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In plants, programmed cell death (PCD) is an important mechanism that controls normal growth and development as well as many defence responses. At present, research on PCD in different plant species is actively carried out due to the possibilities offered by modern methods in molecular biology and the increasing amount of genome data. The pine seed provides a favourable model for PCD because it represents an interesting inheritance of seed tissues as well as an anatomically well-described embryogenesis during which several tissues die via morphologically different PCD processes.
The existing variety of cell death pathways enables diverse functions of PCD in plant development. In the Scots pine, the development of a viable seed includes the strictly co-ordinated action of several cell death programs which, depending on the developmental tasks, result in different magnitudes of cell corpse processing.
In a gymnosperm seed, multiple embryos arise from the same zygote. In the Scots pine, the dominant embryo usually survives and completes its development,1 while the subordinate embryos are eliminated via PCD2 (Fig. 1A and B). In arctic pine seeds, however, the process leading to the death of subordinate embryos is less regular and occasionally results in the survival of several embryos.3 In addition to the elimination of the subordinate embryos, PCD also causes the deletion of the cells in the suspensor tissue, which serve temporary functions during embryo development.2 The suspensor holds the embryo in a fixed position within the seed, anchors the embryo to the maternal tissue and provides nutrition and growth regulators for the developing embryo during early stages of embryogenesis.4 The suspensors of the subordinate embryos deteriorate during later stages of development, leaving remnants of tissues at the micropylar end of the corrosion cavity (CC),5 but the dried and coiled suspensor of the dominant embryo survives, attached to both the root cap and the nucellar cap until germination5,6 (Fig. 1C). In the pine seed, the cells in both subordinate embryos and suspensor tissue display co-operative autolytic and autophagic mechanisms of protoplast degradation, which leads to substantial or complete processing of cell corpses. In the slow phase of the cell dismantling process, the cytoplasm and organelles are gradually engulfed and lysed by a high number of growing vacuoles. The vacuole collapse triggers a brief culmination phase which results in the complete removal of the protoplast.2 Finally, the dying subordinate embryos break down and may be used for the nutrition of the leading embryo via the CC fluid7 (Fig. 1D).
During the Scots pine embryogenesis, the haploid female megagametophyte cells in the narrow embryo surrounding region (ESR) and in the dome-shaped zone in front of the dominant embryo die by sudden necrotic-like cell death, providing nourishment and space to the developing embryo.2,7 The cell wall, plasma membrane and nuclear envelope of the starch-rich cells break down with the release of cell debris and nucleic acids into the CC (Vuosku et al. unpublished), where the remnants form a malleable zone between the ESR and the developing embryo6 (Fig. 1E). The exact function of the pine ESR is still unknown, but in addition to its role in embryo nutrition, the ESR may establish a physical barrier or provide a region for communication between the megagametophyte and the developing embryo, as suggested in the case of endosperm ESR in maize (Zea mays L.).8 The continuation of bidirectional metabolism between the embryo and the megagametophyte during germination is ensured by the mucilaginous remnants of the ESR cells that balance the moisture content of the mature Scots pine seed.6,9
The diploid maternal cells of the nucellar layers face destruction during early embryogenesis7,10 and end up being used as nutrition for the surrounding tissues of the pine seed. The length of this process varies depending on environmental factors, such as photoperiod and temperature.11 The collapsed and phenoliferous nucellar cell walls of the nucellar layers serve another function in the mature coniferous seed, where they form an efficient barrier to the passage of water and against fungal infections.12,13
In gymnosperms, the megagametophyte tissue can be considered a functional homolog of the endosperm in angiosperms due to its role as a nutrient source of the developing embryo.14 PCD in the endosperm during seed development is well described especially in cereals, maize and wheat (Triticum aestivum L.).15 In the Scots pine, a major part of the megagametophyte cells stay alive, and the megagametophyte remains a viable and metabolically active tissue from the early phases of embryo development until early germination7 and even until the completion of germination.16 The megagametophyte develops from a haploid megaspore before the actual fertilization of the plant egg occurs.17 When ovules are fertilised, the chemical composition of the megagametophyte cells undergoes changes as the accumulation of storage reserves begins with the production of starch, lipids and protein bodies, followed by the early phases of maturation drying.17–19 However, if the ovules remain unfertilized or the embryos die, e.g., as a result of self-fertilization, during proembryogeny orearlyembryogeny,themegagametophyte degenerates within a few weeks.20 The degeneration of the Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] megagametophyte is prevented if the seed is parasitized by the chalcid larva (Megastigmus spermotrophus Wachtl.). Moreover, the larva is able to evoke the production of storage reserves,21 which has also been observed in Norway spruce seeds infected by the spruce seed gall midge (Plemeliella abietina Seitn.) (Tillman-Sutela E and Kauppi A, unpublished). According to Chiwocha et al.22 the presence of an insect parasitoid induces hormone profiles analogous to the hormone profiles of a fertilized, normally developing seed, which indicates certain plasticity in the development of conifer seeds as well as the evolutional adaptation of insects to the nutrition source.
Abscisic acid (ABA) and indole-3-acetic acid (IAA) content fluctuate in the zygotic embryo and in the megagametophyte during the different stages of embryogenesis (e.g. refs. 23–25) and are also found to be present in the CC fluid.26 In conifers, somatic embryos are able to produce ABA,27 but additional exogenous ABA with an osmoticum is required for the normal development of somatic embryos and for the proper accumulation of storage reserves.28–34 Therefore, ABA has been considered as a candidate for the megagametophyte-embryo signalling.
Recently, Ungru et al.35 postulated that even though the initiations of endosperm and embryo growth in Arabidopsis thaliana can be regarded as autonomous events, the proper development of both structures requires crosstalk between the developing endosperm and embryo in several checkpoints. In conifer seeds, similar kind of recognition may take place between the megagametophyte and embryos during the different stages of embryogenesis. Expressed sequence tags (ESTs) of loblolly pine (Pinus taeda L.) embryos36 together with identified loblolly pine microRNAs (miRNAs) that modulate in the megagametophyte and zygotic embryo stage-specifically will help to unravel the actual signal cascades directing the development of different conifer seed tissues.37
The main target of PCD in a cell is the nucleus, in which the degradation processes include both chromatin and the nuclear envelope.38 Basic knowledge about the molecular mechanisms of diverse plant PCD machinery is still largely lacking; nonetheless, the importance of nucleases39,40 and proteases, such as vacuolar processing enzymes (VPEs), type II metacaspases and VEIDase activity, has been perceived.41–43 Tat-D, an evolutionary conserved apoptotic nuclease exhibiting endo- and exonuclease activity,44 has been shown to possess decreased gene expression during late embryogeny of the Scots pine embryogenesis.7 This suggests that no large-scale PCD or nucleic acid fragmentation occur in the megagametophyte tissue, but it may also reflect the increasing contribution of the more developed embryo to the total amount of mRNA and, furthermore, the important role of Tat-D in the PCD events during early embryogenesis.7 In white spruce [Picea glauca (Moench) Voss] seeds, VPE activity is associated with PCD of the megagametophyte cells after germination.45 During the Norway spruce [Picea abies (L.) Karst] somatic embryogenesis, type-II metacaspase, mcII-Pa, is activated in the terminally-differentiated cells, where it moves from the cytoplasm to nuclei, causing nuclear envelope disassembly and DNA fragmentation.46 The gene expression of type-II metacaspase from the Scots pine showed a downward trend during zygotic embryogenesis, which suggests that metacaspase plays an important role in PCD at early embryogenesis.7 Bozhkov et al.47 reported that in Norway spruce somatic embryos, VEIDase activity increases at the early stages of embryo development that coincide with massive cell death during shape remodeling. The inhibition of VEIDase prevents normal embryo development by blocking the embryo-suspensor differentiation. The VEIDase activity was found to be sensitive to pH, ionic strength and zinc comparable to human caspase 6. The accumulation of zinc in the embryonal masses, but not in the suspensors, is required for correct embryonic patterning, which suggests that zinc may mediate cell fate specification through its strong anti-cell death effect.48
Gymnosperms have some reproductive features, such as biparental organelle inheritance, haploid megagametophyte and polyembryony, which distinguish them from angiosperms.49,50 In biparental organelle inheritance, the maternal parent contributes the majority of the mitochondrial DNA and the paternal parent contributes the plastid DNA.51,52 An outcrossed Scots pine embryo shares its mitochondrial DNA haplotype with the megagametophyte and its plastid DNA haplotype with the pollen donor.53 The maternal nuclear haplotype of an embryo is identical to the megagametophyte's nuclear haplotype.50 During evolution, polyembryony (i.e., the form of development and reproduction in which a single zygote gives rise to multiple embryos) has developed at least 15 times in six different phyla in animals.54 In plants, the presence of more than one embryo in a young seed is almost universal among gymnosperms, whereas in angiosperms polyembryony is rare.49 Two forms of polyembryony exist in gymnosperms. In monozygotic (or cleavage) polyembryony, genetically identical offspring are derived from the same zygote, whereas in polyzygotic polyembryony (which is also known as non-cleavage or simple polyembryony), multiple embryos arise as a result of independent fertilization of several eggs.55
According to Sarvas,1 in the Scots pine seed the fertilization of many egg nuclei results in several embryos in the same ovule, although some fail to develop to the primary proembryo stage. Later during the development, polyzygotic embryos undergo cleavage polyembryony. The number of monozygotic proembryos for each polyzygotic proembryo vary so that a weak polyzygotic embryo produces fewer monozygotic proembryos than a vigorous one. In the developing seed, the competition between genetically identical cleavage embryos offers no selective advantage, whereas in the case of non-cleavage polyembryony, the competition between embryos can lead to fitness advantage. In the Scots pine seed cone, variation in the growth rate of polyzygotic pro-embryos may be caused by genetic factors.1 However, non-cleavage polyembryony has been found to be a weak barrier against selfed embryos in the western redcedar (Thuja plicata Don ex D. Donn), a conifer with a mixed mating system.56 In a Douglas-fir seed, the embryo closest to the chalazal end of the CC has the advantage in the competition, irrespective of whether it is selfed or outcrossed.57 These observations suggest that the competitive advantage of an embryo is mostly conferred by the embryo position instead of the embryo genotype. In gymnosperms, the megagametophyte undergoes development before fertilization and represents a significant investment of maternal resources, which are wasted if fertilization does not occur.58 Thus, the main adaptive value of both cleavage and non-cleavage polyembryony may be the insurance against the possibility of having no viable offspring. In the Scots pine seed, the death of all the embryos in an ovule leads to the rapid degeneration of the megagametophyte and to the transfer of accumulated nutrients elsewhere, whereas as long as the ovule contains a live embryo, regardless of its vigor or stage of development, the megagametophyte continues to develop normally.1
In the Scots pine seed, the opportunity for embryo replacement is brief and finally only the leading embryo survives while the subordinate embryos are eliminated via autophagic PCD mechanisms.2 PCD is first detected in the suspensor, after which the progression of embryonic PCD follows a rigid basal-apical pattern in a subordinate embryo, first killing the cells adjacent to the suspensor and then proceeding towards the apical region until all cells in the subordinate embryo are dead.2 In the Scots pine, the seed suspensors of subordinate embryos may die when the suspensor has become unnecessary for the further development of the leading embryo. However, the suspensor remnants have a further role during germination: when moistened, they swell and facilitate protrusion of primary root through the micropyle.6,16 There are no reports concerning gymnosperm seeds, but it has been suggested that in angiosperms the continued growth of the suspensor may be inhibited by the embryo proper during early stages of development.4 When suspensors die, subordinate embryos loose their fixed position within the Scots pine seed as well as their source of nutrition, which may lead to starvation if subordinate embryos are not capable of utilizing nutrients from the CC fluid. In plant, animal and yeast systems, autophagy has been associated with nutrient recycling during starvation.59 Due to the intracellular nutritional resources and the ability to control the autophagic process, plant cells can survive for some time after starvation, but finally starvation results in irreversible damages and cell death.59 Thus, in the Scots pine seed, the cells of subordinate embryos may begin autophagy as a survival response to nutritional stress but switch to the cell death pathway later when starvation continues. Compared with the molecular mechanisms underlying apoptosis,60 the regulation of autophagy is poorly known. Especially, the control of the switch between the two types of autophagy, i.e., autophagy that improves the survival of cells under metabolic stress and autophagy that leads to the degradation of the entire cell content, remains to be found.
Initially, necrosis was considered as an uncontrolled form of cell death, but accumulating evidence suggests that necrotic cell death can also be a regulated event that contributes to the development of organisms7,61 and which, under these circumstances, must be under genetic control. The unique occurrence of developmentally regulated necrotic-like cell death in a maternally derived haploid megagametophyte tissue makes the Scots pine seed a favourable model for the study of genetic control of necrotic-like cell death and for the search of an answer to the most basic question: does the genetic control of developmentally programmed necrotic cell death reside in the dying cell itself or outside it?
Both animal and plant cells are able to activate organized self-destruction in development and tissue homeostasis as well as in response to particular abiotic stress conditions, which indicates that PCD may have played an essential role in the development, survival and evolution of most, if not all, multicellular organisms.62,63 Morphologically different cell death processes have proven to be an integral part of pine embryogenesis,2,7,10 and the pine seed has been suggested as a model for studying the origin and evolution of eukaryotic PCD machinery.64 Furthermore, the unique reproductive features of gymnosperms together with the occurrence of cell death in various tissues which differ from each other either in the number or parental origin of the genomes make the pine seed special compared with many other PCD model systems. Thus, the pine seed provides a unique window for studies on the genetic control of developmentally regulated cell death processes in plants.
The work was funded by the Academy of Finland (Project 121994 to T.S.), the Biological Interactions Graduate School (to S.S.) and grants from the Finnish Cultural Foundation and the Niemi Foundation (to J.V.).
Previously published online: www.landesbioscience.com/journals/psb/article/9535