A unique vacuolar processing enzyme responsible for conversion of several proprotein precursors into the mature forms

Seed storage albumins: biosynthesis, trafficking and structures

Seed storage albumins are water-soluble and highly abundant proteins that are broken-down during seed germination to provide nitrogen and sulfur for the developing seedling. During seed maturation these proteins are subject to post-translational modifications and trafficking before they are deposited in great quantity and with great stability in dedicated vacuoles. This review will cover the subcellular movement, biochemical processing and mature structures of seed storage napins.

An asparagine residue at the N-terminus affects the maturation process of low molecular weight glutenin subunits of wheat endosperm

BACKGROUND

Wheat glutenin polymers are made up of two main subunit types, the high- (HMW-GS) and low- (LMW-GS) molecular weight subunits. These latter are represented by heterogeneous proteins. The most common, based on the first amino acid of the mature sequence, are known as LMW-m and LMW-s types. The mature sequences differ as a consequence of three extra amino acids (MET-) at the N-terminus of LMW-m types. The nucleotide sequences of their encoding genes are, however, nearly identical, so that the relationship between gene and protein sequences is difficult to ascertain.It has been hypothesized that the presence of an asparagine residue in position 23 of the complete coding sequence for the LMW-s type might account for the observed three-residue shortened sequence, as a consequence of cleavage at the asparagine by an asparaginyl endopeptidase.

RESULTS

We performed site-directed mutagenesis of a LMW-s gene to replace asparagine at position 23 with threonine and thus convert it to a candidate LMW-m type gene. Similarly, a candidate LMW-m type gene was mutated at position 23 to replace threonine with asparagine. Next, we produced transgenic durum wheat (cultivar Svevo) lines by introducing the mutated versions of the LMW-m and LMW-s genes, along with the wild type counterpart of the LMW-m gene.Proteomic comparisons between the transgenic and null segregant plants enabled identification of transgenic proteins by mass spectrometry analyses and Edman N-terminal sequencing.

CONCLUSIONS

Our results show that the formation of LMW-s type relies on the presence of an asparagine residue close to the N-terminus generated by signal peptide cleavage, and that LMW-GS can be quantitatively processed most likely by vacuolar asparaginyl endoproteases, suggesting that those accumulated in the vacuole are not sequestered into stable aggregates that would hinder the action of proteolytic enzymes. Rather, whatever is the mechanism of glutenin polymer transport to the vacuole, the proteins remain available for proteolytic processing, and can be converted to the mature form by the removal of a short N-terminal sequence.

Uncovering legumain genes in rice

Legumains are Asn specific cysteine proteases physiologically related to the biosynthesis of vacuolar components, degradation of storage proteins and programmed cell death. The present work identifies and characterizes the genic family of legumains in rice (Oryza sativa), which comprises five different loci. Rice legumains (OsaLegs) were ubiquitously detected in all plant tissues analyzed. However, phylogenetic analyses and gene expression studies demonstrated greater association of OsaLeg2 and OsaLeg3 to seed-related legumains, whereas OsaLeg1, 4 and 5 would act as vegetative-related proteases. Additionally, OsaLeg1 mRNA is strongly induced in senescent leaves. All rice legumain genes respond in different ways to environmental conditions such as wounding, salt and abscisic acid treatments. Mainly, wounding is capable of inducing all the four expressed genes OsaLeg1, 2, 3 and 4. Alternative splicing isoforms, with potential to generate pre-activated OsaLeg1 and OsaLeg2 nonvacuolar enzymes under different environmental situations were also observed.

GmNAC30 and GmNAC81 integrate the endoplasmic reticulum stress- and osmotic stress-induced cell death responses through a vacuolar processing enzyme

Prolonged endoplasmic reticulum and osmotic stress synergistically activate the stress-induced N-rich protein-mediated signaling that transduces a cell death signal by inducing GmNAC81 (GmNAC6) in soybean. To identify novel regulators of the stress-induced programmed cell death (PCD) response, we screened a two-hybrid library for partners of GmNAC81. We discovered another member of the NAC (NAM-ATAF1,2-CUC2) family, GmNAC30, which binds to GmNAC81 in the nucleus of plant cells to coordinately regulate common target promoters that harbor the core cis-regulatory element TGTG[TGC]. We found that GmNAC81 and GmNAC30 can function either as transcriptional repressors or activators and cooperate to enhance the transcriptional regulation of common target promoters, suggesting that heterodimerization may be required for the full regulation of gene expression. Accordingly, GmNAC81 and GmNAC30 display overlapping expression profiles in response to multiple environmental and developmental stimuli. Consistent with a role in PCD, GmNAC81 and GmNAC30 bind in vivo to and transactivate hydrolytic enzyme promoters in soybean protoplasts. A GmNAC81/GmNAC30 binding site is located in the promoter of the caspase-1-like vacuolar processing enzyme (VPE) gene, which is involved in PCD in plants. We demonstrated that the expression of GmNAC81 and GmNAC30 fully transactivates the VPE gene in soybean protoplasts and that this transactivation was associated with an increase in caspase-1-like activity. Collectively, our results indicate that the stress-induced GmNAC30 cooperates with GmNAC81 to activate PCD through the induction of the cell death executioner VPE.

Oxidative protein-folding systems in plant cells

Plants are unique among eukaryotes in having evolved organelles: the protein storage vacuole, protein body, and chloroplast. Disulfide transfer pathways that function in the endoplasmic reticulum (ER) and chloroplasts of plants play critical roles in the development of protein storage organelles and the biogenesis of chloroplasts, respectively. Disulfide bond formation requires the cooperative function of disulfide-generating enzymes (e.g., ER oxidoreductase 1), which generate disulfide bonds de novo, and disulfide carrier proteins (e.g., protein disulfide isomerase), which transfer disulfides to substrates by means of thiol-disulfide exchange reactions. Selective molecular communication between disulfide-generating enzymes and disulfide carrier proteins, which reflects the molecular and structural diversity of disulfide carrier proteins, is key to the efficient transfer of disulfides to specific sets of substrates. This review focuses on recent advances in our understanding of the mechanisms and functions of the various disulfide transfer pathways involved in oxidative protein folding in the ER, chloroplasts, and mitochondria of plants.

TaMYB13-1, a R2R3 MYB transcription factor, regulates the fructan synthetic pathway and contributes to enhanced fructan accumulation in bread wheat

Fructans are the major component of temporary carbon reserve in the stem of temperate cereals, which is used for grain filling. Three families of fructosyltransferases are directly involved in fructan synthesis in the vacuole of Triticum aestivum. The regulatory network of the fructan synthetic pathway is largely unknown. Recently, a sucrose-upregulated wheat MYB transcription factor (TaMYB13-1) was shown to be capable of activating the promoter activities of sucrose:sucrose 1-fructosyltransferase (1-SST) and sucrose:fructan 6-fructosyltransferase (6-SFT) in transient transactivation assays. This work investigated TaMYB13-1 target genes and their influence on fructan synthesis in transgenic wheat. TaMYB13-1 overexpression resulted in upregulation of all three families of fructosyltransferases including fructan:fructan 1-fructosyltransferase (1-FFT). A γ-vacuolar processing enzyme (γ-VPE1), potentially involved in processing the maturation of fructosyltransferases in the vacuole, was also upregulated by TaMYB13-1 overexpression. Multiple TaMYB13 DNA-binding motifs were identified in the Ta1-FFT1 and Taγ-VPE1 promoters and were bound strongly by TaMYB13-1. The expression profiles of these target genes and TaMYB13-1 were highly correlated in recombinant inbred lines and during stem development as well as the transgenic and non-transgenic wheat dataset, further supporting a direct regulation of these genes by TaMYB13-1. TaMYB13-1 overexpression in wheat led to enhanced fructan accumulation in the leaves and stems and also increased spike weight and grain weight per spike in transgenic plants under water-limited conditions. These data suggest that TaMYB13-1 plays an important role in coordinated upregulation of genes necessary for fructan synthesis and can be used as a molecular tool to improve the high fructan trait.

A vacuolar processing enzyme RsVPE1 gene of radish is involved in floral bud abortion under heat stress

Radish floral bud abortion (FBA) is an adverse biological phenomenon that occurs during reproduction. Although FBA is a frequent occurrence, its molecular mechanism remains unknown. A transcript-derived fragment (TDF72), which was obtained by cDNA amplified fragment length polymorphism (cDNA-AFLP), was up-regulated in the aborted buds and exhibited 89% sequence homology with the AtγVPE gene. In this study, TDF72 was used to clarify the role of VPE in FBA by isolation of the VPE gene RsVPE1 from radish flower buds. The full-length genomic DNA was 2346 bp including nine exons and eight introns. The full-length cDNA was 1825 bp, containing a complete open reading frame (ORF) of 1470 bp, which encoded a predicted protein containing 489 amino acid residues, with a calculated molecular mass of 53.735 kDa. Expression analysis demonstrated that RsVPE1 was expressed in all tested organs of radish at different levels. Highest expression was detected in aborted flower buds, suggesting that RsVPE1 has a role in FBA. In order to analyze the role of RsVPE1 in FBA, RsVPE1 was overexpressed in transgenic Arabidopsis thaliana plants. Aborted flower buds appeared in transgenic plants subjected to heat stress. In addition, RsVPE1 expression in the transgenic plants reached a maximum when subjected to heat stress for 24 h and increased by 2.1-fold to 2.8-fold in three homozygous transgenic lines. These results indicated that RsVPE1 led to FBA when its expression levels exceeded a particular threshold, and provided evidence for the involvement of RsVPE1 in promoting FBA under heat stress.

Set-point control of RD21 protease activity by AtSerpin1 controls cell death in Arabidopsis

Programmed cell death (PCD) in plants plays a key role in defense response and is promoted by the release of compartmentalized proteases to the cytoplasm. Yet the exact identity and control of these proteases is poorly understood. Serpins are an important group of proteins that uniquely curb the activity of proteases by irreversible inhibition; however, their role in plants remains obscure. Here we show that during cell death the Arabidopsis serpin protease inhibitor, AtSerpin1, exhibits a pro-survival function by inhibiting its target pro-death protease, RD21. AtSerpin1 accumulates in the cytoplasm and RD21 accumulates in the vacuole and in endoplasmic reticulum bodies. Elicitors of cell death, including the salicylic acid agonist benzothiadiazole and the fungal toxin oxalic acid, stimulated changes in vacuole permeability as measured by the changes in the distribution of marker dye. Concomitantly, a covalent AtSerpin1-RD21 complex was detected indicative of a change in protease compartmentalization. Furthermore, mutant plants lacking RD21 or plants with AtSerpin1 over-expression exhibited significantly less elicitor-stimulated PCD than plants lacking AtSerpin1. The necrotrophic fungi Botrytis cinerea and Sclerotina sclerotiorum secrete oxalic acid as a toxin that stimulates cell death. Consistent with a pro-death function for RD21 protease, the growth of these necrotrophs was compromised in plants lacking RD21 but accelerated in plants lacking AtSerpin1. The results indicate that AtSerpin1 controls the pro-death function of compartmentalized protease RD21 by determining a set-point for its activity and limiting the damage induced during cell death.

Activity profiling of vacuolar processing enzymes reveals a role for VPE during oomycete infection

Vacuolar processing enzymes (VPEs) are important cysteine proteases that are implicated in the maturation of seed storage proteins, and programmed cell death during plant-microbe interactions and development. Here, we introduce a specific, cell-permeable, activity-based probe for VPEs. This probe is highly specific for all four Arabidopsis VPEs, and labeling is activity-dependent, as illustrated by sensitivity for inhibitors, pH and reducing agents. We show that the probe can be used for in vivo imaging and displays multiple active isoforms of VPEs in various tissues and in both monocot and dicot plant species. Thus, VPE activity profiling is a robust, simple and powerful tool for plant research for a wide range of applications. Using VPE activity profiling, we discovered that VPE activity is increased during infection with the oomycete pathogen Hyaloperonospora arabidopsidis (Hpa). The enhanced VPE activity is host-derived and EDS1-independent. Sporulation of Hpa is reduced on vpe mutant plants, demonstrating a role for VPE during compatible interactions that is presumably independent of programmed cell death. Our data indicate that, as an obligate biotroph, Hpa takes advantage of increased VPE activity in the host, e.g. to mediate protein turnover and nutrient release.

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.

Recombinant expression and biochemical characterization of sugarcane legumain

Plant legumains, also termed vacuolar processing enzymes (VPEs), are cysteine peptidases that play key roles in plant development, senescence, programmed cell death and defense against pathogens. Despite the increasing number of reports on plant cysteine peptidases, including VPEs, the characterization of sugarcane VPEs and their inhibition by endogenous cystatins have not yet been described. This is the first report of the biochemical characterization of a sugarcane cysteine peptidase. In this work, a recombinant sugarcane legumain was expressed in Pichia pastoris and characterized. Kinetic studies of the recombinant CaneLEG revealed that this enzyme has the main characteristics of VPEs, such as self-activation and activity under acidic pH. CaneLEG activity was strongly inhibited when incubated with sugarcane cystatin 3 (CaneCPI-3). Quantitative analysis of CaneLEG and CaneCPI-3 gene expression indicated a tissue-specific expression pattern for both genes throughout sugarcane growth, with the strong accumulation of CaneLEG transcripts throughout the internode development. Furthermore, the CaneLEG and CaneCPI-3 genes exhibited up-regulation in plantlets treated with abscisic acid (ABA). These results suggest that CaneCPI-3 may be a potential endogenous inhibitor of CaneLEG and these genes may be involved in plant stress response mediated by ABA. Also, the expression analysis provides clues for the putative involvement of CaneLEG and CaneCPI-3 in sugarcane development and phytohormone response.

MAP Kinase 6-mediated activation of vacuolar processing enzyme modulates heat shock-induced programmed cell death in Arabidopsis

• Vacuolar processing enzyme (VPE), a cysteine protease, has been intensively studied in plant hypersensitive cell death, but the role and molecular mechanism of VPEs in response to abiotic stresses remain unclear. This work investigated the involvement of VPEs in Arabidopsis response to heat stress. • Under heat shock (HS), Arabidopsis VPE activity and the transcript level of γVPE were both upregulated, and γVPE deficiency suppressed vacuolar disruption and delayed caspase-3-like activation in HS-induced programmed cell death (PCD). Moreover, the change of VPE activity generally paralleled the alteration of caspase-1-like activity under HS treatment, indicating that HS-induced VPE activity might exhibit the caspase-1-like activity. • Further studies showed that MAP Kinase 6 (MPK6) activity was increased after HS treatment, and experiments with inhibitors and mutants suggested that MPK6 was responsible for the γVPE activation after HS treatment. In response to HS stress, reactive oxygen species (ROS) production, increase of cytoplasmic calcium concentration ([Ca(2+) ](cyt)) and the upregulation of calmodulin 3 (CaM3) transcript level occurred upstream of MPK6 activation. • Our results suggested that activation of Arabidopsis γVPE was mediated by MPK6 and played an important role in HS-induced Arabidopsis PCD, providing new insight into the mechanistic study of plant VPEs.

Natural substrates of plant proteases: how can protease degradomics extend our knowledge?

Despite the key role of proteolysis in various intensively studied biological processes, such as plant immunity, seed development and abiotic stress responses, our knowledge on the identity of natural protease substrates in plants remains scarce. In the genome of the model plant Arabidopsis thaliana, for instance, approximately 700 genes code for proteases. However, only a few natural substrates have been identified, mainly because of the previous lack of sensitive proteomics technologies enabling the identification of low abundant proteins, together with a delay in the implementation of these technologies in the field of plant research. Here, we review the current knowledge on the identity of natural plant protease substrates and describe recently established degradomics technologies that should allow proteome-wide studies of plant proteases in the near future.

Xylem cell death: emerging understanding of regulation and function

Evolutionary, as well as genetic, evidence suggests that vascular development evolved originally as a cell death programme that allowed enhanced movement of water in the extinct protracheophytes, and that secondary wall formation in the water-conducting cells evolved afterwards, providing mechanical support for effective long-distance transport of water. The extant vascular plants possess a common regulatory network to coordinate the different phases of xylem maturation, including secondary wall formation, cell death, and finally autolysis of the cell contents, by the action of recently identified NAC domain transcription factors. Consequently, xylem cell death is an inseparable part of the xylem maturation programme, making it difficult to uncouple cell death mechanistically from secondary wall formation, and thus identify the key factors specifically involved in regulation of cell death. Current knowledge suggests that the necessary components for xylem cell death are produced early during xylem differentiation, and cell death is prevented through the action of inhibitors and storage of hydrolytic enzymes in inactive forms in compartments such as the vacuole. Bursting of the central vacuole triggers autolytic hydrolysis of the cell contents, which ultimately leads to cell death. This cascade of events varies between the different xylem cell types. The water-transporting tracheary elements rely on a rapid cell death programme, with hydrolysis of cell contents taking place for the most part, if not entirely, after vacuolar bursting, while the xylem fibres disintegrate cellular contents at a slower pace, well before cell death. This review includes a detailed description of cell morphology, function of plant growth regulators, such as ethylene and thermospermine, and the action of hydrolytic nucleases and proteases during cell death of the different xylem cell types.

Activation of legumain involves proteolytic and conformational events, resulting in a context- and substrate-dependent activity profile

Localized mainly to endo/lysosomes, legumain plays an important role in exogenous antigen processing and presentation. The cysteine protease legumain, also known as asparaginyl endopepetidase AEP, is synthesized as a zymogen and is known to undergo pH-dependent autoproteolytic activation whereby N-terminal and C-terminal propeptides are released. However, important mechanistic details of this pH-dependent activation as well as the characteristic pH activity profile remain unclear. Here, it is shown that all but one of the autocatalytic cleavage events occur in trans, with only the release of the C-terminal propeptide being relevant to enzymatic activity. An intriguing super-activation event that appears to be exclusively conformational in nature and enhances the enzymatic activity of proteolytically fully processed legumain by about twofold was also found. Accepting asparagines and, to lesser extent, aspartic acid in P1, super-activated legumain exhibits a marked pH dependence that is governed by the P1 residue of its substrate and conformationally stabilizing factors such as temperature or ligands. The crystallization and preliminary diffraction data analysis of active legumain are presented, which form an important basis for further studies that should clarify fundamental aspects of activation, activity and inactivation of legumain, which is a key target in (auto-)immunity and cancer.

Production of plant proteases in vivo and in vitro--a review

In the latest two decades, the interest received by plant proteases has increased significantly. Plant enzymes such as proteases are widely used in medicine and the food industry. Some proteases, like papain, bromelain and ficin are used in various processes such as brewing, meat softening, milk-clotting, cancer treatment, digestion and viral disorders. These enzymes can be obtained from their natural source or through in vitro cultures, in order to ensure a continuous source of plant enzymes. The focus of this review will be the production of plant proteases both in vivo and in vitro, with particular emphasis on the different types of commercially important plant proteases that have been isolated and characterized from naturally grown plants. In vitro approaches for the production of these proteases is also explored, focusing on the techniques that do not involve genetic transformation of the plants and the attempts that have been made in order to enhance the yield of the desired proteases.

Ricin trafficking in plant and mammalian cells

Ricin is a heterodimeric plant protein that is potently toxic to mammalian and many other eukaryotic cells. It is synthesized and stored in the endosperm cells of maturing Ricinus communis seeds (castor beans). The ricin family has two major members, both, lectins, collectively known as Ricinus communis agglutinin ll (ricin) and Ricinus communis agglutinin l (RCA). These proteins are stored in vacuoles within the endosperm cells of mature Ricinus seeds and they are rapidly broken down by hydrolysis during the early stages of post-germinative growth. Both ricin and RCA traffic within the plant cell from their site of synthesis to the storage vacuoles, and when they intoxicate mammalian cells they traffic from outside the cell to their site of action. In this review we will consider both of these trafficking routes.

The role of vacuole in plant cell death

Almost all plant cells have large vacuoles that contain both hydrolytic enzymes and a variety of defense proteins. Plants use vacuoles and vacuolar contents for programmed cell death (PCD) in two different ways: for a destructive way and for a non-destructive way. Destruction is caused by vacuolar membrane collapse, followed by the release of vacuolar hydrolytic enzymes into the cytosol, resulting in rapid and direct cell death. The destructive way is effective in the digestion of viruses proliferating in the cytosol, in susceptible cell death induced by fungal toxins, and in developmental cell death to generate integuments (seed coats) and tracheary elements. On the other hand, the non-destructive way involves fusion of the vacuolar and the plasma membrane, which allows vacuolar defense proteins to be discharged into the extracellular space where the bacteria proliferate. Membrane fusion, which is normally suppressed, was triggered in a proteasome-dependent manner. Intriguingly, both ways use enzymes with caspase-like activity; the membrane-fusion system uses proteasome subunit PBA1 with caspase-3-like activity, and the vacuolar-collapse system uses vacuolar processing enzyme (VPE) with caspase-1-like activity. This review summarizes two different ways of vacuole-mediated PCD and discusses how plants use them to attack pathogens that invade unexpectedly.

Genetic suppression analysis in novel vacuolar processing enzymes reveals their roles in controlling sugar accumulation in tomato fruits

In plant cells, many vacuolar proteins are synthesized as precursors in the endoplasmic reticulum and are subsequently transported to the vacuole. These precursors are subject to post-translational modifications to allow the active mature forms to be produced. Vacuolar processing enzyme (VPE) has been identified as a family of cysteine proteases involved in protein maturation in the vacuole. In this study, novel VPE genes were isolated from tomato (Solanum lycopersicum), and they were designated SlVPE1-SlVPE5. Phylogenic analysis suggested that SlVPE1 and SlVPE2 were categorized as the seed coat type, SlVPE4 was categorized as the seed type, and both SlVPE3 and SlVPE5 were categorized as the vegetative type. Expression analysis demonstrated that these genes were expressed during fruit development, and that their expression profiles agreed with this classification. High VPE enzyme activity was observed during tomato fruit development; the enzyme activity was correlated with the SlVPE mRNA levels, indicating that the SlVPE encoded active VPE proteins. The total sugar content was higher in RNA interference (RNAi) lines compared with the control plants, suggesting negative roles for SlVPE in sugar accumulation. The quantitative expression analysis of each SlVPE gene in the RNAi lines suggested that the suppression of SlVPE5 probably had the strongest effect on the sugar accumulation observed. The suppression of SlVPE did not influence the total amino acid content, suggesting that the molecular targets of SlVPE were mainly involved in sugar accumulation.

The role of vacuolar processing enzymes in plant immunity

Proteases play important roles in plant innate immunity. In this mini-review, we describe the current view on the role of a plant protease, vacuolar processing enzyme (VPE), and the first identified plant caspase-1-like protein, in plant immunity. In the past several years, VPEs were determined to play important roles in various types of cell death in plants. Early studies demonstrated the identification of VPE as a vacuolar hydrolytic protein responsible for maturation of vacuolar proteins. Later, Nicotiana benthamiana VPE was reported to mediate virus-induced hypersensitive response by regulating membrane collapse. The ortholog of VPE in Arabidopsis is also suggested to be involved in both mycotoxin-induced cell death and developmental cell death. However, the role of VPE in elicitor-signaling is still unclear. Our recent studies demonstrated the involvement of VPE in elicitor signal transduction to induce stomatal closure and defense responses, including defense gene expression and hypersensitive cell death.

Two vacuole-mediated defense strategies in plants

As plants lack immune cells, each cell has to defend itself against invading pathogens. Plant cells have a large central vacuole that accumulates a variety of hydrolytic enzymes and antimicrobial compounds, raising the possibility that vacuoles play a role in plant defense. However, how plants use vacuoles to protect against invading pathogens is poorly understood. Recently, we characterized two vacuole-mediated defense strategies associated with programmed cell death (PCD). In one strategy, vacuolar processing enzyme (VPE) mediated the disruption of the vacuolar membrane, resulting in the release of vacuolar contents into the cytoplasm in response to viral infection. In the other strategy, proteasome-dependent fusion of the central vacuole with the plasma membrane caused the discharge of vacuolar antibacterial protease and cell death-promoting contents from the cell in response to bacterial infection. Intriguingly, both strategies relied on enzymes with caspase-like activities: the vacuolar membrane-collapse system required VPE, which has caspase-1-like activity, and the membrane-fusion system required a proteasome that has caspase-3-like activity. Thus, plants may have evolved a cellular immune system that involves vacuolar membrane collapse to prevent the systemic spread of viral pathogens, and membrane fusion to inhibit the proliferation of bacterial pathogens.

Age-related resistance of Nicotiana benthamiana against hemibiotrophic pathogen Phytophthora infestans requires both ethylene- and salicylic acid-mediated signaling pathways

Phytophthora infestans, the agent of late blight disease of potato, is a hemibiotrophic pathogen with biotrophic action during early infection and necrotrophic in the later stage of colonization. Mature Nicotiana benthamiana was resistant to P. infestans, whereas relatively young plants were susceptible to this pathogen. Young plants became resistant following a pretreatment with acibenzolar-S-methyl, a functional analog of salicylic acid (SA), indicating that susceptibility of young plants is due to a lack of induction of SA signaling. Further analysis with virus-induced gene silencing indicated that NbICS1 and NbEIN2, the genes for SA biosynthesis and ethylene (ET) signaling, respectively, are required for the resistance of mature N. benthamiana against P. infestans. Furthermore, these genes are required for the production of reactive oxygen species (ROS) induced by treatment of the INF1 elicitor. In NbICS1-silenced plants, cell death induced by either INF1 or necrosis-inducing protein NPP1.1 was significantly accelerated. Expression of genes for phytoalexin (capsidiol) biosynthesis, NbEAS and NbEAH, were regulated by ET, and gene silencing of either of them compromised resistance of N. benthamiana to P. infestans. Together, these results suggest that resistance of N. benthamiana against hemibiotrophic P. infestans requires both SA-regulated appropriate induction of cell death and ET-induced production of phytoalexin.

Disulfide formation in plant storage vacuoles permits assembly of a multimeric lectin

The ER (endoplasmic reticulum) has long been considered the plant cell compartment within which protein disulfide bond formation occurs. Members of the ER-located PDI (protein disulfide isomerase) family are responsible for oxidizing, reducing and isomerizing disulfide bonds, as well as functioning as chaperones to newly synthesized proteins. In the present study we demonstrate that an abundant 7S lectin of the castor oil seed protein storage vacuole, RCA (Ricinus communis agglutinin 1), is folded in the ER as disulfide bonded A-B dimers in both vegetative cells of tobacco leaf and in castor oil seed endosperm, but that these assemble into (A-B)2 disulfide-bonded tetramers only after Golgi-mediated delivery to the storage vacuoles in the producing endosperm tissue. These observations reveal an alternative and novel site conducive for disulfide bond formation in plant cells.

Identification and functional characterization of legumain in amphioxus Branchiostoma belcheri

Legumain has been reported from diverse sources such as plants, parasites (animals) and mammals, but little is known in the lower chordates. The present study reports the first characterization of legumain cDNA from the protochordate Branchiostoma belcheri. The deduced 435-amino-acid-long protein is structurally characterized by the presence of a putative N-terminal signal peptide, a peptidase_C13 superfamily domain with the conserved Lys123-Gly124-Asp125 motif and catalytic dyad His153 and Cys195 and two potential Asn-glycosylation sites at Asn85 and Asn270. Phylogenetic analysis demonstrates that B. belcheri legumain forms an independent cluster together with ascidian legumain, and is positioned at the base of vertebrate legumains, suggesting that B. belcheri legumain gene may represent the archetype of vertebrate legumain genes. Both recombinant legumain expressed in yeast and endogenous legumain are able to be converted into active protein of ~37 kDa via a C-terminal autocleavage at acid pH values. The recombinant legumain efficiently degrades the legumain-specific substrate Z-Ala-Ala-Asn-MCA (benzyloxycarbonyl-L-alanyl-L-alanyl-L-asparagine-4-methylcoumaryl-7-amide) at optimum pH 5.5; and the enzymatic activity is inhibited potently by iodoacetamide and N-ethylmaleimide, partially by hen's-egg white cystatin, but not by E-64 [trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane], PMSF and pepstatin A. In addition, legumain is expressed in vivo in a tissue-specific manner, with main expression in the hepatic caecum and hind-gut of B. belcheri. Altogether, these results suggest that B. belcheri legumain plays a role in the degradation of macromolecules in food.

Storage reserve accumulation in Arabidopsis: metabolic and developmental control of seed filling

In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.

The origin and evolution of plant cystatins and their target cysteine proteinases indicate a complex functional relationship

BACKGROUND

Cystatins and their putative targets, the families of cysteine proteinases C1A and C13 play key roles in plants. Comparative genomic analyses are powerful tools to obtain valuable insights into the conservation and evolution of the proteinases and their proteinaceous inhibitors, and could aid to elucidate issues concerning the function of these proteins.

RESULTS

We have performed an evolutionary comparative analysis of cysteine proteinases C1A and C13 and their putative inhibitors in representative species of different taxonomic groups that appeared during the evolution of the Viridiplantae. The results indicate that whereas C1A cysteine proteinases are present in all taxonomic groups, cystatins and C13 cysteine proteinases are absent in some basal groups. Moreover, gene duplication events have been associated to the increasing structural and functional complexities acquired in land plants.

CONCLUSION

Comparative genomic analyses have provided us valuable insights into the conservation and evolution of the cystatin inhibitory family and their putative targets, the cysteine proteinases from families C1A and C13. Functionality of both families of proteins in plants must be the result of a coevolutionary process that might have occurred during the evolution of basal and land plants leading to a complex functional relationship among them.

HlLgm2, a member of asparaginyl endopeptidases/legumains in the midgut of the ixodid tick Haemaphysalis longicornis, is involved in blood-meal digestion

Here we describe a cDNA encoding the second asparaginyl endopeptidase/legumain (HlLgm2) from the midgut of the ixodid tick Haemaphysalis longicornis. Endogenous HlLgm2 was expressed in all the developmental stages of the tick, localized mainly in the midgut epithelium and was up-regulated by the host blood-feeding process, as demonstrated by immunoblotting and immunohistochemistry. RT-PCR and real-time PCR showed that the HlLgm2 gene was expressed at a lower level during all phases of blood-feeding than our previously characterized legumain (HlLgm) gene from the same tick. More strikingly, there was no expression of HlLgm2 mRNA beyond 96 h of blood-feeding, while HlLgm mRNA expression continued until full engorgement. Escherichia coli-expressed recombinant HlLgm2 (rHlLgm2) efficiently hydrolysed the legumain-specific synthetic substrate. rHlLgm2 activity was inhibited by iodoacetamide and N-ethylmaleimide and also by Fe(2+), Cu(2+), Co(2+) and Ni(2+). rHlLgm2 digested bovine haemoglobin and exhibited strict specificity for the asparaginyl bonds on the carboxy-terminal side of a peptide, as demonstrated by internal amino acid sequence analysis of the cleaved bovine serum albumin products. Our results suggest that HlLgm2, together with HlLgm, plays a pivotal role in host blood-meal digestion process.

Comparative molecular analysis of two asparaginyl endopeptidases and encoding genes from Fasciola gigantica

In this study we describe the first cysteine proteinases of the MEROPS Clan CD family C13 in Fasciola gigantica. Family C13 contains asparaginyl endopeptidases and glycosylphosphatidylinositol-anchor transamidases and is also called the legumain family due to the discovery of the first asparaginyl endopeptidase in a legume. The cDNAs encoding two asparaginyl endopeptidases, FgLGMN-1 and FgLGMN-2, were cloned and used for the analysis of nucleic acid and protein properties. The deduced amino acid sequences showed 47.4% identity to each other and from 42.2 to 51.1% identity to homologs of other trematode species. The catalytic site residues histidine, cysteine and preceding hydrophobic residues, characteristic for the cysteine proteinase families C11, C13, C14, and C25, were found conserved. Northern and reverse transcription PCR analyses demonstrated that the transcriptional products are present in metacercariae, juveniles and adults. RNA in situ hybridization and immunohistochemistry revealed that RNA and protein products of the two genes are specifically expressed in the intestinal epithelium of juveniles and adults. Immune sera of mice infected with F. gigantica reacted with immunoblotted, bacterially expressed recombinant proteins starting 4 weeks after infection. Polyclonal antisera raised against the recombinant proteins detected 40 and 30 kDa antigens, respectively in crude worm protein extracts but not in the excretion-secretion products of adult parasites. Likewise, legumain-specific activity was found in crude worm protein extracts but not in excretion-secretion products. This study elucidates the molecular characteristics of these proteins in F. gigantica and demonstrates differences in the biology between Fasciola and Schistosoma which may prove useful for the development of vaccines against fasciolosis in domestic livestock.

Characterization of asparaginyl endopeptidase, legumain induced by blood feeding in the ixodid tick Haemaphysalis longicornis

We characterize here a cDNA from the ixodid tick Haemaphysalis longicornis, which encodes an asparaginyl endopeptidase, legumain (HlLgm), that was present as a functional molecule in the midgut of this tick. Endogenous HlLgm was detected as a 38-kDa antigen in H. longicornis extracts and was seen throughout all developmental stages. Endogenous HlLgm was mainly localized in the midgut epithelium by immunohistochemistry, and was shown to be up-regulated by the host blood-feeding process. Recombinant HlLgm (rHlLgm) produced in Escherichia coli was shown to hydrolyze the synthetic substrate Z-Ala-Ala-Asn-MCA at the rate of 6.42x10(-4)mumol/min/mg protein. Its activity was inhibited by the thiol blocking reagents iodoacetamide and N-ethylmaleimide. The enzyme was shown to possess a unique feature of having an autocatalyzed cleavage at asparagines(364-365) at the C-terminus of both endogenous HlLgm and rHlLgm. rHlLgm degraded bovine hemoglobin and bovine serum albumin (BSA) showing its strict specificity for hydrolysis of the peptide on the carboxyl side of the asparagines, as demonstrated by internal amino acid sequence analysis of proteolytic product of BSA cleavage. These results suggest that HlLgm plays an important role in host blood-meal digestion and may be critical for the final process of digestion of blood components.

Carboxy terminal extended phytocystatins are bifunctional inhibitors of papain and legumain cysteine proteinases

Plant legumains are cysteine proteinases putatively involved in processing endogenous proteins. Phytocystatins (PhyCys) have been described as plant inhibitors of papain-like cysteine proteinases. Some PhyCys contain a carboxy terminal extension with an amino acid motif (SNSL) similar to that involved in the inhibition of legumain-like proteins by human cystatins. The role of these carboxy terminal extended PhyCys as inhibitors of legumain-like cysteine proteinases is here shown by in vitro inhibition of human legumain and legumain-like activities from barley extracts. Moreover, site-directed mutagenesis has demonstrated that the asparagine of the SNSL motif is essential in this inhibition. We prove for first time the existence of legumain inhibitors in plants.

Modification of nitrogen remobilization, grain fill and leaf senescence in maize (Zea mays) by transposon insertional mutagenesis in a protease gene

A maize (Zea mays) senescence-associated legumain gene, See2beta, was characterized at the physiological and molecular levels to determine its role in senescence and resource allocation. A reverse-genetics screen of a maize Mutator (Mu) population identified a Mu insertion in See2beta. Maize plants homozygous for the insertion were produced. These See2 mutant and sibling wild-type plants were grown under high or low quantities of nitrogen (N). The early development of both genotypes was similar; however, tassel tip and collar emergence occurred earlier in the mutant. Senescence of the mutant leaves followed a similar pattern to that of wild-type leaves, but at later sampling points mutant plants contained more chlorophyll than wild-type plants and showed a small extension in photosynthetic activity. Total plant weight was higher in the wild-type than in the mutant, and there was a genotype x N interaction. Mutant plants under low N maintained cob weight, in contrast to wild-type plants under the same treatment. It is concluded, on the basis of transposon mutagenesis, that See2beta has an important role in N-use and resource allocation under N-limited conditions, and a minor but significant function in the later stages of senescence.

MAIGO2 is involved in exit of seed storage proteins from the endoplasmic reticulum in Arabidopsis thaliana

Seed storage proteins are synthesized on the endoplasmic reticulum (ER) as precursors and then transported to protein storage vacuoles, where they are processed into mature forms. Here, we isolated an Arabidopsis thaliana mutant, maigo2 (mag2), that accumulated the precursors of two major storage proteins, 2S albumin and 12S globulin, in dry seeds. mag2 seed cells contained many novel structures, with an electron-dense core that was composed of the precursor forms of 2S albumin. 12S globulins were segregated from 2S albumin and were localized in the matrix region of the structures together with the ER chaperones lumenal binding protein and protein disulfide isomerase, which were more abundant in mag2 seeds. The MAG2 gene was identified as At3g47700, and the MAG2 protein had a RINT-1/TIP20 domain in the C-terminal region. We found that some MAG2 molecules were peripherally associated with the ER membrane. MAG2 had an ability to bind to two ER-localized t-SNAREs (for target-soluble NSF [N-ethylmaleimide-sensitive fusion protein] attachment protein receptor; At Sec20 and At Ufe1). Our findings suggest that MAG2 functions in the transport of storage protein precursors between the ER and Golgi complex in plants.

Proteomic and transcriptomic analysis of Arabidopsis seeds: molecular evidence for successive processing of seed proteins and its implication in the stress response to sulfur nutrition

Seed storage proteins are synthesized as sources of carbon, nitrogen and sulfur for the next generation of plants. Their composition changes according to nutritional conditions. Here, we report the precise molecular identification of seed proteins by proteomic analysis of wild-type Arabidopsis thaliana and methionine-over-accumulating mutant mto1-1 plants. The identities of 50 protein spots were determined in the protein extract of mature Arabidopsis seeds by two-dimensional (2D) gel electrophoresis and subsequent mass spectrometric analysis. Of these protein spots, 42 were identified as derived from 12S globulins or 2S albumins. These results indicate that approximately 84% of protein species in Arabidopsis seeds are derived from a few genes coding for 12S globulins and 2S albumins. Extensive mass spectrometric analysis of the 42 spots revealed that successive C-terminal degradation occurred on the 12S globulins. The feasibility of this C-terminal processing was rationalized by molecular modeling of the three-dimensional structure of 12S globulins. The C-terminal degradation at glutamic acid residues of the 12S globulin subunits was repressed under sulfur-deficient conditions. Transcriptome analysis was combined with proteomic analysis to elucidate the mechanism of changes in seed protein composition in response to sulfur deficiency. The results suggest that seed storage proteins in Arabidopsis undergo multi-layer regulation, with emphasis on post-translational modifications that enable the plant to respond to sulfur deficiency.

The N-terminal ricin propeptide influences the fate of ricin A-chain in tobacco protoplasts

The plant toxin ricin is synthesized in castor bean seeds as an endoplasmic reticulum (ER)-targeted precursor. Removal of the signal peptide generates proricin in which the mature A- and B-chains are joined by an intervening propeptide and a 9-residue propeptide persists at the N terminus. The two propeptides are ultimately removed in protein storage vacuoles, where ricin accumulates. Here we have demonstrated that the N-terminal propeptide of proricin acts as a nonspecific spacer to ensure efficient ER import and glycosylation. Indeed, when absent from the N terminus of ricin A-chain, the non-imported material remained tethered to the cytosolic face of the ER membrane, presumably by the signal peptide. This species appeared toxic to ribosomes. The propeptide does not, however, influence catalytic activity per se or the vacuolar targeting of proricin or the rate of retrotranslocation/degradation of A-chain in the cytosol. The likely implications of these findings to the survival of the toxin-producing tissue are discussed.

Participation of proteolytic enzymes in the interaction of plants with phytopathogenic microorganisms

Different forms of participation of proteolytic enzymes in pathogenesis and plant defense are reviewed. Together with extracellular proteinases, phytopathogenic microorganisms produce specific effectors with proteolytic activity and are able to act on proteins inside the plant cell. In turn, plants use both extracellular and intracellular proteinases for defense against phytopathogenic microorganisms. Among the latter, a special role belongs to vacuolar processing enzymes (legumains), which perform the function of caspases in the plant cell.

A cellular suicide strategy of plants: vacuole-mediated cell death

Programmed cell death (PCD) occurs in animals and plants under various stresses and during development. Recently, vacuolar processing enzyme (VPE) was identified as an executioner of plant PCD. VPE is a cysteine protease that cleaves a peptide bond at the C-terminal side of asparagine and aspartic acid. VPE exhibited enzymatic properties similar to that of a caspase, which is a cysteine protease that mediates the PCD pathway in animals, although there is limited sequence identity between the two enzymes. VPE and caspase-1 share several structural properties: the catalytic dyads and three amino acids forming the substrate pockets (Asp pocket) are conserved between VPE and caspase-1. In contrast to such similarities, subcellular localizations of these proteases are completely different from each other. VPE is localized in the vacuoles, while caspases are localized in the cytosol. VPE functions as a key molecule of plant PCD through disrupting the vacuole in pathogenesis and development. Cell death triggered by vacuolar collapse is unique to plants and has not been seen in animals. Plants might have evolved a VPE-mediated vacuolar system as a cellular suicide strategy.

Vacuolar Processing Enzyme Is Essential for Mycotoxin-induced Cell Death in Arabidopsis thaliana

Some compatible pathogens secrete toxins to induce host cell death and promote their growth. The toxin-induced cell death is a pathogen strategy for infection. To clarify the executioner of the toxin-induced cell death, we examined a fungal toxin (fumonisin B1 (FB1))-induced cell death of Arabidopsis plants. FB1-induced cell death was accompanied with disruption of vacuolar membrane followed by lesion formation. The features of FB1-induced cell death were completely abolished in the Arabidopsis vacuolar processing enzyme (VPE)-null mutant, which lacks all four VPE genes of the genome. Interestingly, an inhibitor of caspase-1 abolished FB1-induced lesion formation, as did a VPE inhibitor. The VPE-null mutant had no detectable activities of caspase-1 or VPE in the FB1-treated leaves, although wild-type leaves had the caspase-1 and VPE activities, both of which were inhibited by a caspase-1 inhibitor. gammaVPE is the most essential among the four VPE homologues for FB1-induced cell death in Arabidopsis leaves. Recombinant gammaVPE recognized a VPE substrate with Km = 30.3 microm and a caspase-1 substrate with Km = 44.2 microm, which is comparable with the values for mammalian caspase-1. The gammaVPE precursor was self-catalytically converted into the mature form exhibiting caspase-1 activity. These in vivo and in vitro analyses demonstrate that gammaVPE is the proteinase that exhibits a caspase-1 activity. We show that VPE exhibiting a caspase-1 activity is a key molecule in toxin-induced cell death. Our findings suggest that a susceptible response of toxin-induced cell death is caused by the VPE-mediated vacuolar mechanism similar to a resistance response of hypersensitive cell death (Hatsugai, N., Kuroyanagi, M., Yamada, K., Meshi, T., Tsuda, S., Kondo, M., Nishimura, M., and Hara-Nishimura, I. (2004) Science 305, 855-858).

Vacuolar processing enzyme: an executor of plant cell death

Apoptotic cell death in animals is regulated by cysteine proteinases called caspases. Recently, vacuolar processing enzyme (VPE) was identified as a plant caspase. VPE deficiency prevents cell death during hypersensitive response and cell death of limited cell layers at the early stage of embryogenesis. Because plants do not have macrophages, dying cells must degrade their materials by themselves. VPE plays an essential role in the regulation of the lytic system of plants during the processes of defense and development. VPE is localized in the vacuoles, unlike animal caspases, which are localized in the cytosol. Thus, plants might have evolved a regulated cellular suicide strategy that, unlike animal apoptosis, is mediated by VPE and the vacuoles.

Epidermal differentiation: the role of proteases and their inhibitors

Dermatological diseases range from minor cosmetic problems to life-threatening conditions, as seen in some severe disorders of keratinization and cornification. These disorders are commonly due to abnormal epidermal differentiation processes, which result in disturbed barrier function of human skin. Elucidation of the cellular differentiation programs that regulate the formation and homeostasis of the epidermis is therefore of great importance for the understanding and therapy of these disorders. Much of the barrier function of human epidermis against the environment is provided by the cornified cell envelope (CE), which is assembled by transglutaminase (TGase)-mediated cross-linking of several structural proteins and lipids during the terminal stages of normal keratinocyte differentiation. The major constituents of the stratum corneum and the current knowledge on the formation of the stratum corneum will be briefly reviewed here. The discovery of mutations that underlie several human diseases caused by genetic defects in the protein or lipid components of the CE, and recent analyses of mouse mutants with defects in the structural components of the CE, catalyzing enzymes, and lipid processing, have highlighted their essential function in establishing the epidermal barrier. In addition, recent findings have provided evidence that a disturbed protease-antiprotease balance could cause faulty differentiation processes in the epidermis and hair follicle. The importance of regulated proteolysis in epithelia is well demonstrated by the recent identification of the SPINK5 serine proteinase inhibitor as the defective gene in Netherton syndrome, cathepsin C mutations in Papillon-Lefevre syndrome, cathepsin L deficiency infurless mice, targeted ablation of the serine protease Matriptase/MTSP1, targeted ablation of the aspartate protease cathepsin D, and the phenotype of targeted epidermal overexpression of stratum corneum chymotryptic enzyme in mice. Notably, our recent findings on the role of cystatin M/E and legumain as a functional dyad in skin and hair follicle cornification, a paradigm example of the regulatory functions exerted by epidermal proteases, will be discussed.

Identification of the 23-kDa peptide derived from the precursor of Gly m Bd 28K, a major soybean allergen, as a new allergen

One of the major soybean allergens, Gly m Bd 28K, is suggested to be biosynthesized as a preproprotein form, which would be composed of a signal peptide, Gly m Bd 28K and the C-terminal peptide (the 23-kDa peptide). However, the 23-kDa peptide has never been characterized. In the present study, we prepared a monoclonal antibody (mAb) against a recombinant 23-kDa peptide expressed in Escherichia coli to detect the 23-kDa peptide in soybean. Several proteins were detected by immunoblotting with the mAb. All of the proteins were shown to have the identical N-terminal amino acid sequence, suggesting that the proteins correspond to the C-terminal part of the Gly m Bd 28K precursor. Furthermore, Gly m Bd 28K and the 23-kDa peptide were observed to come out at the 21st day after flowering and to locate in the crystalloid part of protein storage vacuoles in growing cotyledons. Some of the 23-kDa peptides were shown to be glycoproteins with an N-linked glycan moiety and exhibited the binding to IgE antibodies in the sera of patients sensitive to soybean. The binding of the peptides to IgE antibodies was suggested to be predominantly dependent on their glycan moiety. This study proves the occurrence of the 23-kDa peptide in soybean and that it is a new allergen.

A plant vacuolar protease, VPE, mediates virus-induced hypersensitive cell death

Programmed cell death (PCD) in animals depends on caspase protease activity. Plants also exhibit PCD, for example as a response to pathogens, although a plant caspase remains elusive. Here we show that vacuolar processing enzyme (VPE) is a protease essential for a virus-induced hypersensitive response that involves PCD. VPE deficiency prevented virus-induced hypersensitive cell death in tobacco plants. VPE is structurally unrelated to caspases, although VPE has a caspase-1 activity. Thus, plants have evolved a regulated cellular suicide strategy that, unlike PCD of animals, is mediated by VPE and the cellular vacuole.