The involvement of cysteine proteases and protease inhibitor genes in the regulation of programmed cell death in plants

Reactive oxygen species signaling in plants

The evolution of aerobic metabolism such as respiration and photosynthesis resulted in the generation of reactive oxygen species (ROS). A common property of all ROS types is that they can cause oxidative damage to proteins, DNA, and lipids. This toxicity of ROS explains the evolution of complex arrays of nonenzymatic and enzymatic detoxification mechanisms in plants. However, increasing evidence indicates that plants also make use of ROS as signaling molecules for regulating development and various physiological responses. In this review, novel insights into the mechanisms of how plants sense and respond to ROS are discussed in the context of the biological effects and functions of ROS in plants.

Identification of a nuclear-localized nuclease from wheat cells undergoing programmed cell death that is able to trigger DNA fragmentation and apoptotic morphology on nuclei from human cells

PCD (programmed cell death) in plants presents important morphological and biochemical differences compared with apoptosis in animal cells. This raises the question of whether PCD arose independently or from a common ancestor in plants and animals. In the present study we describe a cell-free system, using wheat grain nucellar cells undergoing PCD, to analyse nucleus dismantling, the final stage of PCD. We have identified a Ca2+/Mg2+ nuclease and a serine protease localized to the nucleus of dying nucellar cells. Nuclear extracts from nucellar cells undergoing PCD triggered DNA fragmentation and other apoptotic morphology in nuclei from different plant tissues. Inhibition of the serine protease did not affect DNA laddering. Furthermore, we show that the nuclear extracts from plant cells triggered DNA fragmentation and apoptotic morphology in nuclei from human cells. The inhibition of the nucleolytic activity with Zn2+ or EDTA blocked the morphological changes of the nucleus. Moreover, nuclear extracts from apoptotic human cells triggered DNA fragmentation and apoptotic morphology in nuclei from plant cells. These results show that degradation of the nucleus is morphologically and biochemically similar in plant and animal cells. The implication of this finding on the origin of PCD in plants and animals is discussed.

Identification of a cowpea gamma-thionin with bactericidal activity

Antimicrobial peptides are an abundant group of proteinaceous compounds widely produced in the plant kingdom. Among them, the gamma-thionin family, also known as plant defensins, represents one typical family and comprises low molecular mass cysteine-rich proteins, usually cationic and distributed in different plant tissues. Here, we report the purification and characterization of a novel gamma-thionin from cowpea seeds (Vigna unguiculata), named Cp-thionin II, with bactericidal activity against Gram-positive and Gram-negative bacteria. Once the primary structure was elucidated, molecular modelling experiments were used to investigate the multimerization and mechanism of action of plant gamma-thionins. Furthermore, Cp-thionin II was also localized in different tissues in cowpea seedlings during germination in contrasting conditions, to better understand the plant protection processes. The use of plant defensins in the construction of transgenic plants and also in the production of novel drugs with activity against human pathogens is discussed.

Monitoring programmed cell death triggered by mild heat shock in soybean-cultured cells

Programmed cell death (PCD) is a common form of cellular demise during plant response to environmental stresses. The pathway of PCD has been partially clarified in plants although the underlying molecular mechanisms are still poorly defined. We have investigated the signalling cascade induced by a mild heat treatment causing PCD in soybean cells (Glycine max L.). The data show that heat shock led to the onset of PCD in soybean cells involving H₂O₂ production and mitochondrial damage. Cytochrome c release accompanies the presence of caspase 9-like and caspase 3-like protease activities. Concomitantly, cells were severely damaged with a progressive cell shrinkage, chloroplast alteration and detachment of the plasma membrane from the cell wall. Chromatin condensation and DNA damage were observed. It is proposed that a mild heat stress induces PCD in soybean cells through a caspase-like-dependent pathway.

Gene expression patterns reveal tissue-specific signaling networks controlling programmed cell death and ABA- regulated maturation in developing barley seeds

Gene expression patterns covering over 10,000 seed-expressed sequences were analyzed by macroarray technology in maternal tissue (mainly pericarp) and filial endosperm and embryo during barley seed development from anthesis until late maturation. Defined sets of genes showing distinct expression patterns characterized both tissue type and major developmental phases. The analysis focused on regulatory networks involved in programmed cell death (PCD) and abscisic acid (ABA)-mediated maturation. These processes were similar in the different tissues, but typically involved the expression of alternative members of a common gene family. The analysis of co-expressed gene sets and the identification of cis regulatory elements in orthologous rice gene 'promoter' regions suggest that PCD in the pericarp is mediated by distinct classes of proteases and is under the hormonal control of both jasmonic acid (JA) and ethylene via ethylene-responsive element binding protein (EREBP) transcription factors (TFs). On the other hand, PCD in endosperm apparently involves only the ethylene pathway, but employs distinct gene family members from those active in the pericarp, and a different set of proteases and TFs. JA biosynthetic genes are hardly activated. Accordingly, JA levels are high in the pericarp but low in the endosperm during middle and late developmental stages. Similarly, genes acting in the deduced ABA biosynthetic pathway and signaling network differ between endosperm and embryo. ABA in the endosperm appears to exert an influence over storage product synthesis via SNF1 kinase. In the embryo, ABA seems to influence the acquisition of desiccation tolerance via ABA response element binding factors, but the data also suggest the existence of an ABA-independent but interactive pathway acting via the dehydration-responsive element binding (DREB) 2A TF.

Molecular cloning and characterization of a granulin-containing cysteine protease SPCP3 from sweet potato (Ipomoea batatas) senescent leaves

Granulins are a family of evolutionarily ancient proteins that are involved in regulating cell growth and division in animals. In this report a full-length cDNA, SPCP3, was isolated from senescent leaves of sweet potato (Ipomoea batatas). SPCP3 contains 1389 nucleotides (462 amino acids) in its open reading frame, and exhibits high amino acid sequence homologies (ca. 64-73.6%) with several plant granulin-containing cysteine proteases, including potato, tomato, soybean, kidney bean, pea, maize, rice, cabbage, and Arabidopsis. Gene structural analysis shows that SPCP3 encodes a putative precursor protein. Via cleavage of the N-terminal propeptide, it generates a protein with 324 amino acids (from the 139th to the 462nd amino acid residues), which contains two main domains: the conserved catalytic domain with the putative catalytic residues (the 163rd Cys, 299th His and 319th Asn) and the C-terminal granulin domain (from the 375th to the 462nd amino acid residues). Semi-quantitative RT-PCR and protein gel blot hybridization showed that SPCP3 gene expression was enhanced significantly in natural senescent leaves and in dark- and ethephon-induced senescent leaves, but was almost undetectable in mature green leaves, veins, and roots. Phylogenic analysis showed that SPCP3 displayed close association with a group of plant granulin-containing cysteine proteases which have been implied to be involved in programmed cell death. In conclusion, sweet potato SPCP3 is a functional, senescence-associated gene. Its mRNA and protein levels were significantly enhanced in natural and induced senescing leaves. The physiological role and/or function of SPCP3 associated with programmed cell death during leaf senescence were also discussed.

Cell death in development: shaping the embryo

Cell death in animals is normally classified as type I (apoptotic), type II (autophagic) or necrotic. Of the biologically controlled types of death, in most embryos apoptosis is the most common, although in metamorphosis and in cells with massive cytoplasm type II is often seen, and intermediate forms are seen. For vertebrate embryos other than mammals, apoptosis is not seen prior to gastrulation but thereafter is used to sculpt the organs of the embryo, while overproduction of cells with subsequent death of excess cells is a common means of generating high specificity with low information cost. In zebrafish at least, the inability of embryos prior to the maternal-zygotic transition to undergo apoptosis appears to derive from the inability of the cells to resist lysis once apoptosis begins, rather than any inhibition of apoptosis. In mammalian embryos, apoptosis is seen during cavitation. Thereafter, as in other embryos, cell death plays a major role in shaping and sculpting the embryo. In those situations that have been carefully studied, cell death is under tight genetic control (including regulation of gene products whose function in cell death is not yet known, such as cdk5), with activation of apoptosis sometimes regulated by local environmental variables.

Masterminds or minions? Cysteine proteinases in plant programmed cell deathThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Cysteine proteinases are ubiquitously involved in programmed cell death (PCD) in multicellular organisms. In animals, one group of cysteine proteinases, the cysteine-dependent aspartate-specific proteinases (caspases), are involved in a proteolytic signalling cascade that controls apoptosis, the most studied form of PCD. The enzymes act as both masterminds and executioners in apoptotic cell death. In plants, members of the metacaspase family, as well as those of the papain-like and legumain families, of cysteine proteinases have all been implicated in PCD. These enzymes often belong to sizeable gene families, with Arabidopsis having 9 metacaspase, 32 papain-like, and 4 legumain genes. This redundancy has made it difficult to ascertain the functional importance of any particular enzyme in plant PCD, as many are often expressed in a given tissue undergoing PCD. As yet, mechanisms similar to the apoptotic caspase cascade in animals have not been uncovered in plants and, indeed, may not exist. Are the various cysteine proteinases, so often implicated in plant PCD, merely acting as minions in the process? This review will outline reports of cysteine proteinases associated with plant PCD, discuss problems in determining the function of specific proteases, and suggest avenues for determining how these enzymes might be regulated and how PCD pathways upstream of protease expression and activation might operate.

Studies on low molecular mass phytocystatins purified from Phaseolus mungo (Urd)

In the present study two phytocystatins (thiol protease inhibitors) have been isolated and purified to homogeneity from Phaseolus mungo by a simple two-step procedure using ammonium sulfate fractionation and gel filtration on Sephacryl-100 HR. The latter procedure yielded two peaks of the inhibitors (PMC I and PMC II). The pH optimum of both phytocystatins was pH 7.0; the temperature optima for PMC I and PMC II were 65 and 70 degrees C, respectively. The molecular masses of the purified phytocystatins were 19 and 17 kD, respectively, as determined by SDS-PAGE and mass spectrometry. Antibodies raised against the purified cystatins gave a single precipitin line in Ouchterlony double immunodiffusion. Kinetics of inhibition showed that PMC I and PMC II strongly inhibit papain and ficin but not trypsin and chymotrypsin. Binding stoichiometry of PMC I and PMC II with both papain and ficin was 1 : 2. The effect of urea on PMC I and PMC II was analyzed by fluorescence and circular dichroism spectroscopy. The CD results suggest an unfolding of PMC I and PMC II accompanying a decrease in the amount of extended (hydrated) coil structure and an increase in sheet-like structure. FTIR results show that PMC I is structurally similar to PMC II. Hydrophobic interactions are observed over a long time scale (5-150 min). Furthermore, fluorescence spectroscopy results were found to be in accordance with CD results, by showing quenching of fluorescence intensity of PMC I and PMC II, although to different extents, due to perturbations of the environment of aromatic residues in the protein. Both cystatins showed strong inhibitory activity against Escherichia coli and Staphylococcus aureus.

Two new cysteine proteinases with specific expression patterns in mature and senescent tobacco (Nicotiana tabacum L.) leaves

Cysteine proteinases are involved in various physiological and developmental processes in plants. Two cDNAs from senescent and non-senescent tobacco leaves were isolated with degenerate primers designed from conserved regions of plant senescence-associated cysteine proteinases using rapid amplification of cDNA ends (RACE). Both sequences encode papain-like cysteine proteinases: the 833 bp fragment (NtCP1) encoding a C-terminus partial sequence of a putative tobacco cysteine proteinase gene whereas the 1300 bp fragment (NtCP2) is a full-length cysteine proteinase. On the amino acid sequence level, NtCP1 has a high similarity with other senescence-associated cysteine proteinases. It is expressed only in senescent leaves. It is not induced in mature green leaves upon exposure to drought or heat. These results suggest that it might be a good developmental senescence marker in tobacco. By contrast, NtCP2 has a high similarity to KDEL-tailed cysteine proteinases and is expressed in mature green leaves. Both drought and heat decreased NtCP2 transcript abundance in mature green leaves. It is concluded that NtCP1 is a senescence-specific cysteine proteinase whereas NtCP2 fulfils roles in green leaves that might be similar to those of KDEL-tailed cysteine proteinases involved, for example, in programmed cell death.

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.

Proteomics of Rac GTPase signaling reveals its predominant role in elicitor-induced defense response of cultured rice cells

We have previously shown that a human small GTPase Rac homolog, OsRac1, from rice (Oryza sativa) induces cascades of defense responses in rice plants and cultured cells. Sphingolipid elicitors (SEs) have been similarly shown to activate defense responses in rice. Therefore, to systematically analyze proteins whose expression levels are altered by OsRac1 and/or SE treatment, we performed a differential display analysis of proteins by the use of two-dimensional gel electrophoresis and mass spectrometry. A total of 271 proteins whose expression levels were altered by constitutively active (CA)-OsRac1 or SE were identified. Interestingly, of 100 proteins that were up-regulated by a SE, 87 were also induced by CA-OsRac1, suggesting that OsRac1 plays a pivotal role in defense responses induced by SE in cultured rice cells. In addition, CA-OsRac1 induces the expression of 119 proteins. Many proteins, such as pathogenesis-related proteins, SGT1, and prohibitin, which are known to be involved in the defense response, were found among these proteins. Proteins involved in redox regulation, chaperones such as heat shock proteins, BiP, and chaperonin 60, proteases and protease inhibitors, cytoskeletal proteins, subunits of proteasomes, and enzymes involved in the phenylpropanoid and ethylene biosynthesis pathways were found to be induced by CA-OsRac1 or SE. Results of our proteomic analysis revealed that OsRac1 is able to induce many proteins in various signaling and metabolic pathways and plays a predominant role in the defense response in cultured rice cells.

Plant-mediated interactions between pathogenic microorganisms and herbivorous arthropods

Plant-mediated interactions between pathogenic microorganisms and arthropod herbivores occur when arthropod infestation or pathogen infection changes the shared host plant in ways that affect a subsequent attacker of the opposite type. Interest in such "tripartite" interactions has increased as the ecological and plant physiological framework for understanding and contextualizing them has developed. The outcomes of plant-mediated interactions are variable, and only a few provisional patterns can be identified at present. However, these interactions can have important consequences not only for individual pathogens and herbivores, but also for the population dynamics of both types of organisms in managed and natural ecosystems. Research has focused on the role of two plant response pathways in mediating tripartite interactions, one involving jasmonic acid and the other salicylic acid. Further studies of plant-mediated interactions will facilitate an understanding of how plants coordinate and integrate their defenses against multiple biotic threats.

Effect of trypsin inhibitor from Crotalaria pallida seeds on Callosobruchus maculatus (cowpea weevil) and Ceratitis capitata (fruit fly)

A proteinaceous trypsin inhibitor was purified from Crotalaria pallida seeds by ammonium sulfate precipitation, affinity chromatography on immobilized trypsin-Sepharose and TCA precipitation. The trypsin inhibitor, named CpaTI, had M(r) of 32.5 kDa as determined by SDS-PAGE and was composed of two subunits with 27.7 and 5.6 kDa linked by disulfide bridges. CpaTI was stable at 50 degrees C and lost 40% of activity at 100 degrees C. CpaTI was also stable from pH 2 to 12 at 37 degrees C. CpaTI weakly inhibited chymotrypsin and elastase and its inhibition of papain, a cysteine proteinase, were indicative of its bi-functionality. CpaTI inhibited, in different degrees, digestive enzymes from Spodoptera frugiperda, Alabama argillacea, Plodiainterpunctella, Anthonomus grandis and Zabrotes subfasciatus guts. In vitro and in vivo susceptibility of Callosobruchus maculatus and Ceratitis capitata to CpaTI was evaluated. C. maculatus and C. capitata enzymes were strongly susceptible, 74.4+/-15.8% and 100.0+/-7.3%, respectively, to CpaTI. When CpaTI was added to artificial diets and offered to both insect larvae, the results showed that C. maculatus was more susceptible to CpaTI with an LD(50) of 3.0 and ED(50) of 2.17%. C. capitata larvae were more resistant to CpaTI, in disagreement with the in vitro effects. The larvae were more affected at lower concentrations, causing 27% mortality and 44.4% mass decrease. The action was constant at 2-4% (w/w) with 15% mortality and 38% mass decrease.

Cystatins may confer viral resistance in plants by inhibition of a virus-induced cell death phenomenon in which cysteine proteinases are active: cloning and molecular characterization of a cDNA encoding cysteine-proteinase inhibitor (celostatin) from Celosia cristata (crested cock's comb)

Cystatins (cysteine proteinase inhibitors) have been recently used in plants as antiviral strategy against those viruses whose replication involves cysteine proteinase activity. We proposed an idea that cystatins may confer resistance by inhibition of a virus-induced cell-death phenomenon in which cysteine proteinases are active. To test this idea, a full-length cDNA library was constructed from the preflowering stage of Celosia cristata (crested cock's comb) leaves, and a cDNA clone with cystatin domain was isolated using an oligonucleotide probe designed on the basis of the conserved peptide of plant cystatins. It was expressed in an Escherichia coli expression system as a fusion protein. The purified recombinant product, termed 'celostatin' (Celosia cystatin), inhibited the enzymatic activity of papain indicating its cystatin activity and prevented TMV (tobacco mosaic virus)-induced hypersensitive-response cell death in Nicotiana glutinosa (a wild species of tobacco) leaves by 65-70% at the concentration of approx. 50 ng/ml. It also offered resistance against TMV and caused normal growth of the test plant. Since the activity of cysteine proteinases is not involved in the TMV replication process, we speculated that inhibition of the hypersensitive response by celostatin may be due to the inactivation of proteolysis involved in the plant cell death programme, a phenomenon that has already been reported in animal systems.

Cloning, functional expression in Pichia pastoris, and purification of potato cystatin and multicystatin

In the tubers and leaves of potato, Solanum tuberosum, cysteine protease inhibitors are thought to play roles in the defence against herbivores and in regulating physiological processes like senescence and cell death. The cDNAs for two such inhibitors, potato multicystatin (PMC) with 8 cystatin domains and potato cystatin (PC) with a single domain, were cloned and expressed in the yeast Pichia pastoris. PC yielded on average 100 mg of purified active protein from 1l of culture supernatant. Purification to homogeneity was done in one step by cation exchange. The apparent equilibrium dissociation constant (K(i)) for papain was 0.1 nM. Cloning of the PMC cDNA was successful despite apparent toxicity for Escherichia coli and a high frequency of recombination events in RecA- strains of E. coli. In yeast, the expression of the cloned full length PMC gene was poor compared to that of the single domain.

Thaxtomin A induces programmed cell death in Arabidopsis thaliana suspension-cultured cells

Thaxtomin A is the main phytotoxin produced by Streptomyces scabiei, the causative agent of common scab disease of potato. Pathogenicity of S. scabiei is dependent on the production of thaxtomin A which is required for the development of disease symptoms, such as growth inhibition and cell death. We investigated whether thaxtomin A-induced cell death was similar to the hypersensitive cell death that often occurs in response to specific pathogens or phytotoxins during the so-called hypersensitive response (HR). We demonstrated that thaxtomin A induced in Arabidopsis thaliana suspension-cultured cells a genetically controlled cell death that required active gene expression and de novo protein synthesis, and which involved fragmentation of nuclear DNA, a characteristic hallmark of apoptosis. The thaxtomin A-induced form of programmed cell death (PCD) was not a typical HR, since defence responses generally preceding or associated with the HR, such as rapid medium alkalization, oxidative burst and expression of defence-related genes PR1 and PDF1.2, were not observed in plant cells following addition of thaxtomin A. Thaxtomin A has been shown to inhibit cellulose biosynthesis (Scheible et al. in Plant Cell 15:1781, 2003). We showed that isoxaben, a specific inhibitor of cellulose biosynthesis, also induced in Arabidopsis cell suspensions a PCD similar to that induced by thaxtomin A. These data suggested that rapid changes in the plant cell wall composition and organization can induce PCD in plant cells. We discuss how rapid inhibition of cellulose biosynthesis may trigger this process.

Ethylene-sensitivity regulates proteolytic activity and cysteine protease gene expression in petunia corollas

To investigate ethylene's role in petal senescence, a comparative analysis of age-related changes in total protein, protease activity, and the expression of nine cysteine protease genes in the corollas of ethylene-sensitive Petuniaxhybrida cv. Mitchell Diploid (MD) and ethylene-insensitive (35S:etr1-1; line 44568) transgenic petunias was conducted. The later stages of corolla senescence in MD flowers were associated with decreased fresh weight, decreased total protein, and increased proteolytic activity. Corolla senescence was delayed by approximately 8 d in etr-44568 transgenic petunias, and decreases in corolla fresh weight, protein content, and maximum proteolytic activity were similarly delayed. Protease inhibitor studies indicated that the majority of the protease activity in senescing petals was due to cysteine proteases. Nine cysteine proteases expressed in petals were subsequently identified. Northern blot analysis indicated that six of the nine cysteine proteases showed increased transcript abundance during petal senescence. One of these cysteine proteases, PhCP10, was detected only in senescing tissues. Expression of four of the senescence-associated cysteine proteases was delayed, but not prevented in etr-44568 flowers. The other two senescence associated cysteine proteases had high levels of transcript accumulation in etr-44568 corollas at 8 d after flower opening, when MD flowers were senescing. These patterns suggest that age-related factors, other than ethylene, were regulating the up-regulation of these genes during flower ageing. The delay in visible symptoms and biochemical and molecular indicators of senescence in ethylene-insensitive flowers is consistent with the concept that ethylene modulates the timing of senescence pathways in petals.

Reverse zymography using fluorogenic substrates for protease inhibitor detection

A novel, sensitive method for detecting protease inhibitors by using fluorescent protease substrates in gels is described. The protease inhibitors were separated on sodium dodecyl sulfate (SDS)-polyacrylamide gels containing a copolymerized peptide substrate, namely 4-methyl-coumaryl-7-amide (MCA). As the incorporated substrates in the gel, Boc-Phe Ser-Arg-MCA was used for trypsin, Suc-Ala-Ala-Pro-Phe-MCA for alpha-chymotrypsin, and Z-Phe-Arg-MCA for papain. After electrophoresis, washing and incubating the gel with the target protease solutions allowed the substrate to be cleaved by the protease, and the release of the fluorescent 7 amino-4 methyl-coumarin (AMC), which was detected under a UV transilluminator. The uncleaved peptide-MCA substrate remained where the inhibitors were present, and was visualized as dark blue bands on the light-green fluorescent background gel. This new method offers several advantages over other previous methods including: (i) greatly increased sensitivity can be achieved in a shorter period of time, which may be useful for discovering new protease inhibitors in small amounts of crude material; (ii) the procedure is quite simple and quick since the incubation period is very short and no time is needed for staining and destaining steps; (iii) since these probes using substrate specificity/target proteases, they are excellent tools for detection and discrimination of unknown protease inhibitors for various target proteases.

Biochemical and biophysical changes induced by fungicide sodium diethyl dithiocarbamate (SDD), in phytocystatin purified from Phaseolus mungo (Urd): a commonly used Indian legume

Phytocystatins are the plant thiol protease inhibitors involved in several reaction mechanisms of the plant system like regulation of proteolytic activity and storage of proteins. Biochemical and biophysical changes induced by fungicide SDD in phytocystatin purified from Phaseolus mungo have been investigated in terms of mass spectroscopy, Fourier transform infrared spectroscopy, and fluorescence spectroscopy, at pH 7.0, with varying fungicide concentrations (1-9 mM) and a time of incubation ranging from 2 to 8 h at 37 degrees C, with a fixed cystatin concentration (1.5 mM). Reactive oxygen species responsible for inhibitor damage were also investigated, and thiourea was found to scavenge the free radicals generated by SDD. FTIR analysis indicates a significant conformational transition from alpha-helix to beta-sheet structure; quenching of fluorescence is evident by fluorescence spectroscopy. The activity assay showed a decrease in inhibitory activity, as well as a fragmentation of the inhibitor was observed in electrophoresis. Results obtained implicate that exposure of phytocystatins to SDD involves physicochemical changes in cystatins leading to damage and a decrease in the activity of the inhibitor.

The strawberry gene Cyf1 encodes a phytocystatin with antifungal properties

An EST, encoding a strawberry phytocystatin (PhyCys) obtained from a developing fruit of Fragariaxananassa cv. Elsanta has been characterized. The corresponding gene (Cyf1) had three introns interrupting its ORF that codes for a protein (FaCPI-1) of 235 amino acid residues with a putative signal peptide of 29 residues and an estimated molecular mass for the mature protein of 23.1 kDa. This protein contains, besides a C-terminal extension, several motifs conserved in all members of the PhyCys superfamily: (i) a GG and LARFAV-like motifs towards the N-terminal part of the protein; (ii) the reactive site QVVAG, and (iii) a conserved PW, downstream of the reactive site. Northern blot and in situ hybridization analyses indicated that the Cyf1 gene was expressed in fully expanded leaves, in roots and in achenes, but not in the receptacle (pseudocarp) during fruit development. The recombinant FaCPI-1 protein expressed in E. coli efficiently inhibited papain (K(i) 1.9 x 10(-9) M) and less so cathepsin H (K(i) 4.7 x 10(-7) M) and cathepsin B (K(i) 3.3 x 10(-6) M), and was a good inhibitor of the in vitro growth of phytopathogenic fungi Botrytis cinerea (EC(50): 1.90 microM) and Fusarium oxysporum (EC(50): 2.28 microM).

Maize cystatins respond to developmental cues, cold stress and drought

Comprehensive searches of maize EST data allowed us to identify 8 novel Corn Cystatin (CC) genes in addition to the previously known genes CCI and CCII. The deduced amino acid sequences of all 10 genes contain the typical cystatin family signature. In addition, they show an extended overall similarity with cystatins from other species that belong to several different phyto-cystatin subfamilies. To gain further insight into their respective roles in the maize plant, gene-specific expression profiles were established by semi-quantitative RT-PCR. While 7 CC genes were expressed in two or more tissues varying from gene to gene, CCI was preferentially expressed in immature tassels and CC8 and CC10 in developing kernels. As shown by in situ hybridisation of maize kernels, CC8 was specifically expressed in the basal region of the endosperm and CC10 both in the starchy endosperm and the scutellum of the embryo. The remaining, not kernel-specific genes, all had distinct expression kinetics during kernel development, generally with peaks during the early stages. In addition to developmental regulation, the effect of cold stress and water starvation were tested on cystatin expression. Two genes (CC8 and CC9) were induced by cold stress and 5 genes (CCII, CC3, CC4, CC5 and CC9) were down-regulated in response to water starvation. Taken together our data suggest distinct functions for CC genes in the maize plant.

In vivo bioinsecticidal activity toward Ceratitis capitata (fruit fly) and Callosobruchus maculatus (cowpea weevil) and in vitro bioinsecticidal activity toward different orders of insect pests of a trypsin inhibitor purified from tamarind tree (Tamarindus indica) seeds

A proteinaceous inhibitor with high activity against trypsin-like serine proteinases was purified from seeds of the tamarind tree (Tamarindus indica) by gel filtration on Shephacryl S-200 followed by a reverse-phase HPLC Vidac C18 TP. The inhibitor, called the tamarind trypsin inhibitor (TTI), showed a Mr of 21.42 kDa by mass spectrometry analysis. TTI was a noncompetitive inhibitor with a Ki value of 1.7 x 10(-9) M. In vitro bioinsecticidal activity against insect digestive enzymes from different orders showed that TTI had remarkable activity against enzymes from coleopteran, Anthonomus grandis (29.6%), Zabrotes subfasciatus (51.6%), Callosobruchus maculatus (86.7%), Rhyzopertha dominica(88.2%), and lepidopteron, Plodia interpuncptella (26.7%), Alabama argillacea (53.8%), and Spodoptera frugiperda (75.5%). Also, digestive enzymes from Diptera, Ceratitis capitata (fruit fly), were inhibited (52.9%). In vivo bioinsecticidal assays toward C. capitata and C. maculatus larvae were developed. The concentration of TTI (w/w) in the artificial seed necessary to cause 50% mortality (LD50) of larvae was 3.6%, and that to reduce mass larvae by 50.0% (ED50) was 3.2%. Furthermore, the mass C. capitata larvae were affected at 53.2% and produced approximately 34% mortality at a level of 4.0% (w/w) of TTI incorporated in artificial diets.

Signal role for activation of caspase-3-like protease and burst of superoxide anions during Ce4+-induced apoptosis of cultured Taxus cuspidata cells

The signal events of 1 mM Ce4+ (Ce(NH4)2(NO3)6)-induced apoptosis of cultured Taxus cuspidata cells were investigated. The percentage of apoptotic cells increased from 0.82% to 51.32% within 6 days. Caspase-3-like protease activity became notable during the second day of Ce4+-treatment, and the maximum activity was 5-fold higher than that of control cells at the fourth day. When the experiment system was pretreated with acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO) at 100 microM, caspase-3-like activity resulted in distinct inhibition by 70% and 77.3% after 3 and 4 days of induction. Furthermore, 100 microM Ac-DEVD-CHO partially reduced the apoptotic cells by 58.6% and 60.8% at day 4 and 5 respectively. Ce4+ induced superoxide anions (O2*-) transient burst, and the first peak appeared at around 3.7-4 h, the second appeared at about 7 h. Both O2*- burst and cell apoptosis were effectively suppressed by application of diphenyl iodonium (NADPH oxidase inhibitor). Inhibition of O2*- production attenuated caspase-3-like activation by 49% and 53.6% during day 3 and 4 respectively. In addition, a total of 15 protein spots changed in response to caspase-3-like protease activation were identified by two-dimensional gel electrophoresis. These results suggest that Ce4+ of 1 mM induces apoptosis in suspension cultures of T. cuspidata through O2*- burst as well as caspase-3-like protease activation. The burst of O2*- exerts its activity as an upstream of caspase-3-like activation. Our results also implicate that other signal pathways independent of an O2*- burst possibly participate in mediating caspase-3-like protease activation.

Comparative phylogenetic analysis of cystatin gene families from arabidopsis, rice and barley

The plant cystatins or phytocystatins comprise a family of specific inhibitors of cysteine proteinases. Such inhibitors are thought to be involved in the regulation of several endogenous processes and in defence against pests and pathogens. Extensive searches in the complete rice and Arabidopsis genomes and in barley EST collections have allowed us to predict the presence of twelve different cystatin genes in rice, seven in Arabidopsis, and at least seven in barley. Structural comparisons based on alignments of all the protein sequences using the CLUSTALW program and searches for conserved motifs using the MEME program have revealed broad conservation of the main motifs characteristic of the plant cystatins. Phylogenetic analyses based on their deduced amino acid sequences have allowed us to identify groups of orthologous cystatins, and to establish homologies and define examples of gene duplications mainly among the rice and barley cystatin genes. Moreover, the absence of a counterpart between the two monocots, as well as strong variations in the motifs that interact with the cysteine proteinases, may be related to a species-specific evolutionary process. This cystatin classification should facilitate the assignment of proteinase specificities and functions to other cystatins as new information is obtained.

Identification of a novel gene family, paralogs of inhibitor of apoptosis proteins present in plants, fungi, and animals

Only few orthologs of animal apoptosis regulators have been found in plants. Recently, the ectopic expression of mammalian inhibitor of apoptosis proteins (IAPs) has been shown to affect plant programmed cell death. Here, we identified two novel proteins homologous to Arabidopsis thaliana IAP-like protein (AtILP) 1 and 2 by applying an improved motif searching method. Furthermore, homologs of AtILP1 were found to occur as a novel gene family in other organisms such as fungi and animals including Homo sapiens (HsILP1). Like baculovirus IAP repeats (BIRs) in IAPs, ILPs contain two highly conserved BIR-like domains (BLDs) with a putative C2HC-type zinc finger. Phylogenetic analyses indicated that ILPs are putative paralogs of IAPs. Homology modeling revealed that the three-dimensional structure of BLD in HsILP1 is similar to that of BIR. Transient expression of HsILP1 resulted in inhibition of etoposide-induced apoptosis in HEK293 and HeLaS3 cells. These findings suggest that ILPs are conserved in a wide range of eukaryotes including plants, and that their functions are closely related to those of IAPs.

Cellular re-distribution of flavin-containing polyamine oxidase in differentiating root and mesocotyl of Zea mays L. seedlings

Plant polyamine oxidases (PAOs; EC 1.5.3.11) are hydrogen peroxide-producing enzymes supposedly involved in cell-wall differentiation processes and defence responses. Maize (Zea mays L.) PAO (MPAO) is a 53 kDa secretory glycoprotein, abundant in primary and secondary cell walls of several tissues. Using biochemical, histochemical, ultrastructural and immunocytochemical techniques, the distribution and sub-cellular compartmentalisation of MPAO in the primary root and mesocotyl of seedlings at different maturation stages or after growth under varying light conditions were analysed. In apical root tissues, MPAO immunoreactivity was mainly detected in the cytoplasmic compartment, while a lower immunoreactivity was observed in the cell walls. In the more mature, basal part of the root, intense immunogold labelling was found in the primary and secondary walls of protoxylem precursors and vessels, while endodermal cells and living metaxylem precursors were immunopositive both in their walls and in their thin cytoplasmic compartments. A re-distribution of MPAO protein from the cytoplasm toward the primary and secondary walls was also recognised when immunoreactivity of basal root tissues from 3-day-old seedlings was compared with that detected in 11-day-old tissues. Accordingly, biochemical analyses revealed MPAO entrapment in the extracellular matrix of mature tissues. In the mesocotyl, an enrichment of MPAO immunolabelling in the cell wall of protoxylem, metaxylem and epidermal tissues, as a function of light exposure, was observed. Taken together, these data support the hypothesised role of PAOs in cell-wall maturation. Moreover, the relevant intraprotoplasmic MPAO localisation observed mainly in differentiating root tissues suggests an additional role in intracellular production of hydrogen peroxide.

The Plasmodium circumsporozoite protein is proteolytically processed during cell invasion

The circumsporozoite protein (CSP) is the major surface protein of Plasmodium sporozoites, the infective stage of malaria. Although CSP has been extensively studied as a malaria vaccine candidate, little is known about its structure. Here, we show that CSP is proteolytically cleaved by a papain family cysteine protease of parasite origin. Our data suggest that the highly conserved region I, found just before the repeat region, contains the cleavage site. Cleavage occurs on the sporozoite surface when parasites contact target cells. Inhibitors of CSP processing inhibit cell invasion in vitro, and treatment of mice with E-64, a highly specific cysteine protease inhibitor, completely inhibits sporozoite infectivity in vivo.

Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process

Research on legume nodule development has contributed greatly to our current understanding of plant-microbe interactions. However, the factors that orchestrate root nodule senescence have received relatively little attention. Accumulating evidence suggests that redox signals contribute to the establishment of symbiosis and senescence. Although degenerative in nature, nodule senescence is an active process programmed in development in which reactive oxygen species (ROS), antioxidants, hormones and proteinases have key roles. Nodules have high levels of the redox buffers, ascorbate and glutathione, which are important in the nodulation process and in senescence. These metabolites decline with N-fixation as the nodule ages but the resultant decrease in redox buffering capacity does not necessarily lead to enhanced ROS or oxidative stress. We propose models by which ROS and antioxidants interact with hormones such as abscisic acid in the orchestration of nodule senescence.

Suppression of the cysteine protease, aleurain, delays floret senescence in Brassica oleracea

An aleurain-like protein, BoCP5, is up-regulated during harvest-induced senescence in broccoli floret and leaf tissue. BoCP5 is most closely related to an Arabidopsis protein (91%, AAF43041) and has 71% identity to barley aleurain (P05167). The mRNA for this gene accumulates within 6 h after harvest in broccoli florets, and its expression is reduced in tissue that has been held in senescence-delaying treatments (e.g. water, sucrose feeding, controlled atmosphere). The gene is also expressed in leaves during aging-related and harvest-induced senescence. Analysis of protein bands that cross-react with antibodies raised to the bacterial BoCP5 fusion protein, revealed prominent immunoreactive bands at ca. 26, 28, 31, and 38 kD in floret tissue. The 31 kD band was absent in protein extracts from leaf tissue. Agrobacterium-mediated transformation was used to produce transgenic broccoli plants with down-regulated BoCP5. A reduction in the postharvest expression of BoCP5 in floret tissue was achieved for four transgenic lines in the current study. In three of these lines postharvest floret senescence (yellowing) was delayed, and florets contained significantly greater chlorophyll levels during postharvest storage at 20 degrees C than wild-type plants. Line 4 showed the greatest down-regulation of BoCP5, and in this line postharvest protease activity remained at pre-harvest levels, and the yield of soluble proteins extracted from florets after harvest was significantly greater than that of wild-type tissue.

Structural, biological, and evolutionary relationships of plant food allergens sensitizing via the gastrointestinal tract

The recently completed genome sequence of the model plant species Arabidopsis has been estimated to encode over 25,000 proteins, which, on the basis of their function, can be classified into structural and metabolic (the vast majority of plant proteins), protective proteins, which defend a plant against invasion by pathogens or feeding by pests, and storage proteins, which proved a nutrient store to support germination in seeds. It is now clear that almost all plant food allergens are either protective or storage proteins. It is also becoming evident that those proteins that trigger the development of an allergic response through the gastrointestinal tract belong primarily to two large protein superfamilies: (1) The cereal prolamin superfamily, comprising three major groups of plant food allergens, the 2S albumins, lipid transfer proteins, and cereal alpha-amylase/trypsin inhibitors, which have related structures, and are stable to thermal processing and proteolysis. They include major allergens from Brazil nut, peanuts, fruits, such as peaches, and cereals, such as rice and wheat; (2) The cupin superfamily, comprising the major globulin storage proteins from a number of plant species. The globulins have been found to be allergens in plant foods, such as peanuts, soya bean, and walnut; (3) The cyteine protease C1 family, comprising the papain-like proteases from microbes, plants, and animals. This family contains two notable allergens that sensitize via the GI tract, namely actinidin from kiwi fruit and the soybean allergen, Gly m Bd 30k/P34. This study describes the properties, structures, and evolutionary relationships of these protein families, the allergens that belong to them, and discusses them in relation to the role protein structure may play in determining protein allergenicity.

Identification of the putative staphylococcal AgrB catalytic residues involving the proteolytic cleavage of AgrD to generate autoinducing peptide

The P2 operon of the staphylococcal accessory gene regulator (agr) encodes four genes (agrA, -B, -C, and -D) whose products compose a quorum sensing system: AgrA and AgrC resemble a two-component signal transduction system of which AgrC is a sensor kinase and AgrA is a response regulator; AgrD, a polypeptide that is integrated into the cytoplasmic membrane via an amphipathic alpha-helical motif in its N-terminal region, is the propeptide for an autoinducing peptide that is the ligand for AgrC; and AgrB is a novel membrane protein that involves in the processing of AgrD propeptide and possibly the secretion of the mature autoinducing peptide. In this study, we demonstrated that AgrB had endopeptidase activity, and identified 2 amino acid residues in AgrB (cysteine 84 and histidine 77) that might form a putative cysteine endopeptidase catalytic center in the proteolytic cleavage of AgrD at its C-terminal processing site. Computer analysis revealed that the cysteine and histidine residues were conserved among the potential AgrB homologous proteins, suggesting that the Agr quorum sensing system homologues might also exist in other Gram-positive bacteria.

Toxicity to cotton boll weevil Anthonomus grandis of a trypsin inhibitor from chickpea seeds

Cotton (Gossypium hirsutum L.) is an important agricultural commodity, which is attacked by several pests such as the cotton boll weevil Anthonomus grandis. Adult A. grandis feed on fruits and leaf petioles, reducing drastically the crop production. The predominance of boll weevil digestive serine proteinases has motivated inhibitor screenings in order to discover new ones with the capability to reduce the digestion process. The present study describes a novel proteinase inhibitor from chickpea seeds (Cicer arietinum L.) and its effects against A. grandis. This inhibitor, named CaTI, was purified by using affinity Red-Sepharose Cl-6B chromatography, followed by reversed-phase HPLC (Vydac C18-TP). SDS-PAGE and MALDI-TOF analyses, showed a unique monomeric protein with a mass of 12,877 Da. Purified CaTI showed significant inhibitory activity against larval cotton boll weevil serine proteinases (78%) and against bovine pancreatic trypsin (73%), when analyzed by fluorimetric assays. Although the molecular mass of CaTI corresponded to alpha-amylase/trypsin bifunctional inhibitors masses, no inhibitory activity against insect and mammalian alpha-amylases was observed. In order to observe CaTI in vivo effects, an inhibitor rich fraction was added to an artificial diet at different concentrations. At 1.5% (w/w), CaTI caused severe development delay, several deformities and a mortality rate of approximately 45%. These results suggested that CaTI could be useful in the production of transgenic cotton plants with enhanced resistance toward cotton boll weevil.

A cut above the rest: the regulatory function of plant proteases

Proteolytic enzymes are intricately involved in many aspects of plant physiology and development. On the one hand, they are necessary for protein turnover. Degradation of damaged, misfolded and potentially harmful proteins provides free amino acids required for the synthesis of new proteins. Furthermore, the selective breakdown of regulatory proteins by the ubiquitin/proteasome pathway controls key aspects of plant growth, development, and defense. Proteases are, on the other hand, also responsible for the post-translational modification of proteins by limited proteolysis at highly specific sites. Limited proteolysis results in the maturation of enzymes, is necessary for protein assembly and subcellular targeting, and controls the activity of enzymes, regulatory proteins and peptides. Proteases are thus involved in all aspects of the plant life cycle ranging from the mobilization of storage proteins during seed germination to the initiation of cell death and senescence programs. This article reviews recent findings for the major catalytic classes, i.e. the serine, cysteine, aspartic, and metalloproteases, emphasizing the regulatory function of representative enzymes.

Coordinate involvement of cysteine protease and nuclease in the executive phase of plant apoptosis

We have developed an oat cell-free apoptosis system to investigate the execution mechanisms of plant apoptosis. Cell extracts derived from oat tissues undergoing toxin (victorin)-induced apoptosis caused nuclear collapse and internucleosomal DNA fragmentation in isolated nuclei. Pharmacological studies revealed that cysteine protease, which is E-64-sensitive but insensitive to caspase-specific inhibitors, is a crucial component in the morphological change of isolated nuclei, and that nuclease and the cysteine protease act cooperatively to induce the apoptotic DNA laddering. Interestingly, this finding is contrasted with those in well-studied animal cell-free systems in which an apoptotic endonuclease is solely responsible for the DNA fragmentation.

Chromosome instabilities and programmed cell death in tapetal cells of maize with B chromosomes and effects on pollen viability

B chromosomes (B's), knobbed chromosomes, and chromosome 6 (NOR) of maize undergo nondisjunction and micronucleus formation in binucleate tapetal cells. These chromosome instabilities are regular events in the program of tapetal cell death, but the B's strongly increase A chromosome instability. We studied 1B and 0B plants belonging to selected lines for high or low B transmission rate and their F1 hybrids. These lines are characterized by meiotic conservation or loss of B chromosomes, respectively. The female B transmission (fBtl) allele(s) for low B transmission is dominant, inducing micronucleus formation and B nondisjunction. We hypothesize that the fBtl allele(s) induces knob instability. This instability would be sufficient to produce B loss in both meiocytes and binucleate tapetal cells. B instability could, in turn, produce instabilities in all chromosomes of maize complement. To establish whether the chromosomal instabilities are related to the tapetal programmed cell death (PCD) process, we applied the TUNEL technique. PCD, estimated as the frequency of binucleate tapetal cells with TUNEL label, was significantly correlated with the formation of micronuclei and the frequency of pollen abortion. It can be concluded that the observed chromosome instabilities are important to the PCD process and to the development of microspores to form viable pollen grains.

Induction of programmed cell death in lily by the fungal pathogen Botrytis elliptica

SUMMARY The genus Botrytis contains necrotrophic plant pathogens that have a wide host range (B. cinerea) or are specialized on a single host species, e.g. B. elliptica on lily. In this study, it was found that B. elliptica-induced cell death of lily displays hallmark features of animal programmed cell death or apoptosis including cytoplasmic shrinkage, nuclear DNA fragmentation and the accumulation of NO as well as H(2)O(2). A pharmacological approach showed that B. elliptica-induced cell death could be modulated by serine and cysteine protease inhibitors including one caspase inhibitor. Blocking phosphatase activity stimulated cell death and concomitant lesion formation, suggesting that B. elliptica-induced cell death is mediated by kinase/phosphatase pathways. Blocking Ca(2+) influx restricted cell death. Blocking steps of sphingolipid biosynthesis delayed lily cell death for several days. B. elliptica culture filtrate (CF) was able to induce lily cell death by means of secreted proteins. Induction of cell death is necessary and sufficient for pathogenicity and host specialization because prior infiltration of B. elliptica CF enabled subsequent infection of lily by the otherwise incompatible pathogens B. cinerea and B. tulipae. The secreted B. elliptica proteins also induced cell death in some but not all Arabidopsis accessions and mutants. Arabidopsis accessions that respond to infiltration of B. elliptica CF also display cell death symptoms upon inoculation with B. elliptica conidia.

Protein proteinase inhibitor genes in combat against insects, pests, and pathogens: natural and engineered phytoprotection

The continual need to increase food production necessitates the development and application of novel biotechnologies to enable the provision of improved crop varieties in a timely and cost-effective way. A milestone in this field was the introduction of Bacillus thuringiensis (Bt) entomotoxic proteins into plants. Despite the success of this technology, there is need for development of alternative strategies of phytoprotection. Biotechnology offers sustainable solutions to the problem of pests, pathogens, and plant parasitic nematodes in the form of other insecticidal protein genes. A variety of genes, besides (Bt) toxins that are now available for genetic engineering for pest resistance are genes for vegetative insecticidal proteins, proteinase inhibitors, alpha-amylase inhibitors, and plant lectins. This review presents a comprehensive summary of research efforts that focus on the potential use and advantages of using proteinase inhibitor genes to engineer insect- and pest-resistance. Crop protection by means of PI genes is an important component of Integrated Pest Management programmes.

A multicystatin is induced by drought-stress in cowpea (Vigna unguiculata(L.) Walp.) leaves

Cystatins are protein inhibitors of cystein proteinases belonging to the papain family. In cowpea, cystatin-like polypeptides and a cDNA have been identified from seeds and metabolic functions have been attributed to them. This paper describes VuC1, a new cystatin cDNA isolated from cowpea leaves (Vigna unguiculata (L.) Walp.). Sequence analysis revealed a multicystatin structure with two cystatin-like domains. The recombinant VUC1 protein (rVUC1) was expressed in an heterologous expression system and purified to apparent homogeneity. It appeared to be an efficient inhibitor of papain activity on a chromogenic substrate. Polyclonal antibodies against rVUC1 were obtained. Involvement of the VuC1 cDNA in the cellular response to various abiotic stresses (progressive drought-stress, dessication and application of exogenous abscissic acid) was studied, using Northern blot and Western blot analysis, in the leaf tissues of cowpea plants corresponding to two cultivars with different capacity to tolerate drought-stress. Surprisingly, these abiotic stresses induced accumulation of two VuC1-like messages both translated into VUC1-like polypeptides. Difference in the transcript accumulation patterns was observed between the two cultivars and related to their respective tolerance level. Presence of multiple cystatin-like polypeptides and their possible involvement in the control of leaf protein degradation by cysteine proteinases is discussed.

Spatial and Temporal Tracing of Vessel Differentiation in Young Arabidopsis Seedlings by the Expression of an Immature Tracheary Element-specific Promoter

The vascular system is a complex tissue composed of several vascular cell types. However, little is known about the differentiation process of each vascular cell in situ. In this study, we found that the expression of the Zinnia cysteine protease 4 (ZCP4) promoter is restricted to only immature tracheary elements (TEs) in situ. Therefore, we monitored the early TE differentiation process in young Arabidopsis seedlings using a fusion gene of the ZCP4 promoter and the beta-glucuronidase gene as a molecular marker. This approach revealed unique processes of vessel differentiation during early seedling development, in which discontinuous initiation of vessel element differentiation occurs at distinct regions, followed by the simultaneous differentiation of protoxylem vessels and bidirectional differentiation of metaxylem vessels to form a vessel in the plant body.

Expression of proteinase inhibitor II proteins during floral development in Solanum americanum

The heterologous expression of serine proteinase inhibitor II (PIN2) proteins confers insect resistance in transgenic plants, but little is known of their endogenous roles. We have cloned two cDNAs encoding Solanum americanum PIN2 proteins, SaPIN2a and SaPIN2b. SaPIN2a is highly expressed in stem, particularly in the phloem, suggesting it could possibly regulate proteolysis in the sieve elements. When SaPIN2a was expressed in transgenic lettuce, we observed an inhibition of endogenous trypsin- and chymotrypsin-like activities. Here, we demonstrate that both SaPIN2a and SaPIN2b are expressed in floral tissues that are destined to undergo developmental programmed cell death (PCD), suggesting possible endogenous roles in inhibiting trypsin- and chymotrypsin-like activities during flower development. Northern and western blot analyses revealed that SaPIN2a and SaPIN2b mRNAs and proteins show highest expression early in floral development. In situ hybridization analysis and immunolocalization on floral sections, localized SaPIN2a and SaPIN2b mRNAs and their proteins to tissues that would apparently undergo PCD: the ovules, the stylar transmitting tissue, the stigma and the vascular bundles. Detection of PCD in floral sections was achieved using terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) analysis. Examination of the mid-style before, and 1 day after, pollination revealed that high expression of SaPIN2a and SaPIN2b in the style was inversely correlated with PCD.

Type II Metacaspases Atmc4 and Atmc9 of Arabidopsis thaliana Cleave Substrates after Arginine and Lysine

Nine potential caspase counterparts, designated metacaspases, were identified in the Arabidopsis thaliana genome. Sequence analysis revealed two types of metacaspases, one with (type I) and one without (type II) a proline- or glutamine-rich N-terminal extension, possibly representing a prodomain. Production of recombinant Arabidopsis type II metacaspases in Escherichia coli resulted in cysteine-dependent autocatalytic processing of the proform into large and small subunits, in analogy to animal caspases. A detailed biochemical characterization with a broad range of synthetic oligopeptides and several protease inhibitors of purified recombinant proteins of both metacaspase 4 and 9 showed that both metacaspases are arginine/lysine-specific cysteine proteases and did not cleave caspase-specific synthetic substrates. These findings suggest that type II metacaspases are not directly responsible for earlier reported caspase-like activities in plants.

Bax-induced cell death of Arabidopsis is meditated through reactive oxygen-dependent and -independent processes

An Arabidopsis protoplast system was developed for dissecting plant cell death in individual cells. Bax, a mammalian pro-apoptotic member of the Bcl-2 family, induces apoptotic-like cell death in Arabidopsis. Bax accumulation in Arabidopsis mesophyll protoplasts expressing murine Bax cDNA from a glucocorticoid-inducible promoter results in cytological characteristics of apoptosis, namely DNA fragmentation, increased vacuolation, and loss of plasma membrane integrity. In vivo targeting analysis monitored using jellyfish green fluorescent protein (GFP) reporter indicated full-length Bax was localized to the mitochondria, as it does in animal cells. Deletion of the carboxyl-terminal transmembrane domain of Bax completely abolished targeting to mitochondria. Bax expression was followed by reactive oxygen species (ROS) accumulation. Treatment of protoplasts with the antioxidant N -acetyl- -cysteine (NAC) during induction of Bax expression strongly suppressed Bax-mediated ROS production and the cell death phenotype. However, some population of the ROS depleted cells still induced cell death, indicating that there is a process that Bax-mediated plant cell death is independent of ROS accumulation. Accordingly, suppression of Bax-mediated plant cell death also takes place in two different processes. Over-expression of a key redox-regulator, Arabidopsis nucleoside diphosphate kinase 2 (AtNDPK2) down-regulated ROS accumulation and suppressed Bax-mediated cell death and transient expression of Arabidopsis Bax inhibitor-1 (AtBI-1) substantially suppressed Bax-induced cell death without altering cellular ROS level. Taken together, our results collectively suggest that the Bax-mediated cell death and its suppression in plants is mediated by ROS-dependent and -independent processes.

The plant proteolytic machinery and its role in defence

The diverse roles of plant proteases in defence responses that are triggered by pathogens or pests are becoming clearer. Some proteases, such as papain in latex, execute the attack on the invading organism. Other proteases seem to be part of a signalling cascade, as indicated by protease inhibitor studies. Such a role has also been suggested for the recently discovered metacaspases and CDR1. Some proteases, such as RCR3, even act in perceiving the invader. These exciting recent reports are probably just the first examples of what lies beneath. More roles for plant proteases in defence, as well as the regulation and substrates of these enzymes, are waiting to be discovered.

Activity profiling of papain-like cysteine proteases in plants

Transcriptomic and proteomic technologies are generating a wealth of data that are frequently used by scientists to predict the function of proteins based on their expression or presence. However, activity of many proteins, such as transcription factors, kinases, and proteases, depends on posttranslational modifications that frequently are not detected by these technologies. Therefore, to monitor activity of proteases rather than their abundance, we introduce protease activity profiling in plants. This technology is based on the use of biotinylated, irreversible protease inhibitors that react with active proteases in a mechanism-based manner. Using a biotinylated derivative of the Cys protease inhibitor E-64, we display simultaneous activities of many papain-like Cys proteases in extracts from various tissues and from different plant species. Labeling is pH dependent, stimulated with reducing agents, and inhibited specifically by Cys protease inhibitors but not by inhibitors of other protease classes. Using one-step affinity capture of biotinylated proteases followed by sequencing mass spectrometry, we identified proteases that include xylem-specific XCP2, desiccation-induced RD21, and cathepsin B- and aleurain-like proteases. Together, these results demonstrate that this technology can identify differentially activated proteases and/or characterize the activity of a particular protease within complex mixtures.

Characterization of senescence-associated proteases in postharvest broccoli florets

We characterized the senescence-associated proteases of postharvest broccoli (Brassica oleracea L. var Green King) florets, using class-specific protease inhibitors and gelatin-polyacrylamide gel electrophoresis. Different classes of senescence-associated proteases in broccoli florets were partially characterized for the first time. Protease activity of broccoli florets was depressed by all the inhibitors and showed different inhibition curves during postharvest. The hydrolytic activity of metalloprotease (EC 3.4.24. - ) and serine protease (EC 3.4.21. - ) reached a maximum, 1 day after harvest (DAH), then decreased, while the hydrolytic activity of cysteine protease (EC 3.4.22. - ) and aspartic protease (EC 3.4.23. - ) increased throughout the postharvest senescence based on the calculated inhibition percentage of protease activity. The senescence-associated proteases were separated into seven endoprotease (EP) groups by gelatin-polyacryamide gel electrophoresis and classified into EP1 (metalloprotease), EP2 (metalloprotease and cysteine protease), EP3 (serine protease and aspartic protease), EP4, EP5, EP7 (cysteine protease), and EP6 (serine protease) based on the sensitivity of class-specific protease inhibitors. The proteases EP2, EP3, and EP4 were present throughout the postharvest stages. EP3 was the major EP at all times during senescence; EP4 intensity of activity increased after 2 DAH; EP6 and EP7 clearly increased after 4 DAH. Our results suggest that serine protease activity contributes to early stage (0-1 DAH) and late stage (4-5 DAH) of senescence; metalloprotease activity was involved in the early and intermediate stages (0-3 DAH) of senescence; and cysteine protease and aspartic protease activities participated in the whole process of broccoli senescence.

Tomato phospholipid hydroperoxide glutathione peroxidase inhibits cell death induced by Bax and oxidative stresses in yeast and plants

Using a conditional life or death screen in yeast, we have isolated a tomato (Lycopersicon esculentum) gene encoding a phospholipid hydroperoxide glutathione peroxidase (LePHGPx). The protein displayed reduced glutathione-dependent phospholipid hydroperoxide peroxidase activity, but differs from counterpart mammalian enzymes that instead contain an active seleno-Cys. LePHGPx functioned as a cytoprotector in yeast (Saccharomyces cerevisiae), preventing Bax, hydrogen peroxide, and heat stress induced cell death, while also delaying yeast senescence. When tobacco (Nicotiana tabacum) leaves were exposed to lethal levels of salt and heat stress, features associated with mammalian apoptosis were observed. Importantly, transient expression of LePHGPx protected tobacco leaves from salt and heat stress and suppressed the apoptotic-like features. As has been reported, conditional expression of Bax was lethal in tobacco, resulting in tissue collapse and membrane permeability to Evans blue. When LePHGPx was coexpressed with Bax, little cell death and no vital staining were observed. Moreover, stable expression of LePHGPx in tobacco conferred protection against the fungal phytopathogen Botrytis cinerea. Taken together, our data indicated that LePHGPx can protect plant tissue from a variety of stresses. Moreover, functional screens in yeast are a viable tool for the identification of plant genes that regulate cell death.

Activity profiling of papain-like cysteine proteases in plants

Transcriptomic and proteomic technologies are generating a wealth of data that are frequently used by scientists to predict the function of proteins based on their expression or presence. However, activity of many proteins, such as transcription factors, kinases, and proteases, depends on posttranslational modifications that frequently are not detected by these technologies. Therefore, to monitor activity of proteases rather than their abundance, we introduce protease activity profiling in plants. This technology is based on the use of biotinylated, irreversible protease inhibitors that react with active proteases in a mechanism-based manner. Using a biotinylated derivative of the Cys protease inhibitor E-64, we display simultaneous activities of many papain-like Cys proteases in extracts from various tissues and from different plant species. Labeling is pH dependent, stimulated with reducing agents, and inhibited specifically by Cys protease inhibitors but not by inhibitors of other protease classes. Using one-step affinity capture of biotinylated proteases followed by sequencing mass spectrometry, we identified proteases that include xylem-specific XCP2, desiccation-induced RD21, and cathepsin B- and aleurain-like proteases. Together, these results demonstrate that this technology can identify differentially activated proteases and/or characterize the activity of a particular protease within complex mixtures.

Modification of senescence in ryegrass transformed with IPT under the control of a monocot senescence-enhanced promoter

We report here the genetic modification of ryegrass senescence. Embryogenic cell suspensions of Lolium multiflorum were transformed by microprojectile bombardment with plasmid constructs containing 1.98 kb of the 5' flanking sequence of SEE1 (a maize cysteine protease gene showing enhanced expression during senescence) fused either to the Agrobacterium tumefaciens cytokinin biosynthesis gene IPT (designated PSEE1::IPT) or to the beta-glucuronidase reporter gene UIDA (PSEE1::UIDA). Plants were regenerated under selection for the HPH hygromycin resistance gene in the vector. PSEE1::UIDA transformants confirmed that the SEE1 flanking sequence functioned as a senescence-enhanced promoter in ryegrass. The IPT transgene was detected in 28 regenerants (PSEE1::IPT) from five independent transformation events. PSEE1::IPT leaves displayed a stay-green phenotype. Some PSEE1::IPT lines developed spontaneous lesions.