Erwinia carotovora subsp. carotovora and Erwinia-derived elicitors HrpN and PehA trigger distinct but interacting defense responses and cell death in Arabidopsis

Carrageenans as biostimulants and bio-elicitors: plant growth and defense responses

In the context of climate change, the need to ensure food security and safety has taken center stage. Chemical fertilizers and pesticides are traditionally used to achieve higher plant productivity and improved plant protection from biotic stresses. However, the widespread use of fertilizers and pesticides has led to significant risks to human health and the environment, which are further compounded by the emissions of greenhouse gases during fertilizer and pesticide production and application, contributing to global warming and climate change. The naturally occurring sulfated linear polysaccharides obtained from edible red seaweeds (Rhodophyta), carrageenans, could offer climate-friendly substitutes for these inputs due to their bi-functional activities. Carrageenans and their derivatives, known as oligo-carrageenans, facilitate plant growth through a multitude of metabolic courses, including chlorophyll metabolism, carbon fixation, photosynthesis, protein synthesis, secondary metabolite generation, and detoxification of reactive oxygen species. In parallel, these compounds suppress pathogens by their direct antimicrobial activities and/or improve plant resilience against pathogens by modulating biochemical changes via salicylate (SA) and/or jasmonate (JA) and ethylene (ET) signaling pathways, resulting in increased production of secondary metabolites, defense-related proteins, and antioxidants. The present review summarizes the usage of carrageenans for increasing plant development and defense responses to pathogenic challenges under climate change. In addition, the current state of knowledge regarding molecular mechanisms and metabolic alterations in plants during carrageenan-stimulated plant growth and plant disease defense responses has been discussed. This evaluation will highlight the potential use of these new biostimulants in increasing agricultural productivity under climate change.

Tea (Camellia sinensis): A Review of Nutritional Composition, Potential Applications, and Omics Research

Tea (Camelliasinensis) is the world’s most widely consumed non-alcoholic beverage with essential economic and health benefits since it is an excellent source of polyphenols, catechins, amino acids, flavonoids, carotenoids, vitamins, and polysaccharides. The aim of this review is to summarize the main secondary metabolites in tea plants, and the content and distribution of these compounds in six different types of tea and different organs of tea plant were further investigated. The application of these secondary metabolites on food processing, cosmetics industry, and pharmaceutical industry was reviewed in this study. With the rapid advancements in biotechnology and sequencing technology, omics analyses, including genome, transcriptome, and metabolome, were widely used to detect the main secondary metabolites and their molecular regulatory mechanisms in tea plants. Numerous functional genes and regulatory factors have been discovered, studied, and applied to improve tea plants. Research advances, including secondary metabolites, applications, omics research, and functional gene mining, are comprehensively reviewed here. Further exploration and application trends are briefly described. This review provides a reference for basic and applied research on tea plants.

Calcium Regulation of Bacterial Virulence

Calcium (Ca²⁺) is a universal signaling ion, whose major informational role shaped the evolution of signaling pathways, enabling cellular communications and responsiveness to both the intracellular and extracellular environments. Elaborate Ca²⁺ regulatory networks have been well characterized in eukaryotic cells, where Ca²⁺ regulates a number of essential cellular processes, ranging from cell division, transport and motility, to apoptosis and pathogenesis. However, in bacteria, the knowledge on Ca²⁺ signaling is still fragmentary. This is complicated by the large variability of environments that bacteria inhabit with diverse levels of Ca²⁺. Yet another complication arises when bacterial pathogens invade a host and become exposed to different levels of Ca²⁺ that (1) are tightly regulated by the host, (2) control host defenses including immune responses to bacterial infections, and (3) become impaired during diseases. The invading pathogens evolved to recognize and respond to the host Ca²⁺, triggering the molecular mechanisms of adhesion, biofilm formation, host cellular damage, and host-defense resistance, processes enabling the development of persistent infections. In this review, we discuss: (1) Ca²⁺ as a determinant of a host environment for invading bacterial pathogens, (2) the role of Ca²⁺ in regulating main events of host colonization and bacterial virulence, and (3) the molecular mechanisms of Ca²⁺ signaling in bacterial pathogens.

Using Dynamic Changes of Chlorophyll Fluorescence in Arabidopsis thaliana to Evaluate Plant Immunity-Intensifying Bacillus spp. Strains

The integral defense responses of plants triggered by the small molecules of plant pathogens are regarded as plant immunity. The pathogen-associated molecular pattern-triggered immunity (PTI) occurs on the recognition of a pathogen by receptors on plant cell surfaces as an infection begins. During the activation of PTI, the effectiveness of a plant's photosynthetic system may be altered. In this study, chlorophyll fluorescence was used to assay the dynamic changes of PTI. When we used flg22_Pst as an elicitor, we found that the photosynthetic electron transport rate (ETR) of Arabidopsis thaliana Col-0 was significantly decreased at 2, 4, and 24 h on treatment with a PTI-intensifying protein, plant ferredoxin-like protein (PFLP). In addition, this reduction in the photosynthetic ETR was also carried out with a PTI-intensifying Bacillus amyloliquefaciens strain, PMB05, on the induction of flg22_Pst. The disease resistance against bacterial soft rot caused by Pectobacterium carotovorum subsp. carotovorum (Pcc) was still enhanced by PMB05. Interestingly, among the eight tested Bacillus species strains, the PTI triggered by HrpN_Pcc from P. carotovorum subsp. carotovorum exhibited an ETR that was significantly decreased by PMB05. Furthermore, this decrease was consistent with rapid H₂O₂ generation and callose deposition triggered by HrpN_Pcc and the disease resistance against bacterial soft rot. Taken together, such results led us to conclude that the assay based on the ETR established in this study can be used as a model for evaluating the effectiveness of plant immunity-intensifying microbes for controlling plant diseases.

Advances in research on functional genes of tea plant

Tea plant (Camellia sinensis) is an important leaf-type woody crop used to produce non-alcoholic beverages all over the world. Tea is one of the oldest and most popular non-alcoholic beverages in the world, and long-term tea drinking has numerous healthful for humans due to many of the important secondary metabolites, such as polyphenols and theanine. Theanine and polyphenols are also closely related to tea flavor and tea aroma, which is usually as the standard for judging tea quality. The growth of tea plants and quality of teas are susceptible to adversity abiotic and biotic stresses, such as low temperatures and pests. Consequently, this review focus on the research progress of key genes related to the stress resistance and material metabolism of tea plants in recent years. We aim at comprehensively understanding the growth and metabolism of tea plants and their relationship with the external environment, so as to provide an in-depth and broad theoretical support for the breeding of excellent tea plant varieties.

Comparative transcriptome analysis reveals defense responses against soft rot in Chinese cabbage

Pectobacterium carotovorum ssp. carotovorum (Pcc) is a necrotrophic bacterial species that causes soft rot disease in Chinese cabbage. In this study, plants harboring the resistant mutant sr gene, which confers resistance against Pcc, were screened from an 800 M₂ population mutated by ethyl methane sulfonate (EMS) and scored in vitro and in vivo for lesion size. The transcript profiles showed ~512 differentially expressed genes (DEGs) between sr and WT plants occurring between 6 and 12 h postinoculation (hpi), which corresponded to the important defense regulation period (resistance) to Pcc in Chinese cabbage. The downstream defense genes (CPK, CML, RBOH MPK3, and MPK4) of pathogen pattern-triggered immunity (PTI) were strongly activated during infection at 12 hpi in resistant mutant sr; PTI appears to be central to plant defense against Pcc via recognition by three putative pattern recognition receptors (PRRs; BrLYM1-BrCERK1, BrBKK1/SERK4-PEPR1, BrWAKs). Pcc triggered the upregulation of the jasmonic acid (JA) and ethylene (ET) biosynthesis genes in mutant sr, but auxins and other hormones may have affected some negative signals. Endogenous hormones (auxins, JAs, and SA), as well as exogenous auxins (MEJA and BTH), were also verified as functioning in the immune system. Concurrently, the expression of glucosinolate and lignin biosynthesis genes was increased at 12 hpi in resistant mutant sr, and the accumulation of glucosinolate and lignin also indicated that these genes have a functional defensive role against Pcc. Our study provides valuable information and elucidates the resistance mechanism of Chinese cabbage against Pcc infection.

Harpin-inducible defense signaling components impair infection by the ascomycete Macrophomina phaseolina

Soybean (Glycine max) infection by the charcoal rot (CR) ascomycete Macrophomina phaseolina is enhanced by the soybean cyst nematode (SCN) Heterodera glycines. We hypothesized that G. max genetic lines impairing infection by M. phaseolina would also limit H. glycines parasitism, leading to resistance. As a part of this M. phaseolina resistance process, the genetic line would express defense genes already proven to impair nematode parasitism. Using G. max[DT97-4290/PI 642055], exhibiting partial resistance to M. phaseolina, experiments show the genetic line also impairs H. glycines parasitism. Furthermore, comparative studies show G. max[DT97-4290/PI 642055] exhibits induced expression of the effector triggered immunity (ETI) gene NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) that functions in defense to H. glycines as compared to the H. glycines and M. phaseolina susceptible line G. max[Williams 82/PI 518671]. Other defense genes that are induced in G. max[DT97-4290/PI 642055] include the pathogen associated molecular pattern (PAMP) triggered immunity (PTI) genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), NONEXPRESSOR OF PR1 (NPR1) and TGA2. These observations link G. max defense processes that impede H. glycines parasitism to also potentially function toward impairing M. phaseolina pathogenicity. Testing this hypothesis, G. max[Williams 82/PI 518671] genetically engineered to experimentally induce GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 expression leads to impaired M. phaseolina pathogenicity. In contrast, G. max[DT97-4290/PI 642055] engineered to experimentally suppress the expression of GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 by RNA interference (RNAi) enhances M. phaseolina pathogenicity. The results show components of PTI and ETI impair both nematode and M. phaseolina pathogenicity.

Dickeya dadantii pectic enzymes necessary for virulence are also responsible for activation of the Arabidopsis thaliana innate immune system

Soft-rot diseases of plants attributed to Dickeya dadantii result from lysis of the plant cell wall caused by pectic enzymes released by the bacterial cell by a type II secretion system (T2SS). Arabidopsis thaliana can express several lines of defence against this bacterium. We employed bacterial mutants with defective envelope structures or secreted proteins to examine early plant defence reactions. We focused on the production of AtrbohD-dependent reactive oxygen species (ROS), callose deposition and cell death as indicators of these reactions. We observed a significant reduction in ROS and callose formation with a bacterial mutant in which genes encoding five pectate lyases (Pels) were disrupted. Treatment of plant leaves with bacterial culture filtrates containing Pels resulted in ROS and callose production, and both reactions were dependent on a functional AtrbohD gene. ROS and callose were produced in response to treatment with a cellular fraction of a T2SS-negative mutant grown in a Pels-inducing medium. Finally, ROS and callose were produced in leaves treated with purified Pels that had also been shown to induce the expression of jasmonic acid-dependent defence genes. Pel catalytic activity is required for the induction of ROS accumulation. In contrast, cell death observed in leaves infected with the wild-type strain appeared to be independent of a functional AtrbohD gene. It was also independent of the bacterial production of pectic enzymes and the type III secretion system (T3SS). In conclusion, the work presented here shows that D. dadantii is recognized by the A. thaliana innate immune system through the action of pectic enzymes secreted by bacteria at the site of infection. This recognition leads to AtrbohD-dependent ROS and callose accumulation, but not cell death.

A harpin elicitor induces the expression of a coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene and others functioning during defense to parasitic nematodes

The bacterial effector harpin induces the transcription of the Arabidopsis thaliana NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene. In Glycine max, Gm-NDR1-1 transcripts have been detected within root cells undergoing a natural resistant reaction to parasitism by the syncytium-forming nematode Heterodera glycines, functioning in the defense response. Expressing Gm-NDR1-1 in Gossypium hirsutum leads to resistance to Meloidogyne incognita parasitism. In experiments presented here, the heterologous expression of Gm-NDR1-1 in G. hirsutum impairs Rotylenchulus reniformis parasitism. These results are consistent with the hypothesis that Gm-NDR1-1 expression functions broadly in generating a defense response. To examine a possible relationship with harpin, G. max plants topically treated with harpin result in induction of the transcription of Gm-NDR1-1. The result indicates the topical treatment of plants with harpin, itself, may lead to impaired nematode parasitism. Topical harpin treatments are shown to impair G. max parasitism by H. glycines, M. incognita and R. reniformis and G. hirsutum parasitism by M. incognita and R. reniformis. How harpin could function in defense has been examined in experiments showing it also induces transcription of G. max homologs of the proven defense genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), TGA2, galactinol synthase, reticuline oxidase, xyloglucan endotransglycosylase/hydrolase, alpha soluble N-ethylmaleimide-sensitive fusion protein (α-SNAP) and serine hydroxymethyltransferase (SHMT). In contrast, other defense genes are not directly transcriptionally activated by harpin. The results indicate harpin induces pathogen associated molecular pattern (PAMP) triggered immunity (PTI) and effector-triggered immunity (ETI) defense processes in the root, activating defense to parasitic nematodes.

Response of Vitis vinifera cell cultures to Eutypa lata and Trichoderma atroviride culture filtrates: expression of defence-related genes and phenotypes

Cell suspension cultures of Vitis vinifera cv. Dauphine berries were used to study the response to the vascular pathogen, Eutypa lata, in comparison with a biological control agent, Trichoderma atroviride, that was previously shown to be effective in pruning wound protection. The expression of genes coding for enzymes of the phenylpropanoid pathway and pathogenesis-related (PR) proteins was profiled over a 48-h period using quantitative reverse transcriptase PCR. The cell cultures responded to elicitors of both fungi with a hypersensitive-like response that lead to a decrease in cell viability. Similar genes were triggered by both the pathogen and biocontrol agent, but the timing patterns and magnitude of expression was dependent on the specific fungal elicitor. Culture filtrates of both fungi caused upregulation of phenylalanine ammonia-lyase (PAL), 4-coumaroyl Co-A ligase (CCo-A) and stilbene synthase (STS), and a downregulation of chalcone synthase (CHS) genes. The pathogen filtrate caused a biphasic pattern in the upregulation of PAL and STS genes which was not observed in cells treated with filtrates of the biocontrol agent. Analytical assays showed significantly higher total phenolic content and chitinolytic enzyme activity in the cell cultures treated with the T. atroviride filtrate compared to the pathogen filtrate. These results corresponded well to the higher expression of PAL and chitinase class IV genes. The response of the cell cultures to T. atroviride filtrate provides support for the notion that the wound protection by the biocontrol agent at least partially relies on the induction of grapevine resistance mechanisms.

Expression Profile of Stress-responsive Arabidopsis thaliana miRNAs and their Target Genes in Response to Inoculation with Pectobacterium carotovorum subsp. carotovorum

BACKGROUND

Pectobacterium carotovorum subsp. carotovorum (Pcc) is a soft rot bacterium which upon entry into the plant macerates plant tissues by producing plant cell wall degrading enzymes. It has a wide host range which includes carrot, potato, tomato, leafy greens, squash and other cucurbits, onion, green peppers and cassava. During plant-microbe interactions, one of the ways of plant response to pathogen infection is through the small RNA silencing mechanism. Under pathogen attack the plant utilizes microRNAs to regulate gene expression by means of mediating gene silencing at transcriptional and post-transcriptional level. This study aims to assess for the first time, the expression profile of some stress-responsive miRNA and differential expression pattern of their target genes in Arabidopsis thaliana inoculated with Pcc.

MATERIALS AND METHODS

Leaves of five weeks old Arabidopsis thaliana plants were infected with Pcc and the quantitative real time-PCR, was used to investigate after 0, 24, 48 and 72 h post infection, the expression profiling of the stress-responsive miRNAs which include: miR156, miR159, miR169, miR393, miR396 miR398, miR399 and miR408 along with their target genes which include: Squamosa promoter-binding-like protein, myb domain protein 101, nuclear factor Y subunit A8, concanavalin A-like lectin protein kinase, growth regulating factor 4, copper superoxide dismutase, ubiquitin-protein ligase and plantacyanin respectively.

RESULTS

The findings showed that the overexpression of 6 miRNAs at 24, 48 and 72 h after infection resulted in the repression of their target genes and the expression of 2 miRNAs didn't affect their target genes.

CONCLUSION

These results provide the first indication of the miRNAs role in response to the infection of Pcc in A. thaliana and open new vistas for a better understanding of miRNA regulation of plant response to Pcc.

Morphological features of different polyploids for adaptation and molecular characterization of CC-NBS-LRR and LEA gene families in Agave L

Polyploidy has been widely described in many Agave L. species, but its influence on environmental response to stress is still unknown. With the objective of knowing the morphological adaptations and regulation responses of genes related to biotic (LEA) and abiotic (NBS-LRR) stress in species of Agave with different levels of ploidy, and how these factors contribute to major response of Agave against environmental stresses, we analyzed 16 morphological trials on five accessions of three species (Agave tequilana Weber, Agave angustifolia Haw. and Agave fourcroydes Lem.) with different ploidy levels (2n=2x=60 2n=3x=90, 2n=5x=150, 2n=6x=180) and evaluated the expression of NBS-LRR and LEA genes regulated by biotic and abiotic stress. It was possible to associate some morphological traits (spines, nuclei, and stomata) to ploidy level. The genetic characterization of stress-related genes NBS-LRR induced by pathogenic infection and LEA by heat or saline stresses indicated that amino acid sequence analysis in these genes showed more substitutions in higher ploidy level accessions of A. fourcroydes Lem. 'Sac Ki' (2n=5x=150) and A. angustifolia Haw. 'Chelem Ki' (2n=6x=180), and a higher LEA and NBS-LRR representativeness when compared to their diploid and triploid counterparts. In all studied Agave accessions expression of LEA and NBS-LRR genes was induced by saline or heat stresses or by infection with Erwinia carotovora, respectively. The transcriptional activation was also higher in A. angustifolia Haw. 'Chelem Ki' (2n=6x=180) and A. fourcroydes 'Sac Ki' (2n=5x=150) than in their diploid and triploid counterparts, which suggests higher adaptation to stress. Finally, the diploid accession A. tequilana Weber 'Azul' showed a differentiated genetic profile relative to other Agave accessions. The differences include similar or higher genetic representativeness and transcript accumulation of LEA and NBS-LRR genes than in polyploid (2n=5x=150 and 2n=6x=180) Agave accessions, thus suggesting a differentiated selection pressure for overcoming the lower ploidy level of the diploid A. tequilana Weber 'Azul'.

Evaluation of antibacterial activity of Stenotrophomonas maltophilia against Ralstonia solanacearum under different application conditions

AIM

The aim of this study was the monitoring of different mechanisms involved in the antibacterial activity of the biocontrol agent, Stenotrophomonas maltophilia (PD4560), against Ralstonia solanacearum in vitro and in vivo. Optimization of conditions that favour these mechanisms was the second target of this study.

METHODS AND RESULTS

Proteolytic activity of Sten. maltophilia (PD 4560), was tested on skimmed milk medium. The biocontrol agent was able to produce an alkaline serine protease enzyme with a molecular weight of 40 KDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses. Spraying of salicylic acid (SA) led to an increase in the efficacy of Sten. maltophilia in controlling the Ralstonia potato wilt while spraying of ammonium sulphate (AmS) did not affect the biocontrol efficacy. The efficacy was correlated with the expression of protease enzyme genes; Prt genes (mainly PrtP and Prt4) and PR genes (mainly PR-1 and PRQ) as evaluated using real-time polymerase chain reaction analysis.

CONCLUSIONS

The biocontrol activity of Sten. maltophilia can be attributed to the direct mechanism alkaline serine proteolytic enzyme production and through induction of host systemic acquired resistance as indirect mechanism. Tuber bulking was the most suitable physiological growth stage to apply either SA or the biocontrol agent.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both SA and peat-moss as an organic carrier enhanced the antibacterial efficiency of the biocontrol agent. Application of Sten. maltophilia is more suitable under alkaline soil conditions.

Transcription factors WRKY70 and WRKY11 served as regulators in rhizobacterium Bacillus cereus AR156-induced systemic resistance to Pseudomonas syringae pv. tomato DC3000 in Arabidopsis

The activation of both the SA and JA/ETsignalling pathways may lead to more efficient general and broad resistance to Pst DC3000 by non-pathogenic rhizobacteria. However, the mechanisms that govern this simultaneous activation are unclear. Using Arabidopsis as a model system, two transcription factors, WRKY11 and WRKY70, were identified as important regulators involved in Induced Systemic Resistance (ISR) triggered by Bacillus cereus AR156. The results revealed that AR156 treatment significantly stimulated the transcription of WRKY70, but suppressed that of WRKY11 in Arabidopsis leaves. Furthermore, they were shown to be required for AR156 enhancing the activation of cellular defence responses and the transcription level of the plant defence response gene. Overexpression of the two transcription factors in Arabidopsis also showed that they were essential for AR156 to elicit ISR. AR156-triggered ISR was completely abolished in the double mutant of the two transcription factors, but still partially retained in the single mutants, indicating that the regulation of the two transcription factors depend on two different pathways. The target genes of the two transcription factors and epistasis analysis suggested that WRKY11 regulated AR156-triggered ISR through activating the JA signalling pathway, and WRKY70 regulated the ISR through activating the SA signalling pathway. In addition, both WRKY11 and WRKY70 modulated AR156-triggered ISR in a NPR1-dependent manner. In conclusion, WRKY11 and WRKY70 played an important role in regulating the signalling transduction pathways involved in AR156-triggered ISR. This study is the first to illustrate the mechanism by which a single rhizobacterium elicits ISR by simultaneously activating both the SA and JA/ET signalling pathways.

Salicylic and jasmonic acid pathways are necessary for defence against Dickeya solani as revealed by a novel method for Blackleg disease screening of in vitro grown potato

Potato is major crop ensuring food security in Europe, and blackleg disease is increasingly causing losses in yield and during storage. Recently, one blackleg pathogen, Dickeya solani has been shown to be spreading in Northern Europe that causes aggressive disease development. Currently, identification of tolerant commercial potato varieties has been unsuccessful; this is confounded by the complicated etiology of the disease and a strong environmental influence on disease development. There is currently a lack of efficient testing systems. Here, we describe a system for quantification of blackleg symptoms on shoots of sterile in vitro potato plants, which saves time and space compared to greenhouse and existing field assays. We found no evidence for differences in infection between the described in vitro-based screening method and existing greenhouse assays. This system facilitates efficient screening of blackleg disease response of potato plants independent of other microorganisms and variable environmental conditions. We therefore used the in vitro screening method to increase understanding of plant mechanisms involved in blackleg disease development by analysing disease response of hormone- related (salicylic and jasmonic acid) transgenic potato plants. We show that both jasmonic (JA) and salicylic (SA) acid pathways regulate tolerance to blackleg disease in potato, a result unlike previous findings in Arabidopsis defence response to necrotrophic bacteria. We confirm this by showing induction of a SA marker, pathogenesis-related protein 1 (StPR1), and a JA marker, lipoxygenase (StLOX), in Dickeya solani infected in vitro potato plants. We also observed that tubers of transgenic potato plants were more susceptible to soft rot compared to wild type, suggesting a role for SA and JA pathways in general tolerance to Dickeya.

The Physcomitrella patens unique alpha-dioxygenase participates in both developmental processes and defense responses

BACKGROUND

Plant α-dioxygenases catalyze the incorporation of molecular oxygen into polyunsaturated fatty acids leading to the formation of oxylipins. In flowering plants, two main groups of α-DOXs have been described. While the α-DOX1 isoforms are mainly involved in defense responses against microbial infection and herbivores, the α-DOX2 isoforms are mostly related to development. To gain insight into the roles played by these enzymes during land plant evolution, we performed biochemical, genetic and molecular analyses to examine the function of the single copy moss Physcomitrella patens α-DOX (Ppα-DOX) in development and defense against pathogens.

RESULTS

Recombinant Ppα-DOX protein catalyzed the conversion of fatty acids into 2-hydroperoxy derivatives with a substrate preference for α-linolenic, linoleic and palmitic acids. Ppα-DOX is expressed during development in tips of young protonemal filaments with maximum expression levels in mitotically active undifferentiated apical cells. In leafy gametophores, Ppα-DOX is expressed in auxin producing tissues, including rhizoid and axillary hairs. Ppα-DOX transcript levels and Ppα-DOX activity increased in moss tissues infected with Botrytis cinerea or treated with Pectobacterium carotovorum elicitors. In B. cinerea infected leaves, Ppα-DOX-GUS proteins accumulated in cells surrounding infected cells, suggesting a protective mechanism. Targeted disruption of Ppα-DOX did not cause a visible developmental alteration and did not compromise the defense response. However, overexpressing Ppα-DOX, or incubating wild-type tissues with Ppα-DOX-derived oxylipins, principally the aldehyde heptadecatrienal, resulted in smaller moss colonies with less protonemal tissues, due to a reduction of caulonemal filament growth and a reduction of chloronemal cell size compared with normal tissues. In addition, Ppα-DOX overexpression and treatments with Ppα-DOX-derived oxylipins reduced cellular damage caused by elicitors of P. carotovorum.

CONCLUSIONS

Our study shows that the unique α-DOX of the primitive land plant P. patens, although apparently not crucial, participates both in development and in the defense response against pathogens, suggesting that α-DOXs from flowering plants could have originated by duplication and successive functional diversification after the divergence from bryophytes.

Bio-based resistance inducers for sustainable plant protection against pathogens

An increasing demand for environmentally acceptable alternative for traditional pesticides provides an impetus to conceive new bio-based strategies in crop protection. Employing induced resistance is one such strategy, consisting of boosting the natural plant immunity. Upon infections, plants defend themselves by activating their immune mechanisms. These are initiated after the recognition of an invading pathogen via the microbe-associated molecular patterns (MAMPs) or other microbe-derived molecules. Triggered responses inhibit pathogen spread from the infected site. Systemic signal transport even enables to prepare, i.e. prime, distal uninfected tissues for more rapid and enhanced response upon the consequent pathogen attack. Similar defense mechanisms can be triggered by purified MAMPs, pathogen-derived molecules, signal molecules involved in plant resistance to pathogens, such as salicylic and jasmonic acid, or a wide range of other chemical compounds. Induced resistance can be also conferred by plant-associated microorganisms, including beneficial bacteria or fungi. Treatment with resistance inducers or beneficial microorganisms provides long-lasting resistance for plants to a wide range of pathogens. This study surveys current knowledge on resistance and its mechanisms provided by microbe-, algae- and plant-derived elicitors in different crops. The main scope deals with bacterial substances and fungus-derived molecules chitin and chitosan and algae elicitors, including naturally sulphated polysaccharides such as ulvans, fucans or carageenans. Recent advances in the utilization of this strategy in practical crop protection are also discussed.

Simplification of vacuole structure during plant cell death triggered by culture filtrates of Erwinia carotovora

Vacuoles are suggested to play crucial roles in plant defense-related cell death. During programmed cell death, previous live cell imaging studies have observed vacuoles to become simpler in structure and have implicated this simplification as a prelude to the vacuole's rupture and consequent lysis of the plasma membrane. Here, we examined dynamics of the vacuole in cell cycle-synchronized tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2) cells during cell death induced by application of culture filtrates of Erwinia carotovora. The filtrate induced death in about 90% of the cells by 24 h. Prior to cell death, vacuole shape simplified and endoplasmic actin filaments disassembled; however, the vacuoles did not rupture until after plasma membrane integrity was lost. Instead of facilitating rupture, the simplification of vacuole structure might play a role in the retrieval of membrane components needed for defense-related cell death.

Ectopically expressed sweet pepper ferredoxin PFLP enhances disease resistance to Pectobacterium carotovorum subsp. carotovorum affected by harpin and protease-mediated hypersensitive response in Arabidopsis

Plant ferredoxin-like protein (PFLP) is a photosynthesis-type ferredoxin (Fd) found in sweet pepper. It contains an iron-sulphur cluster that receives and delivers electrons between enzymes involved in many fundamental metabolic processes. It has been demonstrated that transgenic plants overexpressing PFLP show a high resistance to many bacterial pathogens, although the mechanism remains unclear. In this investigation, the PFLP gene was transferred into Arabidopsis and its defective derivatives, such as npr1 (nonexpresser of pathogenesis-related gene 1) and eds1 (enhanced disease susceptibility 1) mutants and NAHG-transgenic plants. These transgenic plants were then infected with the soft-rot bacterial pathogen Pectobacterium carotovorum subsp. carotovorum (Erwinia carotovora ssp. carotovora, ECC) to investigate the mechanism behind PFLP-mediated resistance. The results revealed that, instead of showing soft-rot symptoms, ECC activated hypersensitive response (HR)-associated events, such as the accumulation of hydrogen peroxide (H2 O2 ), electrical conductivity leakage and expression of the HR marker genes (ATHSR2 and ATHSR3) in PFLP-transgenic Arabidopsis. This PFLP-mediated resistance could be abolished by inhibitors, such as diphenylene iodonium (DPI), 1-l-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E64) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), but not by myriocin and fumonisin. The PFLP-transgenic plants were resistant to ECC, but not to its harpin mutant strain ECCAC5082. In the npr1 mutant and NAHG-transgenic Arabidopsis, but not in the eds1 mutant, overexpression of the PFLP gene increased resistance to ECC. Based on these results, we suggest that transgenic Arabidopsis contains high levels of ectopic PFLP; this may lead to the recognition of the harpin and to the activation of the HR and other resistance mechanisms, and is dependent on the protease-mediated pathway.

Overexpression of the Brassica rapa transcription factor WRKY12 results in reduced soft rot symptoms caused by Pectobacterium carotovorum in Arabidopsis and Chinese cabbage

Chinese cabbage (Brassica rapa L. ssp. pekinensis), an important vegetable crop, can succumb to diseases such as bacterial soft rot, resulting in significant loss of crop productivity and quality. Pectobacterium carotovorum ssp. carotovorum (Pcc) causes soft rot disease in various plants, including Chinese cabbage. To overcome crop loss caused by bacterial soft rot, a gene from Chinese cabbage was isolated and characterised in this study. We isolated the BrWRKY12 gene from Chinese cabbage, which is a group II member of the WRKY transcription factor superfamily. The 645-bp coding sequence of BrWRKY12 translates to a protein with a molecular mass of approximately 24.4 kDa, and BrWRKY12 was exclusively localised in the nucleus. Transcripts of BrWRKY12 were induced by Pcc infection in Brassica. Heterologous expression of BrWRKY12 resulted in reduced susceptibility to Pcc but not to Pseudomonas syringae pv. tomato in Arabidopsis. Defence-associated genes, such as AtPDF1.2 and AtPGIP2, were constitutively expressed in transgenic lines overexpressing BrWRKY12. The expression of AtWKRY12, which is the closest orthologue of BrWRKY12, was down-regulated by Pcc in Arabidopsis. However, the Atwrky12-2 mutants did not show any difference in response to Pcc, pointing to a difference in function of WRKY12 in Brassica and Arabidopsis. Furthermore, BrWRKY12 in Chinese cabbage also exhibited enhanced resistance to bacterial soft rot and increased the expression of defence-associated genes. In summary, BrWRKY12 confers enhanced resistance to Pcc through transcriptional activation of defence-related genes.

Harpins, multifunctional proteins secreted by gram-negative plant-pathogenic bacteria

Harpins are glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria. Many studies show that these proteins are mostly targeted to the extracellular space of plant tissues, unlike bacterial effector proteins that act inside the plant cells. Over the two decades since the first harpin of pathogen origin, HrpN of Erwinia amylovora, was reported in 1992 as a cell-free elicitor of hypersensitive response (HR), diverse functional aspects of harpins have been determined. Some harpins were shown to have virulence activity, probably because of their involvement in the translocation of effector proteins into plant cytoplasm. Based on this function, harpins are now considered to be translocators. Their abilities of pore formation in the artificial membrane, binding to lipid components, and oligomerization are consistent with this idea. When harpins are applied to plants directly or expressed in plant cells, these proteins trigger diverse beneficial responses such as induction of defense responses against diverse pathogens and insects and enhancement of plant growth. Therefore, in this review, we will summarize the functions of harpins as virulence factors (or translocators) of bacterial pathogens, elicitors of HR and immune responses, and plant growth enhancers.

Release of proteins from intact chloroplasts induced by reactive oxygen species during biotic and abiotic stress

Plastids sustain life on this planet by providing food, feed, essential biomolecules and oxygen. Such diverse metabolic and biosynthetic functions require efficient communication between plastids and the nucleus. However, specific factors, especially large molecules, released from plastids that regulate nuclear genes have not yet been fully elucidated. When tobacco and lettuce transplastomic plants expressing GFP within chloroplasts, were challenged with Erwinia carotovora (biotic stress) or paraquat (abiotic stress), GFP was released into the cytoplasm. During this process GFP moves gradually towards the envelope, creating a central red zone of chlorophyll fluorescence. GFP was then gradually released from intact chloroplasts into the cytoplasm with an intact vacuole and no other visible cellular damage. Different stages of GFP release were observed inside the same cell with a few chloroplasts completely releasing GFP with detection of only red chlorophyll fluorescence or with no reduction in GFP fluorescence or transitional steps between these two phases. Time lapse imaging by confocal microscopy clearly identified sequence of these events. Intactness of chloroplasts during this process was evident from chlorophyll fluorescence emanated from thylakoid membranes and in vivo Chla fluorescence measurements (maximum quantum yield of photosystem II) made before or after infection with pathogens to evaluate their photosynthetic competence. Hydrogen peroxide and superoxide anion serve as signal molecules for generation of reactive oxygen species and Tiron, scavenger of superoxide anion, blocked release of GFP from chloroplasts. Significant increase in ion leakage in the presence of paraquat and light suggests changes in the chloroplast envelope to facilitate protein release. Release of GFP-RC101 (an antimicrobial peptide), which was triggered by Erwinia infection, ceased after conferring protection, further confirming this export phenomenon. These results suggest a novel signaling mechanism, especially for participation of chloroplast proteins (e.g. transcription factors) in retrograde signaling, thereby offering new opportunities to regulate pathways outside chloroplasts.

Pathogenicity of and plant immunity to soft rot pectobacteria

Soft rot pectobacteria are broad host range enterobacterial pathogens that cause disease on a variety of plant species including the major crop potato. Pectobacteria are aggressive necrotrophs that harbor a large arsenal of plant cell wall-degrading enzymes as their primary virulence determinants. These enzymes together with additional virulence factors are employed to macerate the host tissue and promote host cell death to provide nutrients for the pathogens. In contrast to (hemi)biotrophs such as Pseudomonas, type III secretion systems (T3SS) and T3 effectors do not appear central to pathogenesis of pectobacteria. Indeed, recent genomic analysis of several Pectobacterium species including the emerging pathogen Pectobacterium wasabiae has shown that many strains lack the entire T3SS as well as the T3 effectors. Instead, this analysis has indicated the presence of novel virulence determinants. Resistance to broad host range pectobacteria is complex and does not appear to involve single resistance genes. Instead, activation of plant innate immunity systems including both SA (salicylic acid) and JA (jasmonic acid)/ET (ethylene)-mediated defenses appears to play a central role in attenuation of Pectobacterium virulence. These defenses are triggered by detection of pathogen-associated molecular patterns (PAMPs) or recognition of modified-self such as damage-associated molecular patterns (DAMPs) and result in enhancement of basal immunity (PAMP/DAMP-triggered immunity or pattern-triggered immunity, PTI). In particular plant cell wall fragments released by the action of the degradative enzymes secreted by pectobacteria are major players in enhanced immunity toward these pathogens. Most notably bacterial pectin-degrading enzymes release oligogalacturonide (OG) fragments recognized as DAMPs activating innate immune responses. Recent progress in understanding OG recognition and signaling allows novel genetic screens for OG-insensitive mutants and will provide new insights into plant defense strategies against necrotrophs such as pectobacteria.

Salmonella enterica suppresses Pectobacterium carotovorum subsp. carotovorum population and soft rot progression by acidifying the microaerophilic environment

Although enteric human pathogens are usually studied in the context of their animal hosts, a significant portion of their life cycle occurs on plants. Plant disease alters the phyllosphere, leading to enhanced growth of human pathogens; however, the impact of human pathogens on phytopathogen biology and plant health is largely unknown. To characterize the interaction between human pathogens and phytobacterial pathogens in the phyllosphere, we examined the interactions between Pectobacterium carotovorum subsp. carotovorum and Salmonella enterica or Escherichia coli O157:H7 with regard to bacterial populations, soft rot progression, and changes in local pH. The presence of P. carotovorum subsp. carotovorum enhanced the growth of both S. enterica and E. coli O157:H7 on leaves. However, in a microaerophilic environment, S. enterica reduced P. carotovorum subsp. carotovorum populations and soft rot progression by moderating local environmental pH. Reduced soft rot was not due to S. enterica proteolytic activity. Limitations on P. carotovorum subsp. carotovorum growth, disease progression, and pH elevation were not observed on leaves coinoculated with E. coli O157:H7 or when leaves were coinoculated with S. enterica in an aerobic environment. S. enterica also severely undermined the relationship between the phytobacterial population and disease progression of a P. carotovorum subsp. carotovorum budB mutant defective in the 2,3-butanediol pathway for acid neutralization. Our results show that S. enterica and E. coli O157:H7 interact differently with the enteric phytobacterial pathogen P. carotovorum subsp. carotovorum. S. enterica inhibition of soft rot progression may conceal a rapidly growing human pathogen population. Whereas soft rotted produce can alert consumers to the possibility of food-borne pathogens, healthy-looking produce may entice consumption of contaminated vegetables.

IMPORTANCE

Salmonella enterica and Escherichia coli O157:H7 may use plants to move between animal and human hosts. Their populations are higher on plants cocolonized with the common bacterial soft rot pathogen Pectobacterium carotovorum subsp. carotovorum, turning edible plants into a risk factor for human disease. We inoculated leaves with P. carotovorum subsp. carotovorum and S. enterica or E. coli O157:H7 to study the interactions between these bacteria. While P. carotovorum subsp. carotovorum enhanced the growth of both S. enterica and E. coli O157:H7, these human pathogens affected P. carotovorum subsp. carotovorum fundamentally differently. S. enterica reduced P. carotovorum subsp. carotovorum growth and acidified the environment, leading to less soft rot on leaves; E. coli O157:H7 had no such effects. As soft rot signals a food safety risk, the reduction of soft rot symptoms in the presence of S. enterica may lead consumers to eat healthy-looking but S. enterica-contaminated produce.

Activation of Defense Mechanisms against Pathogens in Mosses and Flowering Plants

During evolution, plants have developed mechanisms to cope with and adapt to different types of stress, including microbial infection. Once the stress is sensed, signaling pathways are activated, leading to the induced expression of genes with different roles in defense. Mosses (Bryophytes) are non-vascular plants that diverged from flowering plants more than 450 million years ago, allowing comparative studies of the evolution of defense-related genes and defensive metabolites produced after microbial infection. The ancestral position among land plants, the sequenced genome and the feasibility of generating targeted knock-out mutants by homologous recombination has made the moss Physcomitrella patens an attractive model to perform functional studies of plant genes involved in stress responses. This paper reviews the current knowledge of inducible defense mechanisms in P. patens and compares them to those activated in flowering plants after pathogen assault, including the reinforcement of the cell wall, ROS production, programmed cell death, activation of defense genes and synthesis of secondary metabolites and defense hormones. The knowledge generated in P. patens together with comparative studies in flowering plants will help to identify key components in plant defense responses and to design novel strategies to enhance resistance to biotic stress.

Priming of the Arabidopsis pattern-triggered immunity response upon infection by necrotrophic Pectobacterium carotovorum bacteria

Boosted responsiveness of plant cells to stress at the onset of pathogen- or chemically induced resistance is called priming. The chemical β-aminobutyric acid (BABA) enhances Arabidopsis thaliana resistance to hemibiotrophic bacteria through the priming of the salicylic acid (SA) defence response. Whether BABA increases Arabidopsis resistance to the necrotrophic bacterium Pectobacterium carotovorum ssp. carotovorum (Pcc) is not clear. In this work, we show that treatment with BABA protects Arabidopsis against the soft-rot pathogen Pcc. BABA did not prime the expression of the jasmonate/ethylene-responsive gene PLANT DEFENSIN 1.2 (PDF1.2), the up-regulation of which is usually associated with resistance to necrotrophic pathogens. Expression of the SA marker gene PATHOGENESIS RELATED 1 (PR1) on Pcc infection was primed by BABA treatment, but SA-defective mutants demonstrated a wild-type level of BABA-induced resistance against Pcc. BABA primed the expression of the pattern-triggered immunity (PTI)-responsive genes FLG22-INDUCED RECEPTOR-LIKE KINASE 1 (FRK1), ARABIDOPSIS NON-RACE SPECIFIC DISEASE RESISTANCE GENE (NDR1)/HAIRPIN-INDUCED GENE (HIN1)-LIKE 10 (NHL10) and CYTOCHROME P450, FAMILY 81 (CYP81F2) after inoculation with Pcc or after treatment with purified bacterial microbe-associated molecular patterns, such as flg22 or elf26. PTI-mediated callose deposition was also potentiated in BABA-treated Arabidopsis, and BABA boosted Arabidopsis stomatal immunity to Pcc. BABA treatment primed the PTI response in the SA-defective mutants SA induction deficient 2-1 (sid2-1) and phytoalexin deficient 4-1 (pad4-1). In addition, BABA priming was associated with open chromatin configurations in the promoter region of PTI marker genes. Our data indicate that BABA primes the PTI response upon necrotrophic bacterial infection and suggest a role for the PTI response in BABA-induced resistance.

Methylobacterium-induced endophyte community changes correspond with protection of plants against pathogen attack

Plant inoculation with endophytic bacteria that normally live inside the plant without harming the host is a highly promising approach for biological disease control. The mechanism of resistance induction by beneficial bacteria is poorly understood, because pathways are only partly known and systemic responses are typically not seen. The innate endophytic community structures change in response to external factors such as inoculation, and bacterial endophytes can exhibit direct or indirect antagonism towards pathogens. Earlier we showed that resistance induction by an endophytic Methylobacterium sp. in potato towards Pectobacterium atrosepticum was dependent on the density of the inoculum, whereas the bacterium itself had no antagonistic activity. To elucidate the role of innate endophyte communities in plant responses, we studied community changes in both in vitro and greenhouse experiments using various combinations of plants, endophyte inoculants, and pathogens. Induction of resistance was studied in several potato (Solanum tuberosum L.) cultivars by Methylobacterium sp. IMBG290 against the pathogens P. atrosepticum, Phytophthora infestans and Pseudomonas syringae pv. tomato DC3000, and in pine (Pinus sylvestris L.) by M. extorquens DSM13060 against Gremmeniella abietina. The capacities of the inoculated endophytic Methylobacterium spp. strains to induce resistance were dependent on the plant cultivar, pathogen, and on the density of Methylobacterium spp. inoculum. Composition of the endophyte community changed in response to inoculation in shoot tissues and correlated with resistance or susceptibility to the disease. Our results demonstrate that endophytic Methylobacterium spp. strains have varying effects on plant disease resistance, which can be modulated through the endophyte community of the host.

Ectopic expression of Hrf1 enhances bacterial resistance via regulation of diterpene phytoalexins, silicon and reactive oxygen species burst in rice

Harpin proteins as elicitor derived from plant gram negative bacteria such as Xanthomonas oryzae pv. oryzae (Xoo), Erwinia amylovora induce disease resistance in plants by activating multiple defense responses. However, it is unclear whether phytoalexin production and ROS burst are involved in the disease resistance conferred by the expression of the harpin(Xoo) protein in rice. In this article, ectopic expression of hrf1 in rice enhanced resistance to bacterial blight. Accompanying with the activation of genes related to the phytoalexin biosynthesis pathway in hrf1-transformed rice, phytoalexins quickly and consistently accumulated concurrent with the limitation of bacterial growth rate. Moreover, the hrf1-transformed rice showed an increased ability for ROS scavenging and decreased hydrogen peroxide (H(2)O(2)) concentration. Furthermore, the localization and relative quantification of silicon deposition in rice leaves was detected by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDS). Finally, the transcript levels of defense response genes increased in transformed rice. These results show a correlation between Xoo resistance and phytoalexin production, H(2)O(2), silicon deposition and defense gene expression in hrf1-transformed rice. These data are significant because they provide evidence for a better understanding the role of defense responses in the incompatible interaction between bacterial disease and hrf1-transformed plants. These data also supply an opportunity for generating nonspecific resistance to pathogens.

Analysis of the plant bos1 mutant highlights necrosis as an efficient defence mechanism during D. dadantii/Arabidospis thaliana interaction

Dickeya dadantii is a broad host range phytopathogenic bacterium provoking soft rot disease on many plants including Arabidopsis. We showed that, after D. dadantii infection, the expression of the Arabidopsis BOS1 gene was specifically induced by the production of the bacterial PelB/C pectinases able to degrade pectin. This prompted us to analyze the interaction between the bos1 mutant and D. dadantii. The phenotype of the infected bos1 mutant is complex. Indeed, maceration symptoms occurred more rapidly in the bos1 mutant than in the wild type parent but at a later stage of infection, a necrosis developed around the inoculation site that provoked a halt in the progression of the maceration. This necrosis became systemic and spread throughout the whole plant, a phenotype reminiscent of that observed in some lesion mimic mutants. In accordance with the progression of maceration symptoms, bacterial population began to grow more rapidly in the bos1 mutant than in the wild type plant but, when necrosis appeared in the bos1 mutant, a reduction in bacterial population was observed. From the plant side, this complex interaction between D. dadantii and its host includes an early plant defence response that comprises reactive oxygen species (ROS) production accompanied by the reinforcement of the plant cell wall by protein cross-linking. At later timepoints, another plant defence is raised by the death of the plant cells surrounding the inoculation site. This plant cell death appears to constitute an efficient defence mechanism induced by D. dadantii during Arabidopsis infection.

Gluconate metabolism is required for virulence of the soft-rot pathogen Pectobacterium carotovorum

Pectobacterium carotovorum is a ubiquitous soft rot pathogen that uses global virulence regulators to coordinate pathogenesis in response to undefined environmental conditions. We characterize an operon in P. carotovorum required for gluconate metabolism and virulence. The operon contains four genes that are highly conserved among proteobacteria (initially annotated ygbJKLM), one of which was misassigned as a type III secreted effector, (ygbK, originally known as hopAN1). A mutant with a deletion-insertion within this operon is unable to metabolize gluconate, a precursor for the pentose phosphate pathway. The mutant exhibits attenuated growth on the leaves of its host of isolation, potato, and those of Arabidopsis thaliana. Notably, the mutant hypermacerates potato tubers and is deficient in motility. Global virulence regulators that are responsive to cell wall pectin breakdown products and other undefined environmental signals, KdgR and FlhD, respectively, are misregulated in the mutant. The alteration of virulence mediated via changes in transcription of known global virulence regulators in our ygbJ-M operon mutant suggests a role for host-derived catabolic intermediates in P. carotovorum pathogenesis. Thus, we rename this operon in P. carotovorum vguABCD for virulence and gluconate metabolism.

Arabidopsis thaliana cells: a model to evaluate the virulence of Pectobacterium carotovorum

Pectobacterium carotovorum are economically important plant pathogens that cause plant soft rot. These enterobacteria display high diversity world-wide. Their pathogenesis depends on production and secretion of virulence factors such as plant cell wall-degrading enzymes, type III effectors, a necrosis-inducing protein, and a secreted virulence factor from Xanthomonas spp., which are tightly regulated by quorum sensing. Pectobacterium carotovorum also present pathogen-associated molecular patterns that could participate in their pathogenicity. In this study, by using suspension cells of Arabidopsis thaliana, we correlate plant cell death and pectate lyase activities during coinfection with different P. carotovorum strains. When comparing soft rot symptoms induced on potato slices with pectate lyase activities and plant cell death observed during coculture with Arabidopsis thaliana cells, the order of strain virulence was found to be the same. Therefore, Arabidopsis thaliana cells could be an alternative tool to evaluate rapidly and efficiently the virulence of different P. carotovorum strains.

Induction of systemic resistance in Arabidopsis thaliana in response to a culture filtrate from a plant growth-promoting fungus, Phoma sp. GS8-3

The plant growth-promoting fungus (PGPF), Phoma sp. GS8-3, isolated from a zoysia grass rhizosphere, is capable of protecting cucumber plants against virulent pathogens. This fungus was investigated in terms of the underlying mechanisms and ability to elicit systemic resistance in Arabidopsis thaliana. Root treatment of Arabidopsis plants with a culture filtrate (CF) from Phoma sp. GS8-3 elicited systemic resistance against the bacterial speck pathogen Pseudomonas syringae pv. tomato DC3000 (Pst), with restricted disease development and inhibited pathogen proliferation. Pathway-specific mutant plants, such as jar1 (jasmonic acid insensitive) and ein2 (ethylene insensitive), and transgenic NahG plants (impaired in salicylate signalling) were protected after application of the CF, demonstrating that these pathways are dispensable (at least individually) in CF-mediated resistance. Similarly, NPR1 interference in npr1 mutants had no effect on CF-induced resistance. Gene expression studies revealed that CF treatment stimulated the systemic expression of both the SA-inducible PR-1 and JA/ET-inducible PDF1.2 genes. However, pathogenic challenge to CF-treated plants was associated with potentiated expression of the PR-1 gene and down-regulated expression of the PDF1.2 gene. The observed down-regulation of the PDF1.2 gene in CF-treated plants indicates that there may be cross-talk between SA- and JA/ET-dependent signalling pathways during the pathogenic infection process. In conclusion, our data suggest that CF of Phoma sp. GS8-3 induces resistance in Arabidopsis in a manner where SA and JA/ET may play a role in defence signalling.

The AtrbohD-mediated oxidative burst elicited by oligogalacturonides in Arabidopsis is dispensable for the activation of defense responses effective against Botrytis cinerea

Oligogalacturonides (OGs) are endogenous elicitors of defense responses released after partial degradation of pectin in the plant cell wall. We have previously shown that, in Arabidopsis (Arabidopsis thaliana), OGs induce the expression of PHYTOALEXIN DEFICIENT3 (PAD3) and increase resistance to the necrotrophic fungal pathogen Botrytis cinerea independently of signaling pathways mediated by jasmonate, salicylic acid, and ethylene. Here, we illustrate that the rapid induction of the expression of a variety of genes by OGs is also independent of salicylic acid, ethylene, and jasmonate. OGs elicit a robust extracellular oxidative burst that is generated by the NADPH oxidase AtrbohD. This burst is not required for the expression of OG-responsive genes or for OG-induced resistance to B. cinerea, whereas callose accumulation requires a functional AtrbohD. OG-induced resistance to B. cinerea is also unaffected in powdery mildew resistant4, despite the fact that callose accumulation was almost abolished in this mutant. These results indicate that the OG-induced oxidative burst is not required for the activation of defense responses effective against B. cinerea, leaving open the question of the role of reactive oxygen species in elicitor-mediated defense.

Kunitz trypsin inhibitor: an antagonist of cell death triggered by phytopathogens and fumonisin b1 in Arabidopsis

Programmed cell death (PCD) is a central regulatory process in both plant development and in plant responses to pathogens. PCD requires a coordinate activation of pro-apoptotic factors such as proteases and suppressors inhibiting and modulating these processes. In plants, various caspase-like cysteine proteases as well as serine proteases have been implicated in PCD. Here, we show that a serine protease (Kunitz trypsin) inhibitor (KTI1) of Arabidopsis acts as a functional KTI when produced in bacteria and in planta. Expression of AtKTI1 is induced late in response to bacterial and fungal elicitors and to salicylic acid. RNAi silencing of the AtKTI1 gene results in enhanced lesion development after infiltration of leaf tissue with the PCD-eliciting fungal toxin fumonisin B1 (FB1) or the avirulent bacterial pathogen Pseudomonas syringae pv tomato DC3000 carrying avrB (Pst avrB). Overexpression of AtKTI1 results in reduced lesion development after Pst avrB and FB1 infiltration. Interestingly, RNAi silencing of AtKTI1 leads to enhanced resistance to the virulent pathogen Erwinia carotovora subsp. carotovora SCC1, while overexpression of AtKTI1 leads to higher susceptibility towards this pathogen. Together, these data indicate that AtKTI1 is involved in modulating PCD in plant-pathogen interactions.

Expression of a harpin-encoding gene in rice confers durable nonspecific resistance to Magnaporthe grisea

Engineering durable nonspecific resistance to phytopathogens is one of the ultimate goals of plant breeding. However, most attempts to reach this goal fail as a result of rapid changes in pathogen populations and the sheer diversity of pathogen infection mechanisms. In this study, we show that the expression of a harpin-encoding gene (hrf1), derived from Xanthomonas oryzae pv. oryzae, confers nonspecific resistance in rice to the blast fungus Magnaporthe grisea. Transgenic plants and their T1-T7 progenies were highly resistant to all major M. grisea races in rice-growing areas along the Yangtze River, China. The expression of defence-related genes was activated in resistant transgenic plants, and the formation of melanized appressoria, which is essential for foliar infection, was inhibited on plant leaves. These results suggest that harpins may offer new opportunities for generating broad-spectrum disease resistance in other crops.

Priming of antimicrobial phenolics during induced resistance response towards Pectobacterium carotovorum in the ornamental monocot calla lily

Calla lilies are herbaceous monocotyledonous plants that are highly sensitive to Pectobacterium carotovorum, the causal agent of soft-rot disease. Results demonstrate that, in response to elicitation using plant defense activators, the calla lily produces elevated levels of antimicrobial phenolics and that these compounds contribute to increased resistance against P. carotovorum, as shown by reduced bacterial proliferation in elicited leaves. The polyphenolic nature of the induced compounds was supported by autofluorescence, absorbance spectra, and reaction with Folin-Ciocalteu reagent. Two plant defense activators, Bion and methyl jasmonate, differed in both their capacity to induce accumulation of polyphenols and their resistance against the pathogen. Methyl jasmonate elicitation brought about higher accumulation of free phenolics relative to Bion, suggesting priming of bioactive polyphenols as a principal factor in the calla lily defense against P. carotovorum. To further characterize the nature of induced compounds, two major compounds were collected and identified as swertisin and isovitexin by mass and nuclear magnetic resonance spectroscopies.

Erwinia carotovora elicitors and Botrytis cinerea activate defense responses in Physcomitrella patens

BACKGROUND

Vascular plants respond to pathogens by activating a diverse array of defense mechanisms. Studies with these plants have provided a wealth of information on pathogen recognition, signal transduction and the activation of defense responses. However, very little is known about the infection and defense responses of the bryophyte, Physcomitrella patens, to well-studied phytopathogens. The purpose of this study was to determine: i) whether two representative broad host range pathogens, Erwinia carotovora ssp. carotovora (E.c. carotovora) and Botrytis cinerea (B. cinerea), could infect Physcomitrella, and ii) whether B. cinerea, elicitors of a harpin (HrpN) producing E.c. carotovora strain (SCC1) or a HrpN-negative strain (SCC3193), could cause disease symptoms and induce defense responses in Physcomitrella.

RESULTS

B. cinerea and E.c. carotovora were found to readily infect Physcomitrella gametophytic tissues and cause disease symptoms. Treatments with B. cinerea spores or cell-free culture filtrates from E.c. carotovoraSCC1 (CF(SCC1)), resulted in disease development with severe maceration of Physcomitrella tissues, while CF(SCC3193) produced only mild maceration. Although increased cell death was observed with either the CFs or B. cinerea, the occurrence of cytoplasmic shrinkage was only visible in Evans blue stained protonemal cells treated with CF(SCC1) or inoculated with B. cinerea. Most cells showing cytoplasmic shrinkage accumulated autofluorescent compounds and brown chloroplasts were evident in a high proportion of these cells. CF treatments and B. cinerea inoculation induced the expression of the defense-related genes: PR-1, PAL, CHS and LOX.

CONCLUSION

B. cinerea and E.c. carotovora elicitors induce a defense response in Physcomitrella, as evidenced by enhanced expression of conserved plant defense-related genes. Since cytoplasmic shrinkage is the most common morphological change observed in plant PCD, and that harpins and B. cinerea induce this type of cell death in vascular plants, our results suggest that E.c. carotovora CFSCC1 containing HrpN and B. cinerea could also induce this type of cell death in Physcomitrella. Our studies thus establish Physcomitrella as an experimental host for investigation of plant-pathogen interactions and B. cinerea and elicitors of E.c. carotovora as promising tools for understanding the mechanisms involved in defense responses and in pathogen-mediated cell death in this simple land plant.

The MAP kinase kinase MKK2 affects disease resistance in Arabidopsis

The Arabidopsis mitogen-activated protein kinase (MAPK) kinase 2 (MKK2) was shown to mediate cold and salt stress responses through activation of the two MAP kinases MPK4 and MPK6. Transcriptome analysis of plants expressing constitutively active MKK2 (MKK2-EE plants) showed altered expression of genes induced by abiotic stresses but also a significant number of genes involved in defense responses. Both MPK4 and MPK6 became rapidly activated upon Pseudomonas syringae pv. tomato DC3000 infection and MKK2-EE plants showed enhanced levels of MPK4 activation. Although MKK2-EE plants shared enhanced expression of genes encoding enzymes of ethylene (ET) and jasmonic acid (JA) synthesis, ET, JA, and salicylic acid (SA) levels did not differ dramatically from those of wild-type or mkk2-null plants under ambient growth conditions. Upon P. syringae pv. tomato DC3000 infection, however, MKK2-EE plants showed reduced increases of JA and SA levels. These results indicate that MKK2 is involved in regulating hormone levels in response to pathogens. MKK2-EE plants were more resistant to infection by P. syringae pv. tomato DC3000 and Erwinia carotovora subsp. carotovora, but showed enhanced sensitivity to the fungal necrotroph Alternaria brassicicola. Our data indicate that MKK2 plays a role in abiotic stress tolerance and plant disease resistance.

AtPTR3, a wound-induced peptide transporter needed for defence against virulent bacterial pathogens in Arabidopsis

Mutation in the wound-induced peptide transporter gene AtPTR3 (At5g46050) of Arabidopsis thaliana has been shown to affect germination on media containing a high salt concentration. The heterologous expression in yeast was utilized to verify that the AtPTR3 protein transports di-and tripeptides. The T-DNA insert in the Atptr3-1 mutant in the Arabidopsis ecotype C24 revealed two T-DNA copies, the whole vector sequence, and the gus marker gene inserted in the second intron of the AtPTR3 gene. An almost identical insertion site was found in the Atptr3-2 mutant of the Col-0 ecotype. The AtPTR3 expression was shown to be regulated by several signalling compounds, most clearly by salicylic acid (SA), but also methyl jasmonate (MeJA) and abscisic acid. Real-time PCR experiments suggested that the wound-induction of the AtPTR3 gene was abolished in the SA and JA signalling mutants. The Atptr3 mutant plants had increased susceptibility to virulent pathogenic bacteria Erwinia carotovora subsp. carotovora and Pseudomonas syringae pv. tomato, and produced more reactive oxygen species when grown on media containing paraquat or rose bengal. Public microarray data suggest that the AtPTR3 expression was induced by Pseudomonas elicitors and by avirulent P. syringae pathovars and type III secretion mutants. This was verified experimentally for the hrpA mutant with real-time PCR. These results suggest that AtPTR3 is one of the defence-related genes whose expression is reduced by virulent bacterium by type III dependent fashion. Our results suggest that AtPTR3 protects the plant against biotic and abiotic stresses.

The role of pectate lyase and the jasmonic acid defense response in Pseudomonas viridiflava virulence

Pseudomonas viridiflava is a common pathogen of Arabidopsis thaliana in wild populations, yet very little is known about mechanisms of resistance and virulence in this interaction. We examined the induced defense response of A. thaliana to several strains of P. viridiflava collected from this host by quantifying the expression of PR-1 and LOX2/PDF1.2, which serve as markers for induction of the salicylic and jasmonic acid (JA) pathways, respectively. Growth of these strains then was assessed on Col-0, the fad3/7/8 and coil-1 mutants deficient in JA- and ethylene (ET)-induced defense responses, and the sid2-1 mutant deficient in salicylic acid-induced defense responses. All strains of P. viridiflava induced high expression of LOX2 and PDF1.2 on Col-0. In contrast, PR-1 expression was delayed and reduced relative to PDF1.2 expression. Additionally, three of four P. viridiflava strains were more virulent on fad3/7/8 relative to Col-0, whereas all strains were more virulent on coil-1 relative to Col-0, indicating that P. viridiflava generally may be suppressed by JA/ET-mediated defense responses. In contrast, no increase in the growth of P. viridiflava strains was observed in the sid2-1 mutant relative to Col-0. Parallel experiments were performed with the closely related P. syringae pv. tomato for comparative purposes. In addition, we assessed the role of pectate lyase and the alternative sigma factor HrpL in P. viridiflava virulence on A. thaliana and found that pectate lyase activity is correlated with virulence, whereas the removal of pectate lyase or HrpL significantly reduced virulence.

Altering glucosinolate profiles modulates disease resistance in plants

Plant diseases are major contributing factors for crop loss in agriculture. Here, we show that Arabidopsis plants with high levels of novel glucosinolates (GSs) as a result of the introduction of single CYP79 genes exhibit altered disease resistance. Arabidopsis expressing CYP79D2 from cassava accumulated aliphatic isopropyl and methylpropyl GS, and showed enhanced resistance against the bacterial soft-rot pathogen Erwinia carotovora, whereas Arabidopsis expressing the sorghum CYP79A1 or over-expressing the endogenous CYP79A2 accumulated p-hydroxybenzyl or benzyl GS, respectively, and showed increased resistance towards the bacterial pathogen Pseudomonas syringae. In addition to the direct toxic effects of GS breakdown products, increased accumulation of aromatic GSs was shown to stimulate salicylic acid-mediated defenses while suppressing jasmonate-dependent defenses, as manifested in enhanced susceptibility to the fungus Alternaria brassicicola. Arabidopsis with modified GS profiles provide important tools for evaluating the biological effects of individual GSs and thereby show potential as biotechnological tools for the generation of plants with tailor-made disease resistance.

WRKY70 modulates the selection of signaling pathways in plant defense

Cross-talk between signal transduction pathways is a central feature of the tightly regulated plant defense signaling network. The potential synergism or antagonism between defense pathways is determined by recognition of the type of pathogen or pathogen-derived elicitor. Our studies have identified WRKY70 as a node of convergence for integrating salicylic acid (SA)- and jasmonic acid (JA)-mediated signaling events during plant response to bacterial pathogens. Here, we challenged transgenic plants altered in WRKY70 expression as well as WRKY70 knockout mutants of Arabidopsis with the fungal pathogens Alternaria brassicicola and Erysiphe cichoracearum to elucidate the role of WRKY70 in modulating the balance between distinct defense responses. Gain or loss of WRKY70 function causes opposite effects on JA-mediated resistance to A. brassicicola and the SA-mediated resistance to E. cichoracearum. While the up-regulation of WRKY70 caused enhanced resistance to E. cichoracearum, it compromised plant resistance to A. brassicicola. Conversely, down-regulation or insertional inactivation of WRKY70 impaired plant resistance to E. cichoracearum. Over-expression of WRKY70 resulted in the suppression of several JA responses including expression of a subset of JA- and A. brassicicola-responsive genes. We show that this WRKY70-controlled suppression of JA-signaling is partly executed by NPR1. The results indicate that WRKY70 has a pivotal role in determining the balance between SA-dependent and JA-dependent defense pathways.

Elevated temperature enhances virulence of Erwinia carotovora subsp. carotovora strain EC153 to plants and stimulates production of the quorum sensing signal, N-acyl homoserine lactone, and extracellular proteins

Erwinia carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, and E. carotovora subsp. carotovora produce high levels of extracellular enzymes, such as pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cel), and protease (Prt), and the quorum-sensing signal N-acyl-homoserine lactone (AHL) at 28 degrees C. However, the production of these enzymes and AHL by these bacteria is severely inhibited during growth at elevated temperatures (31.2 degrees C for E. carotovora subsp. atroseptica and 34.5 degrees C for E. carotovora subsp. betavasculorum and most E. carotovora subsp. carotovora strains). At elevated temperatures these bacteria produce high levels of RsmA, an RNA binding protein that promotes RNA decay. E. carotovora subsp. carotovora strain EC153 is an exception in that it produces higher levels of Pel, Peh, Cel, and Prt at 34.5 degrees C than at 28 degrees C. EC153 also causes extensive maceration of celery petioles and Chinese cabbage leaves at 34.5 degrees C, which correlates with a higher growth rate and higher levels of rRNA and AHL. The lack of pectinase production by E. carotovora subsp. carotovora strain Ecc71 at 34.5 degrees C limits the growth of this organism in plant tissues and consequently impairs its ability to cause tissue maceration. Comparative studies with ahlI (the gene encoding a putative AHL synthase), pel-1, and peh-1 transcripts documented that at 34.5 degrees C the RNAs are more stable in EC153 than in Ecc71. Our data reveal that overall metabolic activity, AHL levels, and mRNA stability are responsible for the higher levels of extracellular protein production and the enhanced virulence of EC153 at 34.5 degrees C compared to 28 degrees C.

Biological activities of purified harpin(Xoo) and harpin(Xoo) detection in transgenic plants using its polyclonal antibody

Many harpins have been found in plant pathogen bacteria that can elicit disease and insect resistance in plants, and promote plant growth. In this work, we overexpressed and purified Xanthomonas oryzae pv. oryzae harpin, harpin(Xoo), in Escherichia coli BL21/pGEX-hpa1. Harpin(Xoo) was fused to the C-terminus of glutathione S-transferase (GST) and purified using the Bulk GST purification module and thrombin cleavage capture kit. Purified harpin(Xoo) protein was sensitive to protease K and stable to heat treatment, and could not induce a hypersensitive response after treatment with various plant metabolic inhibitors; these characteristics were similar to harpin(Ea) of Erwinia amylovora. The purified harpin(Xoo) showed a similar ability to induce tobacco mosaic virus resistance in tobacco as harpin(Ea). Its antibody worked well in detecting the purified harpin(Xoo), harpin(Xoo) in the total protein of E. coli BL21/pGEX-hpa1 and an hpa1 transgenic rice.

Multiple defence signals induced by Erwinia carotovora ssp. carotovora elicitors in potato

SUMMARY Signal pathways involved in Solanum tuberosum-Erwinia carotovora ssp. carotovora(SCC3193) interaction were characterized. To this end, the concentration of several signal molecules implicated in plant defence such as ethylene (ET), jasmonates (JA) and salicylic acid (SA) were measured in potato plants treated by cell-free culture filtrates (CF) from E. c. carotovora(SCC3193). Furthermore, the presence of other potential signalling compounds such as cinnamic acid (CA) and related aromatic compounds was screened in the elicitor-treated plants. The activity of these signal compounds as inducers of defence-related genes such as drd-1 (a defence-related alcohol dehydrogenase), pinII (proteinase inhibitor II), chtB4 (basic chitinase) and chtA2 (acidic chitinase) was characterized. The results demonstrate that ET, JA and CA accumulate in potato tissues in response to CF. These signal molecules were shown to induce differential expression of drd-1, pinII, chtB4 and chtA2. Our data suggest that in addition to ET and JA, CA and possibly other aromatic compounds also may play a role in defence signalling in potato.

Bacterial type two secretion system secreted proteins: double-edged swords for plant pathogens

The type two secretion system (T2S) is important for virulence of a number of gram-negative bacterial plant pathogens. Most of the T2S-secreted proteins that have been characterized to date are involved in degrading different components of plant cell walls. Functional redundancy appears to exist among T2S-secreted proteins because significant effects on virulence are observed only in strains in which multiple secreted proteins are mutated. Several T2S-secreted proteins have been shown to induce plant defense responses, including hypersensitive response-like reactions. Bacterial pathogens can suppress these defense responses, and recent results indicate that suppression is mediated through the type three secretion system.

Rotting softly and stealthily

The soft rot erwiniae, which are plant pathogens on potato and other crops world-wide, synthesize and secrete large quantities of plant cell wall degrading enzymes that are responsible for the soft rot phenotype, earning them the epithet 'brute force' pathogens. They have been distinguished from classic 'stealth' pathogens, such as Pseudomonas syringae, which possesses an extensive battery of Type III secreted effector proteins and phytotoxins to manipulate and suppress host defences. However, recent studies, including whole-genome sequencing, are revealing many components of stealth pathogenesis within the soft rot erwiniae (SRE), suggesting that 'stealth' and 'brute force' should not be regarded as mutually exclusive modes of pathogenesis.

The ABI2-dependent abscisic acid signalling controls HrpN-induced drought tolerance in Arabidopsis

HrpN, a protein produced by the plant pathogenic bacterium Erwinia amylovora, has been shown to stimulate plant growth and resistance to pathogens and insects. Here we report that HrpN activates abscisic acid (ABA) signalling to induce drought tolerance (DT) in Arabidopsis thaliana L. plants grown with water stress. Spraying wild-type plants with HrpN-promoted stomatal closure decreased leaf transpiration rate, increased moisture and proline levels in leaves, and alleviated extents of damage to cell membranes and plant drought symptoms caused by water deficiency. In plants treated with HrpN, ABA levels increased; expression of several ABA-signalling regulatory genes and the important effector gene rd29B was induced or enhanced. Induced expression of rd29B, promotion of stomatal closure, and reduction in drought severity were observed in the abi1-1 mutant, which has a defect in the phosphatase ABI1, after HrpN was applied. In contrast, HrpN failed to induce these responses in the abi2-1 mutant, which is impaired in the phosphatase ABI2. Inhibiting wild-type plants to synthesize ABA eliminated the role of HrpN in promoting stomatal closure and reducing drought severity. Moreover, resistance to Pseudomonas syringae developed in abi2-1 as in wild-type plants following treatment with HrpN. Thus, an ABI2-dependent ABA signalling pathway is responsible for the induction of DT but does not affect pathogen defence under the circumstances of this study.