Apoptosis in Plants

Apoptosis is a feature of animal cells that explains some aspects of programmed cell death in plants. Differences between plant and animal cell development require that concepts be reexamined to signify how plant cells have evolved the need for cell elimination in the meristematic growth habit, life cycle, and alternation of generations. Central to this theme is the regulation of divisional cycles for mitosis, meiosis, apomeiosis, and their related sexual and asexual reproductive processes. Apoptosis depends on the coordinated expression of genes regulating divisional cycles and apoptotic pathways so that irreversible nuclear and cytoplasmic elimination occurs. Cellular degradation products are salvaged to sustain adaptation, viability, structural function, and ontogeny. The cell wall is usually retained and further differentiated or eliminated. A model of factors predisposing apoptosis and comprising checkpoints in cell divisional cycles is presented for comparisons among plant and animal cells.     

前寒武纪瓮安生物群具围卵腔结构的胚胎及其发育序列

通过对贵州瓮安新元古代陡山沱组瓮安微型球状化石SEM的观察,发现代表未分裂卵,和2、4、8、16不同分裂阶段的胚胎。这些未分裂的卵和处于不同发育阶段的胚胎化石不仅大小和卵壳表面装饰相同,而且它们均具明显的围卵腔和球形卵裂等特征,清楚表明这些卵和胚胎均为同一物种的产物。由于这些卵的表面装饰与现生某些甲壳类休眠卵极其相似,曾被解释为动物的休眠卵;当前不同卵裂阶段胚胎的发现表明它们不是休眠卵,而是处于发育过程的卵。围卵腔不仅是现生两侧对称动物早期胚胎的常见构造,而且具围卵腔构造的未分裂卵和不同卵裂阶段的胚胎化石在瓮安动物群也十分常见。除了以上具表面装饰胚胎化石的大量发现外,文中还报道围卵腔构造在表面光滑胚胎化石中的发现。这些具围卵腔结构的胚胎化石主要为两侧对称动物的胚胎。     

Plant cell suspension culture rheology

The results of rheological measurements on 10 different plant cell suspension cultures are presented. Nicotiana tabacum (tobacco) suspension cultures grown in serial batch subculture display high viscosity and power law rheology. This "undesirable" rheology is shown to be a result of elongated cell morphology. The rheology of Papaver somniferum (poppy) cell suspensions is quite different; poppy suspensions behave as Newtonian fluids and have relatively low viscosity (less than 15 cP) at fresh cell densities up to 250 g/L. This flow behavior can be attributed to a lack of elongation in batch-grown poppy cells. A simple correlation for the viscosity as a function of cell density is developed for poppy suspensions up to 300 g fresh weight (FW)/L. It is shown that tobacco cells do not elongate when grown in semicontinuous culture (daily media replacement). These semicontinuously cultured cells have rheological behavior that is indistinguishable from that of poppy, further confirming the dependence of rheology on plant cell morphology. The rheology of a wide variety of other plant suspensions at 200 g FW/L is presented. Most cell suspensions, including soybean, cotton, bindweed, and potato, display low viscosities similar to poppy suspensions. Only carrot and atriplex exhibit slight pseudoplastic behavior which corresponded to a slight degree of cellular elongation for these cultures. This demonstrates that complex rheology associated with elongated cell morphology is much less common than low-viscosity Newtonian behavior. High viscosity in plant cell culture is therefore not an intrinsic characteristic of plant cells but, instead, is a result of the ability to grow cultures to extremely high cell densities due to low biological oxygen demand. (c) 1993 John Wiley & Sons, Inc.     

Programmed cell death in the plant immune system

Cell death has a central role in innate immune responses in both plants and animals. Besides sharing striking convergences and similarities in the overall evolutionary organization of their innate immune systems, both plants and animals can respond to infection and pathogen recognition with programmed cell death. The fact that plant and animal pathogens have evolved strategies to subvert specific cell death modalities emphasizes the essential role of cell death during immune responses. The hypersensitive response (HR) cell death in plants displays morphological features, molecular architectures and mechanisms reminiscent of different inflammatory cell death types in animals (pyroptosis and necroptosis). In this review, we describe the molecular pathways leading to cell death during innate immune responses. Additionally, we present recently discovered caspase and caspase-like networks regulating cell death that have revealed fascinating analogies between cell death control across both kingdoms.     

Bacterial Flagella: Twist and Stick,or Dodge across the Kingdoms

The flagellum organelle is an intricate multiprotein assembly best known for its rotational propulsion of bacteria. However, recent studies have expanded our knowledge of other functions in pathogenic contexts, particularly adherence and immune modulation, e.g., for Salmonella enterica, Campylobacter jejuni, Pseudomonas aeruginosa, and Escherichia coli. Flagella-mediated adherence is important in host colonisation for several plant and animal pathogens, but the specific interactions that promote flagella binding to such diverse host tissues has remained elusive. Recent work has shown that the organelles act like probes that find favourable surface topologies to initiate binding. An emerging theme is that more general properties, such as ionic charge of repetitive binding epitopes and rotational force, allow interactions with plasma membrane components. At the same time, flagellin monomers are important inducers of plant and animal innate immunity: variation in their recognition impacts the course and outcome of infections in hosts from both kingdoms. Bacteria have evolved different strategies to evade or even promote this specific recognition, with some important differences shown for phytopathogens. These studies have provided a wider appreciation of the functions of bacterial flagella in the context of both plant and animal reservoirs.     

Specialized Plant Metabolism Characteristics and Impact on Target Molecule Biotechnological Production

Plant secondary metabolism evolved in the context of highly organized and differentiated cells and tissues, featuring massive chemical complexity operating under tight environmental, developmental and genetic control. Biotechnological demand for natural products has been continuously increasing because of their significant value and new applications, mainly as pharmaceuticals. Aseptic production systems of plant secondary metabolites have improved considerably, constituting an attractive tool for increased, stable and large-scale supply of valuable molecules. Surprisingly, to date, only a few examples including taxol, shikonin, berberine and artemisinin have emerged as success cases of commercial production using this strategy. The present review focuses on the main characteristics of plant specialized metabolism and their implications for current strategies used to produce secondary compounds in axenic cultivation systems. The search for consonance between plant secondary metabolism unique features and various in vitro culture systems, including cell, tissue, organ, and engineered cultures, as well as heterologous expression in microbial platforms, is discussed. Data to date strongly suggest that attaining full potential of these biotechnology production strategies requires being able to take advantage of plant specialized metabolism singularities for improved target molecule yields and for bypassing inherent difficulties in its rational manipulation.     

Comparison of freezing tolerance in cultured plant cells and their respective protoplasts

The possibility that the plant cell wall influences the severity of freezing injury was examined by comparing the freeze stress response of intact cells and protoplasts from four different suspension cultures. In no case did the intact cells suffer more injury than the respective wall-less protoplasts, showing that mechanical strain imposed by the cell wall during freeze-thaw stress is not a major determinant of injury. For three of the four species studied, cells from which the wall was removed showed significantly greater freezing injury, indicating that the plant cell wall may have a protective role. Other researchers have suggested that cell wall rigidity may minimize freezing injury by slowing freeze-induced loss of cell water. We found that decreased enzyme digestibility (perhaps indicating greater rigidity) of cell walls accompanied cold acclimation in various tissues. These results provide impetus to research which will characterize low-temperature-induced cell wall modification in cold acclimating tissues.     

Cloning of the Arabidopsis and Rice Formaldehyde Dehydrogenase Genes: Implications for the Origin of Plant Adh Enzymes

This article reports the cloning of the genes encoding the Arabidopsis and rice class III ADH enzymes, members of the alcohol dehydrogenase or medium chain reductase/dehydrogenase superfamily of proteins with glutathione-dependent formaldehyde dehydrogenase activity (GSH-FDH). Both genes contain eight introns in exactly the same positions, and these positions are conserved in plant ethanol-active Adh genes (class P). These data provide further evidence that plant class P genes have evolved from class III genes by gene duplication and acquisition of new substrate specificities. The position of introns and similarities in the nucleic acid and amino acid sequences of the different classes of ADH enzymes in plants and humans suggest that plant and animal class III enzymes diverged before they duplicated to give rise to plant and animal ethanol-active ADH enzymes. Plant class P ADH enzymes have gained substrate specificities and evolved promoters with different expression properties, in keeping with their metabolic function as part of the alcohol fermentation pathway.     

Plant systems for recognition of pathogen-associated molecular patterns

Research of the last decade has revealed that plant immunity consists of different layers of defense that have evolved by the co-evolutional battle of plants with its pathogens. Particular light has been shed on PAMP- (pathogen-associated molecular pattern) triggered immunity (PTI) mediated by pattern recognition receptors. Striking similarities exist between the plant and animal innate immune system that point for a common optimized mechanism that has evolved independently in both kingdoms. Pattern recognition receptors (PRRs) from both kingdoms consist of leucine-rich repeat receptor complexes that allow recognition of invading pathogens at the cell surface. In plants, PRRs like FLS2 and EFR are controlled by a co-receptor SERK3/BAK1, also a leucine-rich repeat receptor that dimerizes with the PRRs to support their function. Pathogens can inject effector proteins into the plant cells to suppress the immune responses initiated after perception of PAMPs by PRRs via inhibition or degradation of the receptors. Plants have acquired the ability to recognize the presence of some of these effector proteins which leads to a quick and hypersensitive response to arrest and terminate pathogen growth.     

A hypothesis for the mechanism of mycoplasma evolution

Mycoplasmas are wall-less prokaryotes which have small genomes and are known to have evolved from ancestors of Gram-positive bacteria. A model is proposed to explain how mycoplasmas may have evolved from these ancestors which had cell walls and large genomes. It is proposed that the initial step in this process was loss of the cell wall and conversion of the ancestral bacterium to an L-form. Fusion of L-forms would have resulted in a single cell that contained two or more complete genomes. It is thought that this bringing together of multiple genomes by cell fusion resulted in genetic recombination between genomes and loss of DNA segments from the cell. Data from bacterial systems are cited in support of this model.     

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.     

Common infection strategies of plant and animal pathogenic bacteria

Gram-negative bacterial pathogens use common strategies to invade and colonize plant and animal hosts. In many species, pathogenicity depends on a highly conserved type-III protein secretion system that delivers effector proteins into the eukaryotic cell. Effector proteins modulate a variety of host cellular pathways, such as rearrangements of the cytoskeleton and defense responses. The specific set of effectors varies in different bacterial species, but recent studies have revealed structural and functional parallels between some effector proteins from plant and animal pathogenic bacteria. These findings suggest that bacterial pathogens target similar pathways in plant and animal host cells.     

Correlation of reduced chilling injury with Increased spermine and spermidine levels in zucchini squash

By means of a biparametric cytofluorimetric analysis it is possible to distribute meristematic plant cells in a variety of cell cycle sub-compartments, unidentifiable by DNA measurements alone. In this work, an asynchronous proliferating cell population of pea root meristems was divided into different sub-compartments of the cell cycle phases, i.e. G1A, G1B, S. G2A and G2B on the basis of their DNA-nuclear protein content. By means of the same biparametric analysis, differentiated mesophyll cells and quiescent cells of embryo roots, indicated as G0 and G2Q, were distinguished from cycling cells by their low nuclear protein content. These results conform to those of some analyses performed on animal cells in culture and show that it is possible to get a major insight into cell cycle kinetics and its control in a natural system such as root meristem.     

Dynamic Organization of Microtubules and Microfilaments during Cell Cycle Progression in Higher Plant Cells

Abstract: The cytoskeleton, which mainly consists of microtubules (MTs) and actin microfilaments (MFs), plays various significant roles that are indispensable for eukaryotic viability, including determination of cell shape, cell movement, nuclear division, and cytokinesis. In animal cells, MFs appear to be of more importance than MTs, except for spindle formation in nuclear division. In contrast, higher plants have a rigid cell wall around their cells, and have thus evolved elegant systems of MTs to control the direction of cellulose microfibrils (CMFs) deposited in the cell wall, and to divide centrifugally in a physically limited space. Dynamic changes in MTs during cell cycle progression in higher plant cells have been observed over several decades, including cortical MTs (CMTs) during interphase, preprophase bands (PPBs) from late G₂ phase to prophase, spindles from prometaphase to anaphase, and phragmoplasts at telophase. The MFs also show some changes not as obvious as MT dynamics. However, questions regarding the process of formation of these arrays, and the precise mechanisms by which they fulfill their roles, remain unsolved. In this article, we present an outline of the changes in the cytoskeleton based on our studies with highly-synchronized tobacco BY-2 cells. Some candidate molecules that could play roles in cytoskeletal dynamics are discussed. We also hope to draw attention to recent attempts at visualization of cytoskeletons with molecular techniques, and to some examples of genetic approaches in this field.     

A New Fluorescent Test for Cell Vitality Using Calcofluor White M2R

The fluorescent fabric-brightener dye, Calcofluor white M2R (CFW), can be used to distinguish between living and dead cells from a variety of animal and plant sources. CFW does not stain living mouse fibroblasts or trout red blood cells and stains only the cell walls in living cells from the epidermis of onion bulb scale, staminal hairs of Tradescantia, and longitudinal sections of broad bean stems and roots. Heat-killed plant or animal cells are recognized by their lightly stained cytoplasm and brightly stained nuclei. The optimum staining concentrations were very low (0.01% to 0.03%) and nontoxic. Using onion scale epidermis in which some cells had been killed by heating as a test system, and the plasmolysisdeplasmolysis rection as the ultimate test for cell vitality, results from CFW staining correctly predicted cell vitality for about 98% of the cells tested. This success rate was comparable to those for Evans blue, uranin or neutral red in this test system.     

A new fluorescent test for cell vitality using calcofluor white M2R

The fluorescent fabric-brightener dye, Calcofluor white M2R (CFW), can be used to distinguish between living and dead cells from a variety of animal and plant sources. CFW does not stain living mouse fibroblasts or trout red blood cells and stains only the cell walls in living cells from the epidermis of onion bulb scale, staminal hairs of Tradescantia, and longitudinal sections of broad bean stems and roots. Heat-killed plant or animal cells are recognized by their lightly stained cytoplasm and brightly stained nuclei. The optimum staining concentrations were very low (0.01% to 0.03%) and nontoxic. Using onion scale epidermis in which some cells had been killed by heating as a test system, and the plasmolysis-deplasmolysis rection as the ultimate test for cell vitality, results from CFW staining correctly predicted cell vitality for about 98% of the cells tested. This success rate was comparable to those for Evans blue, uranin or neutral red in this test system.     

Cell wall deficiency as an escape mechanism from phage infection

The cell wall plays a central role in protecting bacteria from some environmental stresses, but not against all. In fact, in some cases, an elaborate cell envelope may even render the cell more vulnerable. For example, it contains molecules or complexes that bacteriophages recognize as the first step of host invasion, such as proteins and sugars, or cell appendages such as pili or flagella. In order to counteract phages, bacteria have evolved multiple escape mechanisms, such as restriction-modification, abortive infection, CRISPR/Cas systems or phage inhibitors. In this perspective review, we present the hypothesis that bacteria may have additional means to escape phage attack. Some bacteria are known to be able to shed their cell wall in response to environmental stresses, yielding cells that transiently lack a cell wall. In this wall-less state, the bacteria may be temporarily protected against phages, since they lack the essential entities that are necessary for phage binding and infection. Given that cell wall deficiency can be triggered by clinically administered antibiotics, phage escape could be an unwanted consequence that limits the use of phage therapy for treating stubborn infections.     

Programmed cell death in host-symbiont associations,viewed through the Gene Ontology

Manipulation of programmed cell death (PCD) is central to many host microbe interactions. Both plant and animal cells use PCD as a powerful weapon against biotrophic pathogens, including viruses, which draw their nutrition from living tissue. Thus, diverse biotrophic pathogens have evolved many mechanisms to suppress programmed cell death, and mutualistic and commensal microbes may employ similar mechanisms. Necrotrophic pathogens derive their nutrition from dead tissue, and many produce toxins specifically to trigger programmed cell death in their hosts. Hemibiotrophic pathogens manipulate PCD in a most exquisite way, suppressing PCD during the biotrophic phase and stimulating it during the necrotrophic phase. This mini-review will summarize the mechanisms that have evolved in diverse microbes and hosts for controlling PCD and the Gene Ontology terms developed by the Plant-Associated Microbe Gene Ontology (PAMGO) Consortium for describing those mechanisms.