Tracheary element differentiation uses a novel mechanism coordinating programmed cell death and secondary cell wall synthesis

A proteinase-storing body that prepares for cell death or stresses in the epidermal cells of Arabidopsis

Plants degrade cellular materials during senescence and under various stresses. We report that the precursors of two stress-inducible cysteine proteinases, RD21 and a vacuolar processing enzyme (VPE), are specifically accumulated in approximately 0.5 microm diameter x approximately 5 microm long bodies in Arabidopsis thaliana. Such bodies have previously been observed in Arabidopsis but their function was not known. They are surrounded with ribosomes and thus are assumed to be directly derived from the endoplasmic reticulum (ER). Therefore, we propose to call them the ER bodies. The ER bodies are observed specifically in the epidermal cells of healthy seedlings. These cells are easily wounded and stressed by the external environment. When the seedlings are stressed with a concentrated salt solution, leading to death of the epidermal cells, the ER bodies start to fuse with each other and with the vacuoles, thereby mediating the delivery of the precursors directly to the vacuoles. This regulated, direct pathway differs from the usual case in which proteinases are transported constitutively from the ER to the Golgi complex and then to vacuoles, with intervention of vesicle-transport machinery, such as a vacuolar-sorting receptor or a syntaxin of the SNARE family. Thus, the ER bodies appear to be a novel proteinase-storing system that assists in cell death under stressed conditions.

Signalling in the cereal aleurone: hormones, reactive oxygen and cell death

The cereal aleurone is widely used as a model system to study hormonal signalling. Abscisic acid (ABA) and gibberellins (GAs) elicit distinct responses in aleurone cells, ranging from those occurring within minutes of hormone addition to those that require several hours or days to complete. Programmed cell death is an example of a response in aleurone layers that is hormonally regulated. GAs promote cell death and cells in intact aleurone layers begin to die 24 h after GA treatment, whereas cell death of aleurone protoplasts begins 4 d after GA treatment. ABA prevents aleurone cell death and addition of ABA to cells pretreated with GA can delay cell death. Aleurone cells do not follow the apoptotic route of programmed cell death. Cells treated with GA, but not ABA, develop large, acidic vacuoles containing a spectrum of hydrolases typical of lytic compartments. Enzymes that metabolize reactive oxygen species are also present in aleurone cells, but ascorbate peroxidase, catalase and superoxide dismutase become less abundant after treatment with GA; activity of these enzymes increases or remains unchanged in ABA-treated cells. We propose a model whereby reactive oxygen species accumulate in GA-treated cells and lead to peroxidation of membrane lipids and plasma membrane rupture. ABBREVIATIONS: RO, reactive oxygen species; HR, hypersensitive response; PSV, protein storage vacuole; PCD, programmed cell death; CAT, catalase; SOD, superoxide dismutase; APX, ascorbate peroxidase.

Direct evidence of active and rapid nuclear degradation triggered by vacuole rupture during programmed cell death in Zinnia

Differentiation into a tracheary element (TE) is a typical example of programmed cell death (PCD) in the developmental processes of vascular plants. In the PCD process the TE degrades its cellular contents and becomes a hollow corpse that serves as a water conduct. Using a zinnia (Zinnia elegans) cell culture we obtained serial observations of single living cells undergoing TE PCD by confocal laser scanning microscopy. Vital staining was performed and the relative fluorescence intensity was measured, revealing that the tonoplast of the swollen vacuole in TEs loses selective permeability of fluorescein just before its physical rupture. After the vacuole ruptured the nucleus was degraded rapidly within 10 to 20 min. No prominent chromatin condensation or nuclear fragmentation occurred in this process. Nucleoids in chloroplasts were also degraded in a similar time course to that of the nucleus. Degradations did not occur in non-TEs forced to rupture the vacuole by probenecid treatment. These results demonstrate that TE differentiation involves a unique type of PCD in which active and rapid nuclear degradation is triggered by vacuole rupture.

Inhibition of proteasome activity by the TED4 protein in extracellular space: a novel mechanism for protection of living cells from injury caused by dying cells

In maturation process of tracheary element (TE) differentiation, many hydrolases are activated to execute programmed cell death of TEs. Such hydrolases are released from maturing TEs into extracellular space. The release of hydrolases should be harmful to surrounding cells. The TED4 protein, a tentative plant non-specific lipid transfer protein that is expressed preferentially in TE-induced culture of zinnia (Zinnia elegans L.), is secreted into the apoplastic space prior to and associated with morphological changes of TEs. Our studies on the interrelationship between the TED4 protein and proteolytic activities using an in vitro TE differentiation system of zinnia revealed the following facts. (1) Active proteasome is released into medium at maturation stage of TE differentiation. (2) The TED4 protein forms a complex with proteasome in culture medium. (3) The TED4 protein inhibits proteasome activity in the medium and crude extracts of zinnia cells. (4) The depletion of the TED4 protein from culture medium results in an increase in mortality of other living cells. These results strongly suggest that the secreted TED4 protein acts as an inhibitor of proteasome to protect other cells from undesirable injury due to proteolytic activities exudated from dying TEs.

A calmodulin binding protein from Arabidopsis is induced by ethylene and contains a DNA-binding motif

Calmodulin (CaM), a key calcium sensor in all eukaryotes, regulates diverse cellular processes by interacting with other proteins. To isolate CaM binding proteins involved in ethylene signal transduction, we screened an expression library prepared from ethylene-treated Arabidopsis seedlings with 35S-labeled CaM. A cDNA clone, EICBP (Ethylene-Induced CaM Binding Protein), encoding a protein that interacts with activated CaM was isolated in this screening. The CaM binding domain in EICBP was mapped to the C-terminus of the protein. These results indicate that calcium, through CaM, could regulate the activity of EICBP. The EICBP is expressed in different tissues and its expression in seedlings is induced by ethylene. The EICBP contains, in addition to a CaM binding domain, several features that are typical of transcription factors. These include a DNA-binding domain at the N terminus, an acidic region at the C terminus, and nuclear localization signals. In database searches a partial cDNA (CG-1) encoding a DNA-binding motif from parsley and an ethylene up-regulated partial cDNA from tomato (ER66) showed significant similarity to EICBP. In addition, five hypothetical proteins in the Arabidopsis genome also showed a very high sequence similarity with EICBP, indicating that there are several EICBP-related proteins in Arabidopsis. The structural features of EICBP are conserved in all EICBP-related proteins in Arabidopsis, suggesting that they may constitute a new family of DNA binding proteins and are likely to be involved in modulating gene expression in the presence of ethylene.

Two waves of programmed cell death occur during formation and development of somatic embryos in the gymnosperm, Norway spruce

In the animal life cycle, the earliest manifestations of programmed cell death (PCD) can already be seen during embryogenesis. The aim of this work was to determine if PCD is also involved in the elimination of certain cells during plant embryogenesis. We used a model system of Norway spruce somatic embryogenesis, which represents a multistep developmental pathway with two broad phases. The first phase is represented by proliferating proembryogenic masses (PEMs). The second phase encompasses development of somatic embryos, which arise from PEMs and proceed through the same sequence of stages as described for their zygotic counterparts. Here we demonstrate two successive waves of PCD, which are implicated in the transition from PEMs to somatic embryos and in correct embryonic pattern formation, respectively. The first wave of PCD is responsible for the degradation of PEMs when they give rise to somatic embryos. We show that PCD in PEM cells and embryo formation are closely interlinked processes, both stimulated upon withdrawal or partial depletion of auxins and cytokinins. The second wave of PCD eliminates terminally differentiated embryo-suspensor cells during early embryogeny. During the dismantling phase of PCD, PEM and embryo-suspensor cells exhibit progressive autolysis, resulting in the formation of a large central vacuole. Autolytic degradation of the cytoplasm is accompanied by lobing and budding-like segmentation of the nucleus. Nuclear DNA undergoes fragmentation into both large fragments of about 50 kb and multiples of approximately 180 bp. The tonoplast rupture is delayed until lysis of the cytoplasm and organelles, including the nucleus, is almost complete. The protoplasm then disappears, leaving a cellular corpse represented by only the cell wall. This pathway of cell dismantling suggests overlapping of apoptotic and autophagic types of PCD during somatic embryogenesis in Norway spruce.

An early ethylene up-regulated gene encoding a calmodulin-binding protein involved in plant senescence and death

35S-Labeled calmodulin (CaM) was used to screen a tobacco anther cDNA library. A positive clone (NtER1) with high homology to an early ethylene-up-regulated gene (ER66) in tomato, and an Arabidopsis homolog was isolated and characterized. Based on the helical wheel projection, a 25-mer peptide corresponding to the predicted CaM-binding region of NtER1 (amino acids 796-820) was synthesized. The gel-mobility shift assay showed that the peptide formed a stable complex with CaM only in the presence of Ca(2+). CaM binds to NtER1 with high affinity (K(d) approximately 12 nm) in a calcium-dependent manner. Tobacco flowers at different stages of development were treated with ethylene or with 1-methylcyclopropene for 2 h before treating with ethylene. Northern analysis showed that the NtER1 was rapidly induced after 15 min of exposure to ethylene. However, the 2-h 1-methylcyclopropene treatment totally blocked NtER1 expression in flowers at all stages of development, suggesting that NtER1 is an early ethylene-up-regulated gene. The senescing leaves and petals had significantly increased NtER1 induction as compared with young leaves and petals, implying that NtER1 is developmentally regulated and acts as a trigger for senescence and death. This is the first documented evidence for the involvement of Ca(2+)/CaM-mediated signaling in ethylene action.

Xylogenesis: the birth of a corpse

Xylogenesis is a complex developmental process culminating in programmed cell death as a truly terminal differentiation event. In Arabidopsis, the availability of vascular-patterning mutants, and the identification of genes and their products that are involved in cell specification, secondary-wall deposition and lignification, are providing clues to the functions of some of the sequences in the large expressed sequence tag databases derived from the xylem-rich tissues of trees. An in vitro system, the Zinnia mesophyll cell system, provides an alternative system for those cell-biological experiments that are difficult to tackle in intact plants. In particular, a combination of molecular-genetic and cell-biological approaches has made possible the elucidation of some of the features of plant programmed cell death.

Programmed cell death of tracheary elements as a paradigm in plants

Plant development involves various programmed cell death (PCD) processes. Among them, cell death occurring during differentiation of procambium into tracheary elements (TEs), which are a major component of vessels or tracheids, has been studied extensively. Recent studies of PCD during TE differentiation mainly using an in vitro differentiation system of Zinnia have revealed that PCD of TEs is a plant-specific one in which the vacuole plays a central role. Furthermore, there are recent findings of several factors that may initiate PCD of TEs and that act at autonomous degradation of cell contents. Herein I summarize the present knowledge about cell death program during TE differentiation as an excellent example of PCD in plants.

Programmed cell death in cereal aleurone

Progress in understanding programmed cell death (PCD) in the cereal aleurone is described. Cereal aleurone cells are specialized endosperm cells that function to synthesize and secrete hydrolytic enzymes that break down reserves in the starchy endosperm. Unlike the cells of the starchy endosperm, aleurone cells are viable in mature grain but undergo PCD when germination is triggered or when isolated aleurone layers or protoplasts are incubated in gibberellic acid (GA). Abscisic acid (ABA) slows down the process of aleurone cell death and isolated aleurone protoplasts can be kept alive in media containing ABA for up to 6 months. Cell death in barley aleurone occurs only after cells become highly vacuolated and is manifested in an abrupt loss of plasma membrane integrity. Aleurone cell death does not follow the apoptotic pathway found in many animal cells. The hallmarks of apoptosis, including internucleosomal DNA cleavage, plasma membrane and nuclear blebbing and formation of apoptotic bodies, are not observed in dying aleurone cells. PCD in barley aleurone cells is accompanied by the accumulation of a spectrum of nuclease and protease activities and the loss of organelles as a result of cellular autolysis.

Ethylene induces epidermal cell death at the site of adventitious root emergence in rice

In deepwater rice (Oryza sativa), adventitious root primordia initiate at the nodes as part of normal development. Emergence of the roots is dependent on flooding of the plant and is mediated by ethylene action. Root growth was preceded by the induced death of epidermal cells of the node external to the tip of the root primordium. Cell death proceeded until the epidermis split open. Through this crack the root eventually emerged. Induced death was confined to nodal epidermal cells covering the tip of the primordia. Our results suggest that this process facilitates adventitious root emergence and prevents injury to the growing root. Cell death was inducible not only by submergence but also by application of 1-aminocyclopropane-1-carboxylic acid, the natural precursor of ethylene and it was suppressed in the presence of 2,5-norbornadiene (bicyclo[2.2.1]hepta-2,5-diene), an inhibitor of ethylene action. Adventitious root growth and epidermal cell death are therefore linked to the ethylene signaling pathway, which is activated in response to low oxygen stress.

Programmed cell death in cereal aleurone

Progress in understanding programmed cell death (PCD) in the cereal aleurone is described. Cereal aleurone cells are specialized endosperm cells that function to synthesize and secrete hydrolytic enzymes that break down reserves in the starchy endosperm. Unlike the cells of the starchy endosperm, aleurone cells are viable in mature grain but undergo PCD when germination is triggered or when isolated aleurone layers or protoplasts are incubated in gibberellic acid (GA). Abscisic acid (ABA) slows down the process of aleurone cell death and isolated aleurone protoplasts can be kept alive in media containing ABA for up to 6 months. Cell death in barley aleurone occurs only after cells become highly vacuolated and is manifested in an abrupt loss of plasma membrane integrity. Aleurone cell death does not follow the apoptotic pathway found in many animal cells. The hallmarks of apoptosis, including internucleosomal DNA cleavage, plasma membrane and nuclear blebbing and formation of apoptotic bodies, are not observed in dying aleurone cells. PCD in barley aleurone cells is accompanied by the accumulation of a spectrum of nuclease and protease activities and the loss of organelles as a result of cellular autolysis.

Plant proteolytic enzymes: possible roles during programmed cell death

Proteolytic enzymes are known to be associated with developmentally programmed cell death during organ senescence and tracheary element differentiation. Recent evidence also links proteinases with some types of pathogen- and stress-induced cell suicide. The precise roles of proteinases in these and other plant programmed cell death processes are not understood, however. To provide a framework for consideration of the importance of proteinases during plant cell suicide, characteristics of the best-known proteinases from plants including subtilisin-type and papain-type enzymes, phytepsins, metalloproteinases and the 26S proteasome are summarized. Examples of serine, cysteine, aspartic, metallo- and threonine proteinases linked to animal programmed cell death are cited and the potential for plant proteinases to act as mediators of signal transduction and as effectors of programmed cell death is discussed.

Programmed cell death of tracheary elements as a paradigm in plants

Plant development involves various programmed cell death (PCD) processes. Among them, cell death occurring during differentiation of procambium into tracheary elements (TEs), which are a major component of vessels or tracheids, has been studied extensively. Recent studies of PCD during TE differentiation mainly using an in vitro differentiation system of Zinnia have revealed that PCD of TEs is a plant-specific one in which the vacuole plays a central role. Furthermore, there are recent findings of several factors that may initiate PCD of TEs and that act at autonomous degradation of cell contents. Herein I summarize the present knowledge about cell death program during TE differentiation as an excellent example of PCD in plants.

Hypersensitive response-related death

The hypersensitive response (HR) of plants resistant to microbial pathogens involves a complex form of programmed cell death (PCD) that differs from developmental PCD in its consistent association with the induction of local and systemic defence responses. Hypersensitive cell death is commonly controlled by direct or indirect interactions between pathogen avirulence gene products and those of plant resistance genes and it can be the result of multiple signalling pathways. Ion fluxes and the generation of reactive oxygen species commonly precede cell death, but a direct involvement of the latter seems to vary with the plant-pathogen combination. Protein synthesis, an intact actin cytoskeleton and salicylic acid also seem necessary for cell death induction. Cytological studies suggest that the actual mode and sequence of dismantling the cell contents varies among plant-parasite systems although there may be a universal involvement of cysteine proteases. It seems likely that cell death within the HR acts more as a signal to the rest of the plant rather than as a direct defence mechanism.

Effect of climate conditions and plant developmental stage on the stability of antibodies expressed in transgenic tobacco

Plants are regarded as a promising system for the production of heterologous proteins. However, little is known about the influence of plant physiology and plant development on the yield and quality of the heterologous proteins produced in plants. To investigate this, tobacco (Nicotiana tabacum cv Samsun NN) was transformed with a single construct that contained behind constitutive promotors the light- and heavy-chain genes of a mouse antibody. The in planta stability of the antibody was analyzed in transgenic plants that were grown under high and low irradiation at 15 degrees C and 25 degrees C. High-light conditions favored the production of biomass, of total soluble protein, and of antibody. The plants grown at 25 degrees C developed faster and contained less antibody per amount of leaf tissue than the plants grown at 15 degrees C. Both endogenous protein and antibody content showed a strong decline during leaf development. The heavy chains of the antibody underwent in planta degradation via relatively stable fragments. In vitro incubations of purified plantibody with leaf extracts of wild-type tobacco indicated the involvement of acidic proteases. It is interesting that the same antibody produced by mouse hybridoma cells exhibited higher stability in this in vitro assay. This may be explained by the assumption that the plant type of N-glycosylation contributes less to the stability of the antibody than the mouse-type of N-glycosylation. The results of this study indicate that proteolytic degradation during plant development can be an important factor affecting yield and homogeneity of heterologous protein produced by transgenic plants.

A unique 33-kD cysteine proteinase accumulates in response to larval feeding in maize genotypes resistant to fall armyworm and other Lepidoptera

Plants respond to insect feeding with a number of defense mechanisms. Using maize genotypes derived from Antiquan germ plasm that are resistant to Lepidoptera, we have demonstrated that a unique 33-kD cysteine proteinase accumulates in the whorl in response to larval feeding. The abundance of the proteinase increased dramatically at the site of larval feeding after 1 hr of infestation and continued to accumulate for as long as 7 days. The 33-kD cysteine proteinase was most abundant in the yellow-green portion of the whorl-the normal site of larval feeding and the tissue that has the greatest inhibitory effect on larval growth in bioassays. The proteinase was expressed in response to wounding and was found in senescent leaves. It may be a marker of programmed cell death. The gene coding for the proteinase, mir1, has been transformed into Black Mexican Sweet callus. When larvae were reared on callus expressing the proteinase, their growth was inhibited approximately 60 to 80%. The expression of a cysteine proteinase, instead of a cysteine proteinase inhibitor, may be a novel insect defense mechanism in plants.

Exploiting secondary growth in Arabidopsis. Construction of xylem and bark cDNA libraries and cloning of three xylem endopeptidases

The root-hypocotyl of Arabidopsis produces a relatively large amount of secondary vascular tissue when senescence is delayed by the removal of inflorescences, and plants are grown at low population density. Peptidase zymograms prepared from isolated xylem and phloem revealed the existence of distinct proteolytic enzyme profiles within these tissues. cDNA libraries were constructed from isolated xylem and bark of the root-hypocotyl and screened for cDNAs coding for cysteine, serine, and aspartic peptidases. Three cDNAs, two putative papain-type cysteine peptidases (XCP1 and XCP2) and one putative subtilisin-type serine peptidase (XSP1), were identified from the xylem library for further analysis. Using RNA gel blots it was determined that these peptidases were expressed in the xylem and not in the bark. Quantitative reverse transcriptase-polymerase chain reaction confirmed the RNA gel-blot results and revealed high levels of XCP1 and XCP2 mRNA in stems and flowers of the infloresence. A poly-histidine-tagged version of XCP1 was purified from Escherichia coli by denaturing metal-chelate chromatography. Following renaturation, the 40-kD recombinant XCP1 was not proteolytically active. Activation was achieved by incubation of recombinant XCP1 at pH 5.5 and was dependent on proteolytic processing of the 40-kD inactive polypeptide to a 26-kD active peptidase.

A matrix metalloproteinase gene is expressed at the boundary of senescence and programmed cell death in cucumber

Cell-cell and extracellular cell matrix (ECM) interactions provide cells with information essential for controlling morphogenesis, cell-fate specification, and cell death. In animals, one of the major groups of enzymes that degrade the ECM is the matrix metalloproteinases (MMPs). Here, we report the characterization of the cucumber (Cucumis sativus L. cv Marketmore) Cs1-MMP gene encoding such an enzyme likely to play a role in plant ECM degradation. Cs1-MMP has all the hallmark motif characteristics of animal MMPs and is a pre-pro-enzyme having a signal peptide, propeptide, and zinc-binding catalytic domains. Cs1-MMP also displays functional similarities with animal MMPs. For example, it has a collagenase-like activity that can cleave synthetic peptides and type-I collagen, a major component of animal ECM. Cs1-MMP activity is completely inhibited by a hydroxamate-based inhibitor that binds at the active site of MMPs in a stereospecific manner. The Cs1-MMP gene is expressed de novo at the end stage of developmental senescence, prior to the appearance of DNA laddering in cucumber cotyledons leaf discs and male flowers. As the steady-state level of Cs1-MMP mRNA peaks late in senescence and the pro-enzyme must undergo maturation and activation, the protease is probably not involved in nutrient remobilization during senescence but may have another function. The physiological substrates for Cs1-MMP remain to be determined, but the enzyme represents a good candidate for plant ECM degradation and may be involved in programmed cell death (PCD). Our results suggest that PCD occurs only at the culmination of the senescence program or that the processes are distinct with PCD being triggered at the end of senescence.

Subcellular localization of expansin mRNA in xylem cells

Terminal differentiation of many vascular cells involves cell wall changes. Cells first elongate their primary wall, then lay down a lignified secondary wall, which is often followed by digestion of the primary wall. Expansins are wall proteins that regulate wall changes, but little is known about the specific functions of the many individual expansin isoforms. An in vitro cell culture of synchronously differentiating tracheary elements was used to identify three new expansins and to compare their expression kinetics with the timing of wall changes. The genes encoding these expansins from zinnia (Zinnia elegans), designated ZeExp1, ZeExp2, ZeExp3, are expressed during cell elongation. ZeExp1 and ZeExp2 mRNA decrease at the early stage of secondary wall formation, whereas ZeExp3 does not. In planta, all three ZeExp mRNAs are found predominantly in a single flank of cells adjacent to protoxylem and metaxylem vessels and in cells roughly at the radial position of the fasicular and interfasicular cambium. Furthermore, within these cells, Exp mRNA is localized exclusively either to the apical or basipetal end of cells depending on the expansin gene and organ, providing the first evidence for polar localization of mRNA in plant cells. ZeExp1 and ZeExp3 mRNA are localized at the apical tip, whereas ZeExp2 mRNA is found in the basal tip. These observations indicate that these three expansins are xylem cell specific and possibly involved in the intrusive growth of the primary walls of differentiating xylem cells.

Does the plant mitochondrion integrate cellular stress and regulate programmed cell death?

Research on programmed cell death in plants is providing insight into the primordial mechanism of programmed cell death in all eukaryotes. Much of the attention in studies on animal programmed cell death has focused on determining the importance of signal proteases termed caspases. However, it has recently been shown that cell death can still occur even when the caspase cascade is blocked, revealing that there is an underlying oncotic default pathway. Many programmed plant cell deaths also appear to be oncotic. Shared features of plant and animal programmed cell death can be used to deduce the primordial components of eukaryotic programmed cell death. From this perspective, we must ask whether the mitochondrion is a common factor that can serve in plant and animal cell death as a stress sensor and as a dispatcher of programmed cell death.

Programmed cell death during pollination-induced petal senescence in petunia

Petal senescence, one type of programmed cell death (PCD) in plants, is a genetically controlled sequence of events comprising its final developmental stage. We characterized the pollination-induced petal senescence process in Petunia inflata using a number of cell performance markers, including fresh/dry weight, protein amount, RNA amount, RNase activity, and cellular membrane leakage. Membrane disruption and DNA fragmentation with preferential oligonucleosomal cleavage, events characteristic of PCD, were found to be present in the advanced stage of petal senescence, indicating that plant and animal cell death phenomena share one of the molecular events in the execution phase. As in apoptosis in animals, both single-stranded DNase and double-stranded DNase activities are induced during petal cell death and are enhanced by Ca(2+). In contrast, the release of cytochrome c from mitochondria, one commitment step in signaling of apoptosis in animal cells, was found to be dispensable in petal cell death. Some components of the signal transduction pathway for PCD in plants are likely to differ from those in animal cells.

Programmed cell death and aerenchyma formation in roots

Lysigenous aerenchyma contributes to the ability of plants to tolerate low-oxygen soil environments, by providing an internal aeration system for the transfer of oxygen from the shoot. However, aerenchyma formation requires the death of cells in the root cortex. In maize, hypoxia stimulates ethylene production, which in turn activates a signal transduction pathway involving phosphoinositides and Ca2+. Death occurs in a predictable pattern, is regulated by a hormone (ethylene) and provides an example of programmed cell death.

Regulation of programmed cell death in maize endosperm by abscisic acid

Cereal endosperm undergoes programmed cell death (PCD) during its development, a process that is controlled, in part, by ethylene. Whether other hormones influence endosperm PCD has not been investigated. Abscisic acid (ABA) plays an essential role during late seed development that enables an embryo to survive desiccation. To examine whether ABA is also involved in regulating the onset of PCD during endosperm development, we have used genetic and biochemical means to disrupt ABA biosynthesis or perception during maize kernel development. The onset and progression of cell death, as determined by viability staining and the appearance of internucleosomal DNA fragmentation, was accelerated in developing endosperm of ABA-insensitive vp1 and ABA-deficient vp9 mutants. Ethylene was synthesized in vp1 and vp9 mutant kernels at levels that were 2-4-fold higher than in wild-type kernels. Moreover, the increase and timing of ethylene production correlated with the premature onset and accelerated progression of internucleosomal fragmentation in these mutants. Treatment of developing wild-type endosperm with fluridone, an inhibitor of ABA biosynthesis, recapitulated the increase in ethylene production and accelerated execution of the PCD program that was observed in the ABA mutant kernels. These data suggest that a balance between ABA and ethylene establishes the appropriate onset and progression of programmed cell death during maize endosperm development.

Die and let live - programmed cell death in plants

Cysteine and serine proteases are prominent players in the control of developmental and pathogen-activated cell deaths in plants. Ethylene, salicylic acid, the small G-protein Rac, calcium and reactive oxygen species are recurring mediators of death signaling. Lastly, the mitochondrion has emerged in both plant and animal systems as a 'central depot' that interprets multiple signals and in some instances determines the fate of the cell.

Loss of Tonoplast Integrity Programmed in Tracheary Element Differentiation

A tracheary element (TE) is a typical example of a cell type that undergoes programmed cell death in the developmental processes of vascular plants. The loss of the selective permeability of the tonoplast, which corresponds to tonoplast disintegration, occurred after the cells commenced secondary wall thickening and played a pivotal role in the programmed cell death of TEs in a zinnia (Zinnia elegans L.) cell culture. A search for events specifically associated with the TE vacuole provided an important clue to the understanding of the cell death mechanism. The transport of fluorescein, a fluorescent organic anion, across the tonoplast declined drastically in differentiating TEs. The capacity of the vacuole to accumulate the probe was also impaired. Treatment with probenecid, an inhibitor of organic anion transport, caused rapid cell death of TEs and led to the ultimate disruption of the vacuole even in other types of cultured cells. These changes in vacuolar properties during TE development were suppressed by cycloheximide. Specific mRNA accumulation in cells cultured in a TE differentiation-inductive condition was abolished by probenecid. These results suggest that a change in vacuolar membrane permeability promotes programmed cell death in TEs.

Yariv reagent treatment induces programmed cell death in Arabidopsis cell cultures and implicates arabinogalactan protein involvement

Arabinogalactan proteins (AGPs) are a family of highly glycosylated, hydroxyproline-rich glycoproteins implicated in various aspects of plant growth and development. (beta-D-glucosyl)3 and (beta-D-galactosyl)3 Yariv phenylglycosides, commonly known as Yariv reagents, specifically bind AGPs in a non-covalent manner. Here (beta-D-galactosyl)3 Yariv reagent was added to Arabidopsis thaliana cell suspension cultures and determined to induce programmed cell death (PCD) by three criteria: (i) DNA fragmentation as detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) of DNA 3'-OH groups; (ii) inter- nucleosomal DNA fragmentation as visualized by genomic Southern blotting; and (iii) structural changes characteristic of PCD including cytoplasmic shrinkage and condensation, chromatin condensation and nuclear membrane blebbing. These findings implicate AGP involvement in PCD in plants, presumably by perturbation of AGPs located at the plasma membrane-cell wall interface.

Role of active oxygen species and NO in plant defence responses

Research in the area of active oxygen species is going through a reflective stage. There is controversy whether multiple mechanisms for active oxygen species generation exist and some data may need reassessing since the discovery of a role for NO in defence responses. Important work concerning upstream and downsteam signalling in this area is emerging, and the stage is set for approaches utilising transgenic knockouts and mutants to resolve many questions.