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

The boundary of life and death: changes in mitochondrial and cytosolic proteomes associated with programmed cell death of Arabidopsis thaliana suspension culture cells

Introduction

Despite the critical role of programmed cell death (PCD) in plant development and defense responses, its regulation is not fully understood. It has been proposed that mitochondria may be important in the control of the early stages of plant PCD, but the details of this regulation are currently unknown.

Methods

We used Arabidopsis thaliana cell suspension culture, a model system that enables induction and precise monitoring of PCD rates, as well as chemical manipulation of this process to generate a quantitative profile of the alterations in mitochondrial and cytosolic proteomes associated with early stages of plant PCD induced by heat stress. The cells were subjected to PCD-inducing heat levels (10 min, 54°C), with/without the calcium channel inhibitor and PCD blocker LaCl3. The stress treatment was followed by separation of cytosolic and mitochondrial fractions and mass spectrometry-based proteome analysis.

Results

Heat stress induced rapid and extensive changes in protein abundance in both fractions, with release of mitochondrial proteins into the cytosol upon PCD induction. In our system, LaCl3 appeared to act downstream of cell death initiation signal, as it did not affect the release of mitochondrial proteins, but instead partially inhibited changes occurring in the cytosolic fraction, including upregulation of proteins with hydrolytic activity.

Discussion

We characterized changes in protein abundance and localization associated with the early stages of heat stress-induced PCD. Collectively, the generated data provide new insights into the regulation of cell death and survival decisions in plant cells.

Plant programmed cell death meets auxin signalling

Both auxin signalling and programmed cell death (PCD) are essential components of a normally functioning plant. Auxin underpins plant growth and development, as well as regulating plant defences against environmental stresses. PCD, a genetically controlled pathway for selective elimination of redundant, damaged or infected cells, is also a key element of many developmental processes and stress response mechanisms in plants. An increasing body of evidence suggests that auxin signalling and PCD regulation are often connected. While generally auxin appears to suppress cell death, it has also been shown to promote PCD events, most likely via stimulation of ethylene biosynthesis. Intriguingly, certain cells undergoing PCD have also been suggested to control the distribution of auxin in plant tissues, by either releasing a burst of auxin or creating an anatomical barrier to auxin transport and distribution. These recent findings indicate novel roles of localized PCD events in the context of plant development such as control of root architecture, or tissue regeneration following injury, and suggest exciting possibilities for incorporation of this knowledge into crop improvement strategies.

RNA-Seq analysis reveals potential regulators of programmed cell death and leaf remodelling in lace plant (Aponogeton madagascariensis)

BACKGROUND

The lace plant (Aponogeton madagascariensis) is an aquatic monocot that develops leaves with uniquely formed perforations through the use of a developmentally regulated process called programmed cell death (PCD). The process of perforation formation in lace plant leaves is subdivided into several developmental stages: pre-perforation, window, perforation formation, perforation expansion and mature. The first three emerging "imperforate leaves" do not form perforations, while all subsequent leaves form perforations via developmentally regulated PCD. PCD is active in cells called "PCD cells" that do not retain the antioxidant anthocyanin in spaces called areoles framed by the leaf veins of window stage leaves. Cells near the veins called "NPCD cells" retain a red pigmentation from anthocyanin and do not undergo PCD. While the cellular changes that occur during PCD are well studied, the gene expression patterns underlying these changes and driving PCD during leaf morphogenesis are mostly unknown. We sought to characterize differentially expressed genes (DEGs) that mediate lace plant leaf remodelling and PCD. This was achieved performing gene expression analysis using transcriptomics and comparing DEGs among different stages of leaf development, and between NPCD and PCD cells isolated by laser capture microdissection.

RESULTS

Transcriptomes were sequenced from imperforate, pre-perforation, window, and mature leaf stages, as well as PCD and NPCD cells isolated from window stage leaves. Differential expression analysis of the data revealed distinct gene expression profiles: pre-perforation and window stage leaves were characterized by higher expression of genes involved in anthocyanin biosynthesis, plant proteases, expansins, and autophagy-related genes. Mature and imperforate leaves upregulated genes associated with chlorophyll development, photosynthesis, and negative regulators of PCD. PCD cells were found to have a higher expression of genes involved with ethylene biosynthesis, brassinosteroid biosynthesis, and hydrolase activity whereas NPCD cells possessed higher expression of auxin transport, auxin signalling, aspartyl proteases, cysteine protease, Bag5, and anthocyanin biosynthesis enzymes.

CONCLUSIONS

RNA sequencing was used to generate a de novo transcriptome for A. madagascariensis leaves and revealed numerous DEGs potentially involved in PCD and leaf remodelling. The data generated from this investigation will be useful for future experiments on lace plant leaf development and PCD in planta.

Caspase-like proteases and the phytohormone cytokinin as determinants of S-RNAse-based self-incompatibility-induced PCD in Petunia hybrida L

S-RNAse-based self-incompatibility (SI) in petunia (Petunia hybrida L.) is a self-/non-self-recognition system underlying the pistil rejection of self-pollen. Using different methods, including a TUNEL assay, we have recently shown that programmed cell death (PCD) is a factor of the SI in petunia. Here, we show that the growth of self-incompatible pollen tubes in the style tissues during 4 h after pollination is accompanied by five-sixfold increase in a caspase-like protease (CLP) activity. Exogenous cytokinin (CK) inhibits the pollen tube growth and stimulates the CLP activity in compatible pollen tubes. The actin depolymerization with latrunculin B induces a sharp drop in the CLP activity in self-incompatible pollen tubes and its increase in compatible pollen tubes. Altogether, our results suggest that a CLP is involved in the SI-induced PCD and that CK is a putative activator of the CLP. We assume that CK provokes acidification of the cytosol and thus promotes the activation of a CLP. Thus, our results suggest that CK and CLP are involved in the S-RNAse-based SI-induced PCD in petunia. Potential relations between these components in PCD signaling are discussed.

Cellular oxidative stress in programmed cell death: focusing on chloroplastic 1O2 and mitochondrial cytochrome-c release

The programmed cell death (PCD) occurs when the targeted cells have fulfilled their task or under conditions as oxidative stress generated by ROS species. Thus, plants have to deal with the singlet oxygen ¹O₂ produced in chloroplasts. ¹O₂ is unlikely to act as a primary retrograde signal owing to its high reactivity and short half-life. In addition to its high toxicity, the ¹O₂ generated under an excess or low excitation energy might also act as a highly versatile signal triggering chloroplast-to-nucleus retrograde signaling (ChNRS) and nuclear reprogramming or cell death. Molecular and biochemical studies with the flu mutant, which accumulates protochlorophyllide in the dark, demonstrated that chloroplastic ¹O₂-driven EXECUTER-1 (EX1) and EX2 proteins are involved in the ¹O₂-dependent response. Both EX1 and EX2 are necessary for full suppression of ¹O₂-induced gene expression. That is, EXECUTER proteolysis via the ATP-dependent zinc protease (FtsH) is an integral part of ¹O₂-triggered retrograde signaling. The existence of at least two independent ChNRS involving EX1 and β-cyclocitral, and dihydroactinidiolide and OXI1, respectively, seem clear. Besides, this update also focuses on plant PCD and its relation with mitochondrial cytochrome-c (Cytc) release to cytosol. Changes in the dynamics and morphology of mitochondria were shown during the onset of cell death. The mitochondrial damage and translocation of Cytc may be one of the major causes of PCD triggering. Together, this current overview illustrates the complexity of the cellular response to oxidative stress development. A puzzle with the majority of its pieces still not placed.

Autophagosome Biogenesis in Plants: An Actin Cytoskeleton Perspective

At the subcellular level, the cytoskeleton regulates cell structure, organelle movement, and cytoplasmic streaming. Autophagy is a process to remove unwanted biomaterials or damaged organelles through double membrane compartments known as autophagosomes. Autophagosome biogenesis requires vesicle trafficking between donor and acceptor compartments, membrane expansion, and fusion, which is very likely to be regulated by the cytoskeleton. Recent studies have demonstrated that by knocking out key actin-regulating proteins, autophagosome biogenesis is inhibited. However, the formation of ATG8 positive structures are not affected when the entire actin network is disrupted. Here, we discuss this paradox and propose the function of the actin cytoskeleton in plant autophagy.

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.

Rosmarinic acid inhibits programmed cell death in Solanum tuberosum L. calli under high salinity

Oxidative stress induced by salinity is a prime cause of cell death when Na+ toxicity becomes unbearable. We explored the effect of rosmarinic acid (RA) on the Solanum tuberosum L. cv. Desiree calli against salt-induced programmed cell death (PCD). We showed that PCD events were triggered in calli under 250 mM NaCl by the loss of plasma membrane integrity as measured by the amount of malondialdehyde (MDA) in the cytoplasm, the degree of DNA degradation resulting from the cleavage of nuclear DNA into oligonucleosomal fragments in apoptotic cells, the presence of TUNEL-positive nuclei (90 ± 0.005%) damage in genomic DNA, and activation of caspase 3-like protease. Callus Formation Medium (CFM) supplemented with RA led to the suppression of salt-induced cell death and a dramatic decrease in the MDA level and frequency of TUNEL-positive nuclei under salinity to 4 ± and 7.3 ± % in the presence of 50 and 350 μM RA, respectively. The application of RA also resulted in an increase in GSH content and maintenance of a high GSH/GSSG ratio. Interestingly, these reductions in PCD were accompanied by inhibiting caspase 3-like protease activities due to RA under salinity. Molecular docking predicted high binding energies of RA for binding to subtilisin-like protease (StSCTc-3), which has caspase-3 like activity in Solanum tuberosum, near the active site. This finding supports the notion of a role for RA in PCD protection in plants, which is consistent with earlier reports in animal cells.

Hsp70 plays a role in programmed cell death during the remodelling of leaves of the lace plant (Aponogeton madagascariensis)

Lace plant leaves utilize programmed cell death (PCD) to form perforations during development. The role of heat shock proteins (Hsps) in PCD during lace plant leaf development is currently unknown. Hsp70 amounts were measured throughout lace plant leaf development, and the results indicate that it is highest before and during PCD. Increased Hsp70 amounts correlate with raised anthocyanin content and caspase-like protease (CLP) activity. To investigate the effects of Hsp70 on leaf development, whole plants were treated with either of the known regulators of PCD [reactive oxygen species (ROS) or antioxidants] or an Hsp70 inhibitor, chlorophenylethynylsulfonamide (PES-Cl). ROS treatment significantly increased Hsp70 2-fold and CLP activity in early developing leaves, but no change in anthocyanin and the number of perforations formed was observed. Antioxidant treatment significantly decreased Hsp70, anthocyanin, and CLP activity in early leaves, resulting in the fewest perforations. PES-Cl (25 μM) treatment significantly increased Hsp70 4-fold in early leaves, while anthocyanin, superoxide, and CLP activity significantly declined, leading to fewer perforations. Results show that significantly increased (4-fold) or decreased Hsp70 amounts lead to lower anthocyanin and CLP activity, inhibiting PCD induction. Our data support the hypothesis that Hsp70 plays a role in regulating PCD at a threshold in lace plant leaf development. Hsp70 affects anthocyanin content and caspase-like protease activity, and helps regulate PCD during the remodelling of leaves of lace plant, Aponogeton madagascariensis.

Plant proteases during developmental programmed cell death

Proteases are among the key regulators of most forms of programmed cell death (PCD) in animals. Many PCD processes have also been associated with protease expression or activation in plants, However, functional evidence for the roles and actual modes of action of plant proteases in PCD remains surprisingly limited. In this review, we provide an update on protease involvement in the context of developmentally regulated plant PCD. To illustrate the diversity of protease functions, we focus on several prominent developmental PCD processes, including xylem and tapetum maturation, suspensor elimination, endosperm degradation, and seed coat formation, as well as plant senescence processes. Despite the substantial advances in the field, protease functions are often only correlatively linked to developmental PCD, and the specific molecular roles of proteases in many developmental PCD processes remain to be elucidated.

Vacuolar processing enzymes, AmVPE1 and AmVPE2, as potential executors of ethylene regulated programmed cell death in the lace plant (Aponogeton madagascariensis)

Perforation formation in Aponogeton madagascariensis (Mirb.) H.Bruggen (lace plant) is an excellent model for studying developmentally regulated programmed cell death (PCD). In this study, we isolated and identified two lace plant vacuolar processing enzymes (VPEs) and investigated their involvement in PCD and throughout leaf development. Lace plant VPE transcript levels were determined during seven different stages of leaf development. PCD and non-PCD cells from “window” stage leaves (in which perforations are forming) were separated through laser-capture microscopy and their transcript levels were also determined. VPE activity was also studied between the cell types, through a VPE activity-based probe JOPD1. Additionally, VPE transcript levels were studied in plants treated with an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG). The two isolated VPEs, AmVPE1 and AmVPE2, are vegetative type VPEs. AmVPE1 had higher transcript levels during a pre-perforation developmental stage, immediately prior to visible signs of PCD. AmVPE2 transcript levels were higher later during window and late window stages. Both VPEs had higher transcript and activity levels in PCD compared with the non-PCD cells. AVG treatment inhibited PCD and associated increases in VPE transcript levels. Our results suggested that VPEs are involved in the execution of the ethylene-related PCD in the lace plant.

RNAseq revealed the important gene pathways controlling adaptive mechanisms under waterlogged stress in maize

Waterlogging causes yield penalty in maize-growing countries of subtropical regions. Transcriptome analysis of the roots of a tolerant inbred HKI1105 using RNA sequencing revealed 21,364 differentially expressed genes (DEGs) under waterlogged stress condition. These 21,364 DEGs are known to regulate important pathways including energy-production, programmed cell death (PCD), aerenchyma formation, and ethylene responsiveness. High up-regulation of invertase (49-fold) and hexokinase (36-fold) in roots explained the ATP requirement in waterlogging condition. Also, high up-regulation of expansins (42-fold), plant aspartic protease A3 (19-fold), polygalacturonases (16-fold), respiratory burst oxidase homolog (12-fold), and hydrolases (11-fold) explained the PCD of root cortical cells followed by the formation of aerenchyma tissue during waterlogging stress. We hypothesized that the oxygen transfer in waterlogged roots is promoted by a cross-talk of fermentative, metabolic, and glycolytic pathways that generate ATPs for PCD and aerenchyma formation in root cortical cells. SNPs were mapped to the DEGs regulating aerenchyma formation (12), ethylene-responsive factors (11), and glycolysis (4) under stress. RNAseq derived SNPs can be used in selection approaches to breed tolerant hybrids. Overall, this investigation provided significant evidence of genes operating in the adaptive traits such as ethylene production and aerenchyma formation to cope-up the waterlogging stress.

Comparative transcriptomics provide insight into the morphogenesis and evolution of fistular leaves in Allium

BACKGROUND

Fistular leaves frequently appear in Allium species, and previous developmental studies have proposed that the process of fistular leaf formation involves programmed cell death. However, molecular evidence for the role of programmed cell death in the formation of fistular leaf cavities has yet to be reported.

RESULTS

In this study, we characterized the leaf transcriptomes of nine Allium species, including six fistular- and three solid-leaved species. In addition, we identified orthologous genes and estimated their Ka and Ks values, in order to ascertain their selective pattern. Phylogenetic analysis based on the transcriptomes revealed that A. tuberosum was the most ancestral among the nine species, and analysis of orthologous genes between A. tuberosum and the other eight species indicated that 149 genes were subject to positive selection; whereas >3000 had undergone purifying selection in each species.

CONCLUSIONS

We found that many genes that are potentially related to programmed cell death either exhibited rapid diversification in fistular-leaved species, or were conserved in solid-leaved species in evolutionary history. These genes potentially involved in programmed cell death might play important roles in the formation of fistular leaf cavities in Allium, and the differing selection patterns in fistular- and solid-leaved species may be responsible for the evolution of fistular leaves.

In vivo and in vitro maturation of rabbit oocytes differently affects the gene expression profile, mitochondrial distribution, apoptosis and early embryo development

In vivo-matured cumulus–oocyte complexes are valuable models in which to assess potential biomarkers of rabbit oocyte quality that contribute to enhanced IVM systems. In the present study we compared some gene markers of oocytes and cumulus cells (CCs) from immature, in vivo-matured and IVM oocytes. Moreover, apoptosis in CCs, nuclear maturation, mitochondrial reallocation and the developmental potential of oocytes after IVF were assessed. In relation to cumulus expansion, gene expression of gap junction protein, alpha 1, 43 kDa (Gja1) and prostaglandin-endoperoxide synthase 2 (Ptgs2) was significantly lower in CCs after in vivo maturation than IVM. In addition, there were differences in gene expression after in vivo maturation versus IVM in both oocytes and CCs for genes related to cell cycle regulation and apoptosis (V-Akt murine thymoma viral oncogene homologue 1 (Akt1), tumour protein 53 (Tp53), caspase 3, apoptosis-related cysteine protease (Casp3)), oxidative response (superoxide dismutase 2, mitochondrial (Sod2)) and metabolism (glucose-6-phosphate dehydrogenase (G6pd), glyceraldehyde-3-phosphate dehydrogenase (Gapdh)). In vivo-matured CCs had a lower apoptosis rate than IVM and immature CCs. Meiotic progression, mitochondrial migration to the periphery and developmental competence were higher for in vivo-matured than IVM oocytes. In conclusion, differences in oocyte developmental capacity after IVM or in vivo maturation are accompanied by significant changes in transcript abundance in oocytes and their surrounding CCs, meiotic rate, mitochondrial distribution and apoptotic index. Some of the genes investigated, such as Gja1, could be potential biomarkers for oocyte developmental competence in the rabbit model, helping improve in vitro culture systems in these species.

Flavonoids and darkness lower PCD in senescing Vitis vinifera suspension cell cultures

BACKGROUND

Senescence is a key developmental process occurring during the life cycle of plants that can be induced also by environmental conditions, such as starvation and/or darkness. During senescence, strict control of genes regulates ordered degradation and dismantling events, the most remarkable of which are genetically programmed cell death (PCD) and, in most cases, an upregulation of flavonoid biosynthesis in the presence of light. Flavonoids are secondary metabolites that play multiple essential roles in development, reproduction and defence of plants, partly due to their well-known antioxidant properties, which could affect also the same cell death machinery. To understand further the effect of endogenously-produced flavonoids and their interplay with different environment (light or dark) conditions, two portions (red and green) of a senescing grapevine callus were used to obtain suspension cell cultures. Red Suspension cell Cultures (RSC) and Green Suspension cell Cultures (GSC) were finally grown under either dark or light conditions for 6 days.

RESULTS

Darkness enhanced cell death (mainly necrosis) in suspension cell culture, when compared to those grown under light condition. Furthermore, RSC with high flavonoid content showed a higher viability compared to GSC and were more protected toward PCD, in accordance to their high content in flavonoids, which might quench ROS, thus limiting the relative signalling cascade. Conversely, PCD was mainly occurring in GSC and further increased by light, as it was shown by cytochrome c release and TUNEL assays.

CONCLUSIONS

Endogenous flavonoids were shown to be good candidates for exploiting an efficient protection against oxidative stress and PCD induction. Light seemed to be an important environmental factor able to induce PCD, especially in GSC, which lacking of flavonoids were not capable of preventing oxidative damage and signalling leading to senescence.

Cytochrome c, a hub linking energy, redox, stress and signaling pathways in mitochondria and other cell compartments

Cytochrome c (CYTc) is a soluble redox-active heme protein that transfers electrons from complex III to complex IV in the cyanide-sensitive mitochondrial respiratory pathway. CYTc biogenesis is a complex process that requires multiple steps until the mature active protein is obtained. CYTc levels and activity are finely regulated, revealing the importance of this protein not only as electron carrier but also in many other processes. In this article, we describe the role of CYTc in mitochondrial respiration, from its canonical role as electron carrier for ATP production to its involvement in protein import and the stabilization of respiratory complexes and supercomplexes. In plants, CYTc is connected to the synthesis of the antioxidant ascorbate and the detoxification of toxic compounds. Finally, CYTc is also a multi-functional signaling molecule that influences the balance between life and death, acting in energy provision for cellular functions or triggering programmed cell death. The confluence of several metabolic routes into a single protein that links redox reactions with energy producing pathways seems logical from the point of view of cellular economy, control and organization.

Functions and Regulation of Programmed Cell Death in Plant Development

Programmed cell death (PCD) is a collective term for diverse processes causing an actively induced, tightly controlled cellular suicide. PCD has a multitude of functions in the development and health of multicellular organisms. In comparison to intensively studied forms of animal PCD such as apoptosis, our knowledge of the regulation of PCD in plants remains limited. Despite the importance of PCD in plant development and as a response to biotic and abiotic stresses, the complex molecular networks controlling different forms of plant PCD are only just beginning to emerge. With this review, we provide an update on the considerable progress that has been made over the last decade in our understanding of PCD as an inherent part of plant development. We highlight both functions of developmental PCD and central aspects of its molecular regulation.

Licensed to Kill: Mitochondria, Chloroplasts, and Cell Death

Programmed cell death (PCD) is crucial in plant organogenesis and survival. In this review the involvement of mitochondria and chloroplasts in PCD execution is critically assessed. Recent findings support a central role for mitochondria in PCD, with newly identified components of the mitochondrial electron transport chain (mETC), FOF1 ATP synthase, cardiolipins, and ATPase AtOM66. While chloroplasts received less attention, their contribution to PCD is well supported, suggesting that they possibly contribute by producing reactive oxygen species (ROS) in the presence of light or even contribute through cytochrome f release. Finally we discuss two working models where mitochondria and chloroplasts could cooperatively execute PCD: mitochondria initiate the commitment steps and recruit chloroplasts for swift execution or, alternatively, mitochondria and chloroplasts could operate in parallel.

Only in dying, life: programmed cell death during plant development

Programmed cell death (PCD) is a fundamental process of life. During the evolution of multicellular organisms, the actively controlled demise of cells has been recruited to fulfil a multitude of functions in development, differentiation, tissue homeostasis, and immune systems. In this review we discuss some of the multiple cases of PCD that occur as integral parts of plant development in a remarkable variety of cell types, tissues, and organs. Although research in the last decade has discovered a number of PCD regulators, mediators, and executers, we are still only beginning to understand the mechanistic complexity that tightly controls preparation, initiation, and execution of PCD as a process that is indispensable for successful vegetative and reproductive development of plants.

The Permeability Transition in Plant Mitochondria: The Missing Link

The synthesis of ATP in mitochondria is dependent on a low permeability of the inner membrane. Nevertheless, mitochondria can undergo an increased permeability to solutes, named permeability transition (PT) that is mediated by a permeability transition pore (PTP). PTP opening requires matrix Ca(2+) and leads to mitochondrial swelling and release of intramembrane space proteins (e.g., cytochrome c). This feature has been initially observed in mammalian mitochondria and tentatively attributed to some components present either in the outer or inner membrane. Recent works on mammalian mitochondria point to mitochondrial ATP synthase dimers as physical basis for PT, a finding that has been substantiated in yeast and Drosophila mitochondria. In plant mitochondria, swelling and release of proteins have been linked to programmed cell death, but in isolated mitochondria PT has been observed in only a few cases and in plant cell cultures only indirect evidence is available. The possibility that mitochondrial ATP synthase dimers could function as PTP also in plants is discussed here on the basis of the current evidence. Finally, a hypothetical explanation for the origin of PTP is provided in the framework of molecular exaptation.