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

Germin-like genes are expressed during somatic embryogenesis and early development of conifers

Germins and germin-like proteins (GLPs) are members of a superfamily of proteins widely distributed in plants. Their localization within the extracellular matrix and in some cases their hydrogen peroxide-producing activity suggests that these proteins are involved in cell wall metabolism during stress responses and developmental processes. Several very highly conserved conifer GLPs have been identified in somatic embryo tissues. In order to gain more knowledge on their potential involvement in the development of this particular tissue, we have characterized a new GLP gene, LmGER1 in hybrid larch. Anti-GLP immunserum and in-gel activity analyses suggested the presence of superoxide dismutase activity in apoplastic proteins from larch somatic embryos. These results could indicate a possible role for LmGER1 in this physiological process. The expression of LmGER1 has been followed during the maturation of somatic embryos and in different organs of young plantlets by homologous transformation with a promoter-gus construct. This promoter was activated in the root cap of young embryos and, later on, in the cotyledons and in the vascular procambium and xylem. Furthermore, the importance of this gene in embryo development was evaluated by transforming embryonal masses with a gene construct encoding a hairpin RNA leading to gene silencing. The potential role of LmGER1 in cross-linking of cell wall components is discussed.

AtBAG6, a novel calmodulin-binding protein, induces programmed cell death in yeast and plants

Calmodulin (CaM) influences many cellular processes by interacting with various proteins. Here, we isolated AtBAG6, an Arabidopsis CaM-binding protein that contains a central BCL-2-associated athanogene (BAG) domain. In yeast and plants, overexpression of AtBAG6 induced cell death phenotypes consistent with programmed cell death (PCD). Recombinant AtBAG6 had higher affinity for CaM in the absence of free Ca2 + than in its presence. An IQ motif (IQXXXRGXXXR, where X denotes any amino-acid) was required for Ca2 +-independent CaM complex formation and single amino-acid changes within this motif abrogated both AtBAG6-activated CaM-binding and cell death in yeast and plants. A 134-amino-acid stretch, encompassing both the IQ motif and BAG domain, was sufficient to induce cell death. Agents generating oxygen radicals, which are known to be involved in plant PCD, specifically induced the AtBAG6 transcript. Collectively, these results suggest that AtBAG6 is a stress-upregulated CaM-binding protein involved in plant PCD.

Cysteine protease mcII-Pa executes programmed cell death during plant embryogenesis

Programmed cell death (PCD) is indispensable for eukaryotic development. In animals, PCD is executed by the caspase family of cysteine proteases. Plants do not have close homologues of caspases but possess a phylogenetically distant family of cysteine proteases named metacaspases. The cellular function of metacaspases in PCD is unknown. Here we show that during plant embryogenesis, metacaspase mcII-Pa translocates from the cytoplasm to nuclei in terminally differentiated cells that are destined for elimination, where it colocalizes with the nuclear pore complex and chromatin, causing nuclear envelope disassembly and DNA fragmentation. The cell-death function of mcII-Pa relies on its cysteine-dependent arginine-specific proteolytic activity. Accordingly, mutation of catalytic cysteine abrogates the proteolytic activity of mcII-Pa and blocks nuclear degradation. These results establish metacaspase as an executioner of PCD during embryo patterning and provide a functional link between PCD and embryogenesis in plants. Although mcII-Pa and metazoan caspases have different substrate specificity, they serve a common function during development, demonstrating the evolutionary parallelism of PCD pathways in plants and animals.

Programmed cell death during the transition from multicellular structures to globular embryos in barley androgenesis

Androgenesis represents one of the most fascinating examples of cell differentiation in plants. In barley, the conversion of stressed uninucleate microspores into embryo-like structures is highly efficient. One of the bottlenecks in this process is the successful release of embryo-like structures out of the exine wall of microspores. In the present work, morphological and biochemical studies were performed during the transition from multicellular structures to globular embryos. Exine wall rupture and subsequent globular embryo formation were observed only in microspores that divided asymmetrically. Independent divisions of the generative and the vegetative nuclei gave rise to heterogeneous multicellular structures, which were composed of two different cellular domains: small cells with condensed chromatin structure and large cells with normal chromatin structure. During exine wall rupture, the small cells died and their death marked the site of exine wall rupture. Cell death in the small cell domain showed typical features of plant programmed cell death. Chromatin condensation and DNA degradation preceded cell detachment and cytoplasm dismantling, a process that was characterized by the formation of vesicles and vacuoles that contained cytoplasmic material. This morphotype of programmed cell death was accompanied by an increase in the activity of caspase-3-like proteases. The orchestration of such a death program culminated in the elimination of the small generative domain, and further embryogenesis was carried out by the large vegetative domain. To date, this is the first report to show evidence that programmed cell death takes part in the development of microspore-derived embryos.

Neuronal cell death in transmissible spongiform encephalopathies (prion diseases) revisited: from apoptosis to autophagy

Neuronal autophagy, like apoptosis, is one of the mechanisms of the programmed cell death (PCD). In this review, we summarize the presence of autophagic vacuoles in experimentally induced scrapie, Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Initially, a part of the neuronal cytoplasm was sequestrated by concentric arrays of double membranes; the enclosed cytoplasm appeared relatively normal except that its density was often increased. Next, electron density of the central area dramatically increased. The membranes then proliferated within the cytoplasm in a labyrinth-like manner and the area sequestrated by these membranes enlarged into a more complex structure consisting of vacuoles, electron-dense areas and areas of normally-looking cytoplasm connected by convoluted membranes. Of note, autophagic vacuoles form not only in neuronal perikarya but also in neurites and synapses. Finally, a large area of the cytoplasm was transformed into a collection of autophagic vacuoles of different sizes. On a basis of ultrastructural studies, we suggest that autophagy plays a major role in transmissible spongiform encephalopathies (TSEs) and may even participate in a formation of spongiform change.

Time-lapse tracking of barley androgenesis reveals position-determined cell death within pro-embryos

Following abiotic stress to induce barley (Hordeum vulgare L.) androgenesis, the development of 794 enlarged microspores in culture was monitored by time-lapse tracking. In total, 11% of the microspores tracked developed into embryo-like structures (type-I pathway), 36% formed multicellular structures (type-II pathway) and 53% of the microspores followed gametophytic divisions, accumulated starch and died in the first days of tracking (type-III pathway). Despite the microspore fate, enlarged microspores showed similar morphologies directly after stress treatment. Ultrastructural analysis, however, revealed two morphologically distinct cell types. Cells with a thin intine layer and an undifferentiated cytoplasm after stress treatment were associated with type-I and type-II pathways, whereas the presence of differentiated amyloplasts and a thick intine layer were associated with the type-III pathway. Tracking revealed that the first morphological change associated with embryogenic potential was a star-like morphology, which was a transitory stage between uninucleate vacuolated microspores after stress and the initiation of cell division. The difference between type-I and type-II pathways was observed during the time they displayed the star-like morphology. During the transition phase, embryo-like structures in the type-I pathway were always released out of the exine wall at the opposite side of the pollen germ pore, whereas in the type-II pathway multicellular structures were unable to break the exine and to release embryo-like structures. Moreover, by combining viability studies with cell tracking, we show that release of embryo-like structures was preceded by a decrease in viability of the cells positioned at the site of exine wall rupture. These cells were also positively stained by Sytox orange, a cell death indicator. Thereby, we demonstrate, for the first time, that a position-determined cell death process marks the transition from a multicellular structure into an embryo-like structure during barley androgenesis.

Cell Death and Organ Development in Plants

Programmed cell death (PCD) is an important feature of plant development; however, the mechanisms responsible for its regulation in plants are far less well understood than those operating in animals. In this review data from a wide variety of plant PCD systems is analyzed to compare what is known about the underlying mechanisms. Although senescence is clearly an important part of plant development, only what is known about PCD during senescence is dealt with here. In each PCD system the extracellular and intracellular signals triggering PCD are considered and both cytological and molecular data are discussed to determine whether a unique model for plant PCD can be derived. In the majority of cases reviewed, PCD is accompanied by the formation of a large vacuole, which ruptures to release hydrolytic enzymes that degrade the cell contents, although this model is clearly not universal. DNA degradation and the activation of proteases is also common to most plant PCD systems, where they have been studied; however, breakdown of DNA into nucleosomal units (DNA laddering) is not observed in all systems. Caspase-like activity has also been reported in several systems, but the extent to which it is a necessary feature of all plant PCD has not yet been established. The trigger for tonoplast rupture is not fully understood, although active oxygen species (AOS) have been implicated in several systems. In two systems, self incompatibility and tapetal breakdown as a result of cytoplasmic male sterility, there is convincing evidence for the involvement of mitochondria including release of cytochrome c. However, in other systems, the role of the mitochondrion is not clear-cut. How cells surrounding the cell undergoing PCD protect themselves against death is also discussed as well as whether there is a link between the eventual fate of the cell corpse and the mechanism of its death.

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.

Bax-induced cell death of Arabidopsis is meditated through reactive oxygen-dependent and -independent processes

An Arabidopsis protoplast system was developed for dissecting plant cell death in individual cells. Bax, a mammalian pro-apoptotic member of the Bcl-2 family, induces apoptotic-like cell death in Arabidopsis. Bax accumulation in Arabidopsis mesophyll protoplasts expressing murine Bax cDNA from a glucocorticoid-inducible promoter results in cytological characteristics of apoptosis, namely DNA fragmentation, increased vacuolation, and loss of plasma membrane integrity. In vivo targeting analysis monitored using jellyfish green fluorescent protein (GFP) reporter indicated full-length Bax was localized to the mitochondria, as it does in animal cells. Deletion of the carboxyl-terminal transmembrane domain of Bax completely abolished targeting to mitochondria. Bax expression was followed by reactive oxygen species (ROS) accumulation. Treatment of protoplasts with the antioxidant N -acetyl- -cysteine (NAC) during induction of Bax expression strongly suppressed Bax-mediated ROS production and the cell death phenotype. However, some population of the ROS depleted cells still induced cell death, indicating that there is a process that Bax-mediated plant cell death is independent of ROS accumulation. Accordingly, suppression of Bax-mediated plant cell death also takes place in two different processes. Over-expression of a key redox-regulator, Arabidopsis nucleoside diphosphate kinase 2 (AtNDPK2) down-regulated ROS accumulation and suppressed Bax-mediated cell death and transient expression of Arabidopsis Bax inhibitor-1 (AtBI-1) substantially suppressed Bax-induced cell death without altering cellular ROS level. Taken together, our results collectively suggest that the Bax-mediated cell death and its suppression in plants is mediated by ROS-dependent and -independent processes.

Apoptosis in plants: specific features of plant apoptotic cells and effect of various factors and agents

Apoptosis is an integral part of plant ontogenesis; it is controlled by cellular oxidative status, phytohormones, and DNA methylation. In wheat plants apoptosis appears at early stages of development in coleoptile and initial leaf of 5- to 6-day-old seedlings. Distinct ultrastructural features of apoptosis observed are (1). compaction and vacuolization of cytoplasm in the apoptotic cell, (2). specific fragmentation of cytoplasm and appearance in the vacuole of unique single-membrane vesicles containing active organelles, (3). cessation of nuclear DNA synthesis, (4). condensation and margination of chromatin in the nucleus, (5). internucleosomal fragmentation of nuclear DNA, and (6). intensive synthesis of mitochondrial DNA in vacuolar vesicles. Peroxides, abscisic acid, ethylene releaser ethrel, and DNA methylation inhibitor 5-azacytidine induce and stimulate apoptosis. Modulation of the reactive oxygen species (ROS) level in seedling by antioxidants and peroxides results in tissue-specific changes in the target date for the appearance and the intensity of apoptosis. Antioxidant butylated hydroxytoluene (BHT) reduces the amount of ROS and prevents apoptosis in etiolated seedlings, prolongs coleoptile life span, and prevents the appearance of all apoptotic features mentioned. Besides, BHT induces large structural changes in the organization of all cellular organelles and the formation of new unusual membrane structures in the cytoplasm. BHT distorts mitosis and this results in the appearance of multiblade polyploid nuclei and multinuclear cells. In roots of etiolated wheat seedlings, BHT induces differentiation of plastids with the formation of chloro(chromo)plasts. Therefore, ROS controlled by BHT seems to regulate mitosis, trigger apoptosis, and control plastid differentiation and the organization of various cellular structures formed by endocytoplasmic reticulum.

Expression of Chia4-Pa chitinase genes during somatic and zygotic embryo development in Norway spruce (Picea abies): similarities and differences between gymnosperm and angiosperm class IV chitinases

The developmental pathway of somatic embryogenesis in Norway spruce involves proliferation of proembryogenic masses (PEMs), PEM-to-somatic embryo transition and further development of the somatic embryos. It has previously been shown that extracellular signal molecules, including arabinogalactan proteins, lipo-chitooligosaccharides and chitinases, regulate somatic embryogenesis. The Chia4-Pa1 gene from Norway spruce is described here. The Chia4-Pa1 encodes a typical basic class IV chitinase, although the intron-exon organization of this gymnosperm chitinase is different from that in angiosperm class IV chitinases. The Chia4-Pa1 belongs to a small gene family with highly similar members, and the expression pattern of Chia4-Pa1 cannot be distinguished from that of other Chia4-Pa members. Upon withdrawal of plant growth regulators, i.e. during a treatment that stimulates PEM-to-somatic embryo transition and massive programmed cell death, a significant increase in transcription and translation of Chia4-Pa genes takes place. The expression pattern analysis revealed that Chia4-Pa genes are expressed in a subpopulation of proliferating cells and at the base of the somatic embryo. Furthermore, in seeds, Chia4-Pa genes are expressed in the megagametophyte in the single cell-layered zone surrounding the corrosion cavity. Taken together these results suggest that the Chia4-Pa expressing cells have a megagametophyte signalling function and that CHIA4-Pa stimulates programmed cell death and promotes PEM-to-somatic embryo transition.

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.

Localization of nonspecific lipid transfer proteins correlate with programmed cell death responses during endosperm degradation in Euphorbia lagascae seedlings

When the storage materials have been depleted, the endosperm cells undergo programmed cell death. Very little is known about how the components of the dying cells are recycled and used by the growing seedling. To learn more about endosperm degradation and nutrient recycling, we isolated soluble proteins from the endosperm of Euphorbia lagascae seedlings collected 2, 4, and 6 d after sowing. The protein extracts were subjected to two-dimensional gel electrophoresis. Proteins that increased in amount in the endosperm with time were selected for further analysis with mass spectrometry. We successfully identified 17 proteins, which became more abundant by time during germination. Among these proteins were three E. lagascae lipid transfer proteins (ElLTPs), ElLTP1, ElLTP2, and ElLTP3. Detailed expressional studies were performed on ElLTP1 and ElLTP2. ElLTP1 transcripts were detected in endosperm and cotyledons, whereas ElLTP2 transcripts were only detected in endosperm. Western blots confirmed that ElLTP1 and ElLTP2 accumulate during germination. Immunolocalization experiments showed that ElLTP1 was present in the vessels of the developing cotyledons, and also in the alloplastic space in the endosperm. ElLTP2 formed a concentration gradient in the endosperm, with higher amounts in the inner regions close to the cotyledons, and lesser amounts in the outer regions of the endosperm. On the basis of these data, we propose that ElLTP1 and ElLTP2 are involved in recycling of endosperm lipids, or that they act as protease inhibitors protecting the growing cotyledons from proteases released during programmed cell death.

The intermembrane space of plant mitochondria contains a DNase activity that may be involved in programmed cell death

The key role for mitochondria in mammalian apoptosis, a form of programmed cell death (PCD), is well established, but a similar role for plant mitochondria is just emerging. In order to unravel the molecular mechanisms linking plant mitochondria to the downstream events of PCD, we have developed an Arabidopsis cell-free system that can be used to monitor biochemical and morphological changes in isolated nuclei that are associated with PCD. Using this system, two activities that resulted in nuclear DNA degradation could be distinguished, both of which were facilitated by the addition of mitochondria. One activity mediated the generation of 30 kb DNA fragments within 3 h and chromatin condensation within 6 h, when nuclei were incubated with mitochondria alone. The second activity required cytosolic extract in addition to mitochondria and resulted in oligonucleosome-sized DNA cleavage after >12 h. Submitochondrial fractionation and pharmacological studies suggested the presence of an Mg2+-dependent nuclease activity in the intermembrane space, which is responsible for the former in vitro activity. The evolutionary conservation of the role of mitochondria in PCD in animals and plants is discussed.

Clustering of microarray data reveals transcript patterns associated with somatic embryogenesis in soybean

Globular somatic embryos can be induced from immature cotyledons of soybean (Glycine max L. Merr. cv Jack) placed on high levels of the auxin 2,4-dichlorophenoxyacetic acid (2,4-D). Somatic embryos develop from the adaxial side of the cotyledon, whereas the abaxial side evolves into a callus. Using a 9,280-cDNA clone array, we have compared steady-state RNA from the adaxial side from which embryos develop and from the abaxial callus at five time points over the course of the 4 weeks necessary for the development of globular embryos. In a second set of experiments, we have profiled the expression of each clone in the adaxial side during the same period. A total of 495 genes differentially expressed in at least one of these experiments were grouped according to the similarity of their expression profiles using a nonhierarchical clustering algorithm. Our results indicate that the appearance of somatic embryos is preceded by dedifferentiation of the cotyledon during the first 2 weeks on auxin. Changes in mRNA abundance of genes characteristic of oxidative stress and genes indicative of cell division in the adaxial side of the cotyledons suggest that the arrangement of the new cells into organized structures might depend on a genetically controlled balance between cell proliferation and cell death. Our data also suggest that the formation of somatic globular embryos is accompanied by the transcription of storage proteins and the synthesis of gibberellic acid.

Re-organisation of the cytoskeleton during developmental programmed cell death in Picea abies embryos

Cell and tissue patterning in plant embryo development is well documented. Moreover, it has recently been shown that successful embryogenesis is reliant on programmed cell death (PCD). The cytoskeleton governs cell morphogenesis. However, surprisingly little is known about the role of the cytoskeleton in plant embryogenesis and associated PCD. We have used the gymnosperm, Picea abies, somatic embryogenesis model system to address this question. Formation of the apical-basal embryonic pattern in P. abies proceeds through the establishment of three major cell types: the meristematic cells of the embryonal mass on one pole and the terminally differentiated suspensor cells on the other, separated by the embryonal tube cells. The organisation of microtubules and F-actin changes successively from the embryonal mass towards the distal end of the embryo suspensor. The microtubule arrays appear normal in the embryonal mass cells, but the microtubule network is partially disorganised in the embryonal tube cells and the microtubules disrupted in the suspensor cells. In the same embryos, the microtubule-associated protein, MAP-65, is bound only to organised microtubules. In contrast, in a developmentally arrested cell line, which is incapable of normal embryonic pattern formation, MAP-65 does not bind the cortical microtubules and we suggest that this is a criterion for proembryogenic masses (PEMs) to passage into early embryogeny. In embryos, the organisation of F-actin gradually changes from a fine network in the embryonal mass cells to thick cables in the suspensor cells in which the microtubule network is completely degraded. F-actin de-polymerisation drugs abolish normal embryonic pattern formation and associated PCD in the suspensor, strongly suggesting that the actin network is vital in this PCD pathway.

Up, down and up again is a signature global gene expression pattern at the beginning of gymnosperm embryogenesis

Somatic embryogenesis of a gymnosperm, Picea abies, represents a sequence of specifically regulated developmental stages including proembryogenic mass (PEM), PEM-to-embryo transition, and early and late embryogeny. Here, we report cDNA array analysis of expression patterns of 373 genes in the beginning of P. abies embryo development. The analysis revealed a group of 107 genes (29% of arrayed cDNAs) which were upregulated upon PEM-to-embryo transition, then downregulated during early embryogeny and finally upregulated again at the beginning of late embryogeny. This major gene expression pattern was abrogated in a developmentally arrested cell line that is unable to pass through the PEM-to-embryo transition. Thirty-five genes (9.4% of arrayed cDNAs) were found to be differentially expressed during normal embryonic pattern formation. Among them, 22 genes (5.9% of arrayed cDNAs) were directly associated with embryo pattern formation and can be considered as marker genes for early stages of P. abies embryogenesis. The majority of the marker genes encode for proteins involved in translation and posttranslational modification. Among them, 18 genes displayed the major expression pattern.

Programmed cell death eliminates all but one embryo in a polyembryonic plant seed

Development of multiple embryos from a single zygote, the phenomenon called monozygotic polyembryony, is a widespread reproductive strategy found in higher plants and especially in gymnosperms. The enigma of plant monozygotic polyembryony is that only one embryo in a polyembryonic seed usually survives while the others are eliminated at an early stage. Here we report that programmed cell death (PCD) is the major mechanism responsible for elimination of subordinate embryos in a polyembryonic seed. Using post-fertilized pine (Pinus sylvestris) ovules, we show that once the dominant embryo is selected and, subsequently, the entire female gametophyte is affected by PCD, the cells of subordinate embryos initiate an autolytic self-destruction program. The progression of embryonic PCD follows a rigid basal-apical pattern, first killing the most basally situated cells, adjacent to the suspensor, and then proceeding towards the apical region until all cells in the embryonal mass are doomed. Our data demonstrate that during polyembryony, PCD serves to halt competition among monozygotic embryos in order to ensure survival of one embryo.

A key developmental switch during Norway spruce somatic embryogenesis is induced by withdrawal of growth regulators and is associated with cell death and extracellular acidification

The biotechnology of somatic embryogenesis holds considerable promise for clonal propagation and breeding programs in forestry. To efficiently regulate the whole process of plant regeneration through somatic embryogenesis, it is of outmost importance to understand early developmental events when somatic embryos are just formed. In Norway spruce, somatic embryos transdifferentiate from proembryogenic masses (PEMs). This work describes the developmental dynamics (frequency distribution of PEMs and early somatic embryos) of the whole embryogenic suspension culture growing in the presence and absence of plant growth regulators (PGRs), auxin and cytokinin. The experiments have shown that PEM-to-somatic embryo transition is a key developmental switch that determines the yield and quality of mature somatic embryos and ultimately plant production. This switch was induced by the withdrawal of PGRs in cell suspension leading to a rapid accumulation of early somatic embryos (to a maximum of 75% of the entire population of suspension culture) and concomitant degradation of PEMs. The latter was evident from increased level of cell death measured through spectrophotometric Evans blue staining assay. Proembryogenic mass-to-embryo transition and concomitant activation of cell death were mediated by strong extracellular acidification. Therefore, buffering PGR-free culture medium at high (pH 5.8) or low (pH 4.5) levels of pH inhibited both PEM-to-embryo transition and cell death. The yield of mature somatic embryos on abscisic acid (ABA)-containing medium was increased up to 10-fold if the suspension culture had been pretreated for 1 to 9 days in unbuffered PGR-free medium. In this case a large proportion (75%) of the total number of mature embryos was formed within a short, 5-week, contact with ABA. The latter is practically important because prolonged contact with ABA suppresses the growth of somatic embryo plants. Based on these results, an improved method for regulating somatic embryogenesis was set up and tested for nine genotypes of Norway spruce. Over 800 plants regenerated from all tested genotypes demonstrated a good performance in the greenhouse and they were transferred to the field.

Endogenous Nod-factor-like signal molecules promote early somatic embryo development in Norway spruce

Embryogenic cultures of Norway spruce (Picea abies) are composed of pro-embryogenic masses (PEMs) and somatic embryos of various developmental stages. Auxin is important for PEM formation and proliferation. In this report we show that depletion of auxin blocks PEM development and causes large-scale cell death. Extracts of the media conditioned by embryogenic cultures stimulate development of PEM aggregates in auxin-deficient cultures. Partial characterization of the conditioning factor shows that it is a lipophilic, low-molecular-weight molecule, which is sensitive to chitinase and contains GlcNAc residues. On the basis of this information, we propose that the factor is a lipophilic chitin oligosaccharide (LCO). The amount of LCO correlates to the developmental stages of PEMs and embryos, with the highest level in the media conditioned by developmentally blocked cultures. LCO is not present in nonembryogenic cultures. Cell death, induced by withdrawal of auxin, is suppressed by extra supply of endogenous LCO or Nod factor from Rhizobium sp. NGR234. The effect can be mimicked by a chitotetraose or chitinase from Streptomyces griseus. Taken together, our data suggest that endogenous LCO acts as a signal molecule stimulating PEM and early embryo development in Norway spruce.

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

Recent studies have expanded our view of the interactions between small molecule signals that regulate the hypersensitive response and other forms of cell suicide in plants. The mitochondrion has received increasing support as a mediator of at least some forms of programmed cell death in plants. In addition, new information provides a glimpse of how plant hormone signaling may be integrated with extensive autolysis, sensitivity to reactive oxygen intermediates and cell death.

Regulation of in vitro somatic embryogenesis with emphasis on to the role of endogenous hormones

Different aspects of the in vitro somatic embryogenesis regulation are reviewed in this paper.work. A description of g General aspects, such as terminology, uses, stages of development and factors associated with the somatic embryogenesis, are described. is carried out. Although a brief description ofn the effects of the addition of different plant growth regulators to the culture medium wasis given, the article is centereds itself on the effect that the endogenous hormone concentrations in the initial explants and in the tissue cultures derived from them could play oin the induction and expression of somatic embryogenesis. It is significant that few to emphasize the low amount of systematic studies have been conducted, in this subject, in which different species and hormone groups were compared in cultures with and without embryogenic capacity. Moreover, the lack of correlation between the results presented in different studies the distinct works indicates that the hormone content of the cultures is not the only factor involved.