Apoptosis in Plants

Morphogenesis of fruit as a subject matter for the carpological studies

In this review, the concept of fruit morphogenesis is treated in the context of implementation of the evo-devo approach in carpology. A new viewpoint on the fruit morphogenesis is proposed and justified, comprising the pre-anthetic, as well as post-anthetic periods of fruit development, id est, development of the gynoecium, and development of the fruit itself. It is proposed to recognize ontogenetical (individual) and evolutionary (historical) aspects of fruit morphogenesis, the first of them we can study directly, while the second aspect can be only hypothesized or treated as a theoretical model of fruit evolution in consequence of some presumed changes in the individual fruit morphogenesis. In this article these aspects are named as "ontomorphogenesis" and "phylomorphogenesis" of the fruit, correspondingly. Our concept of ontomorphogenesis of the fruit involves four components that could not be brought together, such as changes in the morphological structure of the gynoecium, abscission of the extragynecial floral parts and the style, histogenesis of the fruit wall and other fruit parts, and terminal stages of the fruit morphogenesis (dehiscence, splitting, or abscission). The current state of studies of these components in the individual and evolutionary contexts is discussed. By examining the patterns of fruit evolution, we should consider factors acting at both the post-anthetic and pre-anthetic periods of fruit ontomorphogenesis.

Are Early Somatic Embryos of the Norway Spruce (Picea abies (L.) Karst.) Organised?

Background

Somatic embryogenesis in conifer species has great potential for the forestry industry. Hence, a number of methods have been developed for their efficient and rapid propagation through somatic embryogenesis. Although information is available regarding the previous process-mediated generation of embryogenic cells to form somatic embryos, there is a dearth of information in the literature on the detailed structure of these clusters.

Methodology/Principal Findings

The main aim of this study was to provide a more detailed structure of the embryogenic tissue clusters obtained through the in vitro propagation of the Norway spruce (Picea abies (L.) Karst.). We primarily focused on the growth of early somatic embryos (ESEs). The data on ESE growth suggested that there may be clear distinctions between their inner and outer regions. Therefore, we selected ESEs collected on the 56^th day after sub-cultivation to dissect the homogeneity of the ESE clusters. Two colourimetric assays (acetocarmine and fluorescein diacetate/propidium iodide staining) and one metabolic assay based on the use of 2,3,5-triphenyltetrazolium chloride uncovered large differences in the metabolic activity inside the cluster. Next, we performed nuclear magnetic resonance measurements. The ESE cluster seemed to be compactly aggregated during the first four weeks of cultivation; thereafter, the difference between the ¹H nuclei concentration in the inner and outer clusters was more evident. There were clear differences in the visual appearance of embryos from the outer and inner regions. Finally, a cluster was divided into six parts (three each from the inner and the outer regions of the embryo) to determine their growth and viability. The innermost embryos (centripetally towards the cluster centre) could grow after sub-cultivation but exhibited the slowest rate and required the longest time to reach the common growth rate. To confirm our hypothesis on the organisation of the ESE cluster, we investigated the effect of cluster orientation on the cultivation medium and the influence of the change of the cluster’s three-dimensional orientation on its development. Maintaining the same position when transferring ESEs into new cultivation medium seemed to be necessary because changes in the orientation significantly affected ESE growth.

Conclusions and Significance

This work illustrated the possible inner organisation of ESEs. The outer layer of ESEs is formed by individual somatic embryos with high metabolic activity (and with high demands for nutrients, oxygen and water), while an embryonal group is directed outside of the ESE cluster. Somatic embryos with depressed metabolic activity were localised in the inner regions, where these embryonic tissues probably have a very important transport function.

Female parthenogenetic apomixis and androsporogenetic parthenogenesis in embryonal cells of Araucaria angustifolia: interpolation of progenesis and asexual heterospory in an artificial sporangium

Cell fate, development timing and occurrence of reproductive versus apomictic development in gymnosperms are shown to be influenced by culture conditions in vitro. In this study, female parthenogenetic apomixis (fPA), androsporogenetic parthenogenesis (mAP) and progenesis were demonstrated using embryonal initials of Araucaria angustifolia in scaled-up cell suspensions passing through a single-cell bottleneck in darkness and in an artificial sporangium (AS). Expression was based on defined nutrition, hormones and feedforward-adaptive feedback process controls at 23-25 °C and in darkness. In fPA, the nucleus of an embryonal initial undergoes endomitosis and amitosis, forming a diploid egg-equivalent and an apoptotic ventral canal nucleus in a transdifferentiated archegonial tube. Discharge of egg-equivalent cells as parthenospores and their dispersal into the aqueous culture medium were followed by free-nuclear conifer-type proembryogenesis. This replaced the plesiomorphic and central features of proembryogenesis in Araucariaceae. Protoplasmic fusions of embryonal initials were used to reconstruct heterokaryotic expressions of fPA in multiwell plates. In mAP, restitutional meiosis (automixis) was responsible for androsporogenesis and the discharge of monads, dyads, tetrads and polyads. In a display of progenesis, reproductive development was brought to an earlier ontogenetic stage and expressed by embryonal initials. Colchicine increased polyploidy, but androspore formation became aberrant and fragmented. Aberrant automixis led to the formation of chromosomal bouquets, which contributed to genomic silencing in embryonal initials, cytomixis and the formation of pycnotic micronucleated cells. Dispersal of female and male parthenospores displayed heteromorphic asexual heterospory in an aqueous environment.

Female parthenogenetic apomixis and androsporogenesis in Douglas-fir embryonal initials in an artificial sporangium

Control of female parthenogenetic apomixis and androsporogenesis of Douglas-fir embryonal initials was studied using an experimental culture system in which changes in growth condition can mediate changes in cell identity and outcomes. This culture system constitutes an artificial sporangium in which myriad culture conditions can be simulated and should be applicable for research on a variety of gymnosperms. In this study, embryonal initials from developing seeds from two Douglas-fir trees were rescued and became reprogrammed for female parthenogenetic apomixis (fPA) and parthenogenetic androsporogenesis (mPA). Female PA was initiated by endomitosis forming a binucleate cell with a diploid egg-equivalent and an apoptotic ventral canal nucleus in an archegonial tube. Egg-equivalent nuclei formed cells (parthenotes) that were discharged into an aqueous culture medium. Parthenotes developed axial tiers atypical of early embryogenesis in seeds. Earlier in the year, androsporangial tubes were parthenogenetically differentiated and released monads, dyads, triads, and tetrads into the culture medium. Spores showed chromosomal aberrations. PA demonstrated a temporal separation in gender expression (dichogamy). Embryonal initials brought forward and by-passed the long juvenile phases normally needed for cells to develop into trees and express reproductive maturity. Expressions of fPA and mPA indicated that the specialized culture flasks served as an artificial sporangium (AS). Awareness is raised for the value of an AS for research in gymnosperm life cycles and as a teaching and research laboratory.

Arginine, scurvy and Cartier's "tree of life"

Several conifers have been considered as candidates for "Annedda", which was the source for a miraculous cure for scurvy in Jacques Cartier's critically ill crew in 1536. Vitamin C was responsible for the cure of scurvy and was obtained as an Iroquois decoction from the bark and leaves from this "tree of life", now commonly referred to as arborvitae. Based on seasonal and diurnal amino acid analyses of candidate "trees of life", high levels of arginine, proline, and guanidino compounds were also probably present in decoctions prepared in the severe winter. The semi-essential arginine, proline and all the essential amino acids, would have provided additional nutritional benefits for the rapid recovery from scurvy by vitamin C when food supply was limited. The value of arginine, especially in the recovery of the critically ill sailors, is postulated as a source of nitric oxide, and the arginine-derived guanidino compounds as controlling factors for the activities of different nitric oxide synthases. This review provides further insights into the use of the candidate "trees of life" by indigenous peoples in eastern Canada. It raises hypotheses on the nutritional and synergistic roles of arginine, its metabolites, and other biofactors complementing the role of vitamin C especially in treating Cartier's critically ill sailors.

Timing is everything: regulatory overlap in plant cell death

Plant development and defence are intimately connected to programmed cell death (PCD). PCD can occur after environmental cues such as pathogen infection, mechanical damage or abiotic stress. However, PCD also constitutes an essential feature of various aspects of growth and development. Despite the differences in stimuli, the subsequent steps leading to programmed cellular death show considerable commonality, reflecting the essential and overlapping roles of individual regulatory components in these processes. These components can function as positive or negative regulators and can have contrasting functions depending on the form of cell death.

Plant programmed cell death: can't live with it; can't live without it

The decision of whether a cell should live or die is fundamental for the wellbeing of all organisms. Despite intense investigation into cell growth and proliferation, only recently has the essential and equally important idea that cells control/programme their own demise for proper maintenance of cellular homeostasis gained recognition. Furthermore, even though research into programmed cell death (PCD) has been an extremely active area of research there are significant gaps in our understanding of the process in plants. In this review, we discuss PCD during plant development and pathogenesis, and compare/contrast this with mammalian apoptosis.

Cadmium-induced genotoxicity, cytotoxicity and lipid peroxidation in Allium sativum and Vicia faba

Cadmium (Cd) is one of the most toxic environmental pollutants affecting cytogenetically the various organisms. The cytogenetic damage in root tip cells exposed to cadmium nitrate (CdNO3) solutions at four different concentrations (1, 10, 100 and 200 microM) was evaluated with biological tests based on micronucleus (MN) assay in two plant species, Allium sativum and Vicia faba. Additionally to the cytogenetic analysis, lipid peroxidation analyses were performed in both A.sativum and V.faba roots. Cd enhanced the MN frequency in both A.sativum and V.faba root tip cells, but no dose-dependent. Induction of MN is not depending on CdNO3 concentrations. Besides, high concentrations of Cd decreased the mitotic index and caused the delay in mitosis stages in both plants, mainly in V.faba. On the other hand, lipid peroxidation was significantly enhanced with external Cd in V.faba. The results clearly indicate that high concentrations of cadmium induce the lipid peroxidation resulting in oxidative stress that may contribute to the genotoxicity and cytotoxicity of Cd ions.

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.

Programmed cell death (PCD) processes begin extremely early in Alstroemeria petal senescence

•  In the Liliaceous species Alstroemeria, petal senescence is characterized by wilting and inrolling, terminating in abscission 8-10 d after flower opening. •  In many species, flower development and senescence involves programmed cell death (PCD). PCD in Alstroemeria petals was investigated by light (LM) and transmission electron microscopy (TEM) (to study nuclear degradation and cellular integrity), DNA laddering and the expression programme of the DAD-1 gene. •  TEM showed nuclear and cellular degradation commenced before the flowers were fully open and that epidermal cells remained intact whilst the mesophyll cells degenerated completely. DNA laddering increased throughout petal development. Expression of the ALSDAD-1 partial cDNA was shown to be downregulated after flower opening. •  We conclude that some PCD processes are started extremely early and proceed throughout flower opening and senescence, whereas others occur more rapidly between stages 4-6 (i.e. postanthesis). The spatial distribution of PCD across the petals is discussed. Several molecular and physiological markers of PCD are present during Alstroemeria petal senescence.

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.

Nitric oxide induces cell death in Taxus cells

Sodium nitroprusside (SNP), a nitric oxide donor, or centrifugation at 150 times unit gravity, caused a nitric oxide burst in oocyte-derived Taxus brevifolia haploid cultures. This burst, visualized by the specific fluorescent probe 4,5-diaminofluorescein diacetate (DAF-2 DA), preceded a significant increase in nuclear DNA fragmentation and cell death. DNA fragmentation was detected in situ by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) of DNA 3'-OH groups. Nitric oxide formation and cell death were significantly decreased by N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide-synthase (NOS; EC 1.14.13.39) inhibitor. Our results show that nitric oxide leads to irreversible DNA fragmentation and cell death under stressful conditions, and that its effect can be prevented by L-NMMA.

The role of cytosolic potassium and pH in the growth of barley roots

In an earlier paper we showed that in fully developed barley (Hordeum vulgare L.) root epidermal cells a decrease in cytosolic K+ was associated with an acidification of the cytosol (D.J. Walker, R. A. Leigh, A.J. Miller [1996] Proc Natl Acad Sci USA 93: 10510-10514). To show that these changes in cytosolic ion concentrations contributed to the decreased growth of K+-starved roots, we first measured whether similar changes occurred in cells of the growing zone. Triple-barreled ion-selective microelectrodes were used to measure cytosolic K+ activity and pH in cells 0.5 to 1.0 mm from the root tip. In plants growing from 7 to 21 d after germination under K+-replete conditions, the mean values did not change significantly, with values ranging from 80 to 84 mM for K+ and 7.3 to 7.4 for pH. However, in K+-starved plants (external [K+], 2 &mgr;M), the mean cytosolic K+ activity and pH had declined to 44 mM and 7.0, respectively, after 14 d. For whole roots, sap osmolality was always lower in K+-starved than in K+-replete plants, whereas elongation rate and dry matter accumulation were significantly decreased after 14 and 16 d of K+ starvation. The rate of protein synthesis in root tips did not change for K+-replete plants but declined significantly with age in K+-starved plants. Butyrate treatment decreased cytosolic pH and diminished the rate of protein synthesis in K+-replete roots. Procaine treatment of K+-starved roots gave an alkalinization of the cytosol and increased protein synthesis rate. These results show that changes in both cytosolic pH and K+ can be significant factors in inhibiting protein synthesis and root growth during K+ deficiency.

Effects of mevinolin on cell cycle progression and viability of tobacco BY-2 cells

Mevinolin, an inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, was used to study the importance of mevalonic acid (MVA) for cell cycle progression of tobacco (Nicotiana tabacum L.) BY-2 cells. After treatment with 5 microM mevinolin, the cell cycle progression was completely blocked and two cell populations accumulated (80% in phase G0/G1 and 20% in G2/M). The arrest could be released by subsequent addition of MVA. Effects were compared to those caused by aphidicolin, an inhibitor of alpha-like DNA polymerases that blocks cell cycle at the entry of the S phase. The 80% proportion of mevinolin-treated TBY-2 cells was clearly arrested before the aphidicolin-inducible block. By the aid of a double-blocking technique, it was shown that the mevinolin-induced cell arrest of highly synchronized cells was due to interaction with a control point located at the mitotic telophase/entry G1 phase. Depending on the developmental stage, mevinolin induced rapid cell death in a considerable percentage of cells. Mevinolin treatment led to a partial synchronization, as shown by the increase in mitotic index. The following decrease was correlated with the above-mentioned induction of cell death.

Microscopic and Ultrastructural Examination of Vegetative Incompatibility in Partial Diploids Heterozygous at het Loci in Neurospora crassa

Vegetative (or heterokaryon) incompatibility is often characterized by cell death after anastomosis. In Neurospora crassa, partial diploid strains heterozygous for a single heterokaryon incompatibility (het) gene are viable, but grow at a significantly inhibited rate. Strains heterozygous for het-6 or het-c were examined microscopically for evidence of cell death; approximately 15% of cells randomly distributed within such colonies were dead or dying. Electron microscopy revealed extensive organelle degradation and plasmolysis. Ultimately, the cytoplasm fragmented into small membrane-bound bodies. Hyphal regrowth into dying cells from adjacent healthy cells was common. Ultrastructure and cell size measurements indicated no differences in death processes between incompatibility caused by het-6 and het-c. Linear growth rate was the only measured parameter which correlated with the observed macroscopic differences in colony morphology between het genes. The ultrastructural changes in dying cells were consistent with descriptions of apoptosis in plants and animals. However, designating vegetative incompatibility as apoptosis is premature without further study. Copyright 1998 Academic Press.

Programmed cell death in plant disease: the purpose and promise of cellular suicide

The interaction of pathogens with plants leads to a disruption in cellular homeostasis, often leading to cell death, in both compatible and incompatible relationships. The mechanistic basis of this cellular disruption and consequent death is complex and poorly characterized, but it is established that host responses to pathogens are dependent on gene expression, involve signal transduction, and require energy. Recent data suggest that in animals, a genetically regulated, signal transduction-dependent programmed cell death process, commonly referred to as apoptosis, is conserved over a wide range of phyla. The basic function of apoptosis is to direct the selective elimination of certain cells during development, but it also is a master template that is involved in host responses to many pathogens. Programmed cell death in plants, while widely observed, has not been studied extensively at either the biochemical or genetic level. Current data suggest that activation or suppression of programmed cell death may underlie diseases in plants as it does in animals. This review describes some of the fundamental characteristics of apoptosis in animals and points to a number of connections to programmed cell death in plants that may lead to both a better understanding of disease processes and novel strategies for engineering disease resistance in plants.

Taxane recovery from cells of Taxus in micro- and hypergravity

Cell suspension cultures of Taxus cuspidata produce taxanes that are released from the outer surface of cells into the culture medium as free and bound alkaloids. Paclitaxel (Taxol (TM)), is an anti-cancer drug in short supply. It has a taxane ring derived from baccatin III and a C-13 phenylisoserine side-chain. This drug is produced over a wide range of gravitational forces. Monoclonal and polyclonal antibodies to paclitaxel, baccatin III, and the C-13 phenylisoserine side chain were combined in multiple-labeling studies to localize taxanes and paclitaxel on cell surfaces or on particles released into the culture medium. Bioreactor vessel design altered the composition of taxanes recovered from cells in simulated microgravity. At 10(-2) and 2x10(-4)g, taxane recovery was reduced but biomass growth and percent paclitaxel was significantly increased. At 1 to 24g, growth was reduced with a significant recovery of total taxanes with low percent paclitaxel. Bound paclitaxel was also localized in endonuclease-rich fragmenting nuclei of individual apoptotic cells. A model is presented comprising TCH (touch) genes encoding enzymes that modify taxane-bearing xylan residues in cell walls, the calcium-sensing of gravitational forces by the cytoplasm, and the predisposition of nuclei to apoptosis. This integrates the adaptive physiological and biochemical responses of drug-producing genomes with gravitational forces.

Detection of a homologue of bcl-2 in plant cells

An emerging family of bcl-2-like genes has been identified from nematode to humans. These genes play a role in the maintenance of homeostasis. Its members have highly conserved domains that are important for their dimerization. Since nothing is known about the importance of these genes in plant cells, we have investigated their presence in an alga as well as in three higher plants both by Western analysis and by immunocytochemistry. Immunoblots revealed the presence of a protein immunoreacting with the anti-bcl-2 polyclonal antibody in leaves of tobacco plants. Furthermore, immunocytochemical localization has shown that this protein is mainly associated with mitochondria, plastids, and nuclei of plant cells. Taken together, our results suggest that bcl-2 is a protein highly conserved throughout evolution.