HrpZPsph from the plant pathogen Pseudomonas syringae pv. phaseolicola binds to lipid bilayers and forms an ion-conducting pore in vitro

Importance of microbial amphiphiles: interaction potential of biosurfactants, amyloids, and other exo-polymeric-substances

Microorganisms produce a diverse group of biomolecules having amphipathic nature (amphiphiles). Microbial amphiphiles, including amyloids, bio-surfactants, and other exo-polymeric substances, play a crucial role in various biological processes and have gained significant attention recently. Although diverse in biochemical composition, these amphiphiles have been reported for common microbial traits like biofilm formation and pathogenicity due to their ability to act as surface active agents with active interfacial properties essential for microbes to grow in various niches. This enables microbes to reduce surface tension, emulsification, dispersion, and attachment at the interface. In this report, the ecological importance and biotechnological usage of important amphiphiles have been discussed. The low molecular weight amphiphiles like biosurfactants, siderophores, and peptides showing helical and antimicrobial activities have been extensively reported for their ability to work as quorum-sensing mediators. While high molecular weight amphiphiles make up amyloid fibers, exopolysaccharides, liposomes, or magnetosomes have been shown to have a significant influence in deciding microbial physiology and survival. In this report, we have discussed the functional similarities and biochemical variations of several amphipathic biomolecules produced by microbes, and the present report shows these amphiphiles showing polyphyletic and ecophysiological groups of microorganisms and hence can `be replaced in biotechnological applications depending on the compatibility of the processes.

Pseudomonas syringae Type III Secretion Protein HrpP Manipulates Plant Immunity To Promote Infection

The bacterial plant pathogen Pseudomonas syringae deploys a type III secretion system (T3SS) to deliver effector proteins into plant cells to facilitate infection, for which many effectors have been characterized for their interactions. However, few T3SS Hrp (hypersensitive response and pathogenicity) proteins from the T3SS secretion apparatus have been studied for their direct interactions with plants. Here, we show that the P. syringae pv. tomato DC3000 T3SS protein HrpP induces host cell death, suppresses pattern-triggered immunity (PTI), and restores the effector translocation ability of the hrpP mutant. The hrpP-transgenic Arabidopsis lines exhibited decreased PTI responses to flg22 and elf18 and enhanced disease susceptibility to P. syringae pv. tomato DC3000. Transcriptome analysis reveals that HrpP sensing activates salicylic acid (SA) signaling while suppressing jasmonic acid (JA) signaling, which correlates with increased SA accumulation and decreased JA biosynthesis. Both yeast two-hybrid and bimolecular fluorescence complementation assays show that HrpP interacts with mitogen-activated protein kinase kinase 2 (MKK2) on the plant membrane and in the nucleus. The HrpP truncation HrpP1-119, rather than HrpP1-101, retains the ability to interact with MKK2 and suppress PTI in plants. In contrast, HrpP1-101 continues to cause cell death and electrolyte leakage. MKK2 silencing compromises SA signaling but has no effect on cell death caused by HrpP. Overall, our work highlights that the P. syringae T3SS protein HrpP facilitates effector translocation and manipulates plant immunity to facilitate bacterial infection. IMPORTANCE The T3SS is required for the virulence of many Gram-negative bacterial pathogens of plants and animals. This study focuses on the sensing and function of the T3SS protein HrpP during plant interactions. Our findings show that HrpP and its N-terminal truncation HrpP1-119 can interact with MKK2, promote effector translocation, and manipulate plant immunity to facilitate bacterial infection, highlighting the P. syringae T3SS component involved in the fine-tuning of plant immunity.

Bacillus cereus AR156 Extracellular Polysaccharides Served as a Novel Micro-associated Molecular Pattern to Induced Systemic Immunity to Pst DC3000 in Arabidopsis

Non-host resistance (NHR) is a broad-spectrum plant defense. Upon colonizing on the surface on the root or leaves of non-host species, pathogens initial encounter preform and induce defense response in plant, such as induced hypersensitive response, PAMPs triggered immunity (PTI), and effector triggered immunity (ETI). The ability of plants to develop an induced systemic response (ISR) in reaction to the colonization by non-pathogenic rhizobacterium depends on interactions between host plants and the colonizing rhizobacterium, and the ISR also can be defined as a NHR. However, how the colonization signal is and how systemic resistance to pathogens is developed is still unclear. In this study, we demonstrated that the extracellular polysaccharides (EPSs) of Bacillus cereus AR156 could act as novel microbe-associated molecular patterns (MAMPs) and function in the early perception status of the ISR of B. cereus AR156. The results revealed that B. cereus AR156 EPS could induce systemic resistance to Pst DC3000 in Arabidopsis. Cellular defense response markers such as hydrogen peroxide accumulation, callose deposition, and defense-associated enzyme were induced upon challenge inoculation in the leaves primed by EPS. Moreover, the defense-related genes PR1, PR2, and PR5 and mitogen-activated kinases (MAPK) cascade marker gene MPK6 were concurrently expressed in the leaves of EPS-treated plants and induced higher resistance to Pst DC3000 in Col-0 than that in the jar1 or etr1 mutants. The protection was absent in the NahG transgenic plants and npr1 mutant, suggesting an activation of the salicylic acid (SA)- and the MAPK-dependent signaling pathways with NPR1-dependent by B. cereus AR156 EPS. In conclusion, B. cereus AR156 EPS play an important role in MAMP perception during the process of rhizobacteria-triggered NHR. This study is the first to illustrate how AR156 induces systemic resistance to Pst DC3000 in Arabidopsis. It also provides the first explanation of how plants perceive colonization of non-pathogenic bacteria and how rhizobacteria trigger ISR to plant pathogens.

Horizontal Gene Transfer from Bacteria Has Enabled the Plant-Parasitic Nematode Globodera pallida to Feed on Host-Derived Sucrose

The evolution of plant-parasitic nematodes (PPN) is unusual in that these organisms have acquired a range of genes from bacteria via horizontal gene transfer (HGT). The proteins encoded by most of these genes are involved in metabolism of various components of the plant cell wall during invasion of the host. Recent genome sequencing projects for PPN have shown that Glycosyl Hydrolase Family 32 (GH32) sequences are present in several PPN species. These sequences are absent from almost all other animals. Here, we show that the GH32 sequences from an economically important cyst nematode species, Globodera pallida are functional invertases, are expressed during feeding and are restricted in expression to the nematode digestive system. These data are consistent with a role in metabolizing host-derived sucrose. In addition, a detailed phylogenetic analysis shows that the GH32 sequences from PPN and those present in some insect species have distinct bacterial origins and do not therefore derive from a gene present in the last common ancestor of ecdysozoan species. HGT has therefore played at least two critical roles in the evolution of PPN, enabling both invasion of the host and feeding on the main translocation carbohydrate of the plant.

Commonalities and differences of T3SSs in rhizobia and plant pathogenic bacteria

Plant pathogenic bacteria and rhizobia infect higher plants albeit the interactions with their hosts are principally distinct and lead to completely different phenotypic outcomes, either pathogenic or mutualistic, respectively. Bacterial protein delivery to plant host plays an essential role in determining the phenotypic outcome of plant-bacteria interactions. The involvement of type III secretion systems (T3SSs) in mediating animal- and plant-pathogen interactions was discovered in the mid-80's and is now recognized as a multiprotein nanomachine dedicated to trans-kingdom movement of effector proteins. The discovery of T3SS in bacteria with symbiotic lifestyles broadened its role beyond virulence. In most T3SS-positive bacterial pathogens, virulence is largely dependent on functional T3SSs, while in rhizobia the system is dispensable for nodulation and can affect positively or negatively the mutualistic associations with their hosts. This review focuses on recent comparative genome analyses in plant pathogens and rhizobia that uncovered similarities and variations among T3SSs in their genetic organization, regulatory networks and type III secreted proteins and discusses the evolutionary adaptations of T3SSs and type III secreted proteins that might account for the distinguishable phenotypes and host range characteristics of plant pathogens and symbionts.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Inhibition of fungal and bacterial plant pathogens in vitro and in planta with ultrashort cationic lipopeptides

Plant diseases constitute an emerging threat to global food security. Many of the currently available antimicrobial agents for agriculture are highly toxic and nonbiodegradable and cause extended environmental pollution. Moreover, an increasing number of phytopathogens develop resistance to them. Recently, we have reported on a new family of ultrashort antimicrobial lipopeptides which are composed of only four amino acids linked to fatty acids (A. Makovitzki, D. Avrahami, and Y. Shai, Proc. Natl. Acad. Sci. USA 103:15997-16002, 2006). Here, we investigated the activities in vitro and in planta and the modes of action of these short lipopeptides against plant-pathogenic bacteria and fungi. They act rapidly, at low micromolar concentrations, on the membranes of the microorganisms via a lytic mechanism. In vitro microscopic analysis revealed wide-scale damage to the microorganism's membrane, in addition to inhibition of pathogen growth. In planta potent antifungal activity was demonstrated on cucumber fruits and leaves infected with the pathogen Botrytis cinerea as well as on corn leaves infected with Cochliobolus heterostrophus. Similarly, treatment with the lipopeptides of Arabidopsis leaves infected with the bacterial leaf pathogen Pseudomonas syringae efficiently and rapidly reduced the number of bacteria. Importantly, in contrast to what occurred with many native lipopeptides, no toxicity was observed on the plant tissues. These data suggest that the ultrashort lipopeptides could serve as native-like antimicrobial agents economically feasible for use in plant protection.

Downstream divergence of the ethylene signaling pathway for harpin-stimulated Arabidopsis growth and insect defense

Ethylene (ET) signal transduction may regulate plant growth and defense, depending on which components are recruited into the pathway in response to different stimuli. We report here that the ET pathway controls both insect resistance (IR) and plant growth enhancement (PGE) in Arabidopsis (Arabidopsis thaliana) plants responding to harpin, a protein produced by a plant pathogenic bacterium. PGE may result from spraying plant tops with harpin or by soaking seeds in harpin solution; the latter especially enhances root growth. Plants treated similarly develop resistance to the green peach aphid (Myzus persicae). The salicylic acid pathway, although activated by harpin, does not lead to PGE and IR. By contrast, PGE and IR are induced in both wild-type plants and genotypes that have defects in salicylic acid signaling. In response to harpin, levels of jasmonic acid (JA) decrease, and the COI1 gene, which is indispensable for JA signal transduction, is not expressed in wild-type plants. However, PGE and IR are stimulated in the JA-resistant mutant jar1-1. In the wild type, PGE and IR develop coincidently with increases in ET levels and the expression of several genes essential for ET signaling. The ET receptor gene ETR1 is required because both phenotypes are arrested in the etr1-1 mutant. Consistently, inhibition of ET perception nullifies the induction of both PGE and IR. The signal transducer EIN2 is required for IR, and EIN5 is required for PGE because IR and PGE are impaired correspondingly in the ein2-1 and ein5-1 mutants. Therefore, harpin activates ET signaling while conscribing EIN2 and EIN5 to confer IR and PGE, respectively.

Pseudomonas syringae pv. tomato cells encounter inhibitory levels of water stress during the hypersensitive response of Arabidopsis thaliana

During plant defense against bacterial pathogens, the hypersensitive response (HR) functions to restrict pathogen growth and spread. The mechanisms driving this growth restriction are poorly understood. We used a water stress-responsive transcriptional fusion to quantify the water potential sensed by individual Pseudomonas syringae pv. tomato DC3000 cells during infection of Arabidopsis thaliana leaves. A nonpathogenic DC3000 hrcC mutant defective in type III secretion, as well as the saprophyte Pseudomonas fluorescens A506, sensed water potentials of -0.3 to -0.4 MPa at 48 h postinfiltration (hpi). During pathogenesis, DC3000 sensed lower water potentials (-0.4 to -0.9 MPa), demonstrating that it can modify the intercellular environment, and these water potentials were associated with optimal DC3000 growth in culture. During the HR, DC3000 cells sensed water potentials (-1.6 to -2.2 MPa) that were low enough to prevent cell division in the majority of cells in culture. This water potential decrease occurred within only 4 hpi and was influenced by avirulence gene expression, with avrRpm1 expression associated with lower water potentials than avrRpt2 or avrB expression at 48 hpi. The population sizes of the DC3000 variants tested were significantly correlated with the apoplastic water potential at 48 hpi, with a decrease of -0.9 MPa associated with a 10-fold decrease in cells per gram of leaf. These results suggest that the apoplastic water potential is a determinant of endophytic bacterial population size, and water stress, resulting from high osmolarity or tissue desiccation, is at least one factor restricting bacterial growth during the HR.

Altered gene expression in three plant species in response to treatment with Nep1, a fungal protein that causes necrosis

Nep1 is an extracellular fungal protein that causes necrosis when applied to many dicotyledonous plants, including invasive weed species. Using transmission electron microscopy, it was determined that application of Nep1 (1.0 micro g mL(-)(1), 0.1% [v/v] Silwet-L77) to Arabidopsis and two invasive weed species, spotted knapweed (Centaurea maculosa) and dandelion (Taraxacum officinale), caused a reduction in the thickness of the cuticle and a breakdown of chloroplasts 1 to 4 h after treatment. Membrane breakdown was most severe in cells closest to the surface of application. Differential display was used to isolate cDNA clones from the three species showing differential expression in response to Nep1 treatment. Differential gene expression was observed for a putative serpin (CmSER-1) and a calmodulin-like (CmCAL-1) protein from spotted knapweed, and a putative protein phosphatase 2C (ToPP2C-1) and cytochrome P-450 (ToCYP-1) protein from dandelion. In addition, differential expression was observed for genes coding for a putative protein kinase (AtPK-1), a homolog (AtWI-12) of wound-induced WI12, a homolog (AtLEA-1) of late embryogenesis abundant LEA-5, a WRKY-18 DNA-binding protein (AtWRKY-18), and a phospholipase D (AtPLD-1) from Arabidopsis. Genes showing elevated mRNA levels in Nep1-treated (5 micro g mL(-)(1), 0.1% [v/v] Silwet-L77) leaves 15 min after Nep1 treatment included CmSER-1 and CmCAL-1 for spotted knapweed, ToCYP-1 and CmCAL-1 for dandelion, and AtPK-1, AtWRKY-18, AtWI-12, and AtLEA-1 for Arabidopsis. Levels of mRNA for AtPLD-1 (Arabidopsis) and ToPP2C-1 (dandelion) decreased rapidly in Silwet-L77-treated plants between 15 min and 4 h of treatment, but were maintained or decreased more slowly over time in Nep1-treated (5 micro g mL(-)(1), 0.1% [v/v] Silwet-L77) leaves. In general, increases in mRNA band intensities were in the range of two to five times, with only ToCYP-1 in dandelion exceeding an increase of 10 times. The identified genes have been shown to be involved or are related to gene families that are involved in plant stress responses, including wounding, drought, senescence, and disease resistance.