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

The wheat mitogen-activated protein kinases TaMPK3 and TaMPK6 are differentially regulated at multiple levels during compatible disease interactions with Mycosphaerella graminicola

Many race- or isolate-specific disease resistance responses of plants toward pathogens (incompatible interactions) invoke hypersensitive response (HR)-like programmed cell death (PCD) and the coordinated activation of mitogen-activated protein kinases homologous with Arabidopsis (Arabidopsis thaliana) AtMPK6 and AtMPK3 (or tobacco [Nicotiana tabacum] SIPK and WIPK), respectively. Resistance of wheat (Triticum aestivum) leaves to the necrotrophic fungal pathogen Mycosphaerella graminicola can also operate at an isolate/cultivar-specific level. We confirm here that resistance is achieved without any sign of HR-like PCD during the incompatible interaction. Instead, PCD is strictly associated with the compatible interaction and is triggered during disease symptom expression. A strong transcriptional activation of TaMPK3, the wheat homolog of Arabidopsis AtMPK3, was observed immediately preceding PCD and symptom development in the compatible interaction. Generation and use of TaMPK3- and TaMPK6-specific antibodies on western blots and in coupled immunoprecipitation-protein kinase assays demonstrated that the TaMPK3 protein also accumulated, and was subsequently posttranslationally activated, during the compatible interaction in parallel to PCD. In contrast, no increase in expression, protein levels, or posttranslational activation of TaMPK6 was observed at any stage of either compatible or incompatible interactions. However, the protein levels of TaMPK6 became markedly reduced during the compatible interaction coincident with the onset of TaMPK3 protein accumulation. These data highlight the emerging similarity between the signaling pathways triggered in a host plant during successful infection by a necrotrophic fungal pathogen and the resistance responses normally effective against biotrophs.

Evidence for transcriptional and post-translational regulation of sucrose synthase in pea nodules by the cellular redox state

Nitrogen fixation (NF) in legume nodules is very sensitive to environmental constraints. Nodule sucrose synthase (SS; EC 2.4.1.13) has been suggested to play a crucial role in those circumstances because its downregulation leads to an impaired glycolytic carbon flux and, therefore, a depletion of carbon substrates for bacteroids. In the present study, the likelihood of SS being regulated by oxidative signaling has been addressed by the in vivo supply of paraquat (PQ) to nodulated pea plants and the in vitro effects of oxidizing and reducing agents on nodule SS. PQ produced cellular redox imbalance leading to an inhibition of NF. This was preceded by the downregulation of SS gene expression, protein content, and activity. In vitro, oxidizing agents were able to inhibit SS activity and this inhibition was completely reversed by the addition of dithiothreitol. The overall results are consistent with a regulation model of nodule SS exerted by the cellular redox state at both the transcriptional and post-translational levels. The importance of such mechanisms for the regulation of NF in response to environmental stresses are discussed.

Identification of a consensus DNA-binding site for the Arabidopsis thaliana SBP domain transcription factor, AtSPL14, and binding kinetics by surface plasmon resonance

Proteins with a conserved Cys- and His-rich SQUAMOSA promoter binding protein (SBP) domain are transcription factors restricted to photosynthetic organisms that possess a novel two Zn-finger structure DNA-binding domain. Despite the fact that altered expression of some SBP-encoding genes has profound effects on organism growth and development, little is known about SBP domain protein target genes. Misexpression of the Arabidopsis thaliana AtSPL14 SBP domain gene confers resistance to programmed cell death and modifies plant architecture. A consensus DNA-binding motif for AtSPL14 was identified by systematic evolution of ligands by exponential enrichment (SELEX) or random binding site selection (RBSS). DNA recognized by AtSPL14 contained the core binding motif (GTAC) found for other SBP domain proteins, but mutational analyses indicated that at least one additional flanking nucleotide is necessary for effective AtSPL14-DNA interaction. Comparison of several SBP domain amino acid sequences allows us to hypothesize which specific amino acids might participate in this sequence-specific DNA recognition. Electrophoretic mobility shift assays (EMSA) with mutant AtSPL14 DNA-binding domain proteins indicated that not all of the Zn (2+) ion coordinating ligands in the second Zn structure are strictly required for DNA binding. Surface plasmon resonance (SPR) was used to evaluate AtSPL14 in vitro binding kinetics for comparison of equilibrium binding constants with other SBP domain proteins. These data provide a strong basis for further experiments aimed at defining and distinguishing the sets of genes regulated by the closely related SBP domain family members.

Molecular diversity at the plant-pathogen interface

Plants have evolved a robust innate immune system that exhibits striking similarities as well as significant differences with various metazoan innate immune systems. For example, plants are capable of perceiving pathogen-associated molecular patterns through pattern recognition receptors that bear structural similarities to animal Toll-like receptors. In addition, plants have evolved a second surveillance system based on cytoplasmic "NB-LRR" proteins (nucleotide-binding, leucine-rich repeat) that are structurally similar to animal nucleotide-binding and oligomerization domain (NOD)-like receptors. Plant NB-LRR proteins do not detect PAMPs; rather, they perceive effector proteins that pathogens secrete into plant cells to promote virulence. This review summarizes the current state of knowledge about the molecular functionality and evolution of these immune surveillance genes.

From plants to animals; the role of plant cell death in ruminant herbivores

Plant cell death occurring as a result of adverse environmental conditions is known to limit crop production. It is less well recognized that plant cell death processes can also contribute to the poor environmental footprint of ruminant livestock production. Although the forage cells ingested by grazing ruminant herbivores will ultimately die, the lack of oxygen, elevated temperature, and challenge by microflora experienced in the rumen induce regulated plant stress responses resulting in DNA fragmentation and autolytic protein breakdown during the cell death process. Excessive ruminal proteolysis contributes to the inefficient conversion of plant to microbial and animal protein which results in up to 70% of the ingested nitrogen being returned to the land as the nitrogenous pollutants ammonia and urea. This constitutes a significant challenge for sustainable livestock production. As it is estimated that 25% of cultivated land worldwide is assigned to livestock production, it is clear that understanding the fundamental biology underlying cell death in ingested forage will have a highly significant role in minimizing the impact of human activities. This review examines our current understanding of plant metabolism in the rumen and explores opportunities for exploitation of plant genetics to advance sustainable land use.

Aspects of programmed cell death during early senescence of barley leaves: possible role of nitric oxide

Leaf senescence is a highly coordinated process which involves programmed cell death (PCD). Early stages of leaf senescence occurring during normal leaf ontogenesis, but not triggered by stress factors, are less well known. In this study, we correlated condensation of chromatin and nuclear DNA (nDNA) fragmentation, two main features of PCD during early senescence in barley leaves, with the appearance of nitric oxide (NO) within leaf tissue. With the help of the alkaline version of the comet assay, together with measurements of nDNA fluorescence intensity, we performed a detailed analysis of the degree of nDNA fragmentation. We localised NO in vivo and in situ within the leaf and photometrically measured its concentration with the NO-specific fluorochrome 4-amino-5-methylamino-2',7'-difluorofluorescein. We found that both nDNA fragmentation and chromatin condensation occurred quite early during barley leaf senescence and always in the same order: first nDNA fragmentation, in leaves of 6-day-old seedlings, and later chromatin condensation, in the apical part of leaves from 10-day-old seedlings. PCD did not start simultaneously even in neighbouring cells and probably did not proceed at the same rate. NO was localised in vivo and in situ within the cytoplasm, mainly in mitochondria, in leaves at the same stage as those in which chromatin condensation was observed. Localisation of NO in vascular tissue and in a large number of mesophyll cells during the senescence process might imply its transport to other parts of the leaf and its involvement in signalling between cells. The fact that the highest concentration of NO was found in the cytoplasm of mesophyll cells in the earliest stage of senescence and lower concentrations were found during later stages might suggest that NO plays an inductive role in PCD.

Copper and manganese induce yeast apoptosis via different pathways

Metal ions are essential as well as toxic to the cell. The mechanism of metal-induced toxicity is not well established. Here, for the first time we studied two essential nutritional elements, copper and manganese, for their apoptotic effects in yeast Saccharomyces cerevisiae. Although beneficial at subtoxic levels, we demonstrated that at moderately toxic levels, both metals induce extensive apoptosis in yeast cells. At even higher concentrations, necrosis takes over. Furthermore, we investigated the molecular pathways mediating Cu- and Mn-mediated apoptotic action. Mitochondria-defective yeast exhibit a much reduced apoptotic marker expression and better survival under Cu and Mn stress, indicating mitochondria are involved in both Cu- and Mn-induced apoptosis. Reactive oxygen species (ROS) are generated in high amounts in Cu- but not in Mn-induced cell death, and Cu toxicity can be alleviated by overexpression of superoxide dismutase 2, suggesting ROS mediate Cu but not Mn toxicity. Yeast metacaspase Yca1p is not involved in Cu-induced apoptosis, although it plays an important role in the Mn-induced process. A genetic screen identified Cpr3p, a yeast cyclophilin D homologue, as mediating the Cu-induced apoptotic program. Cpr3p mutant seems to eliminate Cu-induced apoptosis without affecting ROS production, while leaving necrosis intact. These results may provide important insight into a detailed understanding at the molecular and cellular level of metal toxicity and metal accumulation diseases.

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.

The leucine-rich repeat (LRR) protein, CaLRR1, interacts with the hypersensitive induced reaction (HIR) protein, CaHIR1, and suppresses cell death induced by the CaHIR1 protein

Leucine-rich repeat proteins (LRRs) function in a number of signal transduction pathways via protein-protein interactions. The gene encoding a small protein of pepper, CaLRR1, is specifically induced upon pathogen challenge and treatment with pathogen-associated molecular patterns (PAMPs). We identified a pepper hypersensitive induced reaction (CaHIR1) protein that interacts with the LRR domain of the CaLRR1 protein using yeast two-hybrid screening. Ectopic expression of the pepper CaHIR1 gene induces cell death in tobacco and Arabidopsis, indicating that the CaHIR1 protein may be a positive regulator of HR-like cell death. Because transformation is very difficult in pepper plants, we over-expressed CaLRR1 and CaHIR1 in Arabidopsis to determine cellular functions of the two genes. The over-expression of the CaHIR1 gene, but not the CaLRR1 gene, in transgenic Arabidopsis confers disease resistance in response to Pseudomonas syringae infection, accompanied by the strong expression of PR genes, the accumulation of both salicylic acid and H(2)O(2), and K(+) efflux in plant cells. In Arabidopsis and tobacco plants over-expressing both CaHIR1 and CaLRR1, the CaLRR1 protein suppresses not only CaHIR1-induced cell death, but also PR gene expression elicited by CaHIR1 via its association with HIR protein. We propose that the CaLRR1 protein functions as a novel negative regulator of CaHIR1-mediated cell death responses in plants.

Dinitro-o-cresol induces apoptosis-like cell death but not alternative oxidase expression in soybean cells

In plants, programmed cell death is thought to be activated during differentiation and in response to biotic and abiotic stresses. Although its mechanisms are far less clear, several morphological and biochemical features have been described in different experimental systems, including DNA laddering and cytosolic protease activation. Moreover, plant mitochondria have an alternative terminal oxidase (AOX), which is thought to be involved in protection against increased reactive oxygen species production, perhaps representing a mechanism to prevent programmed cell death. In this study, we analysed cell death induced by the herbicide dinitro-o-cresol (DNOC) in soybean (Glycine max) suspension cell cultures and evaluated biochemical and molecular events associated with programmed cell death. AOX capacity and expression were also determined. DNOC-treated cells showed fragmented nuclear DNA as assessed by an in situ assay that detects 3'-OH ends. In addition, specific colorimetric assays and immunoblot analysis revealed activation of caspase-3-like proteins and release of cytochrome c from mitochondria, respectively, confirming the apoptotic-like phenotype. Surprisingly, AOX capacity and protein levels decreased in DNOC-treated cells, suggesting no association between cell death and AOX under these experimental conditions. In conclusion, the results show that DNOC induces programmed cell death in soybean cells, suggesting that plants and animals might share similar pathways. Further, the role of AOX in cell death has not been confirmed, and may depend on the nature and intensity of stress conditions.

A chickpea Kunitz trypsin inhibitor is located in cell wall of elongating seedling organs and vascular tissue

Kunitz proteinase inhibitors in legumes have mainly been described as defence and storage proteins. Here, we report a Kunitz trypsin inhibitor, encoded by the CaTPI-1 gene from Cicer arietinum. The transcription of this gene mainly occurs in seedling vegetative organs, and is affected by the light and growth stages. The recombinant TPI-1 protein expressed in E. coli was seen to be an efficient inhibitor of trypsin. After the generation of polyclonal antibodies against recombinant TPI-1 protein, the protein was located in the cell wall of elongating epicotyls and radicles by Western-blot experiments, in agreement with the transcription pattern. These results, together with the fact that both CaTPI-1 mRNA and protein levels decreased with epicotyl growth, suggest a possible role in the elongation of seedling epicotyls and radicles. Immunolocalization analyses of the TPI-1 protein indicated that it is abundant in the cell walls of both immature primary xylem cells and surrounding parenchyma cells. This location has led us to explore potential functions for TPI-1 protein in vascular tissue during seedling elongation.

A high-throughput screen of cell-death-inducing factors in Nicotiana benthamiana identifies a novel MAPKK that mediates INF1-induced cell death signaling and non-host resistance to Pseudomonas cichorii

A high-throughput overexpression screen of Nicotiana benthamiana cDNAs identified a gene for a mitogen-activated protein kinase kinase (MAPKK) as a potent inducer of the hypersensitive response (HR)-like cell death. NbMKK1 protein is localized to the nucleus, and the N-terminal putative MAPK docking site of NbMKK1 is required for its function as a cell-death inducer. NbMKK1-mediated leaf-cell death was compromised in leaves where NbSIPK expression was silenced by virus-induced gene silencing. A yeast two-hybrid assay showed that NbMKK1 and NbSIPK physically interact, suggesting that NbSIPK is one of the downstream targets of NbMKK1. Phytophthora infestans INF1 elicitor-mediated HR was delayed in NbMKK1-silenced plants, indicating that NbMKK1 is involved in this HR pathway. Furthermore, the resistance of N. benthamiana to a non-host pathogen Pseudomonas cichorii was compromised in NbMKK1-silenced plants. These results demonstrate that MAPK cascades involving NbMKK1 control non-host resistance including HR cell death.

DNA degradation during programmed cell death in Phaseolus coccineus suspensor

The suspensor of Phaseolus coccineus L. degenerates at the cotyledonary stage of embryogenesis when it is no longer necessary for continued embryonic development; this degeneration is considered to be a typical example of the so-called developmental programmed cell death (PCD) in plants. The presence of specific hallmarks of PCD as it occurs during the degeneration of P. coccineus suspensor was investigated in the current study. By using the TUNEL assay and electrophoretic analysis, we found evidence of nuclear DNA degradation, a known feature of PCD, in the endosperm and degenerating suspensors. Degeneration of the suspensor begins after degeneration in the endosperm and it starts in the neck region, spreading basipetally towards the knob. We conclude from this study that suspensor degeneration in P. coccineus occurs by means of PCD and displays typical hallmarks of PCD, such as DNA fragmentation. PCD in the suspensor is a highly asynchronous process, originating first in the neck cells and subsequently spreading to the basal cells.

Transcriptional adaptation of Mycosphaerella graminicola to programmed cell death (PCD) of its susceptible wheat host

Many important fungal pathogens of plants spend long periods (days to weeks) of their infection cycle in symptomless association with living host tissue, followed by a sudden transition to necrotrophic feeding as host tissue death occurs. Little is known about either the host responses associated with this sudden transition or the specific adaptations made by the pathogen to invoke or tolerate it. We are studying a major host-specific fungal pathogen of cultivated wheat, Septoria tritici (teleomorph Mycosphaerella graminicola). Here, we describe the host responses of wheat leaves infected with M. graminicola during the development of disease symptoms and use microarray transcription profiling to identify adaptive responses of the fungus to its changing environment. We show that symptom development on a susceptible host genotype has features reminiscent of the hypersensitive response, a rapid and strictly localized form of host programmed cell death (PCD) more commonly associated with disease-resistance mechanisms. The initiation and advancement of this host response is associated with a loss of cell-membrane integrity and dramatic increases in apoplastic metabolites and the rate of fungal growth. Microarray analysis of the fungal genes differentially expressed before and after the onset of host PCD supports a transition to more rapid growth. Specific physiological adaptation of the fungus is also revealed with respect to membrane transport, chemical and oxidative stress mechanisms, and metabolism. Our data support the hypothesis that host plant PCD plays an important role in susceptibility towards fungal pathogens with necrotrophic lifestyles.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

The reactive oxygen species network pathways:an essential prerequisite for perception of pathogen attack and the acquired disease resistance in plants

Availability of complete Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) genome sequences, together with molecular recourses of functional genomics and proteomics have revolutionized our understanding of reactive oxygen species (ROS) signalling network mediating disease resistance in plants. So far, ROS have been associated with aging, cellular and molecular alteration in animal and plant cells. Recently,concluding evidences suggest that ROS network is essential to induce disease resistance and even to mediate resistance to multiple stresses in plants. ROS are obligatory by-products emerging as a result of normal metabolic reactions. They have the potential to be both beneficial and harmful to cellular metabolism. Their dual effects on metabolic reactions are dosage specific. In this review we focus our attention on cellular ROS level to trigger beneficial effects on plant cells responding to pathogen attack. By exploring the research related contributions coupled with data of targeted gene disruption, and RNA interference approaches, we show here that ROS are ubiquitous molecules of redox-pathways that play a crucial role in plant defence mechanism. The molecular prerequisites of ROS network to activate plant defence system in response to pathogen infections are here underlined. Bioinformatic tools are now available to scientists for high throughput analysis of cellular metabolisms. These tools are used to illustrate crucial ROS-related genes that are involved in the defence mechanism of plants. The review describes also the emerging findings of ROS network pathways to modulate multiple stress resistance in plants.

Monitoring programmed cell death triggered by mild heat shock in soybean-cultured cells

Programmed cell death (PCD) is a common form of cellular demise during plant response to environmental stresses. The pathway of PCD has been partially clarified in plants although the underlying molecular mechanisms are still poorly defined. We have investigated the signalling cascade induced by a mild heat treatment causing PCD in soybean cells (Glycine max L.). The data show that heat shock led to the onset of PCD in soybean cells involving H₂O₂ production and mitochondrial damage. Cytochrome c release accompanies the presence of caspase 9-like and caspase 3-like protease activities. Concomitantly, cells were severely damaged with a progressive cell shrinkage, chloroplast alteration and detachment of the plasma membrane from the cell wall. Chromatin condensation and DNA damage were observed. It is proposed that a mild heat stress induces PCD in soybean cells through a caspase-like-dependent pathway.

Rice GTPase OsRacB: potential accessory factor in plant salt-stress signaling

As the sole ubiquitous signal small guanosine triphosphate-binding protein in plants, Rop gene plays an important role in plant growth and development. In this study, we focus on the relationship between the novel rice Rop gene OsRacB and plant salt tolerance. Results show that OsRacB transcription is highly accumulated in roots after treatment with salinity, but only slightly accumulated in stems and leaves under the same treatment. Promoter analysis showed that OsRacB promoter is induced by salinity and exogenous salicylic acid, not abscisic acid. To elucidate its physiological function, we generated OsRacB sense and antisense transgenic tobacco and rice. Under proper salinity treatment, sense transgenic plants grew much better than the control. This suggests that overexpression of OsRacB in tobacco and rice can improve plant salt tolerance. But under the same treatment, no difference could be observed between OsRacB antisense plants and the control. The results indicated that OsRacB is only an accessory factor in plant salt tolerance.

Upregulation of a tonoplast-localized cytochrome P450 during petal senescence in Petunia inflata

BACKGROUND

Gene expression in Petunia inflata petals undergoes major changes following compatible pollination. Severe flower wilting occurs reproducibly within 36 hours, providing an excellent model for investigation of petal senescence and programmed cell death. Expression of a number of genes and various enzyme activities involved in the degradation and remobilization of macromolecules have been found to be upregulated during the early stages of petal senescence.

RESULTS

By performing differential display of cDNAs during Petunia inflata petal senescence, a highly upregulated gene encoding a cytochrome P450 was identified. Analysis of the complete cDNA sequence revealed that the predicted protein is a member of the CYP74C family (CYP74C9) and is highly similar to a tomato CYP74C allene oxide synthase (AOS) that is known to be active on 9-hydroperoxides. Cloning of the petunia genomic DNA revealed an intronless gene with a promoter region that carries signals found in stress-responsive genes and potential binding sites for Myb transcription factors. Transcripts were present at detectable levels in root and stem, but were 40 times more abundant in flowers 36 hours after pollination. Ethylene and jasmonate treatment resulted in transitory increases in expression in detached flowers. A protein fusion of the CYP74C coding region to a C-terminal GFP was found to be located in the tonoplast.

CONCLUSION

Though oxylipins, particularly jasmonates, are known to be involved in stress responses, the role of other products of CYP74 enzymes is less well understood. The identification of a CYP74C family member as a highly upregulated gene during petal senescence suggests that additional products of fatty acid metabolism may play important roles during programmed cell death. In contrast to the chloroplast localization of AOS proteins in the CYP74A subfamily, GFP fusion data indicates that the petunia CYP74C9 enzyme is in the tonoplast. This result suggests that the highly similar CYP74C enzymes that have been identified in two other Solanaceous plants may also be associated with the vacuole, an organelle known to have a prominent role in programmed cell death.

Arabidopsis Bax inhibitor-1 functions as an attenuator of biotic and abiotic types of cell death

Programmed cell death (PCD) is a common process in eukaryotes during development and in response to pathogens and stress signals. Bax inihibitor-1 (BI-1) is proposed to be a cell death suppressor that is conserved in both animals and plants, but the physiological importance of BI-1 and the impact of its loss of function in plants are still unclear. In this study, we identified and characterized two independent Arabidopsis mutants with a T-DNA insertion in the AtBI1 gene. The phenotype of atbi1-1 and atbi1-2, with a C-terminal missense mutation and a gene knockout, respectively, was indistinguishable from wild-type plants under normal growth conditions. However, these two mutants exhibit accelerated progression of cell death upon infiltration of leaf tissues with a PCD-inducing fungal toxin fumonisin B1 (FB1) and increased sensitivity to heat shock-induced cell death. Under these conditions, expression of AtBI1 mRNA was up-regulated in wild-type leaves prior to the activation of cell death, suggesting that increase of AtBI1 expression is important for basal suppression of cell death progression. Over-expression of AtBI1 transgene in the two homozygous mutant backgrounds rescued the accelerated cell death phenotypes. Together, our results provide direct genetic evidence for a role of BI-1 as an attenuator for cell death progression triggered by both biotic and abiotic types of cell death signals in Arabidopsis.

Analysis of the wheat andPuccinia triticina (leaf rust) proteomes during a susceptible host-pathogen interaction

Wheat leaf rust is caused by the fungus Puccinia triticina. The genetics of resistance follows the gene-for-gene hypothesis, and thus the presence or absence of a single host resistance gene renders a plant resistant or susceptible to a leaf rust race bearing the corresponding avirulence gene. To investigate some of the changes in the proteomes of both host and pathogen during disease development, a susceptible line of wheat infected with a virulent race of leaf rust were compared to mock-inoculated wheat using 2-DE (with IEF pH 4-8) and MS. Up-regulated protein spots were excised and analyzed by MALDI-QqTOF MS/MS, followed by cross-species protein identification. Where possible MS/MS spectra were matched to homologous proteins in the NCBI database or to fungal ESTs encoding putative proteins. Searching was done using the MASCOT search engine. Remaining unmatched spectra were then sequenced de novo and queried against the NCBInr database using the BLAST and MS BLAST tools. A total of 32 consistently up-regulated proteins were examined from the gels representing the 9-day post-infection proteome in susceptible plants. Of these 7 are host proteins, 22 are fungal proteins of known or hypothetical function and 3 are unknown proteins of putative fungal origin.

Drought effects on carbon and nitrogen metabolism of pea nodules can be mimicked by paraquat: evidence for the occurrence of two regulation pathways under oxidative stresses

Biological nitrogen fixation (BNF) is dramatically affected by environmental constraints such as water stress or heavy metals. It has been reported that these stresses induce the over-production of reactive oxygen species (ROS) and, in turn, oxidative stress that may be responsible for the above-mentioned BNF decline at the molecular level. Oxidative stress, occurring under different environmental stresses, has been widely related to physiological damage. However, a direct relationship between oxidative stress and the decline of BNF, independently from any other cellular damage resulting from adverse environmental situations, has yet to be demonstrated. In order to study the likely in vivo relationship between ROS and BNF inhibition in the legume-Rhizobium symbiosis, two paraquat (PQ) doses, 1 (LPQ) and 10 (HPQ) mmol m(-3), were applied to pea roots for 96 h in order to exacerbate ROS production. Whole-plant physiology and nodule metabolism parameters were determined every 24 h to monitor the evolution of plant responses to ROS. LPQ provoked BNF decline, which was preceded by a prior decrease in sucrose synthase (SS) activity. However, HPQ gave rise to a faster and more pronounced BNF inhibition, which coincided with a decline in SS and also with a reduction in leghaemoglobin (Lb) content. These results indicate a likely involvement of ROS in the effects of environmental stresses on BNF. Furthermore, these results support the occurrence of two regulation pathways for BNF under oxidative stress, one of these involving carbon shortage and the other involving Lb/oxygen flux.

Rice proteomics: a cornerstone for cereal food crop proteomes

Proteomics-a systematic study of proteins present in a cell, tissue, organ, or organism at a particular moment during the life cycle-that began with classical two-dimensional electrophoresis and its advancement during the 1990s, has been revolutionized by a series of tremendous technological developments in mass spectrometry (MS), a core technology. Proteomics is exerting its influence on biological function of genes and genomes in the era (21st century) of functional genomics, and for this reason yeast, bacterial, and mammalian systems are the best examples. Although plant proteomics is still in its infancy, evolving proteomic technologies and the availability of the genome sequences of Arabidopsis thaliana (L.) Heyhn, and rice (Oryza sativa L.), model dicotyledoneous and monocotyledoneous (monocot) species, respectively, are propelling it towards new heights, as evidenced by the rapid spurt in worldwide plant proteome research. Rice, with an immense socio-economic impact on human civilization, is a representative model of cereal food crops, and we consider it as a cornerstone for functional genomics of cereal plants. In this review, we look at the history and the current state of monocot proteomes, including barley, maize, and wheat, with a central focus on rice, which has the most extensive proteomic coverage to date. On one side, we highlight advances in technologies that have generated enormous amount of interest in plant proteomics, and the other side summarizes the achievements made towards establishing proteomes during plant growth & development and challenge to environmental factors, including disease, and for studying genetic relationships. In light of what we have learned from the proteomic journey in rice and other monocots, we finally reveal and assess their impact in our continuous strive towards completion of their full proteomes.

Reactive Oxygen Species and Root Hairs in Arabidopsis Root Response to Nitrogen, Phosphorus and Potassium Deficiency

Plant root sensing and adaptation to changes in the nutrient status of soils is vital for long-term productivity and growth. Reactive oxygen species (ROS) have been shown to play a role in root response to potassium deprivation. To determine the role of ROS in plant response to nitrogen and phosphorus deficiency, studies were conducted using wild-type Arabidopsis and several root hair mutants. The expression of several nutrient-responsive genes was determined by Northern blot, and ROS were quantified and localized in roots. The monitored genes varied in intensity and timing of expression depending on which nutrient was deficient. In response to nutrient deprivation, ROS concentrations increased in specific regions of the Arabidopsis root. Changes in ROS localization in Arabidopsis and in a set of root hair mutants suggest that the root hair cells are important for response to nitrogen and potassium. In contrast, the response to phosphorus deprivation occurs in the cortex where an increase in ROS was measured. Based on these results, we put forward the hypothesis that root hair cells in Arabidopsis contain a sensing system for nitrogen and potassium deprivation.

An endopolygalacturonase from Sclerotinia sclerotiorum induces calcium-mediated signaling and programmed cell death in soybean cells

A basic endopolygalacturonase (PG) isoform, produced early by Sclerotinia sclerotiorum when infecting soybean seedlings, was used to examine the signaling role of the enzyme in aequorin-expressing soybean cells. A cytosolic Ca2+ elevation was induced, with a rapid increase (phase 1) and a very slow decrease (phase 2) of Ca2+ concentration, indicating the involvement of Ca2+ ions in PG signaling. Within 1 h of PG-cell contact a remarkable level of cell death was recorded, significantly higher than the control cell culture turnover. The observed morphological and biochemical changes were indicative of the activation of programmed cell death; in particular, cytochrome c release in the cytoplasm and activation of both caspase 9-like and caspase 3-like proteases were found. When a polygalacturonase-inhibiting protein (PGIP) and the PG were simultaneously applied to cells, both the Ca2+ increase and cell death were annulled. The possible roles of prolonged sustained cytosolic Ca2+ concentrations in inducing cell death and of the PG-PGIP interaction in preventing PG signaling are discussed.

Fatty acid hydroperoxides and H2O2 in the execution of hypersensitive cell death in tobacco leaves

We initially compared lipid peroxidation profiles in tobacco (Nicotiana tabacum) leaves during different cell death events. An upstream oxylipin assay was used to discriminate reactive oxygen species (ROS)-mediated lipid peroxidation from 9- and 13-lipoxygenase (LOX)-dependent lipid peroxidation. Free radical-mediated membrane peroxidation was measured during H(2)O(2)-dependent cell death in leaves of catalase-deficient plants. Taking advantage of these transgenic plants, we demonstrate that, under light conditions, H(2)O(2) plays an essential role in the execution of cell death triggered by an elicitor, cryptogein, which provokes a similar ROS-mediated lipid peroxidation. Under dark conditions, however, cell death induction by cryptogein was independent of H(2)O(2) and accompanied by products of the 9-LOX pathway. In the hypersensitive response induced by the avirulent pathogen Pseudomonas syringae pv syringae, both 9-LOX and oxidative processes operated concurrently, with ROS-mediated lipid peroxidation prevailing in the light. Our results demonstrate, therefore, the tight interplay between H(2)O(2) and lipid hydroperoxides and underscore the importance of light during the hypersensitive response.

Identification of arabidopsis loci required for susceptibility to the downy mildew pathogen Hyaloperonospora parasitica

Plants are susceptible to a limited number of pathogens. Most infections fail due to active defense or absence of compatibility. Many components of the plant's surveillance system and defense arsenal have been identified in the last decades. However, knowledge is limited on compatibility; in particular, the role of plant factors in the infection process. To gain insight into these processes, we have initiated an Arabidopsis thaliana mutant screen for reduced susceptibility to the downy mildew pathogen Hyaloperonospora parasitica. Ethyl methane sulfonate (EMS) mutants were generated in the highly susceptible Arabidopsis line Ler eds1-2. Eight downy mildew-resistant (dmr) mutants were analyzed in detail, corresponding to six different loci. Microscopic analysis showed that, in all mutants, H. parasitica growth was severely reduced. Resistance of dmr3, dmr4, and dmr5 was associated with constitutive expression of PR-1. Furthermore, dmr3 and dmr4, but not dmr5, also were resistant to Pseudomonas syringae and Golovinomyces orontii, respectively. However, enhanced activation of plant defense was not observed in dmr1, dmr2, and dmr6. We postulate that, in these susceptibility mutants, cellular processes are disrupted which are required for H. parasitica infection. This interesting new set of mutants provides a basis to elucidate the molecular processes underlying susceptibility to downy mildew in Arabidopsis.

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

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

Arabidopsis SHMT1, a serine hydroxymethyltransferase that functions in the photorespiratory pathway influences resistance to biotic and abiotic stress

We found that a recessive mutation, shmt1-1, causes aberrant regulation of cell death resulting in chlorotic and necrotic lesion formation under a variety of environmental conditions. Salicylic acid-inducible genes and genes involved in H(2)O(2) detoxification were expressed constitutively in shmt1-1 plants in direct correlation with the severity of the lesions. The shmt1-1 mutants were more susceptible than control plants to infection with biotrophic and necrotrophic pathogens, developing severe infection symptoms in a high percentage of infected leaves. In addition, mutants carrying shmt1-1 or a loss-of-function shmt1-2 allele, were smaller and showed a greater loss of chlorophyll and greater accumulation of H(2)O(2) than wild-type plants when subjected to salt stress. SHMT1 was map-based cloned and found to encode a serine hydroxymetyltransferase (SHMT1) involved in the photorespiratory pathway. Our results indicate that this enzymatic activity plays a critical role in controlling the cell damage provoked by abiotic stresses such as high light and salt and in restricting pathogen-induced cell death, supporting the notion that photorespiration forms part of the dissipatory mechanisms of plants to minimize production of reactive oxygen species (ROS) at the chloroplast and to mitigate oxidative damage. Moreover, results shown here indicate that whereas production of ROS is an essential component of the hypersensitive defense response, the excessive accumulation of these toxic compounds impairs cell death containment and counteracts the effectiveness of the plant defenses to restrict pathogen infection.

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

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

Programmed cell death in plant embryogenesis

Successful embryonic development in plants, as in animals, requires a strict coordination of cell proliferation, cell differentiation, and cell-death programs. The role of cell death is especially critical for the establishment of polarity at early stages of plant embryogenesis, when the differentiation of the temporary structure, the suspensor, is followed by its programmed elimination. Here, we review the emerging knowledge of this and other functions of programmed cell death during plant embryogenesis, as revealed by developmental analyses of Arabidopsis embryo-specific mutants and gymnosperm (spruce and pine) model embryonic systems. Cell biological studies in these model systems have helped to identify and order the cellular processes occurring during self-destruction of the embryonic cells. While metazoan embryos can recruit both apoptotic and autophagic cell deaths, the ultimate choice depending on the developmental task and conditions, plant embryos use autophagic cell disassembly as a single universal cell-death pathway. Dysregulation of this pathway leads to aberrant or arrested embryo development. We address the role of distinct cellular components in the execution of the autophagic cell death, and outline an overall mechanistic view of how cells are eliminated during plant embryonic pattern formation. Finally, we discuss the possible roles of some of the candidate plant cell-death proteins in the regulation of developmental cell death.

Chromosome instabilities and programmed cell death in tapetal cells of maize with B chromosomes and effects on pollen viability

B chromosomes (B's), knobbed chromosomes, and chromosome 6 (NOR) of maize undergo nondisjunction and micronucleus formation in binucleate tapetal cells. These chromosome instabilities are regular events in the program of tapetal cell death, but the B's strongly increase A chromosome instability. We studied 1B and 0B plants belonging to selected lines for high or low B transmission rate and their F1 hybrids. These lines are characterized by meiotic conservation or loss of B chromosomes, respectively. The female B transmission (fBtl) allele(s) for low B transmission is dominant, inducing micronucleus formation and B nondisjunction. We hypothesize that the fBtl allele(s) induces knob instability. This instability would be sufficient to produce B loss in both meiocytes and binucleate tapetal cells. B instability could, in turn, produce instabilities in all chromosomes of maize complement. To establish whether the chromosomal instabilities are related to the tapetal programmed cell death (PCD) process, we applied the TUNEL technique. PCD, estimated as the frequency of binucleate tapetal cells with TUNEL label, was significantly correlated with the formation of micronuclei and the frequency of pollen abortion. It can be concluded that the observed chromosome instabilities are important to the PCD process and to the development of microspores to form viable pollen grains.

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.

Tomato phospholipid hydroperoxide glutathione peroxidase inhibits cell death induced by Bax and oxidative stresses in yeast and plants

Using a conditional life or death screen in yeast, we have isolated a tomato (Lycopersicon esculentum) gene encoding a phospholipid hydroperoxide glutathione peroxidase (LePHGPx). The protein displayed reduced glutathione-dependent phospholipid hydroperoxide peroxidase activity, but differs from counterpart mammalian enzymes that instead contain an active seleno-Cys. LePHGPx functioned as a cytoprotector in yeast (Saccharomyces cerevisiae), preventing Bax, hydrogen peroxide, and heat stress induced cell death, while also delaying yeast senescence. When tobacco (Nicotiana tabacum) leaves were exposed to lethal levels of salt and heat stress, features associated with mammalian apoptosis were observed. Importantly, transient expression of LePHGPx protected tobacco leaves from salt and heat stress and suppressed the apoptotic-like features. As has been reported, conditional expression of Bax was lethal in tobacco, resulting in tissue collapse and membrane permeability to Evans blue. When LePHGPx was coexpressed with Bax, little cell death and no vital staining were observed. Moreover, stable expression of LePHGPx in tobacco conferred protection against the fungal phytopathogen Botrytis cinerea. Taken together, our data indicated that LePHGPx can protect plant tissue from a variety of stresses. Moreover, functional screens in yeast are a viable tool for the identification of plant genes that regulate cell death.

Catalase deficiency drastically affects gene expression induced by high light in Arabidopsis thaliana

In plants, hydrogen peroxide (H(2)O(2)) plays a major signaling role in triggering both a defense response and cell death. Increased cellular H(2)O(2) levels and subsequent redox imbalances are managed at the production and scavenging levels. Because catalases are the major H(2)O(2) scavengers that remove the bulk of cellular H(2)O(2), altering their levels allows in planta modulation of H(2)O(2) concentrations. Reduced peroxisomal catalase activity increased sensitivity toward both ozone and photorespiratory H(2)O(2)-induced cell death in transgenic catalase-deficient Arabidopsis thaliana. These plants were used as a model system to build a comprehensive inventory of transcriptomic variations, which were triggered by photorespiratory H(2)O(2) induced by high-light (HL) irradiance. In addition to an H(2)O(2)-dependent and -independent type of transcriptional response during light stress, microarray analysis on both control and transgenic catalase-deficient plants, exposed to 0, 3, 8, and 23 h of HL, revealed several specific regulatory patterns of gene expression. Thus, photorespiratory H(2)O(2) has a direct impact on transcriptional programs in plants.

Endoplasmic reticulum stress-induced programmed cell death in soybean cells

In animal cells, the endoplasmic reticulum may participate in programmed cell death by sensing and transducing apoptotic signals. In an attempt to analyze the role of the endoplasmic reticulum in plant programmed cell death we investigated the effect of cyclopiazonic acid, a specific blocker of plant endoplasmic reticulum-type IIA Ca2+-pumps, in soybean cells. Cyclopiazonic acid treatment elicited endoplasmic reticulum stress and a biphasic increase in cytosolic Ca2+ concentration, followed by the induction of a cell death program. Cyclopiazonic acid-induced programmed cell death occurred with accumulation of H2O2, cytochrome c release from mitochondria, caspase 9- and caspase 3-like protease activation, cytoplasmic shrinkage and chromatin condensation. Chelation of cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethil ester) failed to inhibit cyclopiazonic acid-induced cell death. Taken together, our results provide evidence for a role of the endoplasmic reticulum and mitochondria in regulating cyclopiazonic acid-induced programmed cell death in soybean cells, probably via a cross-talk between the two organelles.

Arabidopsis DND2, a second cyclic nucleotide-gated ion channel gene for which mutation causes the "defense, no death" phenotype

A previous mutant screen identified Arabidopsis dnd1 and dnd2 "defense, no death" mutants, which exhibit loss of hypersensitive response (HR) cell death without loss of gene-for-gene resistance. The dnd1 phenotype is caused by mutation of the gene encoding cyclic nucleotide-gated (CNG) ion channel AtCNGC2. This study characterizes dnd2 plants. Even in the presence of high titers of Pseudomonas syringae expressing avrRpt2, most leaf mesophyll cells in the dnd2 mutant exhibited no HR. These plants retained strong RPS2-, RPM1-, or RPS4-mediated restriction of P. syringae pathogen growth. Mutant dnd2 plants also exhibited enhanced broad-spectrum resistance against virulent P. syringae and constitutively elevated levels of salicylic acid, and pathogenesis-related (PR) gene expression. Unlike the wild type, dnd2 plants responding to virulent and avirulent P. syringae exhibited elevated expression of both salicylate-dependent PR-1 and jasmonate and ethylene-dependent PDF1.2. Introduction of nahG+ (salicylate hydroxylase) into the dnd2 background, which removes salicylic acid and causes other defense alterations, eliminated constitutive disease resistance and PR gene expression but only weakly impacted the HR- phenotype. Map-based cloning revealed that dnd2 phenotypes are caused by mutation of a second CNG ion channel gene, AtCNGC4. Hence, loss of either of two functionally nonredundant CNG ion channels can cause dnd phenotypes. The dnd mutants provide a unique genetic background for dissection of defense signaling.

Production of reactive oxygen species, alteration of cytosolic ascorbate peroxidase, and impairment of mitochondrial metabolism are early events in heat shock-induced programmed cell death in tobacco Bright-Yellow 2 cells

To gain some insight into the mechanisms by which plant cells die as a result of abiotic stress, we exposed tobacco (Nicotiana tabacum) Bright-Yellow 2 cells to heat shock and investigated cell survival as a function of time after heat shock induction. Heat treatment at 55 degrees C triggered processes leading to programmed cell death (PCD) that was complete after 72 h. In the early phase, cells undergoing PCD showed an immediate burst in hydrogen peroxide (H2O2) and superoxide (O2*-) anion production. Consistently, death was prevented by the antioxidants ascorbate (ASC) and superoxide dismutase (SOD). Actinomycin D and cycloheximide, inhibitors of transcription and translation, respectively, also prevented cell death, but with a lower efficiency. Induction of PCD resulted in gradual oxidation of endogenous ASC; this was accompanied by a decrease in both the amount and the specific activity of the cytosolic ASC peroxidase (cAPX). A reduction in cAPX gene expression was also found in the late PCD phase. Moreover, changes of cAPX kinetic properties were found in PCD cells. Production of ROS in PCD cells was accompanied by early inhibition of glucose (Glc) oxidation, with a strong impairment of mitochondrial function as shown by an increase in cellular NAD(P)H fluorescence, and by failure of mitochondria isolated from cells undergoing PCD to generate membrane potential and to oxidize succinate in a manner controlled by ADP. Thus, we propose that in the early phase of tobacco Bright-Yellow 2 cell PCD, ROS production occurs, perhaps because of damage of the cell antioxidant system, with impairment of the mitochondrial oxidative phosphorylation.

Chitosan induces Ca2+ -mediated programmed cell death in soybean cells

•  Chitosan, a component of the cell wall of many fungi, has been widely used to mimic pathogen attack and has been shown to induce several defence responses. •  Here we show that low concentrations (50 µg ml^-1 ) of chitosan are able to induce an increase in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ), accumulation of H₂ O₂ in the culture medium, induction of the defence gene chalcone synthase (chs), and cell death in soybean cells (Glycine max). •  Chitosan-induced cell death occurred through cytoplasmic shrinkage, chromatin condensation and activation of caspase 3-like protease, suggesting the activation of a programmed cell death (PCD) pathway. Buffering extracellular Ca²⁺ with the Ca²⁺ chelator EGTA prevents [Ca²⁺ ]_cyt elevation, H₂ O₂ production and all downstream PCD features, but not cell death. •  Higher doses (200 µg ml^-1 ) of chitosan evoked neither Ca²⁺ transient and H₂ O₂ production nor caspase 3-like activation, but caused cell death, possibly as a result of plasma membrane disturbance.

Chromosome Instabilities and Programmed Cell Death in Tapetal Cells of Maize With B Chromosomes and Effects on Pollen Viability

B chromosomes (B’s), knobbed chromosomes, and chromosome 6 (NOR) of maize undergo nondisjunction and micronucleus formation in binucleate tapetal cells. These chromosome instabilities are regular events in the program of tapetal cell death, but the B’s strongly increase A chromosome instability. We studied 1B and 0B plants belonging to selected lines for high or low B transmission rate and their F1 hybrids. These lines are characterized by meiotic conservation or loss of B chromosomes, respectively. The female B transmission (fBtl) allele(s) for low B transmission is dominant, inducing micronucleus formation and B nondisjunction. We hypothesize that the fBtl allele(s) induces knob instability. This instability would be sufficient to produce B loss in both meiocytes and binucleate tapetal cells. B instability could, in turn, produce instabilities in all chromosomes of maize complement. To establish whether the chromosomal instabilities are related to the tapetal programmed cell death (PCD) process, we applied the TUNEL technique. PCD, estimated as the frequency of binucleate tapetal cells with TUNEL label, was significantly correlated with the formation of micronuclei and the frequency of pollen abortion. It can be concluded that the observed chromosome instabilities are important to the PCD process and to the development of microspores to form viable pollen grains.

Yeast lacking the SRO7/SOP1-encoded tumor suppressor homologue show increased susceptibility to apoptosis-like cell death on exposure to NaCl stress

Yeast cells deleted for the SRO7/SOP1 encoded tumor suppressor homologue show increased sensitivity to NaCl stress. On exposure to growth-inhibiting NaCl concentrations, sro7Delta mutants display a rapid loss in viability that is associated with markers of apoptosis: accumulation of reactive oxygen species, DNA breakage, and nuclear fragmentation. Additional deletion of the yeast metacaspase gene YCA1 prevents the primary fast drop in viability and diminishes nuclear fragmentation and DNA breakage. We also observed that NaCl induced loss in viability of wild-type cells is Yca1p dependent. However, a yeast strain deleted for both SRO7 and its homologue SRO77 exhibits NaCl-induced cell death that is independent on YCA1. Likewise, sro77Delta single mutants do not survive better after additional deletion of the YCA1 gene, and both sro77Delta and sro77Deltayca1Delta mutants display apoptotic characteristics when exposed to growth-inhibiting salinity, suggesting that yeast possesses Yca1p-independent pathway(s) for apoptosis-like cell death. The activity of Yca1p increases with increasing NaCl stress and sro7Delta mutants achieve levels that are higher than in wild-type cells. However, mutants lacking SRO77 do not enhance caspase activity when subject to NaCl stress, suggesting that Sro7p and Sro77p exert opposing effects on the cellular activity of Yca1p.

Recent advance in the study of caspase-like proteases and Bax inhibitor-1 in plants: their possible roles as regulator of programmed cell death

SUMMARY Programmed cell death (PCD) is a common process in multicellular organisms during development and in response to pathogens and stress signals. In animal systems, it is well established that a class of cysteine protease called caspase plays a central role in signalling and executing PCD. No caspase orthologue has been uncovered from any plant genomic database so far, but recent structural element-based iterative searches have uncovered the presence of a putative caspase-related protease family designated as metacaspases in plants. Furthermore, a structural homologue of Bcl-2-like proteins such as Bax and Bcl-2, which act as important regulators to modulate caspase action in animal cells, has not been found in any plant genomic databases. Clearly, the molecular components of plant PCD machinery and many key interactions in the regulatory network of plant PCD remain obscure. However, recent discoveries from numerous laboratories have provided good evidence for the involvement of caspase-like proteases in the control of cell death activation in plants, and there is accumulating evidence for a plant homologue of Bax inhibitor-1, which was first identified as an antagonist of Bax action in yeast, as an evolutionarily conserved protein and that may act as a key regulator for a common cell death pathway in plants and animals.

Deciphering host resistance and pathogen virulence: the Arabidopsis/Pseudomonas interaction as a model

SUMMARY The last decade has witnessed steady progress in deciphering the molecular basis of plant disease resistance and pathogen virulence. Although contributions have been made using many different plant and pathogen species, studies of the interactions between Arabidopsis thaliana and Pseudomonas syringae have yielded a particularly significant body of information. The present review focuses on recent findings regarding R gene products and the guard hypothesis, RAR1/SGT1 and other examples where protein processing activity is implicated in disease resistance or susceptibility, the use of microarray expression profiling to generate information and experimental leads, and important molecular- and genome-level discoveries regarding P. syringae effectors that mediate bacterial virulence. The development of the Arabidopsis-Pseudomonas model system is also reviewed briefly, and we close with a discussion of characteristics to consider when selecting other pathosystems as experimentally tractable models for future research.

VEIDase is a principal caspase-like activity involved in plant programmed cell death and essential for embryonic pattern formation

Plant embryogenesis is intimately associated with programmed cell death. The mechanisms of initiation and control of programmed cell death during plant embryo development are not known. Proteolytic activity associated with caspase-like proteins is paramount for control of programmed cell death in animals and yeasts. Caspase family of proteases has unique strong preference for cleavage of the target proteins next to asparagine residue. In this work, we have used synthetic peptide substrates containing caspase recognition sites and corresponding specific inhibitors to analyse the role of caspase-like activity in the regulation of programmed cell death during plant embryogenesis. We demonstrate that VEIDase is a principal caspase-like activity implicated in plant embryogenesis. This activity increases at the early stages of embryo development that coincide with massive cell death during shape remodeling. The VEIDase activity exhibits high sensitivity to pH, ionic strength and Zn(2+) concentration. Altogether, biochemical assays show that VEIDase plant caspase-like activity resembles that of both mammalian caspase-6 and yeast metacaspase, YCA1. In vivo, VEIDase activity is localised specifically in the embryonic cells during both the commitment and in the beginning of the execution phase of programmed cell death. Inhibition of VEIDase prevents normal embryo development via blocking the embryo-suspensor differentiation. Our data indicate that the VEIDase activity is an integral part in the control of plant developmental cell death programme, and that this activity is essential for the embryo pattern formation.

A developmental response to pathogen infection in Arabidopsis

We present evidence that susceptible Arabidopsis plants accelerate their reproductive development and alter their shoot architecture in response to three different pathogen species. We infected 2-week-old Arabidopsis seedlings with two bacterial pathogens, Pseudomonas syringae and Xanthomonas campestris, and an oomycete, Peronospora parasitica. Infection with each of the three pathogens reduced time to flowering and the number of aerial branches on the primary inflorescence. In the absence of competition, P. syringae and P. parasitica infection also increased basal branch development. Flowering time and branch responses were affected by the amount of pathogen present. Large amounts of pathogen caused the most dramatic changes in the number of branches on the primary inflorescence, but small amounts of P. syringae caused the fastest flowering and the production of the most basal branches. RPS2 resistance prevented large changes in development when it prevented visible disease symptoms but not at high pathogen doses and when substantial visible hypersensitive response occurred. These experiments indicate that phylogenetically disparate pathogens cause similar changes in the development of susceptible Arabidopsis. We propose that these changes in flowering time and branch architecture constitute a general developmental response to pathogen infection that may affect tolerance of and/or resistance to disease.

Reactive oxygen species and hormonal control of cell death

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