CaBI-1 transgenic plants displayed markedly improved tolerance to high temperature, water deficit, and high salinity in comparison to the control plants. The results indicate that CaBI-1 is a BI-1 gene of which expression induced under various abiotic stresses and endows tolerance to several types of environmental stresses.

Loss of calmodulin binding to Bax inhibitor-1 affects Pseudomonas-mediated hypersensitive response-associated cell death in Arabidopsis thaliana

Bax inhibitor-1 (BI-1) is a cell death suppressor protein conserved across a variety of organisms. The Arabidopsis atbi1-1 plant is a mutant in which the C-terminal 6 amino acids of the expressed BI-1 protein have been replaced by T-DNA insertion. This mutant BI-1 protein (AtBI-CM) produced in Escherichia coli can no longer bind to calmodulin. A promoter-reporter assay demonstrated compartmentalized expression of BI-1 during hypersensitive response, introduced by the inoculation of Pseudomonas syringae possessing the avrRTP2 gene, Pst(avrRPT2). In addition, both BI-1 knockdown plants and atbi1-1 showed increased sensitivity to Pst(avrRPT2)-induced cell death. The results indicated that the loss of calmodulin binding reduces the cell death suppressor activity of BI-1 in planta.

Bax Inhibitor-1, a conserved cell death suppressor, is a key molecular switch downstream from a variety of biotic and abiotic stress signals in plants

In Nature plants are constantly challenged by a variety of environmental stresses that could lead to disruptions in cellular homeostasis. Programmed cell death (PCD) is a fundamental cellular process that is often associated with defense responses to pathogens, during development and in response to abiotic stresses in fungi, animals and plants. Although there are many characteristics shared between different types of PCD events, it remains unknown whether a common mechanism drives various types of PCD in eukaryotes. One candidate regulator for such a mechanism is Bax Inhibitor-1 (BI-1), an evolutionary conserved, endoplasmic reticulum (ER)-resident protein that represents an ancient cell death regulator that potentially regulates PCD in all eukaryotes. Recent findings strongly suggested that BI-1 plays an important role in the conserved ER stress response pathway to modulate cell death induction in response to multiple types of cell death signals. As ER stress signaling pathways has been suggested to play important roles not only in the control of ER homeostasis but also in other biological processes such as the response to pathogens and abiotic stress in plants, BI-1 might function to control the convergence point that modulates the level of the “pro-survival and pro-death” signals under multiple stress conditions.

Functional association of cell death suppressor, Arabidopsis Bax inhibitor-1, with fatty acid 2-hydroxylation through cytochrome b₅

Bax inhibitor-1 (BI-1) is a widely conserved cytoprotective protein localized in the endoplasmic reticulum (ER) membrane. We identified Arabidopsis cytochrome b(5) (AtCb5) as an interactor of Arabidopsis BI-1 (AtBI-1) by screening the Arabidopsis cDNA library with the split-ubiquitin yeast two-hybrid (suY2H) system. Cb5 is an electron transfer protein localized mainly in the ER membrane. In addition, a bimolecular fluorescence complementation (BiFC) assay and fluorescence resonance energy transfer (FRET) analysis confirmed that AtBI-1 interacted with AtCb5 in plants. On the other hand, we found that the AtBI-1-mediated suppression of cell death in yeast requires Saccharomyces cerevisiae fatty acid hydroxylase 1 (ScFAH1), which had a Cb5-like domain at the N terminus and interacted with AtBI-1. ScFAH1 is a sphingolipid fatty acid 2-hydroxylase localized in the ER membrane. In contrast, AtFAH1 and AtFAH2, which are functional ScFAH1 homologues in Arabidopsis, had no Cb5-like domain, and instead interacted with AtCb5 in plants. These results suggest that AtBI-1 interacts with AtFAHs via AtCb5 in plant cells. Furthermore, the overexpression of AtBI-1 increased the level of 2-hydroxy fatty acids in Arabidopsis, indicating that AtBI-1 is involved in fatty acid 2-hydroxylation.

Transgenic suppression of cell death limits penetration success of the soybean rust fungus Phakopsora pachyrhizi into epidermal cells of barley

The basidiomycete Phakopsora pachyrhizi (P. pachyrhizi) causes Asian soybean rust, one of the most devastating plant diseases on soybean. When inoculated on the nonhost barley P. pachyrhizi caused only very small necrotic spots, typical for an incompatible interaction, which involves a hypersensitive cell death reaction. A microscopic inspection of the interaction of barley with P. pachyrhizi revealed that the fungus germinated on barley and formed functional appressoria on epidermal cells. The fungus attempted to directly penetrate through periclinal cell walls but often failed, arrested in plant cell wall appositions that stained positively for callose. Penetration resistance depends on intact ROR1(REQUIRED FOR mlo-SPECIFIED RESISTANCE 1) and ROR2 genes of barley. If the fungus succeeded in penetration, epidermal cell death took place. Dead epidermal cells did not generally restrict fungal development but allowed for mesophyll invasion, which was followed by mesophyll cell death and fungal arrest. Transient or stable over expression of the barley cell death suppressor BAX inhibitor-1 reduced both epidermal cell death and fungal penetration success. Data suggest that P. pachyrhizi provokes a programmed cell death facilitating fungal entry into epidermal cells of barley.

The LCB2 subunit of the sphingolip biosynthesis enzyme serine palmitoyltransferase can function as an attenuator of the hypersensitive response and Bax-induced cell death

Previous results showed that expression of the gene encoding the LONG-CHAIN BASE2 (LCB(2)) subunit of serine palmitoyltransferase (SPT), designated BcLCB(2), from nonheading Chinese cabbage (Brassica campestris ssp. chinensis) was up-regulated during hypersensitive cell death (HCD) induced by the Phytophthora boehmeriae elicitor PB90. Overexpression of BcLCB(2) in Nicotiana tabacum leaves suppressed the HCD normally initiated by elicitors and PB90-triggered H(2)O(2) accumulation. BcLCB(2) also functioned as a suppressor of mouse Bcl-2 associated X (Bax) protein-mediated HCD and cell death caused by Ralstonia solanacearum. BcLCB(2) overexpression suppressed Bax- and oxidant stress-triggered yeast cell death. Reactive oxygen species (ROS) accumulation induced by Bax was compromised in BcLCB(2)-overexpressing yeast cells. The findings that NbLCB(2) silencing in Nicotiana benthamiana enhanced elicitor-triggered HCD, combined with the fact that myriocin, a potent inhibitor of SPT, had no effect on Bax-induced programmed cell death, suggested that suppression of cell death was not involved in the dominant-negative effect that resulted from BcLCB(2) overexpression. A BcLCB(2) mutant assay showed that the suppression was not involved in SPT activity. The results suggest that plant HCD and stress-induced yeast cell death might share a common signal transduction pathway involving LCB(2), and that LCB(2) protects against cell death by inhibiting ROS accumulation, this inhibition being independent of SPT activity.

Comparative expression analysis of genes induced during development of bacterial rot and induction of hypersensitive cell death in lettuce

The development of bacterial rot disease caused by Pseudomonas cichorii is closely associated with programmed cell death. To investigate the molecular events occurring during the development of bacterial rot, we isolated 20 P. cichorii-responsive genes (PcRGs) in lettuce by differential display. Among these PcRGs, signal transduction-, transcription/translation- and defense/stress responses-related PcRGs were subjected to a comparative expression study. We used RNA samples isolated from lettuce leaves inoculated with P. cichorii and hypersensitive response-inducing Pseudomonas syringae pv. syringae. Expression of PcRG1-5-5 (spliceosomal protein), 2-9-2 (protein kinase) and 1-6-2 (ACC oxidase), 7-5 (alternative oxidase) and BI-I (bax inhibitor I) significantly increased in lettuce leaves inoculated with both P. cichorii and P. syringae pv. syringae. Intriguingly, PcRG 1-2-6 (protein phosphatase 2C) and 4-D-5 (protein kinase) were only up-regulated in P. cichorii-inoculated lettuce, whereas expression of PcRG1-3-2 (ribonucleoprotein) was only enhanced in P. syringae pv. syringae-inoculated lettuce. Expressions of PcRG1-3-2, 1-5-5, 1-6-2, 2-9-2, 7-5 and BI-I were induced by treatments with salicylic acid and/or methyl jasmonate. However, expression of PcRG1-2-6 and 4-D-5, which were specifically up-regulated by P. cichorii, were scarcely affected by these chemicals. Pharmacological studies suggested that ethylene and alternative oxidase were commonly related to disease development and hypersensitive responses. By contrast, there may be a different role for protein synthesis and protein kinase during disease development and in hypersensitive responses. These results suggested the overall similarity of genes expressed during disease development and in hypersensitive responses. However, there were differences not only in induction kinetics and the level of gene expression but also in the signal transduction pathway between hypersensitive responses and disease development.

Initiation of programmed cell death in self-incompatibility: role for cytoskeleton modifications and several caspase-like activities

Programmed cell death (PCD) is an important and universal process regulating precise death of unwanted cells in eukaryotes. In plants, the existence of PCD has been firmly established for about a decade, and many components shown to be involved in apoptosis/PCD in mammalian systems are found in plant cells undergoing PCD. Here, we review work from our lab demonstrating the involvement of PCD in the self-incompatibility response in Papaver rhoeas pollen. This utilization of PCD as a consequence of a specific pollen-pistil interaction provides a very neat way to destroy unwanted 'self', but not 'non-self' pollen. We discuss recent data providing evidence for SI-induced activation of several caspase-like activities and suggest that an acidification of the cytosol may be a key turning point in the activation of caspase-like proteases executing PCD. We also review data showing the involvement of the actin and microtubule cytoskeletons as well as that of a MAPK in signalling to caspase-mediated PCD. Potential links between these various components in signalling to PCD are discussed. Together, this begins to build a picture of PCD in a single cell system, triggered by a receptor-ligand interaction.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Genome-wide expression profiling Arabidopsis at the stage of Golovinomyces cichoracearum haustorium formation

Compatibility between plants and obligate biotrophic fungi requires fungal mechanisms for efficiently obtaining nutrients and counteracting plant defenses under conditions that are expected to induce changes in the host transcriptome. A key step in the proliferation of biotrophic fungi is haustorium differentiation. Here we analyzed global gene expression patterns in Arabidopsis thaliana leaves during the formation of haustoria by Golovinomyces cichoracearum. At this time, the endogenous levels of salicylic acid (SA) and jasmonic acid (JA) were found to be enhanced. The responses of wild-type, npr1-1, and jar1-1 plants were used to categorize the sensitivity of gene expression changes to NPR1 and JAR1, which are components of the SA and JA signaling pathways, respectively. We found that the infection process was the major source of variation, with 70 genes identified as having similarly altered expression patterns regardless of plant genotype. In addition, principal component analysis (PCA) identified genes responding both to infection and to lack of functional JAR1 (17 genes) or NPR1 (18 genes), indicating that the JA and SA signaling pathways function as secondary sources of variation. Participation of these genes in the SA or JA pathways had not been described previously. We found that some of these genes may be sensitive to the balance between the SA and JA pathways, representing novel markers for the elucidation of cross-talk points between these signaling cascades. Conserved putative regulatory motifs were found in the promoter regions of each subset of genes. Collectively, our results indicate that gene expression changes in response to infection by obligate biotrophic fungi may support fungal nutrition by promoting alterations in host metabolism. In addition, these studies provide novel markers for the characterization of defense pathways and susceptibility features under this infection condition.

BI-1 regulates endoplasmic reticulum Ca2+ homeostasis downstream of Bcl-2 family proteins

BI-1 (Bax inhibitor-1) is an evolutionarily conserved multitransmembrane protein that resides in the endoplasmic reticulum (ER) and that has documented cytoprotective functions in both animals and plants. Recent studies indicate that BI-1 shares in common with Bcl-2/Bax family proteins the ability to regulate the amounts of Ca(2+) that can be released from the ER by agents, such as the ER-Ca(2+)-ATPase (SERCA) inhibitor thapsigargin (TG). Using an ER-targeted, Ca(2+) indicator (cameleon), with characteristics optimized for measuring ER Ca(2+) ([Ca(2+)](er)), we studied the effects of BI-1 on [Ca(2+)](er) in resting and TG-treated cells. Similar to cells overexpressing antiapoptotic Bcl-2 or Bcl-X(L), overexpression of BI-1 resulted in lower resting [Ca(2+)](er), with concomitantly less Ca(2+) released into the cytosol upon stimulation by TG and with a higher rate of Ca(2+) leakage from the ER. Co-expression of SERCA restored levels of [Ca(2+)](er) to normal, showing opposing actions of the ER-Ca(2+)ATPase and BI-1 on ER Ca(2+) homeostasis. Conversely, cells with deficient BI-1 have increased [Ca(2+)](er), and release more Ca(2+) into the cytosol when challenged with TG. In BI-1-deficient cells, Bcl-X(L) fails to reduce [Ca(2+)](er), indicating that BI-1 functions downstream of Bcl-X(L). In bax(-/-)bak(-/-) double knock-out cells, both BI-1 and Bcl-X(L) retained their ability to reduce [Ca(2+)](er), suggesting that BI-1 and Bcl-X(L) operate downstream of or parallel to Bax/Bak. The findings reveal a hierarchy of functional interactions of BI-1 with Bcl-2/Bax family proteins in regulating ER Ca(2+) homeostasis.