Elicitor-induced cytoskeletal rearrangement relates to vacuolar dynamics and execution of cell death: in vivo imaging of hypersensitive cell death in tobacco BY-2 cells

Basally distributed actin array drives embryonic hypocotyl elongation during the seed-to-seedling transition in Arabidopsis

Seed germination is a vital developmental transition for the production of progeny by sexual reproduction in spermatophytes. The seed-to-seedling transition is predominately driven by hypocotyl cell elongation. However, the mechanism that underlies hypocotyl growth remains largely unknown. In this study, we characterized the actin array reorganization in embryonic hypocotyl epidermal cells. Live-cell imaging revealed a basally organized actin array formed during hypocotyl cell elongation. This polarized actin assembly is a barrel-shaped network, which comprises a backbone of longitudinally aligned actin cables and a fine actin cap linking these cables. We provide genetic evidence that the basal actin array formation requires formin-mediated actin polymerization and directional movement of actin filaments powered by myosin XIs. In fh1-1 and xi3ko mutants, actin filaments failed to reorganize into the basal actin array, and the hypocotyl cell elongation was inhibited compared with wild-type plants. Collectively, our work uncovers the molecular mechanisms for basal actin array assembly and demonstrates the connection between actin polarization and hypocotyl elongation during seed-to-seedling transition.

Differential effects of the recombinant type 1 ribosome-inactivating protein, OsRIP1, on growth of PSB-D and BY-2 cells

Plant suspension cells were treated with recombinant OsRIP1, a type 1 ribosome-inactivating protein (RIP) from rice (Oryza sativa L.). OsRIP1 triggered cell death in tobacco BY-2 cells but not in Arabidopsis PSB-D cells. Phenotypic changes in BY-2 cells exposed to OsRIP1, included loss of growth capacity, loss of integrity of the plasma membrane and vacuolar collapse. These effects were also accompanied by RNA degradation and DNA fragmentation. Targeting of exogenous OsRIP1 to plant vacuoles and OsRIP1-induced accumulation of transcripts for vacuolar processing enzymes (VPEs) indicated that OsRIP1 provoked plant cell death in tobacco BY-2 cells through the activation of VPEs and subsequent vacuolar disruption, which was probably independent of its N-glycosylase activity on cytosolic ribosomes. Necrosis with limited production of H₂O₂ was observed after infiltration of high concentrations of OsRIP1 in epidermal cells of Nicotiana tabacum cv. Samsun NN plants. Our study provides the first evidence that OsRIP1 exerts differential effects on the growth of PSB-D and BY-2 cells. The vacuole-dependent cell death pathway is associated with the lethal effect of the exogenously applied OsRIP1 on BY-2 cells.

Distinct Roles for KASH Proteins SINE1 and SINE2 in Guard Cell Actin Reorganization, Calcium Oscillations, and Vacuolar Remodeling

The linker of nucleoskeleton and cytoskeleton (LINC) complex is a protein complex spanning the inner and outer membranes of the nuclear envelope. Outer nuclear membrane KASH proteins interact in the nuclear envelope lumen with inner nuclear membrane SUN proteins. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and ABA. Previous studies have shown F-actin organization, cytoplasmic calcium (Ca²⁺) oscillations, and vacuolar morphology changes are involved in ABA-induced stomatal closure. Here, we show that SINE1 and SINE2 are both required for actin pattern changes during ABA-induced stomatal closure, but influence different, temporally distinguishable steps. External Ca²⁺ partially overrides the mutant defects. ABA-induced cytoplasmic Ca²⁺ oscillations are diminished in sine2-1 but not sine1-1, and this defect can be rescued by both exogenous Ca²⁺ and F-actin depolymerization. We show first evidence for nuclear Ca²⁺ oscillations during ABA-induced stomatal closure, which are disrupted in sine2-1. Vacuolar fragmentation is impaired in both mutants and is partially rescued by F-actin depolymerization. Together, these data indicate distinct roles for SINE1 and SINE2 upstream of this network of players involved in ABA-based stomatal closure, suggesting a role for the nuclear surface in guard cell ABA signaling.

Implication of actin in the uptake of sucrose and valine in the tap root and leaf of sugar beet

Actin microfilaments (F-actin) are major components of the cytoskeleton essential for many cellular dynamic processes (vesicle trafficking, cytoplasmic streaming, organelle movements). The aim of this study was to examine whether cortical actin microfilaments might be implicated in the regulation of nutrient uptake in root and leaf cells of Beta vulgaris. Using antibodies raised against actin and the AtSUC1 sucrose transporter, immunochemical assays demonstrated that the expression of actin and a sucrose transporter showed different characteristics, when detected on plasma membrane vesicles (PMVs) purified from roots and from leaves. The in situ immunolabeling of actin and AtSUC1 sites in PMVs and tissues showed their close proximity to the plasma membrane. Using co-labeling in protoplasts, actin and sucrose transporters were localized along the internal border and in the outermost part of the plasma membrane, respectively. This respective membrane co-localization was confirmed on PMVs and in tissues using transmission electronic microscopy. The possible functional role of actin in sucrose uptake (and valine uptake, comparatively) by PMVs and tissues from roots and leaves was examined using the pharmacological inhibitors, cytochalasin B (CB), cytochalasin D (CD), and phalloidin (PH). CB and CD inhibited the sucrose and valine uptake by root tissues in a concentration-dependent manner above 1 μM, whereas PH had no such effect. Comparatively, the toxins inhibited the sucrose and valine uptake in leaf discs to a lesser extent. The inhibition was not due to a hindering of the proton pumping and H⁺ -ATPase catalytic activity determined in PMVs incubated in presence of these toxins.

Discovery, characterization and functional improvement of kumamonamide as a novel plant growth inhibitor that disturbs plant microtubules

The discovery and useful application of natural products can help improve human life. Chemicals that inhibit plant growth are broadly utilized as herbicides to control weeds. As various types of herbicides are required, the identification of compounds with novel modes of action is desirable. In the present study, we discovered a novel N-alkoxypyrrole compound, kumamonamide from Streptomyces werraensis MK493-CF1 and established a total synthesis procedure. Resulted in the bioactivity assays, we found that kumamonamic acid, a synthetic intermediate of kumamonamide, is a potential plant growth inhibitor. Further, we developed various derivatives of kumamonamic acid, including a kumamonamic acid nonyloxy derivative (KAND), which displayed high herbicidal activity without adverse effects on HeLa cell growth. We also detected that kumamonamic acid derivatives disturb plant microtubules; and additionally, that KAND affected actin filaments and induced cell death. These multifaceted effects differ from those of known microtubule inhibitors, suggesting a novel mode of action of kumamonamic acid, which represents an important lead for the development of new herbicides.

OsVPE3 Mediates GA-induced Programmed Cell Death in Rice Aleurone Layers via Interacting with Actin Microfilaments

BACKGROUND

Vacuolar processing enzymes (VPEs) have been identified as the enzymes that regulate vacuole-mediated programmed cell death (PCD) in plants. The mechanism that VPE regulates the PCD in rice aleurone layers remains unknown.

RESULTS

The aleurone layers treated with distilled water exerted caspase-1 and VPE activity, both of which were inhibited by the caspase-1 specific inhibitor Ac-YVAD-CMK but not by the caspase-3 specific inhibitor Ac-DEVD-CHO. However, the caspase-1 and caspase-3 inhibitors weakened the activity of caspase-3. Combined with the effects of endogenous gibberellin (GA) on the induction of OsVPEs, we suggest that the OsVPE3 in the aleurone layers, which exhibits caspase-1-like activity, is a key molecule in GA-induced PCD via regulating the protease with caspase-3-like activity. Many studies have confirmed that vacuolar fusion is an important feature of vacuole-mediated PCD in plants. In this experiment, the process of vacuole fusion was accompanied by changes in the structure of actin filaments (AFs), specifically, their depolymerization and polymerization. The process of vacuolar fusion was accelerated or delayed by the promotion or inhibition of the depolymerization of AFs, respectively. Here, the inhibition of OsVPE3 blocked the depolymerization of AFs and delayed the fusion of vacuoles, indicating that OsVPE3 can regulate the fusion of vacuoles in rice aleurone layers via mediating AFs. Furthermore, the depolymerization of AFs contributed to the up-regulation of OsVPE3 gene expression and VPE activity, resulting in accelerated PCD in rice aleurone layers. However, the inhibitor of VPE reversed the effects of AF depolymerization on the activity of VPE, then postponing the process of PCD, implying that AF can involve in GA-induced PCD of rice aleurone layers by mediating OsVPE3.

CONCLUSIONS

Together, activation of OsVPE3 and depolymerization of AFs shortened the process of vacuolation and PCD in rice aleurone layers, and OsVPE3 interacted with AFs during regulation.

Vacuolar membrane structures and their roles in plant-pathogen interactions

Short review focussing on the role and targeting of vacuolar substructure in plant immunity and pathogenesis. Plants lack specialized immune cells, therefore each plant cell must defend itself against invading pathogens. A typical plant defense strategy is the hypersensitive response that results in host cell death at the site of infection, a process largely regulated by the vacuole. In plant cells, the vacuole is a vital organelle that plays a central role in numerous fundamental processes, such as development, reproduction, and cellular responses to biotic and abiotic stimuli. It shows divergent membranous structures that are continuously transforming. Recent technical advances in visualization and live-cell imaging have significantly altered our view of the vacuolar structures and their dynamics. Understanding the active nature of the vacuolar structures and the mechanisms of vacuole-mediated defense responses is of great importance in understanding plant-pathogen interactions. In this review, we present an overview of the current knowledge about the vacuole and its internal structures, as well as their role in plant-microbe interactions. There is so far limited information on the modulation of the vacuolar structures by pathogens, but recent research has identified the vacuole as a possible target of microbial interference.

Understanding Cytoskeletal Dynamics During the Plant Immune Response

The plant cytoskeleton is a dynamic framework of cytoplasmic filaments that rearranges as the needs of the cell change during growth and development. Incessant turnover mechanisms allow these networks to be rapidly redeployed in defense of host cytoplasm against microbial invaders. Both chemical and mechanical stimuli are recognized as danger signals to the plant, and these are perceived and transduced into cytoskeletal dynamics and architecture changes through a collection of well-recognized, previously characterized players. Recent advances in quantitative cell biology approaches, along with the powerful molecular genetics techniques associated with Arabidopsis, have uncovered two actin-binding proteins as key intermediaries in the immune response to phytopathogens and defense signaling. Certain bacterial phytopathogens have adapted to the cytoskeletal-based defense mechanism during the basal immune response and have evolved effector proteins that target actin filaments and microtubules to subvert transcriptional reprogramming, secretion of defense-related proteins, and cell wall-based defenses. In this review, we describe current knowledge about host cytoskeletal dynamics operating at the crossroads of the molecular and cellular arms race between microbes and plants.

Reassessing apoptosis in plants

Cell death can be driven by a genetically programmed signalling pathway known as programmed cell death (PCD). In plants, PCD occurs during development as well as in response to environmental and biotic stimuli. Our understanding of PCD regulation in plants has advanced significantly over the past two decades; however, the molecular machinery responsible for driving the system remains elusive. Thus, whether conserved PCD regulatory mechanisms include plant apoptosis remains enigmatic. Animal apoptotic regulators, including Bcl-2 family members, have not been identified in plants but expression of such regulators can trigger or suppress plant PCD. Moreover, plants exhibit nearly all of the biochemical and morphological features of apoptosis. One difference between plant and animal PCD is the absence of phagocytosis in plants. Evidence is emerging that the vacuole may be key to removal of unwanted plant cells, and may carry out functions that are analogous to animal phagocytosis. Here, we provide context for the argument that apoptotic-like cell death occurs in plants.

Autophagy, programmed cell death and reactive oxygen species in sexual reproduction in plants

Autophagy is one of the major cellular processes of recycling of proteins, metabolites and intracellular organelles, and plays crucial roles in the regulation of innate immunity, stress responses and programmed cell death (PCD) in many eukaryotes. It is also essential in development and sexual reproduction in many animals. In plants, although autophagy-deficient mutants of Arabidopsis thaliana show phenotypes in abiotic and biotic stress responses, their life cycle seems normal and thus little had been known until recently about the roles of autophagy in development and reproduction. Rice mutants defective in autophagy show sporophytic male sterility and immature pollens, indicating crucial roles of autophagy during pollen maturation. Enzymatic production of reactive oxygen species (ROS) by respiratory burst oxidase homologues (Rbohs) play multiple roles in regulating anther development, pollen tube elongation and fertilization. Significance of autophagy and ROS in the regulation of PCD of transient cells during plant sexual reproduction is discussed in comparison with animals.

Stomatal Closure and Rise in ROS/NO of Arabidopsis Guard Cells by Tobacco Microbial Elicitors: Cryptogein and Harpin

Plants use stomatal closure mediated by elicitors as the first step of the innate immune response to restrict the microbial entry. We present a comprehensive study of the effect of cryptogein and harpin, two elicitors from microbial pathogens of tobacco, on stomatal closure and guard cell signaling components in Arabidopsis thaliana, a model plant. Cryptogein as well as harpin induced stomatal closure, while elevating the levels of reactive oxygen species (ROS) and nitric oxide (NO) in the guard cells of A. thaliana. Kinetic studies with fluorescent dyes revealed that the rise in ROS levels preceded that of NO in guard cells, when treated with these two elicitors. The restriction of NO levels in guard cells, even by ROS modulators indicates the essentiality of ROS for NO production during elicitor-triggered stomatal closure. The signaling events during elicitor-induced stomatal closure appear to converge at NADPH oxidase and ROS production. Our results provide the first report on stomatal closure associated with rise in ROS/NO of guard cells by cryptogein and harpin in A. thaliana. Our results establish that A. thaliana can be used to study stomatal responses to the typical elicitors from microbial pathogens of other plants. The suitability of Arabidopsis opens up an excellent scope for further studies on signaling events leading to stomatal closure by microbial elicitors.

Differential responses to high- and low-dose ultraviolet-B stress in tobacco Bright Yellow-2 cells

Ultraviolet (UV)-B irradiation leads to DNA damage, cell cycle arrest, growth inhibition, and cell death. To evaluate the UV-B stress-induced changes in plant cells, we developed a model system based on tobacco Bright Yellow-2 (BY-2) cells. Both low-dose UV-B (low UV-B: 740 J m(-2)) and high-dose UV-B (high UV-B: 2960 J m(-2)) inhibited cell proliferation and induced cell death; these effects were more pronounced at high UV-B. Flow cytometry showed cell cycle arrest within 1 day after UV-B irradiation; neither low- nor high-UV-B-irradiated cells entered mitosis within 12 h. Cell cycle progression was gradually restored in low-UV-B-irradiated cells but not in high-UV-B-irradiated cells. UV-A irradiation, which activates cyclobutane pyrimidine dimer (CPD) photolyase, reduced inhibition of cell proliferation by low but not high UV-B and suppressed high-UV-B-induced cell death. UV-B induced CPD formation in a dose-dependent manner. The amounts of CPDs decreased gradually within 3 days in low-UV-B-irradiated cells, but remained elevated after 3 days in high-UV-B-irradiated cells. Low UV-B slightly increased the number of DNA single-strand breaks detected by the comet assay at 1 day after irradiation, and then decreased at 2 and 3 days after irradiation. High UV-B increased DNA fragmentation detected by the terminal deoxynucleotidyl transferase dUTP nick end labeling assay 1 and 3 days after irradiation. Caffeine, an inhibitor of ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) checkpoint kinases, reduced the rate of cell death in high-UV-B-irradiated cells. Our data suggest that low-UV-B-induced CPDs and/or DNA strand-breaks inhibit DNA replication and proliferation of BY-2 cells, whereas larger contents of high-UV-B-induced CPDs and/or DNA strand-breaks lead to cell death.

Self-incompatibility-induced programmed cell death in field poppy pollen involves dramatic acidification of the incompatible pollen tube cytosol

Self-incompatibility (SI) is an important genetically controlled mechanism to prevent inbreeding in higher plants. SI involves highly specific interactions during pollination, resulting in the rejection of incompatible (self) pollen. Programmed cell death (PCD) is an important mechanism for destroying cells in a precisely regulated manner. SI in field poppy (Papaver rhoeas) triggers PCD in incompatible pollen. During SI-induced PCD, we previously observed a major acidification of the pollen cytosol. Here, we present measurements of temporal alterations in cytosolic pH ([pH]cyt); they were surprisingly rapid, reaching pH 6.4 within 10 min of SI induction and stabilizing by 60 min at pH 5.5. By manipulating the [pH]cyt of the pollen tubes in vivo, we show that [pH]cyt acidification is an integral and essential event for SI-induced PCD. Here, we provide evidence showing the physiological relevance of the cytosolic acidification and identify key targets of this major physiological alteration. A small drop in [pH]cyt inhibits the activity of a soluble inorganic pyrophosphatase required for pollen tube growth. We also show that [pH]cyt acidification is necessary and sufficient for triggering several key hallmark features of the SI PCD signaling pathway, notably activation of a DEVDase/caspase-3-like activity and formation of SI-induced punctate actin foci. Importantly, the actin binding proteins Cyclase-Associated Protein and Actin-Depolymerizing Factor are identified as key downstream targets. Thus, we have shown the biological relevance of an extreme but physiologically relevant alteration in [pH]cyt and its effect on several components in the context of SI-induced events and PCD.

Simplification of vacuole structure during plant cell death triggered by culture filtrates of Erwinia carotovora

Vacuoles are suggested to play crucial roles in plant defense-related cell death. During programmed cell death, previous live cell imaging studies have observed vacuoles to become simpler in structure and have implicated this simplification as a prelude to the vacuole's rupture and consequent lysis of the plasma membrane. Here, we examined dynamics of the vacuole in cell cycle-synchronized tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2) cells during cell death induced by application of culture filtrates of Erwinia carotovora. The filtrate induced death in about 90% of the cells by 24 h. Prior to cell death, vacuole shape simplified and endoplasmic actin filaments disassembled; however, the vacuoles did not rupture until after plasma membrane integrity was lost. Instead of facilitating rupture, the simplification of vacuole structure might play a role in the retrieval of membrane components needed for defense-related cell death.

Roles of autophagy in male reproductive development in plants

Autophagy, a major catabolic pathway in eukaryotic cells, is essential in development, maintenance of cellular homeostasis, immunity and programmed cell death (PCD) in multicellular organisms. In plant cells, autophagy plays roles in recycling of proteins and metabolites including lipids, and is involved in many physiological processes such as abiotic and biotic stress responses. However, its roles during reproductive development had remained poorly understood. Quantitative live cell imaging techniques for the autophagic flux and genetic studies in several plant species have recently revealed significant roles of autophagy in developmental processes, regulation of PCD and lipid metabolism. We here review the novel roles of autophagic fluxes in plant cells, and discuss their possible significance in PCD and metabolic regulation, with particular focus on male reproductive development during the pollen maturation.

An S-type anion channel SLAC1 is involved in cryptogein-induced ion fluxes and modulates hypersensitive responses in tobacco BY-2 cells

Pharmacological evidence suggests that anion channel-mediated plasma membrane anion effluxes are crucial in early defense signaling to induce immune responses and hypersensitive cell death in plants. However, their molecular bases and regulation remain largely unknown. We overexpressed Arabidopsis SLAC1, an S-type anion channel involved in stomatal closure, in cultured tobacco BY-2 cells and analyzed the effect on cryptogein-induced defense responses including fluxes of Cl(-) and other ions, production of reactive oxygen species (ROS), gene expression and hypersensitive responses. The SLAC1-GFP fusion protein was localized at the plasma membrane in BY-2 cells. Overexpression of SLAC1 enhanced cryptogein-induced Cl(-) efflux and extracellular alkalinization as well as rapid/transient and slow/prolonged phases of NADPH oxidase-mediated ROS production, which was suppressed by an anion channel inhibitor, DIDS. The overexpressor also showed enhanced sensitivity to cryptogein to induce downstream immune responses, including the induction of defense marker genes and the hypersensitive cell death. These results suggest that SLAC1 expressed in BY-2 cells mediates cryptogein-induced plasma membrane Cl(-) efflux to positively modulate the elicitor-triggered activation of other ion fluxes, ROS as well as a wide range of defense signaling pathways. These findings shed light on the possible involvement of the SLAC/SLAH family anion channels in cryptogein signaling to trigger the plasma membrane ion channel cascade in the plant defense signal transduction network.

Unveiling interactions among mitochondria, caspase-like proteases, and the actin cytoskeleton during plant programmed cell death (PCD)

Aponogeton madagascariensis produces perforations over its leaf surface via programmed cell death (PCD). PCD begins between longitudinal and transverse veins at the center of spaces regarded as areoles, and continues outward, stopping several cells from these veins. The gradient of PCD that exists within a single areole of leaves in an early stage of development was used as a model to investigate cellular dynamics during PCD. Mitochondria have interactions with a family of proteases known as caspases, and the actin cytoskeleton during metazoan PCD; less is known regarding these interactions during plant PCD. This study employed the actin stain Alexa Fluor 488 phalloidin, the actin depolymerizer Latrunculin B (Lat B), a synthetic caspase peptide substrate and corresponding specific inhibitors, as well as the mitochondrial pore inhibitor cyclosporine A (CsA) to analyze the role of these cellular constituents during PCD. Results depicted that YVADase (caspase-1) activity is higher during the very early stages of perforation formation, followed by the bundling and subsequent breakdown of actin. Actin depolymerization using Lat B caused no change in YVADase activity. In vivo inhibition of YVADase activity prevented PCD and actin breakdown, therefore substantiating actin as a likely substrate for caspase-like proteases (CLPs). The mitochondrial pore inhibitor CsA significantly decreased YVADase activity, and prevented both PCD and actin breakdown; therefore suggesting the mitochondria as a possible trigger for CLPs during PCD in the lace plant. To our knowledge, this is the first in vivo study using either caspase-1 inhibitor (Ac-YVAD-CMK) or CsA, following which the actin cytoskeleton was examined. Overall, our findings suggest the mitochondria as a possible upstream activator of YVADase activity and implicate these proteases as potential initiators of actin breakdown during perforation formation via PCD in the lace plant.

Centrality of host cell death in plant-microbe interactions

Programmed cell death (PCD) is essential for proper growth, development, and cellular homeostasis in all eukaryotes. The regulation of PCD is of central importance in plant-microbe interactions; notably, PCD and features associated with PCD are observed in many host resistance responses. Conversely, pathogen induction of inappropriate cell death in the host results in a susceptible phenotype and disease. Thus, the party in control of PCD has a distinct advantage in these battles. PCD processes appear to be of ancient origin, as indicated by the fact that many features of cell death strategy are conserved between animals and plants; however, some of the details of death execution differ. Mammalian core PCD genes, such as caspases, are not present in plant genomes. Similarly, pro- and antiapoptotic mammalian regulatory elements are absent in plants, but, remarkably, when expressed in plants, successfully impact plant PCD. Thus, subtle structural similarities independent of sequence homology appear to sustain operational equivalence. The vacuole is emerging as a key organelle in the modulation of plant PCD. Under different signals for cell death, the vacuole either fuses with the plasmalemma membrane or disintegrates. Moreover, the vacuole appears to play a key role in autophagy; evidence suggests a prosurvival function for autophagy, but other studies propose a prodeath phenotype. Here, we describe and discuss what we know and what we do not know about various PCD pathways and how the host integrates signals to activate salicylic acid and reactive oxygen pathways that orchestrate cell death. We suggest that it is not cell death as such but rather the processes leading to cell death that contribute to the outcome of a given plant-pathogen interaction.

Changes in cytosolic ATP levels and intracellular morphology during bacteria-induced hypersensitive cell death as revealed by real-time fluorescence microscopy imaging

Hypersensitive cell death is known to involve dynamic remodeling of intracellular structures that uses energy released during ATP hydrolysis. However, the relationship between intracellular structural changes and ATP levels during hypersensitive cell death remains unclear. Here, to visualize ATP dynamics directly in real time in individual living plant cells, we applied a genetically encoded Förster resonance energy transfer (FRET)-based fluorescent ATP indicator, ATeam1.03-nD/nA, for plant cells. Intracellular ATP levels increased approximately 3 h after inoculation with the avirulent strain DC3000/avrRpm1 of Pseudomonas syringae pv. tomato (Pst), which was accompanied by the simultaneous disappearance of transvacuolar strands and appearance of bulb-like structures within the vacuolar lumen. Approximately 5 h after bacterial inoculation, the bulb-like structures disappeared and ATP levels drastically decreased. After another 2 h, the large central vacuole was disrupted. In contrast, no apparent changes in intracellular ATP levels were observed in the leaves inoculated with the virulent strain Pst DC3000. The Pst DC3000/avrRpm1-induced hypersensitive cell death was strongly suppressed by inhibiting ATP synthesis after oligomycin A application within 4 h after bacterial inoculation. When the inhibitor was applied 7 h after bacterial inoculation, cell death was unaffected. These observations show that changes in intracellular ATP levels correlate with intracellular morphological changes during hypersensitive cell death, and that ATP is required just before vacuolar rupture in response to bacterial infection.

Non-apoptotic programmed cell death with paraptotic-like features in bleomycin-treated plant cells is suppressed by inhibition of ATM/ATR pathways or NtE2F overexpression

In plants, different forms of programmed cell death (PCD) have been identified, but they only partially correspond to those described for animals, which is most probably due to structural differences between animal and plant cells. Here, the results show that in tobacco BY-2 cells, bleomycin (BLM), an inducer of double-strand breaks (DSBs), triggers a novel type of non-apoptotic PCD with paraptotic-like features. Analysis of numerous PCD markers revealed an extensive vacuolization, vacuolar rupture, and chromatin condensation, but no apoptotic DNA fragmentation, fragmentation of the nuclei, or sensitivity to caspase inhibitors. BLM-induced PCD was cell cycle regulated, occurring predominantly upon G(2)/M cell cycle checkpoint activation. In addition, this paraptotic-like PCD was at least partially inhibited by caffeine, a known inhibitor of DNA damage sensor kinases ATM and ATR. Interestingly, overexpression of one NtE2F transcriptional factor, whose homologues play a dual role in animal apoptosis and DNA repair, reduced PCD induction and modulated G(2)/M checkpoint activation in BY-2 cells. These observations provide a solid ground for further investigations into the paraptotic-like PCD in plants, which might represent an ancestral non-apoptotic form of PCD conserved among animals, protists, and plants.

Confocal microscopy in plant-pathogen interactions

The development of confocal microscopy and its application to studies of plant-pathogen interactions have revolutionised research into the role of selected molecules and cell components in pathogen infection strategies and plant defence responses. Confocal microscopy allows high-resolution visualisation of a variety of fluorescent and fluorescently tagged molecules in both fixed and living cells, not only in single cells but also in intact tissues. Confocal microscopes greatly improve image quality by reducing interference by out-of-focus light and can capture high-resolution serial optical sections through samples in the z-axis. In combination with a range of computational image analysis techniques, confocal microscopy provides a powerful tool by which molecules, molecular interactions, and cell components can be localised and studied.

Organisation and regulation of the cytoskeleton in plant programmed cell death

Programmed cell death (PCD) involves precise integration of cellular responses to extracellular and intracellular signals during both stress and development. In recent years much progress in our understanding of the components involved in PCD in plants has been made. Signalling to PCD results in major reorganisation of cellular components. The plant cytoskeleton is known to play a major role in cellular organisation, and reorganization and alterations in its dynamics is a well known consequence of signalling. There are considerable data that the plant cytoskeleton is reorganised in response to PCD, with remodelling of both microtubules and microfilaments taking place. In the majority of cases, the microtubule network depolymerises, whereas remodelling of microfilaments can follow two scenarios, either being depolymerised and then forming stable foci, or forming distinct bundles and then depolymerising. Evidence is accumulating that demonstrate that these cytoskeletal alterations are not just a consequence of signals mediating PCD, but that they also may have an active role in the initiation and regulation of PCD. Here we review key data from higher plant model systems on the roles of the actin filaments and microtubules during PCD and discuss proteins potentially implicated in regulating these alterations.

Dynamic intracellular reorganization of cytoskeletons and the vacuole in defense responses and hypersensitive cell death in plants

Plants have evolved various means for controlled and organized cell destruction, known as programmed cell death (PCD). In plant immune responses against microbial infection, hypersensitive cell death as a form of PCD is a crucial event to prevent the spread of biotrophic pathogens. Recent live cell imaging techniques have revealed dynamic features and significant roles of cytoskeletons and the vacuole during defense responses and the PCD. Actin microfilaments (MFs) focus on the infection sites and function as tracks for the polar transport of antimicrobial materials. To accomplish hypersensitive cell death, further dynamic changes in cytoskeletons are induced. MFs play a role in the structural and functional regulation of the vacuole, leading to execution of the PCD. We here overview spatiotemporal dynamic changes in the cytoskeletons and the vacuoles triggered by signals from pathogens, and propose a hypothetical model for MF-regulated vacuole-mediated PCD in plant immunity.

Cryptogein-induced cell cycle arrest at G2 phase is associated with inhibition of cyclin-dependent kinases, suppression of expression of cell cycle-related genes and protein degradation in synchronized tobacco BY-2 cells

Induction of defense responses by pathogens or elicitors is often accompanied by growth inhibition in planta, but its molecular mechanisms are poorly understood. In this report, we characterized the molecular events that occur during cryptogein-induced cell cycle arrest at G(2) phase in synchronously cultured tobacco Bright Yellow-2 (BY-2) cells. Concomitant with the proteinaceous elicitor-induced G(2) arrest, we observed inhibition of the histone H1 kinase activity of cyclin-dependent kinases (CDKs), which correlated with a decrease in mRNA and protein levels of CDKB1. In contrast, the amount of CDKA was almost unaffected by cryptogein even at M phase. Cryptogein rapidly inhibited the expression not only of positive, e.g. A- and B-type cyclins and NtCAK, but also of negative cell cycle regulators such as WEE1, suggesting that cryptogein affects multiple targets to inactivate CDKA to induce G(2) arrest by mechanisms distinct from known checkpoint regulation. Moreover, we show that CDKB1 and cyclin proteins are also rapidly degraded by cryptogein and that the proteasome-dependent protein degradation has a crucial role in the control of cryptogein-induced hypersensitive cell death.

Actin microfilaments and vacuoles are downstream targets of H2O2 signalling pathways in hyperosmotic stress-induced stomatal closure

Changes in osmotic pressure can induce stomatal closure to reduce transpirational water loss from plants. In the present work, we investigated the mechanism underlying the perception and transduction of extracellular changes in osmotic pressure in Vicia faba L. guard cells. Using an epidermal strip bioassay and laser-scanning confocal microscopy, we provide evidence that hyperosmotic stress treatment led to stomatal closure and the rapid promotion of hydrogen peroxide (H2O2) production in V. faba guard cells. The effects were largely reduced by H2O2 scavengers ASA, CAT, NADPH oxidase inhibitor DPI and cell wall peroxidase inhibitor SHAM. These results indicate that hyperosmotic stress induces stomatal closure by promoting H2O2 production. Cytochalasin B (CB), latrunculin B (Lat B) and jasplakinolide (JK) inhibited stomatal closure induced by hyperosmotic stress but didn't prevent the increase of endogenous H2O2 levels, suggesting that microfilaments reorganisation participates in stomatal closure induced by hyperosmotic stress, and may act downstream of H2O2 signalling processes. In addition, we observed splitting of big vacuoles into many small vacuoles in response to hyperosmotic stress and H2O2 treatment, and CB inhibited these changes of vacuoles; stomatal closure was also inhibited. Taken together these results indicate that the stomatal closure in response to hyperosmotic stress may initiate H2O2 generation, and that reorganisation of microfilaments and the changing of vacuoles occurs downstream of H2O2 signalling processes.

The AAA-type ATPase AtSKD1 contributes to vacuolar maintenance of Arabidopsis thaliana

The vacuole is the most prominent organelle of plant cells. Despite its importance for many physiological and developmental aspects of plant life, little is known about its biogenesis and maintenance. Here we show that Arabidopsis plants expressing a dominant-negative version of the AAA (ATPase associated with various cellular activities) ATPase AtSKD1 (SUPPRESSOR OF K+ TRANSPORT GROWTH DEFECT1) under the control of the trichome-specific GLABRA2 (GL2) promoter exhibit normal vacuolar development in early stages of trichome development. Shortly after its formation, however, the large central vacuole is fragmented and finally disappears completely. Secretion assays with amylase fused to the vacuolar sorting signal of Sporamin show that dominant-negative AtSKD1 inhibits vacuolar trafficking of the reporter that is instead secreted. In addition, trichomes expressing dominant-negative AtSKD1 frequently contain multiple nuclei. Our results suggest that AtSKD1 contributes to vacuolar protein trafficking and thereby to the maintenance of the large central vacuole of plant cells, and might play a role in cell-cycle regulation.

The cytoskeleton enhances gene expression in the response to the Harpin elicitor in grapevine

The cytoskeleton undergoes dramatic reorganization during plant defence. This response is generally interpreted as part of the cellular repolarization establishing physical barriers against the invading pathogen. To gain insight into the functional significance of cytoskeletal responses for defence, two Vitis cell cultures that differ in their microtubular dynamics were used, and the cytoskeletal response to the elicitor Harpin in parallel to alkalinization of the medium as a fast response, and the activation of defence-related genes were followed. In one cell line derived from the grapevine cultivar 'Pinot Noir', microtubules contained mostly tyrosinylated alpha-tubulin, indicating high microtubular turnover, whereas in another cell line derived from the wild grapevine V. rupestris, the alpha-tubulin was strongly detyrosinated, indicating low microtubular turnover. The cortical microtubules were disrupted and actin filaments were bundled in both cell lines, but the responses were elevated in V. rupestris as compared with V. vinifera cv. 'Pinot Noir'. The cytoskeletal responsiveness correlated with elicitor-induced alkalinization and the expression of defence genes. Using resveratrol synthase and stilbene synthase as examples, it could be shown that pharmacological manipulation of microtubules could induce gene expression in the absence of elicitor. These findings are discussed with respect to a role for microtubules as positive regulators of defence-induced gene expression.

Roles of calcineurin B-like protein-interacting protein kinases in innate immunity in rice

Cytosolic free Ca(2+) mobilization induced by microbe/pathogen-associated molecular patterns (MAMPs/PAMPs) play key roles in plant innate immunity. However, components involved in Ca(2+) signaling pathways still remain to be identified and possible involvement of the CBL (calcineurin B-like proteins)-CIPK (CBL-interacting protein kinases) system in biotic defense signaling has yet to be clarified. Recently we identified two CIPKs, OsCIPK14 and OsCIPK15, which are rapidly induced by MAMPs, involved in various MAMP-induced immune responses including defense-related gene expression, phytoalexin biosynthesis and hypersensitive cell death. MAMP-induced production of reactive oxygen species as well as cell browning were also suppressed in OsCIPK14/15-RNAi transgenic cell lines. Possible molecular mechanisms and physiological functions of the CIPKs in plant innate immunity are discussed.

A kiss of death--proteasome-mediated membrane fusion and programmed cell death in plant defense against bacterial infection

Eukaryotes have evolved various means for controlled and organized cellular destruction, known as programmed cell death (PCD). In plants, PCD is a crucial regulatory mechanism in multiple physiological processes, including terminal differentiation, senescence, and disease resistance. In this issue of Genes & Development, Hatsugai and colleagues (pp. 2496-2506) demonstrate a novel plant defense strategy to trigger bacteria-induced PCD, involving proteasome-dependent tonoplast and plasma membrane fusion followed by discharge of vacuolar antimicrobial and death-inducing contents into the apoplast.

Dynamics of vacuoles and actin filaments in guard cells and their roles in stomatal movement

Vacuoles and actin filaments are important cytoarchitectures involved in guard cell function. The changes in the morphology and number of vacuoles and the regulation of ion channel activity in tonoplast of guard cells are essential for stomatal movement. A number of studies have investigated the regulation of ion channels in animal and plant cells; however, little is known about the regulating mechanism for vacuolar dynamics in stomatal movement. Actin filaments of guard cells are remodelling with the changes in the stomatal aperture; however, the dynamic functions of actin filaments in stomatal movement remain elusive. In this paper, we summarize the recent developments in the understanding of the dynamics of actin filaments and vacuoles of guard cells during stomatal movement. All relevant studies suggest that actin filaments might be involved in stomatal movement by regulating vacuolar dynamics and the ion channels in tonoplast. The future study could be focused on the linker protein mediating the interaction between actin filaments and tonoplast, which will provide insights into the interactive function of actin and vacuole in stomatal movement regulation.

Chapter 3. New insights into plant vacuolar structure and dynamics

The plant vacuole is a multifunctional organelle and is essential for plant development and growth. The most distinctive feature of the plant vacuole is its size, which usually occupies over 80-90% of the cell volume in well-developed somatic cells, and is therefore highly involved in cell growth and plant body size. Recent progress in the visualization of the vacuole, together with developments in image analysis, has revealed the highly organized and complex morphology of the vacuole, as well as its dynamics. The plant vacuolar membrane (VM) forms not only a typically large vacuole but also other structures, such as tubular structures, transvacuolar strands, bulbs, and sheets. In higher plant cells, actin microfilaments are mainly located near the VM and are involved in vacuolar shape changes with the actin-myosin systems. Most recently, microtubule-dependent regulation of vacuolar structures in moss plant cells was reported, suggesting a diversity of mechanisms regulating vacuolar morphogenesis.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Vacuolar and cytoskeletal dynamics during elicitor-induced programmed cell death in tobacco BY-2 cells

Responses of plant cells to environmental stresses often involve morphological changes, differentiation and redistribution of various organelles and cytoskeletal network. Tobacco BY-2 cells provide excellent model system for in vivo imaging of these intracellular events. Treatment of the cell cycle-synchronized BY-2 cells with a proteinaceous oomycete elicitor, cryptogein, induces highly synchronous programmed cell death (PCD) and provide a model system to characterize vacuolar and cytoskeletal dynamics during the PCD. Sequential observation revealed dynamic reorganization of the vacuole and actin microfilaments during the execution of the PCD. We further characterized the effects cryptogein on mitotic microtubule organization in cell cycle-synchronized cells. Cryptogein treatment at S phase inhibited formation of the preprophase band, a cortical microtubule band that predicts the cell division site. Cortical microtubules kept their random orientation till their disruption that gradually occurred during the execution of the PCD twelve hours after the cryptogein treatment. Possible molecular mechanisms and physiological roles of the dynamic behavior of the organelles and cytoskeletal network in the pathogenic signal-induced PCD are discussed.

Rapid and dynamic subcellular reorganization following mechanical stimulation of Arabidopsis epidermal cells mimics responses to fungal and oomycete attack

BACKGROUND

Plant cells respond to the presence of potential fungal or oomycete pathogens by mounting a basal defence response that involves aggregation of cytoplasm, reorganization of cytoskeletal, endomembrane and other cell components and development of cell wall appositions beneath the infection site. This response is induced by non-adapted, avirulent and virulent pathogens alike, and in the majority of cases achieves penetration resistance against the microorganism on the plant surface. To explore the nature of signals that trigger this subcellular response and to determine the timing of its induction, we have monitored the reorganization of GFP-tagged actin, microtubules, endoplasmic reticulum (ER) and peroxisomes in Arabidopsis plants - after touching the epidermal surface with a microneedle.

RESULTS

Within 3 to 5 minutes of touching the surface of Arabidopsis cotyledon epidermal cells with fine glass or tungsten needles, actin microfilaments, ER and peroxisomes began to accumulate beneath the point of contact with the needle. Formation of a dense patch of actin was followed by focusing of actin cables on the site of contact. Touching the cell surface induced localized depolymerization of microtubules to form a microtubule-depleted zone surrounding a dense patch of GFP-tubulin beneath the needle tip. The concentration of actin, GFP-tubulin, ER and peroxisomes remained focused on the contact site as the needle moved across the cell surface and quickly dispersed when the needle was removed.

CONCLUSION

Our results show that plant cells can detect the gentle pressure of a microneedle on the epidermal cell surface and respond by reorganizing subcellular components in a manner similar to that induced during attack by potential fungal or oomycete pathogens. The results of our study indicate that during plant-pathogen interactions, the basal defence response may be induced by the plant's perception of the physical force exerted by the pathogen as it attempts to invade the epidermal cell surface.