A plant defense response effector induces microbial apoptosis

Osmotin from Calotropis procera latex: new insights into structure and antifungal properties

This study aimed at investigating the structural properties and mechanisms of the antifungal action of CpOsm, a purified osmotin from Calotropis procera latex. Fluorescence and CD assays revealed that the CpOsm structure is highly stable, regardless of pH levels. Accordingly, CpOsm inhibited the spore germination of Fusarium solani in all pH ranges tested. The content of the secondary structure of CpOsm was estimated as follows: α-helix (20%), β-sheet (33%), turned (19%) and unordered (28%), RMSD 1%. CpOsm was stable at up to 75°C, and thermal denaturation (T(m)) was calculated to be 77.8°C. This osmotin interacted with the negatively charged large unilamellar vesicles (LUVs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-1-glycerol (POPG), inducing vesicle permeabilization by the leakage of calcein. CpOsm induced the membrane permeabilization of spores and hyphae from Fusarium solani, allowing for propidium iodide uptake. These results show that CpOsm is a stable protein, and its antifungal activity involves membrane permeabilization, as property reported earlier for other osmotins and thaumatin-like proteins.

The small molecular mass antifungal protein of Penicillium chrysogenum--a mechanism of action oriented review

The β-lactam producing filamentous fungus Penicillium chrysogenum secretes a 6.25 kDa small molecular mass antifungal protein, PAF, which has a highly stable, compact 3D structure and is effective against a wide spectrum of plant and zoo pathogenic fungi. Its precise physiological functions and mode of action need to be elucidated before considering possible biomedical, agricultural or food technological applications. According to some more recent experimental data, PAF plays an important role in the fine-tuning of conidiogenesis in Penicillium chrysogenum. PAF triggers apoptotic cell death in sensitive fungi, and cell death signaling may be transmitted through two-component systems, heterotrimeric G protein coupled signal transduction and regulatory networks as well as via alteration of the Ca(2+) -homeostasis of the cells. Possible biotechnological applications of PAF are also outlined in the review.

The activity of antimicrobial peptide S-thanatin is independent on multidrug-resistant spectrum of bacteria

In this study, the activity of S-thanatin (an analog of antimicrobial peptide derived from thanatin) against different bacterial pathogens frequently which can cause therapeutic problems was tested. The result showed minimal inhibitory concentrations (MICs) of S-thanatin against all isolates of the Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Enterobacter aerogenes, Klebsiella ornithinolytica and Klebsiella oxytoca were in the range of 4-16μg/ml, no matter which antibiotic the bacterial was resistant or susceptible, while almost all MICs to Gram-positive bacterial were >128μg/ml except Enterococcus faecium. S-thanatin was more effective toward Gram-negative strains, especially for Enterobacter and Klebsiella. The MICs of S-thanatin were no significantly different in the same species regardless of antibiotic sensitive or -resistant isolates to single or multiple antibiotic (P>0.05). Likewise, no notable difference could be observed between E. coli, K. pneumoniae, E. cloacae, E. aerogenes, K. ornithinolytica which were sensitive to S-thanatin (P>0.05). It was implied that the antimicrobial activity of S-thanatin was independent on multi-drug resistance spectrum of bacteria.

The Antifungal Plant Defensin HsAFP1 from Heuchera sanguinea Induces Apoptosis in Candida albicans

Plant defensins are active against plant and human pathogenic fungi (such as Candida albicans) and baker's yeast. However, they are non-toxic to human cells, providing a possible source for treatment of fungal infections. In this study, we characterized the mode of action of the antifungal plant defensin HsAFP1 from coral bells by screening the Saccharomyces cerevisiae deletion mutant library for mutants with altered HsAFP1 sensitivity and verified the obtained genetic data by biochemical assays in S. cerevisiae and C. albicans. We identified 84 genes, which when deleted conferred at least fourfold hypersensitivity or resistance to HsAFP1. A considerable part of these genes were found to be implicated in mitochondrial functionality. In line, sodium azide, which blocks the respiratory electron transport chain, antagonized HsAFP1 antifungal activity, suggesting that a functional respiratory chain is indispensable for HsAFP1 antifungal action. Since mitochondria are the main source of cellular reactive oxygen species (ROS), we investigated the ROS-inducing nature of HsAFP1. We showed that HsAFP1 treatment of C. albicans resulted in ROS accumulation. As ROS accumulation is one of the phenotypic markers of apoptosis in yeast, we could further demonstrate that HsAFP1 induced apoptosis in C. albicans. These data provide novel mechanistic insights in the mode of action of a plant defensin.

Expression of truncated tobacco osmotin in Escherichia coli: purification and antifungal activity

PURPOSE OF WORK

Tobacco osmotin is a functional homolog of mammalian adiponectin, and has antifungal activity. This work was undertaken to produce recombinant osmotin that has previously been unsuccessful because of its toxicity. Expression of recombinant tobacco osmotin (rOSM) in Escherichia coli inclusion bodies has been achieved. The optimal pH for rOSM expression in ZYM 505 medium is 7.0 at OD(650) of 1.5 of culture growth. The rOSM from the inclusion body was extracted with 8 M urea, and purified using CM-cellulose and cobalt-agarose bead affinity chromatography to a high purity. Approximately 80% of the rOSM remained bound to CM-cellulose and Cobalt-agarose beads after initial elution. The yield of purified rOSM was between 40 and 50 mg from 2 l of culture. Repeated elution of protein from CM-cellulose and Co-agarose increased the yield of rOSM to 200 mg from 2 l culture. The purified rOSM showed variable antifungal activities against two pathogenic yeast strains; Cryptococcus neoformans, Candida albicans, and non-pathogenic strains; Saccharomyces cerevisiae and Pichia methanolica.

2-amino-nonyl-6-methoxyl-tetralin muriate activity against Candida albicans augments endogenous reactive oxygen species production --a microarray analysis study

Candida infections have become an increasingly significant problem, mainly because of the widespread nature of Candida and drug resistance. There is an urgent need to develop new classes of drugs for the treatment of opportunistic Candida infections, especially in medically complex patients. Previous studies have confirmed that 2-amino-nonyl-6-methoxyl-tetralin muriate (10b) possesses powerful antifungal activity in vitro against Candia albicans. To clarify the underlying action mechanism, an oligonucleotide microarray study was performed in C. albicans SC5314 without and with 10b treatment. The analytical results showed that energy metabolism-related genes, including glycolysis-related genes (PFK1, CDC19 and HXK2), fermentation-related genes (PDC11, ALD5 and ADH1) and respiratory electron transport chain-related genes (CBP3, COR1 and QCR8), were downregulated significantly. Functional analysis revealed that 10b treatment increased the generation of endogenous reactive oxygen species, and decreased mitochondrial membrane potential, ubiquinone-cytochrome c reductase (complex III) activity and intracellular ATP levels in C. albicans SC5314. Also, addition of the antioxidant ascorbic acid reduced the antifungal activity of 10b significantly. These results suggest that mitochondrial aerobic respiration shift and endogenous reactive oxygen species augmentation might contribute to the antifungal activity of 10b against C. albicans. This information may prove to be useful for the development of new strategies to treat Candida infections.

Genome-wide analysis of eukaryote thaumatin-like proteins (TLPs) with an emphasis on poplar

BACKGROUND

Plant inducible immunity includes the accumulation of a set of defense proteins during infection called pathogenesis-related (PR) proteins, which are grouped into families termed PR-1 to PR-17. The PR-5 family is composed of thaumatin-like proteins (TLPs), which are responsive to biotic and abiotic stress and are widely studied in plants. TLPs were also recently discovered in fungi and animals. In the poplar genome, TLPs are over-represented compared with annual species and their transcripts strongly accumulate during stress conditions.

RESULTS

Our analysis of the poplar TLP family suggests that the expansion of this gene family was followed by diversification, as differences in expression patterns and predicted properties correlate with phylogeny. In particular, we identified a clade of poplar TLPs that cluster to a single 350 kb locus of chromosome I and that are up-regulated by poplar leaf rust infection. A wider phylogenetic analysis of eukaryote TLPs - including plant, animal and fungi sequences - shows that TLP gene content and diversity increased markedly during land plant evolution. Mapping the reported functions of characterized TLPs to the eukaryote phylogenetic tree showed that antifungal or glycan-lytic properties are widespread across eukaryote phylogeny, suggesting that these properties are shared by most TLPs and are likely associated with the presence of a conserved acidic cleft in their 3D structure. Also, we established an exhaustive catalog of TLPs with atypical architectures such as small-TLPs, TLP-kinases and small-TLP-kinases, which have potentially developed alternative functions (such as putative receptor kinases for pathogen sensing and signaling).

CONCLUSION

Our study, based on the most recent plant genome sequences, provides evidence for TLP gene family diversification during land plant evolution. We have shown that the diverse functions described for TLPs are not restricted to specific clades but seem to be universal among eukaryotes, with some exceptions likely attributable to atypical protein structures. In the perennial plant model Populus, we unravelled the TLPs likely involved in leaf rust resistance, which will provide the foundation for further functional investigations.

Structural and functional similarities between osmotin from Nicotiana tabacum seeds and human adiponectin

Osmotin, a plant protein, specifically binds a seven transmembrane domain receptor-like protein to exert its biological activity via a RAS2/cAMP signaling pathway. The receptor protein is encoded in the gene ORE20/PHO36 and the mammalian homolog of PHO36 is a receptor for the human hormone adiponectin (ADIPOR1). Moreover it is known that the osmotin domain I can be overlapped to the β-barrel domain of adiponectin. Therefore, these observations and some already existing structural and biological data open a window on a possible use of the osmotin or of its derivative as adiponectin agonist. We have modelled the three-dimensional structure of the adiponectin trimer (ADIPOQ), and two ADIPOR1 and PHO36 receptors. Moreover, we have also modelled the following complexes: ADIPOQ/ADIPOR1, osmotin/PHO36 and osmotin/ADIPOR1. We have then shown the structural determinants of these interactions and their physico-chemical features and analyzed the related interaction residues involved in the formation of the complexes. The stability of the modelled structures and their complexes was always evaluated and controlled by molecular dynamics. On the basis of these results a 9 residues osmotin peptide was selected and its interaction with ADIPOR1 and PHO36 was modelled and analysed in term of energetic stability by molecular dynamics. To confirm in vivo the molecular modelling data, osmotin has been purified from nicotiana tabacum seeds and its nine residues peptide synthesized. We have used cultured human synovial fibroblasts that respond to adiponectin by increasing the expression of IL-6, TNF-alpha and IL-1beta via ADIPOR1. The biological effect on fibroblasts of osmotin and its peptide derivative has been found similar to that of adiponectin confirming the results found in silico.

A genomic approach highlights common and diverse effects and determinants of susceptibility on the yeast Saccharomyces cerevisiae exposed to distinct antimicrobial peptides

Background

The mechanism of action of antimicrobial peptides (AMP) was initially correlated with peptide membrane permeation properties. However, recent evidences indicate that action of a number of AMP is more complex and involves specific interactions at cell envelopes or with intracellular targets. In this study, a genomic approach was undertaken on the model yeast Saccharomyces cerevisiae to characterize the antifungal effect of two unrelated AMP.

Results

Two differentiated peptides were used: the synthetic cell-penetrating PAF26 and the natural cytolytic melittin. Transcriptomic analyses demonstrated distinctive gene expression changes for each peptide. Quantitative RT-PCR confirmed differential expression of selected genes. Gene Ontology (GO) annotation of differential gene lists showed that the unique significant terms shared by treatment with both peptides were related to the cell wall (CW). Assays with mutants lacking CW-related genes including those of MAPK signaling pathways revealed genes having influence on sensitivity to peptides. Fluorescence microscopy and flow cytometry demonstrated PAF26 interaction with cells and internalization that correlated with cell killing in sensitive CW-defective mutants such as Δecm33 or Δssd1. GO annotation also showed differential responses between peptides, which included ribosomal biogenesis, ARG genes from the metabolism of amino groups (specifically induced by PAF26), or the reaction to unfolded protein stress. Susceptibility of deletion mutants confirmed the involvement of these processes. Specifically, mutants lacking ARG genes from the metabolism of arginine pathway were markedly more resistant to PAF26 and had a functional CW. In the deletant in the arginosuccinate synthetase (ARG1) gene, PAF26 interaction occurred normally, thus uncoupling peptide interaction from cell killing. The previously described involvement of the glycosphingolipid gene IPT1 was extended to the peptides studied here.

Conclusions

Reinforcement of CW is a general response common after exposure to distinct AMP, and likely contributes to shield cells from peptide interaction. However, a weakened CW is not necessarily indicative of a higher sensitivity to AMP. Additional processes modulate susceptibility to specific peptides, exemplified in the involvement of the metabolism of amino groups in the case of PAF26. The relevance of the response to unfolded protein stress or the sphingolipid biosynthesis, previously reported for other unrelated AMP, was also independently confirmed.

Lipopeptide induces apoptosis in fungal cells by a mitochondria-dependent pathway

Bacillus amyloliquefaciens WH1 inhibit the growth of fungi by producing a new surfactin called as WH1fungin. WH1fungin plays an anti-fungal role by two models: high concentration to elicit pores on cell membrane and low concentration to induce apoptosis. WH1fungin can also inhibits the glucan synthase resulting in a decreased synthesis of callose on fungal cell wall. Further detection revealed that classical apoptotic markers such as reactive oxygen species (ROS) accumulation, phosphatidylserine (PS) externalization, DNA strand breaks and caspase-like activities could be found in fungal cells after treated by WH1fungin. Oligomycin was used as an inhibitor to block the mitochondria-dependent apoptosis in fungal cells, and results showed it could not inhibit but enhance the apoptosis induced by WH1fungin. After isolation by affinity chromatography, WH1fungin was found to bind with ATPase on the mitochondrial membrane and result in a decreased ATPase activity in fungal cells. This was further verified by treating fungal cells with FITC-labeled WH1fungin, which could bind to the mitochondrial membrane showing green fluorescence in fungal cells. After that, cytochrome C was released from the mitochondria, which then acted with caspase 9 to induce apoptosis by an intracellular pathway. High caspase 8 activity was also detectable in apoptotic fungal cells, indicating that an extracellular pathway might also be responsible for apoptosis induced by WH1fungin. Taken together, we report that lipopeptide can induce apoptosis in fungal cells, and induction of apoptosis by lipopeptide might be a common anti-fungal mechanism of Bacillus in the natural habitat.

The superfamily of thaumatin-like proteins: its origin, evolution, and expression towards biological function

Thaumatin-like proteins (TLPs) are the products of a large, highly complex gene family involved in host defence and a wide range of developmental processes in fungi, plants, and animals. Despite their dramatic diversification in organisms, TLPs appear to have originated in early eukaryotes and share a well-defined TLP domain. Nonetheless, determination of the roles of individual members of the TLP superfamily remains largely undone. This review summarizes recent advances made in elucidating the varied TLP activities related to host resistance to pathogens and other physiological processes. Also discussed is the current state of knowledge on the origins and types of TLPs, regulation of gene expression, and potential biotechnological applications for TLPs.

Use of the plant defense protein osmotin to identify Fusarium oxysporum genes that control cell wall properties

Fusarium oxysporum is the causative agent of fungal wilt disease in a variety of crops. The capacity of a fungal pathogen such as F. oxysporum f. sp. nicotianae to establish infection on its tobacco (Nicotiana tabacum) host depends in part on its capacity to evade the toxicity of tobacco defense proteins, such as osmotin. Fusarium genes that control resistance to osmotin would therefore reflect coevolutionary pressures and include genes that control mutual recognition, avoidance, and detoxification. We identified FOR (Fusarium Osmotin Resistance) genes on the basis of their ability to confer osmotin resistance to an osmotin-sensitive strain of Saccharomyces cerevisiae. FOR1 encodes a putative cell wall glycoprotein. FOR2 encodes the structural gene for glutamine:fructose-6-phosphate amidotransferase, the first and rate-limiting step in the biosynthesis of hexosamine and cell wall chitin. FOR3 encodes a homolog of SSD1, which controls cell wall composition, longevity, and virulence in S. cerevisiae. A for3 null mutation increased osmotin sensitivity of conidia and hyphae of F. oxysporum f. sp. nicotianae and also reduced cell wall beta-1,3-glucan content. Together our findings show that conserved fungal genes that determine cell wall properties play a crucial role in regulating fungal susceptibility to the plant defense protein osmotin.

Fungal apoptosis: function, genes and gene function

Cells of all living organisms are programmed to self-destruct under certain conditions. The most well known form of programmed cell death is apoptosis, which is essential for proper development in higher eukaryotes. In fungi, apoptotic-like cell death occurs naturally during aging and reproduction, and can be induced by environmental stresses and exposure to toxic metabolites. The core apoptotic machinery in fungi is similar to that in mammals, but the apoptotic network is less complex and of more ancient origin. Only some of the mammalian apoptosis-regulating proteins have fungal homologs, and the number of protein families is drastically reduced. Expression in fungi of animal proteins that do not have fungal homologs often affects apoptosis, suggesting functional conservation of these components despite the absence of protein-sequence similarity. Functional analysis of Saccharomyces cerevisiae apoptotic genes, and more recently of those in some filamentous species, has revealed partial conservation, along with substantial differences in function and mode of action between fungal and human proteins. It has been suggested that apoptotic proteins might be suitable targets for novel antifungal treatments. However, implementation of this approach requires a better understanding of fungal apoptotic networks and identification of the key proteins regulating apoptotic-like cell death in fungi.

Rice blast fungus (Magnaporthe oryzae) infects Arabidopsis via a mechanism distinct from that required for the infection of rice

Magnaporthe oryzae is a hemibiotrophic fungal pathogen that causes rice (Oryza sativa) blast. Although M. oryzae as a whole infects a wide variety of monocotyledonous hosts, no dicotyledonous plant has been reported as a host. We found that two rice pathogenic strains of M. oryzae, KJ201 and 70-15, interacted differentially with 16 ecotypes of Arabidopsis (Arabidopsis thaliana). Strain KJ201 infected all ecotypes with varying degrees of virulence, whereas strain 70-15 caused no symptoms in certain ecotypes. In highly susceptible ecotypes, small chlorotic lesions appeared on infected leaves within 3 d after inoculation and subsequently expanded across the affected leaves. The fungus produced spores in susceptible ecotypes but not in resistant ecotypes. Fungal cultures recovered from necrotic lesions caused the same symptoms in healthy plants, satisfying Koch's postulates. Histochemical analyses showed that infection by the fungus caused an accumulation of reactive oxygen species and eventual cell death. Similar to the infection process in rice, the fungus differentiated to form appressorium and directly penetrated the leaf surface in Arabidopsis. However, the pathogenic mechanism in Arabidopsis appears distinct from that in rice; three fungal genes essential for pathogenicity in rice played only limited roles in causing disease symptoms in Arabidopsis, and the fungus seems to colonize Arabidopsis as a necrotroph through the secretion of phytotoxic compounds, including 9,12-octadecadienoic acid. Expression of PR-1 and PDF1.2 was induced in response to infection by the fungus, suggesting the activation of salicylic acid- and jasmonic acid/ethylene-dependent signaling pathways. However, the roles of these signaling pathways in defense against M. oryzae remain unclear. In combination with the wealth of genetic and genomic resources available for M. oryzae, this newly established pathosystem allows comparison of the molecular and cellular mechanisms underlying pathogenesis and host defense in two well-studied model plants.

Apoptotic death of ageing yeast

Yeast has been a valuable model to study replicative and chronological ageing processes. Replicative ageing is defined by the number of daughter cells a mother can give birth to and hence reflects the ageing situation in proliferating cells, whereas chronological ageing is widely accepted as a model for postmitotic tissue ageing. Since both ageing forms end in yeast programmed death (necrotic and apoptotic), and abrogation of cell death by deletion of the apoptotic machinery or diminishment of oxidative radicals leads to longevity, apoptosis and ageing seem closely connected. This review focuses on ageing as a physiological way to induce yeast apoptosis, which unexpectedly defines apoptosis as a pro- and not an anti-ageing mechanism.

Human lactoferrin induces apoptosis-like cell death in Candida albicans: critical role of K+-channel-mediated K+ efflux

Human lactoferrin (hLf) induced an apoptosis-like phenotype in Candida albicans cells, which includes phosphatidylserine externalization, nuclear chromatin condensation, DNA degradation, and increased reactive oxygen species (ROS) production. Intracellular ROS accumulation was seen to correlate with candidacidal activity in hLf-treated cells. Mitochondrial activity was involved as indicated by mitochondrial depolarization and increased hLf resistance of cells preincubated with sordarin or erythromycin, the latter of which inhibits protein synthesis in mitoribosomes. Interestingly, Cl(-)- and K(+)-channel blockers prevented the hLf antimicrobial activity, but only when cells were pretreated with the blocking agent (tetraethylammonium) prior to the hLf-induced K(+)-release period. These results indicate for the first time that K(+)-channel-mediated K(+) efflux is required for the progression of apoptosis-like process in yeast, suggesting that this essential apoptotic event of higher eukaryotes has been evolutionary conserved among species ranging from yeasts to humans.

Kex1 protease is involved in yeast cell death induced by defective N-glycosylation, acetic acid, and chronological aging

N-glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to humans. The key step of this pathway is the transfer of the lipid-linked core oligosaccharide to the nascent polypeptide chain, catalyzed by the oligosaccharyltransferase complex. Temperature-sensitive oligosaccharyltransferase mutants of Saccharomyces cerevisiae at the restrictive temperature, such as wbp1-1, as well as wild-type cells in the presence of the N-glycosylation inhibitor tunicamycin display typical apoptotic phenotypes like nuclear condensation, DNA fragmentation, phosphatidylserine translocation, caspase-like activity, and reactive oxygen species accumulation. Since deletion of the yeast metacaspase YCA1 did not abrogate this death pathway, we postulated a different proteolytic process to be responsible. Here, we show that Kex1 protease is involved in the programmed cell death caused by defective N-glycosylation. Its disruption decreases caspase-like activity, production of reactive oxygen species, and fragmentation of mitochondria and, conversely, improves growth and survival of cells. Moreover, we demonstrate that Kex1 contributes also to the active cell death program induced by acetic acid stress or during chronological aging, suggesting that Kex1 plays a more general role in cellular suicide of yeast.

Apoptosis pathways in fungal growth, development and ageing

Apoptosis is one type of programmed cell death with great importance for development and homeostasis of multicellular organisms. Unexpectedly, during the past decade, evidence has been obtained for the existence of a basal apoptosis machinery in yeast, as unicellular fungus, and in some filamentous fungi, a group of microorganisms that are neither true unicellular nor true multicellular biological systems but something in between. Here, we review evidence for a role of apoptotic processes in fungal pathogenicity, competitiveness, propagation, ageing and lifespan control.

Chronological aging-induced apoptosis in yeast

Saccharomyces cerevisiae is the simplest among the major eukaryotic model organisms for aging and diseases. Longevity in the chronological life span paradigm is measured as the mean and maximum survival period of populations of non-dividing yeast. This paradigm has been used successfully to identify several life-regulatory genes and three evolutionary conserved pro-aging pathways. More recently, Schizosaccharomyces pombe has been shown to age chronologically in a manner that resembles that of S. cerevisiae and that depends on the activity of the homologues of two pro-aging proteins previously identified in the budding yeast. Both yeast show features of apoptotic death during chronological aging. Here, we review some fundamental aspects of the genetics of chronological aging and the overlap between yeast aging and apoptotic processes with particular emphasis on the identification of an aging/death program that favors the dedifferentiation and regrowth of a few better adapted mutants generated within populations of aging S. cerevisiae. We also describe the use of a genome-wide screening technique to gain further insights into the mechanisms of programmed death in populations of chronologically aging S. cerevisiae.

Negative regulation of apoptosis in yeast

In recent years, yeast has been proven to be a useful model organism for studying programmed cell death. It not only exhibits characteristic markers of apoptotic cell death when heterologous inducers of apoptosis are expressed or when treated with apoptosis inducing drugs such as hydrogen peroxide (H(2)O(2)) or acetic acid, but contains homologues of several components of the apoptotic machinery identified in mammals, flies and nematodes, such as caspases, apoptosis inducing factor (AIF), Omi/HtrA2 and inhibitor-of-apoptosis proteins (IAPs). In this review, we focus on the role of negative regulators of apoptosis in yeasts. Bir1p is the only IAP protein in Saccharomyces cerevisiae and has long been known to play a role in cell cycle progression by acting as kinetochore and chromosomal passenger protein. Recent data established Bir1p's protective function against programmed cell death induced by H(2)O(2) treatment and in chronological ageing. Other factors that have a direct or indirect influence on intracellular levels of reactive oxygen species (ROS) and thus lead to apoptosis if they are misregulated or non-functional will be discussed.

Program-like aging and mitochondria: instead of random damage by free radicals

As recently suggested, the target of rapamycin (TOR) pathway, rather than molecular damage by free radicals, drives aging and diseases of aging. But may mitochondria nevertheless contribute to aging? Here, I discuss aimless program-like aging (versus altruistic program), conflict between the cell and mitochondria, cell murder (versus cell suicide) and the role of mitochondria in aging. In particular, life-long selection among mitochondria may yield "selfish" (malignant) mitochondria resistant to autophagy. And TOR may create an intra-cellular environment that is permissive for such selfish mitochondria. In theory, pharmacologic inhibitors of the TOR pathway may reverse accumulation of defective mitochondria, while also inhibiting the aging process.

Expression in Escherichia coli, purification, refolding and antifungal activity of an osmotin from Solanum nigrum

BACKGROUND

Heterologous protein expression in microorganisms may contribute to identify and demonstrate antifungal activity of novel proteins. The Solanum nigrum osmotin-like protein (SnOLP) gene encodes a member of pathogenesis-related (PR) proteins, from the PR-5 sub-group, the last comprising several proteins with different functions, including antifungal activity. Based on deduced amino acid sequence of SnOLP, computer modeling produced a tertiary structure which is indicative of antifungal activity.

RESULTS

To validate the potential antifungal activity of SnOLP, a hexahistidine-tagged mature SnOLP form was overexpressed in Escherichia coli M15 strain carried out by a pQE30 vector construction. The urea solubilized His6-tagged mature SnOLP protein was affinity-purified by immobilized-metal (Ni2+) affinity column chromatography. As SnOLP requires the correct formation of eight disulfide bonds, not correctly formed in bacterial cells, we adapted an in vitro method to refold the E. coli expressed SnOLP by using reduced:oxidized gluthatione redox buffer. This method generated biologically active conformations of the recombinant mature SnOLP, which exerted antifungal action towards plant pathogenic fungi (Fusarium solani f. sp.glycines, Colletotrichum spp., Macrophomina phaseolina) and oomycete (Phytophthora nicotiana var. parasitica) under in vitro conditions.

CONCLUSION

Since SnOLP displays activity against economically important plant pathogenic fungi and oomycete, it represents a novel PR-5 protein with promising utility for biotechnological applications.

Drug-induced apoptosis in yeast

In order to alter the impact of diseases on human society, drug development has been one of the most invested research fields. Nowadays, cancer and infectious diseases are leading targets for the design of effective drugs, in which the primary mechanism of action relies on the modulation of programmed cell death (PCD). Due to the high degree of conservation of basic cellular processes between yeast and higher eukaryotes, and to the existence of an ancestral PCD machinery in yeast, yeasts are an attractive tool for the study of affected pathways that give insights into the mode of action of both antitumour and antifungal drugs. Therefore, we covered some of the leading reports on drug-induced apoptosis in yeast, revealing that in common with mammalian cells, antitumour drugs induce apoptosis through reactive oxygen species (ROS) generation and altered mitochondrial functions. The evidence presented suggests that yeasts may be a powerful model for the screening/development of PCD-directed drugs, overcoming the problem of cellular specificity in the design of antitumour drugs, but also enabling the design of efficient antifungal drugs, targeted to fungal-specific apoptotic regulators that do not have major consequences for human cells.

The antifungal activity of RsAFP2, a plant defensin from raphanus sativus, involves the induction of reactive oxygen species in Candida albicans

RsAFP2 (Raphanus sativus antifungal peptide 2), an antifungal plant defensin isolated from seed of R. sativus, interacts with glucosylceramides (GlcCer) in membranes of susceptible yeast and fungi and induces membrane permeabilization and fungal cell death. However, using carboxyfluorescein-containing small unilamellar vesicles containing purified GlcCer, we could not observe permeabilization as a consequence of insertion of RsAFP2 in such vesicles. Therefore, we focused on a putative RsAFP2-induced signaling cascade downstream of RsAFP2-binding to GlcCer in fungal membranes. We show that RsAFP2 induces reactive oxygen species (ROS) in Candida albicans wild type in a dose-dependent manner, but not at all in an RsAFP2-resistant DeltagcsC. albicans mutant that lacks the RsAFP2-binding site in its membranes. These findings indicate that upstream binding of RsAFP2 to GlcCer is needed for ROS production leading to yeast cell death. Moreover, the antioxidant ascorbic acid blocks RsAFP2-induced ROS generation, as well as RsAFP2 antifungal activity. These data point to the presence of an intracellular plant defensin-induced signaling cascade, which involves ROS generation and leads to fungal cell growth arrest.

Isolation and characterization of novel peptides from chilli pepper seeds: Antimicrobial activities against pathogenic yeasts

Different types of antimicrobial peptides have been identified in seeds from different plant species. The aim of this study was to isolate and characterize peptides present in chilli pepper seeds (Capsicum annuum L.) and evaluate their toxic activities against some yeast species. Initially, proteins from seed flour were extracted in phosphate buffer, pH 5.4, for 3 h at 4 degrees C and the pellet obtained at 90% saturation with ammonium sulfate was heated at 80 degrees C for 15 min. The resulting suspension was clarified by centrifugation and the supernatant was extensively dialyzed against water; the peptide-rich extract was then named F/0-90. Cation-exchange chromatography was performed to separate low molecular mass proteins. One of the resulting fractions, named F3, enriched with basic proteins of 6-16 kDa, was submitted to reverse-phase chromatography in a C2/C18 column by HPLC, resulting in four fractions denominated RP1, RP2, RP3 and RP4. When these fractions were submitted to N-terminal sequencing, the comparative analysis in databanks revealed homology for two of these peptides, isolated from fractions RP3 and RP4, with sequences of proteinase inhibitors and 2S albumins, respectively. The F3 fraction, rich in peptides, inhibited the growth of yeasts Saccharomyces cerevisiae, Candida albicans, Candida parapsilosis, Candida tropicalis, Pichia membranifaciens, Kluyveromyces marxiannus and Candida guilliermondii. The RP3 and RP4 fractions showed high inhibitory activity against the growth of the yeast S. cerevisiae. The F3 fraction was also able to inhibit glucose-stimulated acidification of the medium by yeast cells of S. cerevisiae and to cause several morphological changes in different yeasts, such as cell wall disorganization, bud formation as well as the formation of pseudohyphae.

Apoptosis in yeast--mechanisms and benefits to a unicellular organism

Initial observations that the budding yeast Saccharomyces cerevisiae can be induced to undergo a form of cell death exhibiting typical markers of apoptosis has led to the emergence of a thriving new field of research. Since this discovery, a number of conserved pro- and antiapoptotic proteins have been identified in yeast. Indeed, early experiments have successfully validated yeasts as a powerful genetic tool with which to investigate mechanisms of apoptosis. However, we still have little understanding as to why programmes of cell suicide exist in unicellular organisms and how they may be benefit such organisms. Recent research has begun to elucidate pathways that regulate yeast apoptosis in response to environmental stimuli. These reports strengthen the idea that physiologically relevant mechanisms of programmed cell death are present, and that these function as important regulators of yeast cell populations.

Arsenic induces caspase- and mitochondria-mediated apoptosis in Saccharomyces cerevisiae

In recent years, it has been shown that yeast, a unicellular organism, undergoes apoptosis in response to various factors. Here we demonstrate that the highly effective anticancer agent arsenic induces apoptotic process in yeast cells. Reactive oxygen species (ROS) production was observed in the process. Moreover, mitochondrial membrane potential decreased after arsenic treatment. Resistance of the rho(0) mutant strain (lacking mtDNA) to arsenic provides further evidence that this death process involves mitochondria. In addition, hypersensitivity of Deltasod1 to arsenic suggests the critical role of ROS. Cell death and DNA fragmentation decreased in a Deltayca1 deletion mutant, indicating the participation of yeast caspase-1 protein in apoptosis. The implications of these findings for arsenic-induced apoptosis are discussed.

An amphibian-derived, cationic, ?-helical antimicrobial peptide kills yeast by caspase-independent but AIF-dependent programmed cell death

The dermaseptins are a family of antimicrobial peptides from the tree-frog Phyllomedusa sauvagii. Yeast exposed to dermaseptin S3(1-16), a truncated derivative of dermaseptin S3 with full activity, showed diagnostic markers of yeast apoptosis: the appearance of reactive oxygen species and fragmentation of nuclear DNA. This process was independent of the yeast caspase, Yca1p. Screening of a non-essential gene deletion collection in yeast identified genes that conferred resistance to dermaseptin S3(1-16): izh2Delta, izh3Delta, stm1Delta and aif1Delta, all known to be involved in regulating yeast apoptosis. The appearance of apoptotic markers was reduced in these strains when exposed to the peptide. Dermaseptin S3(1-16) was shown to interact with DNA, and cause DNA damage in vivo, a process known to trigger apoptosis. Supporting this, a dermaseptin S3(1-16) affinity column specifically purified Stm1p, Mre11p and Htb2p; DNA-binding proteins implicated in yeast apoptosis and DNA repair. Thus, amphibians may have evolved a mechanism to induce cell suicide in invading fungal pathogens.

alpha-Tomatine, the major saponin in tomato, induces programmed cell death mediated by reactive oxygen species in the fungal pathogen Fusarium oxysporum

The tomato saponin alpha-tomatine has been proposed to kill sensitive cells by binding to cell membranes followed by leakage of cell components. However, details of the modes of action of the compound on fungal cells are poorly understood. In the present study, mechanisms involved in alpha-tomatine-induced cell death of fungi were examined using a filamentous pathogenic fungus Fusarium oxysporum. alpha-Tomatine-induced cell death of F. oxysporum (TICDF) occurred only under aerobic conditions and was blocked by the mitochondrial F(0)F(1)-ATPase inhibitor oligomycin, the caspase inhibitor D-VAD-fmk, and protein synthesis inhibitor cycloheximide. Fungal cells exposed to alpha-tomatine showed TUNEL-positive nuclei, depolarization of transmembrane potential of mitochondria, and reactive oxygen species (ROS) accumulation. These results suggest that TICDF occurs through a programmed cell death process in which mitochondria play a pivotal role. Pharmacological studies using inhibitors suggest that alpha-tomatine activates phosphotyrosine kinase and monomeric G-protein signaling pathways leading to Ca(2+) elevation and ROS burst in F. oxysporum cells.

Probing the mechanism of FET3 repression by Izh2p overexpression

We previously reported a role for the IZH2 gene product in metal ion metabolism. Subsequently, Izh2p was also identified as a member of the PAQR family of receptors and, more specifically, as the receptor for the plant protein osmotin. In this report, we investigate the effect of Izh2p on iron homeostasis. We show that overproduction of Izh2p prevents the iron-dependent induction of the Fet3p component of the high-affinity iron-uptake system and is deleterious for growth in iron-limited medium. We demonstrate that the effect of Izh2p requires cAMP-dependent kinase and AMP-dependent kinase and is not mediated by general inhibition of the Aft1p iron-responsive transcriptional activator. We also show that Izh2p-overproduction negatively regulates Nrg1p/Nrg2p- and Msn2p/Msn4p-dependent reporters. Furthermore, we show that the Nrg1p/Nrg2p and Msn2p/Msn4p pairs are epistatic to each other with respect to their effects on FET3 expression. Finally, we show that the mechanism by which PAQR receptors activate signal transduction pathways is likely to be conserved from yeast to humans.

Osmotin induces cold protection in olive trees by affecting programmed cell death and cytoskeleton organization

Osmotin is a pathogenesis-related protein exhibiting cryoprotective functions. Our aim was to understand whether it is involved in the cold acclimation of the olive tree (Olea europaea L.), a frost-sensitive species lacking dormancy. We exposed olive trees expressing tobacco osmotin gene under the 35S promoter (35S:osm) [in the same manner as wild type (wt) plants] to cold shocks in the presence/absence of cold acclimation, and monitored changes in programmed cell death (PCD), cytoskeleton, and calcium ([Ca2+]c) signalling. In the wt, osmotin was immunolocalized only in cold-acclimated plants, and in the tissues showing PCD. In the 35S:osm clones, the protein was detected also in the non-acclimated plants, and always in the tissues exhibiting PCD. In the non-acclimated wt protoplasts exposed to cold shock, a transient decrease in phallotoxin signal suggests a temporary disassembly of F-actin, a transient increase occurred instead in 35S:osm protoplasts exposed to the same shock. Transient increases in [Ca2+]c were observed only in the wt protoplasts. However, when F-actin was depolymerized by cytochalasin or latrunculin, and microtubules by colchicine, increase in [Ca2+]c also occurred in the 35S:osm protoplasts. Successive cold shocks caused transient rises in [Ca2+]c and transient decreases in the phallotoxin signal in wt protoplasts. No change occurred in [Ca2+]c occurred in the 35S:osm protoplasts. The phallotoxin signal transiently increased at the first shock, but did not change after the subsequent shocks, and an overall signal reduction occurred with shock repetition. Following acclimation, no cold shock-induced change in [Ca2+]c levels and F-actin signal occurred either in wt or 35S:osm protoplasts. The results show that osmotin is positively involved in the acclimation-related PCD, in blocking the cold-induced calcium signalling, and in affecting cytoskeleton in response to cold stimuli.

Isolation of an osmotin-like protein gene from strawberry and analysis of the response of this gene to abiotic stresses

A strawberry genomic clone containing an osmotin-like protein (OLP) gene, designated FaOLP2, was isolated and sequenced. FaOLP2 is predicted to encode a precursor protein of 229 amino acid residues, and its sequence shares high degrees of homology with a number of other OLPs. Genomic DNA hybridization analysis indicated that FaOLP2 represents a multi-gene family. The expression of FaOP2 in different strawberry organs was analyzed using real-time PCR. The results showed that FaOLP2 expressed at different levels in leaves, crowns, roots, green fruits and ripe red fruits. In addition, the expression of FaOLP2 under different abiotic stresses was analyzed at different time points. All of the three tested abiotic stimuli, abscisic acid, salicylic acid and mechanical wounding, triggered a significant induction of FaOLP2 within 2-6h post-treatment. Moreover, FaOLP2 was more prominently induced by salicylic acid than by abscisic acid or mechanical wounding. The positive responses of FaOLP2 to the three abiotic stimuli suggested that strawberry FaOLP2 may help to protect against osmotic-related environmental stresses and that it may also be involved in plant defense system against pathogens.

TheAspergillus fumigatusmetacaspases CasA and CasB facilitate growth under conditions of endoplasmic reticulum stress

We have examined the contribution of metacaspases to the growth and stress response of the opportunistic human mould pathogen, Aspergillus fumigatus, based on increasing evidence implicating the yeast metacaspase Yca1p in apoptotic-like programmed cell death. Single metacaspase-deficient mutants were constructed by targeted disruption of each of the two metacaspase genes in A. fumigatus, casA and casB, and a metacaspase-deficient mutant, DeltacasA/DeltacasB, was constructed by disrupting both genes. Stationary phase cultures of wild-type A. fumigatus were associated with the appearance of typical markers of apoptosis, including elevated proteolytic activity against caspase substrates, phosphatidylserine exposure on the outer leaflet of the membrane, and loss of viability. By contrast, phosphatidylserine exposure was not observed in stationary phase cultures of the DeltacasA/DeltacasB mutant, although caspase activity and viability was indistinguishable from wild type. The mutant retained wild-type virulence and showed no difference in sensitivity to a range of pro-apoptotic stimuli that have been reported to initiate yeast apoptosis. However, the DeltacasA/DeltacasB mutant showed a growth detriment in the presence of agents that disrupt endoplasmic reticulum homeostasis. These findings demonstrate that metacaspase activity in A. fumigatus contributes to the apoptotic-like loss of membrane phospholipid asymmetry at stationary phase, and suggest that CasA and CasB have functions that support growth under conditions of endoplasmic reticulum stress.

The antifungal properties of a 2S albumin-homologous protein from passion fruit seeds involve plasma membrane permeabilization and ultrastructural alterations in yeast cells

Different types of antimicrobial proteins were purified from plant seeds, including chitinases, β-1,3-glucanases, defensins, thionins, lipid transfer proteins and 2S albumins. It has become clear that these groups of proteins play an important role in the protection of plants from microbial infection. Recent results from our laboratory have shown that the defense-related proteins from passion fruit seeds, named Pf1 and Pf2 (which show sequence homology with 2S albumins), inhibit fungal growth and glucose-stimulated acidification of the medium by Saccharomyces cerevisiae cells. The aim of this study was to determine whether 2S albumins from passion fruit seeds induce plasma membrane permeabilization and cause morphological alterations in yeast cells. Initially, we used an assay based on the uptake of SYTOX Green, an organic compound that fluoresces upon interaction with nucleic acids and penetrates cells with compromised plasma membranes, to investigate membrane permeabilization in S. cerevisiae cells. When viewed with a confocal laser microscope, S. cervisiae cells showed strong SYTOX Green fluorescence in the cytosol, especially in the nuclei. 2S albumins also inhibited glucose-stimulated acidification of the medium by S. cerevisiae cells, which indicates a probable impairment of fungal metabolism. The microscopical analysis of the yeast cells treated with 2S albumins demonstrated several morphological alterations in cell shape, cell surface, cell wall and bud formation, as well as in the organization of intracellular organelles.

Peptide antimicrobial agents

Antimicrobial host defense peptides are produced by all complex organisms as well as some microbes and have diverse and complex antimicrobial activities. Collectively these peptides demonstrate a broad range of antiviral and antibacterial activities and modes of action, and it is important to distinguish between direct microbicidal and indirect activities against such pathogens. The structural requirements of peptides for antiviral and antibacterial activities are evaluated in light of the diverse set of primary and secondary structures described for host defense peptides. Peptides with antifungal and antiparasitic activities are discussed in less detail, although the broad-spectrum activities of such peptides indicate that they are important host defense molecules. Knowledge regarding the relationship between peptide structure and function as well as their mechanism of action is being applied in the design of antimicrobial peptide variants as potential novel therapeutic agents.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

Bioenergetic aspects of apoptosis, necrosis and mitoptosis

In this review I summarize interrelations between bioenergetic processes and such programmed death phenomena as cell suicide (apoptosis and necrosis) and mitochondrial suicide (mitoptosis). The following conclusions are made. (I) ATP and rather often mitochondrial hyperpolarization (i.e. an increase in membrane potential, delta psi) are required for certain steps of apoptosis and necrosis. (II) Apoptosis, even if it is accompanied by delta psi and [ATP] increases at its early stage, finally results in a delta psi collapse and ATP decrease. (III) Moderate (about three-fold) lowering of [ATP] for short and long periods of time induces apoptosis and necrosis, respectively. In some types of apoptosis and necrosis, the cell death is mediated by a delta psi-dependent overproduction of ROS by the initial (Complex I) and the middle (Complex III) spans of the respiratory chain. ROS initiate mitoptosis which is postulated to rid the intracellular population of mitochondria from those that are ROS overproducing. Massive mitoptosis can result in cell death due to release to cytosol of the cell death proteins normally hidden in the mitochondrial intermembrane space.

Mating-type locus control of killer toxins fromKluyveromyces lactisandPichia acaciae

Killer-toxin complexes produced by Kluyveromyces lactis and Pichia acaciae inhibit cell proliferation of Saccharomyces cerevisiae. Analysis of their actions in haploid MATalpha cells revealed that introduction of the opposite mating-type locus (MATa) significantly suppressed antizymosis. Together with resistance expressed by MATa/MATalpha diploids, the reciprocal action of MATa or MATalpha in haploids of opposite mating types suggests that these killer toxins may be subject to MAT locus control. Congruently, derepressing the silent mating-type loci, HMR and HML, by removing individual components of the histone deacetylase complex Sir1-4, either by transposon-tagging or by chemically inactivating the histone deacetylase catalytic subunit Sir2, yields toxin resistance. Consistent with MAT control of toxin action, killer-toxin-insensitive S. cerevisiae mutants (kti) become mating-compromised despite resisting the toxins' cell-cycle effects. Mating inhibition largely depends on the time point of toxin application to the mating mixtures and is less pronounced in Elongator mutants, whose resistance to the toxins' cell-cycle effects is the result of toxin-target process deficiencies. In striking contrast, non-Elongator mutants defective in early-response events such as toxin import/activation hardly recover from toxin-induced mating inhibition. This study reveals a novel effect of yeast killer toxins on mating and sexual reproduction that is independent of their impact on cellular proliferation and cell-cycle progression.

Ras pathway signaling accelerates programmed cell death in the pathogenic fungus Candida albicans

A better understanding of the molecular basis of programmed cell death (PCD) in fungi could provide information that is useful in the design of antifungal drugs that combat life-threatening fungal infections. Harsh environmental stresses, such as acetic acid or hydrogen peroxide, have been shown to induce PCD in the pathogenic fungus Candida albicans. In this study, we show that dying cells progress from an apoptotic state to a secondary necrotic state and that the rate at which this change occurs is proportional to the intensity of the stimulus. Also, we found that the temporal response is modulated by Ras-cAMP-PKA signals. Mutations that block Ras-cAMP-PKA signaling (ras1Delta, cdc35Delta, tpk1Delta, and tpk2Delta) suppress or delay the apoptotic response, whereas mutations that stimulate signaling (RAS1(val13) and pde2Delta) accelerate the rate of entry of cells into apoptosis. Pharmacological stimulation or inhibition of Ras signaling reinforces these findings. Transient increases in endogenous cAMP occur under conditions that stimulate apoptosis but not growth arrest. Death-specific changes in the abundance of different isoforms of the PKA regulatory subunit, Bcy1p, are also observed. Activation of Ras signals may regulate PCD of C. albicans, either by inhibiting antiapoptotic functions (such as stress responses) or by activating proapoptotic functions.

Programmed and altruistic ageing

Ageing is widely believed to be a non-adaptive process that results from a decline in the force of natural selection. However, recent studies in Saccharomyces cerevisiae are consistent with the existence of a programme of altruistic ageing and death. We suggest that the similarities between the molecular pathways that regulate ageing in yeast, worms, flies and mice, together with evidence that is consistent with programmed death in salmon and other organisms, raise the possibility that programmed ageing or death can also occur in higher eukaryotes.

Old yellow enzyme interferes with Bax-induced NADPH loss and lipid peroxidation in yeast

The yeast transcriptional response to murine Bax expression was compared with the changes induced by H(2)O(2) treatment via microarray technology. Although most of the Bax-responsive genes were also triggered by H(2)O(2) treatment, OYE3, ICY2, MLS1 and BTN2 were validated to have a Bax-specific transcriptional response not shared with the oxidative stress trigger. In knockout experiments, only deletion of OYE3, coding for yeast Old yellow enzyme, attenuated the rate of Bax-induced growth arrest, cell death and NADPH decrease. Lipid peroxidation was completely absent in DeltaOYE3 expressing Bax. However, the absence of OYE3 sensitized yeast cells to H(2)O(2)-induced cell death, and increased the rate of NADPH decrease and lipid peroxidation. Our results clearly indicate that OYE3 interferes with Bax- and H(2)O(2)-induced lipid peroxidation and cell death in Saccharomyces cerevisiae.

Plant-induced cell death in the oomycete pathogen Phytophthora parasitica

The activation of programmed cell death in the host during plant-pathogen interactions is an important component of the plant disease resistance mechanism. In this study we show that activation of programmed cell death in microorganisms also regulates plant-pathogen interactions. We found that a form of vacuolar cell death is induced in the oomycete Phytophthora parasitica--the agent that causes black shank disease in Nicotiana tabacum--by extracellular stimuli from resistant tobacco. The single-celled zoospores underwent cell death characterized by dynamic membrane rearrangements, cell shrinkage, formation of numerous large vacuoles in the cytoplasm and degradation of cytoplasmic components before plasma membrane disruption. Phytophthora cell death required protein synthesis but not caspase activation, and was associated with the production of intracellular reactive oxygen species. This characterization of plant-mediated cell death signalling in pathogens will enhance our understanding of the biological processes regulating plant-pathogen interactions, and improve our ability to control crop diseases.

Antifungal protein PAF severely affects the integrity of the plasma membrane of Aspergillus nidulans and induces an apoptosis-like phenotype

The small, basic, and cysteine-rich antifungal protein PAF is abundantly secreted into the supernatant by the beta-lactam producer Penicillium chrysogenum. PAF inhibits the growth of various important plant and zoopathogenic filamentous fungi. Previous studies revealed the active internalization of the antifungal protein and the induction of multifactorial detrimental effects, which finally resulted in morphological changes and growth inhibition in target fungi. In the present study, we offer detailed insights into the mechanism of action of PAF and give evidence for the induction of a programmed cell death-like phenotype. We proved the hyperpolarization of the plasma membrane in PAF-treated Aspergillus nidulans hyphae by using the aminonaphtylethenylpyridinium dye di-8-ANEPPS. The exposure of phosphatidylserine on the surface of A. nidulans protoplasts by Annexin V staining and the detection of DNA strand breaks by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) gave evidence for a PAF-induced apoptotic-like mechanism in A. nidulans. The localization of reactive oxygen species (ROS) by dichlorodihydrofluorescein diacetate and the abnormal cellular ultrastructure analyzed by transmission electron microscopy suggested that ROS-elicited membrane damage and the disintegration of mitochondria played a major role in the cytotoxicity of PAF. Finally, the reduced PAF sensitivity of A. nidulans strain FGSC1053, which carries a dominant-interfering mutation in fadA, supported our assumption that G-protein signaling was involved in PAF-mediated toxicity.

G-protein beta subunit of Cochliobolus heterostrophus involved in virulence, asexual and sexual reproductive ability, and morphogenesis

Previous work established that mutations in mitogen-activated protein (MAP) kinase (CHK1) and heterotrimeric G-protein alpha (Galpha) subunit (CGA1) genes affect the development of several stages of the life cycle of the maize pathogen Cochliobolus heterostrophus. The effects of mutating a third signal transduction pathway gene, CGB1, encoding the Gbeta subunit, are reported here. CGB1 is the sole Gbeta subunit-encoding gene in the genome of this organism. cgb1 mutants are nearly wild type in vegetative growth rate; however, Cgb1 is required for appressorium formation, female fertility, conidiation, regulation of hyphal pigmentation, and wild-type virulence on maize. Young hyphae of cgb1 mutants grow in a straight path, in contrast to those of the wild type, which grow in a wavy pattern. Some of the phenotypes conferred by mutations in CGA1 are found in cgb1 mutants, suggesting that Cgb1 functions in a heterotrimeric G protein; however, there are also differences. In contrast to the deletion of CGA1, the loss of CGB1 is not lethal for ascospores, evidence that there is a Gbeta subunit-independent signaling role for Cga1 in mating. Furthermore, not all of the phenotypes conferred by mutations in the MAP kinase CHK1 gene are found in cgb1 mutants, implying that the Gbeta heterodimer is not the only conduit for signals to the MAP kinase CHK1 module. The additional phenotypes of cgb1 mutants, including severe loss of virulence on maize and of the ability to produce conidia, are consistent with CGB1 being unique in the genome. Fluorescent DNA staining showed that there is often nuclear degradation in mature hyphae of cgb1 mutants, while comparable wild-type cells have intact nuclei. These data may be genetic evidence for a novel cell death-related function of the Gbeta subunit in filamentous fungi.