Search for host defense markers uncovers an apple agglutination factor corresponding with fire blight resistance

Pathenogenesis-related (PR) proteins are extensively used as molecular markers to dissect the signaling cascades leading to plant defense responses. However, studies focusing on the biochemical or biological properties of these proteins remain rare. Here, we identify and characterize a class of apple (Malus domestica) PR proteins, named M. domestica AGGLUTININS (MdAGGs), belonging to the amaranthin-like lectin family. By combining molecular and biochemical approaches, we show that abundant production of MdAGGs in leaf tissues corresponds with enhanced resistance to the bacterium Erwinia amylovora, the causal agent of the disease fire blight. We also show that E. amylovora represses the expression of MdAGG genes by injecting the type 3 effector DspA/E into host cells and by secreting bacterial exopolysaccharides. Using a purified recombinant MdAGG, we show that the protein agglutinates E. amylovora cells in vitro and binds bacterial lipopolysaccharides at low pH, conditions reminiscent of the intercellular pH occurring in planta upon E. amylovora infection. We finally provide evidence that negatively charged polysaccharides, such as the free exopolysaccharide amylovoran progressively released by the bacteria, act as decoys relying on charge-charge interaction with the MdAGG to inhibit agglutination. Overall, our results suggest that the production of this particular class of PR proteins may contribute to apple innate immunity mechanisms active against E. amylovora.

Plant nitrate supply regulates Erwinia amylovora virulence gene expression in Arabidopsis

We showed previously that nitrogen (N) limitation decreases Arabidopsis resistance to Erwinia amylovora (Ea). We show that decreased resistance to bacteria in low N is correlated with lower apoplastic reactive oxygen species (ROS) accumulation and lower jasmonic acid (JA) pathway expression. Consistently, pretreatment with methyl jasmonate (Me-JA) increased the resistance of plants grown under low N. In parallel, we show that in planta titres of a nonvirulent type III secretion system (T3SS)-deficient Ea mutant were lower than those of wildtype Ea in low N, as expected, but surprisingly not in high N. This lack of difference in high N was consistent with the low expression of the T3SS-encoding hrp virulence genes by wildtype Ea in plants grown in high N compared to plants grown in low N. This suggests that expressing its virulence factors in planta could be a major limiting factor for Ea in the nonhost Arabidopsis. To test this hypothesis, we preincubated Ea in an inducing medium that triggers expression of hrp genes in vitro, prior to inoculation. This preincubation strongly enhanced Ea titres in planta, independently of the plant N status, and was correlated to a significant repression of JA-dependent genes. Finally, we identify two clusters of metabolites associated with resistance or with susceptibility to Ea. Altogether, our data showed that high susceptibility of Arabidopsis to Ea, under low N or following preincubation in hrp-inducing medium, is correlated with high expression of the Ea hrp genes in planta and low expression of the JA signalling pathway, and is correlated with the accumulation of specific metabolites.

CRISPR genotyping as complementary tool for epidemiological surveillance of Erwinia amylovora outbreaks

Fire blight is a destructive plant disease caused by Erwinia amylovora affecting pome fruit trees, and responsible for large yield declines, long phytosanitary confinements, and high economic losses. In Portugal, the first major fire blight outbreaks occurred in 2010 and 2011, and although later considered eradicated, the emergence of other outbreaks in recent years stressed the need to characterize the E. amylovora populations associated with these outbreaks. In this regard, CRISPR genotyping, assessment of three virulence markers, and semi-quantitative virulence bioassays, were carried out to determine the genotype, and assess the virulence of thirty-six E. amylovora isolates associated with outbreaks occurring between 2010 and 2017 and affecting apple and pear orchards located in the country central-west, known as the main producing region of pome fruits in Portugal. The data gathered reveal that 35 E. amylovora isolates belong to one of the widely-distributed CRISPR genotypes (5-24-38 / D-a-α) regardless the host species, year and region. Ea 680 was the single isolate revealing a new CRISPR genotype due to a novel CR2 spacer located closer to the leader sequence and therefore thought to be recently acquired. Regarding pathogenicity, although dot-blot hybridization assays showed the presence of key virulence factors, namely hrpL (T3SS), hrpN (T3E) and amsG from the amylovoran biosynthesis operon in all E. amylovora isolates studied, pathogenicity bioassays on immature pear slices allowed to distinguish four virulence levels, with most of the isolates revealing an intermediate to severe virulence phenotype. Regardless the clonal population structure of the E. amylovora associated to the outbreaks occurring in Portugal between 2010 and 2017, the different virulence phenotypes, suggests that E. amylovora may have been introduced at different instances into the country. This is the first study regarding E. amylovora in Portugal, and it discloses a novel CRISPR genotype for this bacterium.

Orchestration of virulence factor expression and modulation of biofilm dispersal in Erwinia amylovora through activation of the Hfq-dependent small RNA RprA

Erwinia amylovora is the causative agent of the devastating disease fire blight of pome fruit trees. After infection of host plant leaves at apple shoot tips, E. amylovora cells form biofilms in xylem vessels, restrict water flow, and cause wilting symptoms. Although E. amylovora is well known to be able to cause systemic infection, how biofilm cells of E. amylovora transit from the sessile mode of growth in xylem to the planktonic mode of growth in cortical parenchyma remains unknown. Increasing evidence has suggested the important modulatory roles of Hfq-dependent small RNAs (sRNAs) in the pathogenesis of E. amylovora. Here, we demonstrate that the sRNA RprA acts as a positive regulator of amylovoran exopolysaccharide production, the type III secretion system (T3SS), and flagellar-dependent motility, and as a negative regulator of levansucrase activity and cellulose production. We also show that RprA affects the promoter activity of multiple virulence factor genes and regulates hrpS, a critical T3SS regulator, at the posttranscriptional level. We determined that rprA expression can be activated by the Rcs phosphorelay, and that expression is active during T3SS-mediated host infection in an immature pear fruit infection model. We further showed that overexpression of rprA activated the in vitro dispersal of E. amylovora cells from biofilms. Thus, our investigation of the varied role of RprA in affecting E. amylovora virulence provides important insights into the functions of this sRNA in biofilm control and systemic infection.

Reduced fire blight susceptibility in apple cultivars using a high-efficiency CRISPR/Cas9-FLP/FRT-based gene editing system

The bacterium Erwinia amylovora, the causal agent of fire blight disease in apple, triggers its infection through the DspA/E effector which interacts with the apple susceptibility protein MdDIPM4. In this work, MdDIPM4 knockout has been produced in two Malus × domestica susceptible cultivars using the CRISPR/Cas9 system delivered via Agrobacterium tumefaciens. Fifty-seven transgenic lines were screened to identify CRISPR/Cas9-induced mutations. An editing efficiency of 75% was obtained. Seven edited lines with a loss-of-function mutation were inoculated with the pathogen. Highly significant reduction in susceptibility was observed compared to control plants. Sequencing of five potential off-target sites revealed no mutation event. Moreover, our construct contained a heat-shock inducible FLP/FRT recombination system designed specifically to remove the T-DNA harbouring the expression cassettes for CRISPR/Cas9, the marker gene and the FLP itself. Six plant lines with reduced susceptibility to the pathogen were heat-treated and screened by real-time PCR to quantify the exogenous DNA elimination. The T-DNA removal was further validated by sequencing in one plant line. To our knowledge, this work demonstrates for the first time the development and application of a CRISPR/Cas9-FLP/FRT gene editing system for the production of edited apple plants carrying a minimal trace of exogenous DNA.

The RNA-Binding Protein CsrA Controls Virulence in Erwinia amylovora by Regulating RelA, RcsB, and FlhD at the Posttranscriptional Level

CsrA, an RNA-binding protein, binds to target transcripts and alters their translation or stability. In Erwinia amylovora, CsrA positively regulates the expression of type III secretion system (T3SS), exopolysaccharide amylovoran, and motility. In this study, the global effect of CsrA and its noncoding small RNA (ncsRNA) csrB in E. amylovora was determined by RNA-seq, and potential molecular mechanisms of CsrA-dependent virulence regulation were examined. Transcriptomic analyses under the T3SS-inducing condition revealed that mutation in the csrA gene led to differential expression of more than 20% of genes in the genome. Among them, T3SS genes and those required for cell growth and viability were significantly downregulated. On the other hand, the csrB mutant exhibited significant upregulation of most major virulence genes, suggesting an antagonistic effect of csrB on CsrA targets. Direct interaction between CsrA protein and csrB was further confirmed through the RNA electrophoretic mobility shift assay (REMSA). However, no direct interaction between CsrA and hrpL and hrpS transcripts was detected, suggesting that HrpL and HrpS are not targets of CsrA, whereas three CsrA targets (relA, rcsB, and flhD) were identified and confirmed by REMSA, site-directed mutagenesis, and LacZ reporter gene assays. These findings might partially explain how CsrA positively controls E. amylovora virulence by targeting major regulators at the posttranscriptional level.

Development of a viability digital PCR protocol for the selective detection and quantification of live Erwinia amylovora cells in cankers

Fire blight is a devastating disease of apple and pear caused by the bacterium Erwinia amylovora. One of its main symptoms is canker formation on perennial tissues which may lead to the death of limbs and/or the entire tree. E. amylovora overwinters in cankers which play an important role in initiating fire blight epidemics. However, knowledge of pathogen biology in cankers is scarce, in part due to limitations of classical microbiology methods and the inability of most molecular techniques to distinguish live from dead cells. In this work, a viability digital PCR (v-dPCR) protocol using propidium monoazide (PMA) was developed, allowing for the first time the selective detection and absolute quantification of E. amylovora live cells in apple and pear cankers collected in two time periods. Some key factors affecting the v-dPCR performance were the maceration buffer composition, the target DNA amplicon length, the thermal cycle number and the use of sodium dodecyl sulfate or PMA enhancer for Gram-negative bacteria to improve the effect of PMA. In the future, this methodology could shed light on E. amylovora population dynamics in cankers and provide clues on the effect of management practices, host cultivar, host water/nutritional status, etc., on bacterial survival.

The Apple Fruitlet Model System for Fire Blight Disease

Fire blight is a perennial disease affecting apple and pear production worldwide. Development of resistant cultivars and disease control measures are crucial aspects of managing fire blight. Furthermore, the study of the causal agent, the Gram-negative bacterium Erwinia amylovora, has led to important insights into molecular plant-microbe interactions. However, fire blight does not have a suitable model host, since its host range is limited to plants with complex genetics and relatively limited resources for genetic analysis. Here, we present a rationale for using apple fruitlets as a potential fire blight model host system, and describe a protocol for quantitative fruit inoculation, bacterial growth measurement, and symptom assessment. The use of apple fruitlets is applicable to the molecular genetic analysis of E. amylovora, including high-throughput genetic screens for E. amylovora virulence-defective mutants, and is potentially useful to study host resistance and responses to E. amylovora as well.

A Single Effector Protein, AvrRpt2EA, from Erwinia amylovora Can Cause Fire Blight Disease Symptoms and Induces a Salicylic Acid-Dependent Defense Response

The AvrRpt2_EA effector protein of Erwinia amylovora is important for pathogen recognition in the fire blight-resistant crabapple Malus × robusta 5; however, little is known about its role in susceptible apples. To study its function in planta, we expressed a plant-optimized version of AvrRpt2_EA driven by a heat shock-inducible promoter in transgenic plants of the fire blight-susceptible cultivar Pinova. After induced expression of AvrRpt2_EA, transgenic lines showed shoot necrosis and browning of older leaves, with symptoms similar to natural fire blight infections. Transgenic expression of this effector protein resulted in an increase in the expression of the salicylic acid (SA)-responsive PR-1 gene but, also, in the levels of SA and its derivatives, with diverse kinetics in leaves of different ages. In contrast, no increase of expression levels of VSP2 paralogs, used as marker genes for the activation of the jasmonic acid (JA)-dependent defense pathway, could be detected, which is in agreement with metabolic profiling of JA and its derivatives. Our work demonstrates that AvrRpt2_EA acts as a virulence factor and induces the formation of SA and SA-dependent systemic acquired resistance.

An Erwinia amylovora yjeK mutant exhibits reduced virulence, increased chemical sensitivity and numerous environmentally dependent proteomic alterations

The Gram-negative bacterium Erwinia amylovora causes fire blight, an economically important disease of apples and pears. Elongation factor P (EF-P) is a highly conserved protein that stimulates the formation of the first peptide bond of certain proteins and facilitates the translation of certain proteins, including those with polyproline motifs. YjeK and YjeA are two enzymes involved in the essential post-translational β-lysylation of EF-P at a conserved lysine residue, K34. EF-P, YjeA and YjeK have been shown to be essential for the full virulence of Escherichia coli, Salmonella species and Agrobacterium tumefaciens, with efp, yjeA and yjeK mutants having highly similar phenotypes. Here, we identified an E. amylovora yjeK::Tn5 transposon mutant with decreased virulence in apple fruit and trees. The yjeK::Tn5 mutant also showed pleiotropic phenotypes, including reduced growth in rich medium, lower extracellular polysaccharide production, reduced swimming motility and increased chemical sensitivity compared with the wild-type, whilst maintaining wild-type level growth in minimal medium. All yjeK::Tn5 mutant phenotypes were complemented in trans with a plasmid bearing a wild-type copy of yjeK. Comprehensive, quantitative proteomics analyses revealed numerous, environmentally dependent changes in the prevalence of a wide range of proteins, in higher abundance and lower abundance, in yjeK::Tn5 compared with the wild-type, and many of these alterations could be linked to yjeK::Tn5 mutant phenotypes. The environmental dependence of the yjeK::Tn5 mutant proteomic alterations suggests that YjeK could be required for aspects of the environmentally dependent regulation of protein translation. YjeK activity may be critical to overcoming stress, including the challenging host environment faced by invading pathogenic bacteria.

Comparative genomics of Spiraeoideae-infecting Erwinia amylovora strains provides novel insight to genetic diversity and identifies the genetic basis of a low-virulence strain

Erwinia amylovora is the causal agent of fire blight, one of the most devastating diseases of apple and pear. Erwinia amylovora is thought to have originated in North America and has now spread to at least 50 countries worldwide. An understanding of the diversity of the pathogen population and the transmission to different geographical regions is important for the future mitigation of this disease. In this research, we performed an expanded comparative genomic study of the Spiraeoideae-infecting (SI) E. amylovora population in North America and Europe. We discovered that, although still highly homogeneous, the genetic diversity of 30 E. amylovora genomes examined was about 30 times higher than previously determined. These isolates belong to four distinct clades, three of which display geographical clustering and one of which contains strains from various geographical locations ('Widely Prevalent' clade). Furthermore, we revealed that strains from the Widely Prevalent clade displayed a higher level of recombination with strains from a clade strictly from the eastern USA, which suggests that the Widely Prevalent clade probably originated from the eastern USA before it spread to other locations. Finally, we detected variations in virulence in the SI E. amylovora strains on immature pear, and identified the genetic basis of one of the low-virulence strains as being caused by a single nucleotide polymorphism in hfq, a gene encoding an important virulence regulator. Our results provide insights into the population structure, distribution and evolution of SI E. amylovora in North America and Europe.

Lon protease modulates virulence traits in Erwinia amylovora by direct monitoring of major regulators and indirectly through the Rcs and Gac-Csr regulatory systems

Lon, an ATP-dependent protease in bacteria, influences diverse cellular processes by degrading damaged, misfolded and short-lived regulatory proteins. In this study, we characterized the effects of lon mutation and determined the molecular mechanisms underlying Lon-mediated virulence regulation in Erwinia amylovora, an enterobacterial pathogen of apple. Erwinia amylovora depends on the type III secretion system (T3SS) and the exopolysaccharide (EPS) amylovoran to cause disease. Our results showed that mutation of the lon gene led to the overproduction of amylovoran, increased T3SS gene expression and the non-motile phenotype. Western blot analyses showed that mutation in lon directly affected the accumulation and stability of HrpS/HrpA and RcsA. Mutation in lon also indirectly influenced the expression of flhD, hrpS and csrB through the accumulation of the RcsA/RcsB proteins, which bind to the promoter of these genes. In addition, lon expression is under the control of CsrA, possibly at both the transcriptional and post-transcriptional levels. Although mutation in csrA abolished both T3SS and amylovoran production, deletion of the lon gene in the csrA mutant only rescued amylovoran production, but not T3SS. These results suggest that CsrA might positively control both T3SS and amylovoran production partly by suppressing Lon, whereas CsrA may also play a critical role in T3SS by affecting unknown targets.

Erwinia amylovora catalases KatA and KatG are virulence factors and delay the starvation-induced viable but non-culturable (VBNC) response

The life cycle of the plant pathogen Erwinia amylovora comprises periods inside and outside the host in which it faces oxidative stress caused by hydrogen peroxide (H₂ O₂ ) and other compounds. The sources of this stress are plant defences, other microorganisms and/or exposure to starvation or other environmental challenges. However, the functional roles of H₂ O₂ -neutralizing enzymes, such as catalases, during plant-pathogen interactions and/or under starvation conditions in phytopathogens of the family Erwiniaceae or closely related families have not yet been investigated. In this work, the contribution of E. amylovora catalases KatA and KatG to virulence and survival in non-host environments was determined using catalase gene mutants and expression, as well as catalase activity analyses. The participation of E. amylovora exopolysaccharides (EPSs) in oxidative stress protection was also investigated. Our study revealed the following: (i) a different growth phase regulation of each catalase, with an induction by H₂ O₂ and host tissues; (ii) the significant role of E. amylovora catalases as virulence and survival factors during plant-pathogen interactions; (iii) the induction of EPSs by H₂ O₂ despite the fact that apparently they do not contribute to protection against this compound; and (iv) the participation of both catalases in the detoxification of the starvation-induced intracellular oxidative stress, favouring the maintenance of culturability, and hence delaying the development of the viable but non-culturable (VBNC) response.

New potential bacterial antagonists for the biocontrol of fire blight disease (Erwinia amylovora) in Morocco

The effectiveness of antagonistic bacteria to control Erwinia amylovora was evaluated under in vitro and field conditions. Among 61 bacteria isolated from soil and flowers of fire blight host plants of different Moroccan areas, 20 bacterial isolates showed higher antagonistic activity against the pathogen during agar-diffusion-test, attached blossoms assay and in a bioassay on immature pear fruits. Effective isolates were identified by using biochemical tests and 16 S rRNA genes sequencing. These isolates were grouped into the following genera: Alcaligenes (ACBC1), Pantoea (ACBC2, ACBP1, and ACBP2), Serratia (HC4), Brevibacterium (SF3, SF4, SF7, and SF15), Pseudomonas (SP9), and Bacillus (CPa12, CPa2, HF6, JB2, LMR2, SF14, SF16, SP10, SP13, and SP18). Furthermore, isolates were reported in the NCBI nucleotide sequence database (Genbank) under the accession numbers from KY357285 to KY357304. A 2-year field trials consisted of spray treatments with different bacterial antagonists was conducted on the susceptible apple cultivars 'Gala', 'Golden Parsi' and 'Golden Smoothee'. Their efficacies were evaluated 15 days post-inoculation on detached blossoms and were ranged from 54.6 to 95.0% for 11 strains, most of them were slightly better or better than that obtained with commercial bacterial strains P10c (66%) and QST713 (63%). In field trials, the most effective were P. agglomerans ACBP2, B. amyloliquefaciens LMR2, B. halotolerans (SF3 and SF4), and B. mojarvensis SF16. In addition, effective bacterial isolates did not show the pathogenicity signs towards plant tissue and are, therefore, considered as potential candidates to be integrated in actives ingredients of microbial formulation for the effective control of Fire Blight.

Cellulose production, activated by cyclic di-GMP through BcsA and BcsZ, is a virulence factor and an essential determinant of the three-dimensional architectures of biofilms formed by Erwinia amylovora Ea1189

Bacterial biofilms are multicellular aggregates encased in an extracellular matrix mainly composed of exopolysaccharides (EPSs), protein and nucleic acids, which determines the architecture of the biofilm. Erwinia amylovora Ea1189 forms a biofilm inside the xylem of its host, which results in vessel plugging and water transport impairment. The production of the EPSs amylovoran and levan is critical for the formation of a mature biofilm. In addition, cyclic dimeric GMP (c-di-GMP) has been reported to positively regulate amylovoran biosynthesis and biofilm formation in E. amylovora Ea1189. In this study, we demonstrate that cellulose is synthesized by E. amylovora Ea1189 and is a major modulator of the three-dimensional characteristics of biofilms formed by this bacterium, and also contributes to virulence during systemic host invasion. In addition, we demonstrate that the activation of cellulose biosynthesis in E. amylovora is a c-di-GMP-dependent process, through allosteric binding to the cellulose catalytic subunit BcsA. We also report that the endoglucanase BcsZ is a key player in c-di-GMP activation of cellulose biosynthesis. Our results provide evidence of the complex composition of the extracellular matrix produced by E. amylovora and the implications of cellulose biosynthesis in shaping the architecture of the biofilm and in the expression of one of the main virulence phenotypes of this pathogen.

Biological relevance of volatile organic compounds emitted during the pathogenic interactions between apple plants and Erwinia amylovora

Volatile organic compounds emitted during the infection of apple (Malus pumila var. domestica) plants by Erwinia amylovora or Pseudomonas syringae pv. syringae were studied by gas chromatography-mass spectrometry and proton transfer reaction-mass spectrometry, and used to treat uninfected plants. Infected plants showed a disease-specific emission of volatile organic compounds, including several bio-active compounds, such as hexenal isomers and 2,3-butanediol. Leaf growth promotion and a higher resistance to the pathogen, expressed as a lower bacterial growth and migration in plant tissues, were detected in plants exposed to volatile compounds from E. amylovora-infected plants. Transcriptional analysis revealed the activation of salicylic acid synthesis and signal transduction in healthy plants exposed to volatiles produced by E. amylovora-infected neighbour plants. In contrast, in the same plants, salicylic acid-dependent responses were repressed after infection, whereas oxylipin metabolism was activated. These results clarify some metabolic and ecological aspects of the pathogenic adaptation of E. amylovora to its host.

The virulence-associated protein HsvA from the fire blight pathogen Erwinia amylovora is a polyamine amidinotransferase

Studies of virulence determinants in the bacterial phytopathogen Erwinia amylovora, the cause of devastating fire blight disease in apple and pear, have shown that HsvA, a putative amidinotransferase enzyme located in the Hrp pathogenicity island, is required for systemic infection in apple. However, the mechanism by which HsvA contributes to virulence is unclear. To investigate the role of HsvA in virulence, we carried out a series of biochemical and structural studies to characterize the amidinotransferase activity of HsvA. We found that HsvA displays a preference for linear aliphatic polyamines as the amidino acceptor substrate, especially for spermidine and putrescine (Km values of 33 μm and 3.9 mm, respectively). The three-dimensional structure, determined at 2.30 Å resolution using X-ray crystallography, revealed that the overall architecture of HsvA is similar to that of the human arginine-glycine amidinotransferase in the creatine biosynthesis pathway. The active site is located in the core of the protein at the base of a long, narrow substrate access channel. Specific amino acids near the entrance of the channel may serve as major determinants of the substrate specificity, including a glutamate residue at the rim of the channel entrance that appears to be positioned to interact with the distal primary amine in the putrescine substrate as well as the internal and distal amines in the spermidine substrate. These results suggest potential in vivo functions for HsvA as a virulence factor in fire blight and may also provide a basis for strategies to control fire blight by inhibiting HsvA activity.

Conservation of Erwinia amylovora pathogenicity-relevant genes among Erwinia genomes

The Erwinia genus comprises species that are plant pathogens, non-pathogen, epiphytes, and opportunistic human pathogens. Within the genus, Erwinia amylovora ranks among the top 10 plant pathogenic bacteria. It causes the fire blight disease and is a global threat to commercial apple and pear production. We analyzed the presence/absence of the E. amylovora genes reported to be important for pathogenicity towards Rosaceae within various Erwinia strains genomes. This simple bottom-up approach, allowed us to correlate the analyzed genes to pathogenicity, host specificity, and make useful considerations to drive targeted studies.

ClpXP-Dependent RpoS Degradation Enables Full Activation of Type III Secretion System, Amylovoran Production, and Motility in Erwinia amylovora

Erwinia amylovora, the causal agent of fire blight disease of apple and pear, employs intracellular proteases, including Lon and ClpXP, for posttranslational regulation of various cellular proteins. It has been shown that Lon plays a critical role in E. amylovora virulence by directly targeting type III secretion system (T3SS) proteins and the Rcs phosphorelay system. In this study, we genetically examined the role of ClpXP and its potential interaction with Lon in E. amylovora. Mutation in clpXP diminished the expression of the T3SS, reduced exopolysaccharide amylovoran production and motility, and resulted in delayed disease progress. Western blot analyses showed highly accumulated RpoS proteins in the clpXP mutant. Moreover, mutation of rpoS in the clpXP mutant background rescued the expression of the T3SS and amylovoran production, suggesting that ClpXP-dependent RpoS degradation positively affects virulence traits. Interestingly, lack of both ClpXP and Lon resulted in significantly reduced virulence but increased expression of the T3SS and amylovoran production. However, this phenomenon was independent of RpoS accumulation, suggesting that ClpXP and Lon are indispensable for full virulence in E. amylovora.

Type VI Secretion Systems of Erwinia amylovora Contribute to Bacterial Competition, Virulence, and Exopolysaccharide Production

The type VI secretion system (T6SS) plays a major role in mediating interbacterial competition and might contribute to virulence in plant pathogenic bacteria. However, the role of T6SS in Erwinia amylovora remains unknown. In this study, 33 deletion mutants within three T6SS clusters were generated in E. amylovora strain NCPPB1665. Our results showed that all 33 mutants displayed reduced antibacterial activities against Escherichia coli as compared with that of the wild-type (WT) strain, indicating that Erwinia amylovora T6SS are functional. Of the 33 mutants, 19 exhibited reduced virulence on immature pear fruit as compared with that of the WT strain. Among them, 6, 1, and 12 genes belonged to T6SS-1, T6SS-2, and T6SS-3 clusters, respectively. Interestingly, these 19 mutants also produced less amylovoran or levan or both. These findings suggest that E. amylovora T6SS play a role in bacterial competition and virulence possibly by influencing exopolysaccharide production.

Microbiological Examination of Erwinia amylovora Exopolysaccharide Ooze

Fire blight, caused by the pathogen Erwinia amylovora, is the most devastating bacterial disease of pome fruit in North America and worldwide. The primary method of dispersal for E. amylovora is through ooze, a mass of exopolysaccharides and bacterial cells that is exuded as droplets from infected host tissue. During the 2013 and 2014 field seasons, 317 ooze droplets were collected from field-inoculated apple trees. Populations of E. amylovora in ooze droplets were 10⁸ CFU/μl on average. Ooze droplets harboring larger (>10⁸ CFU/μl) cell populations were typically smaller in total volume and had darker coloring, such as orange, red, or dark red hues. Examination of apple host tissue at the site of emergence of ooze droplets using scanning electron microscopy revealed that ooze was not exuding through natural openings; instead, it was found on erumpent mounds and small (10-μm) tears in tissue. These observations suggested that E. amylovora-induced wounds in tissue provided the exit holes for ooze extrusion from the host. Analyses of E. amylovora populations in ooze droplets and within the stems from which ooze droplets emerged indicated that approximately 9% of the total bacterial population from infected stems is diverted to ooze. Gene expression analyses indicated that E. amylovora cells in stem sections located above ooze droplets and in ooze droplets were actively expressing critical pathogenicity genes such as hrpL, dspE, and amsK. Thus, our study identified ooze as a source of large, concentrated populations of E. amylovora that emerged from the host by rupturing host tissue. Because the cells in ooze droplets are expressing genes required for pathogenesis, they are already primed for infection should they be dispersed from ooze to new infection courts.

Directed Evolution of FLS2 towards Novel Flagellin Peptide Recognition

Microbe-associated molecular patterns (MAMPs) are molecules, or domains within molecules, that are conserved across microbial taxa and can be recognized by a plant or animal immune system. Although MAMP receptors have evolved to recognize conserved epitopes, the MAMPs in some microbial species or strains have diverged sufficiently to render them unrecognizable by some host immune systems. In this study, we carried out in vitro evolution of the Arabidopsis thaliana flagellin receptor FLAGELLIN-SENSING 2 (FLS2) to isolate derivatives that recognize one or more flagellin peptides from bacteria for which the wild-type Arabidopsis FLS2 confers little or no response. A targeted approach generated amino acid variation at FLS2 residues in a region previously implicated in flagellin recognition. The primary screen tested for elevated response to the canonical flagellin peptide from Pseudomonas aeruginosa, flg22. From this pool, we then identified five alleles of FLS2 that confer modest (quantitatively partial) recognition of an Erwinia amylovora flagellin peptide. Use of this Erwinia-based flagellin peptide to stimulate Arabidopsis plants expressing the resulting FLS2 alleles did not lead to a detectable reduction of virulent P. syringae pv. tomato growth. However, combination of two identified mutations into a single allele further increased FLS2-mediated responses to the E. amylovora flagellin peptide. These studies demonstrate the potential to raise the sensitivity of MAMP receptors toward particular targets.

Transgenic apple plants overexpressing the chalcone 3-hydroxylase gene of Cosmos sulphureus show increased levels of 3-hydroxyphloridzin and reduced susceptibility to apple scab and fire blight

MAIN CONCLUSION

Overexpression of chalcone-3-hydroxylase provokes increased accumulation of 3-hydroxyphloridzin in Malus . Decreased flavonoid concentrations but unchanged flavonoid class composition were observed. The increased 3-hydroxyphlorizin contents correlate well with reduced susceptibility to fire blight and scab. The involvement of dihydrochalcones in the apple defence mechanism against pathogens is discussed but unknown biosynthetic steps in their formation hamper studies on their physiological relevance. The formation of 3-hydroxyphloretin is one of the gaps in the pathway. Polyphenol oxidases and cytochrome P450 dependent enzymes could be involved. Hydroxylation of phloretin in position 3 has high similarity to the B-ring hydroxylation of flavonoids catalysed by the well-known flavonoid 3'-hydroxylase (F3'H). Using recombinant F3'H and chalcone 3-hydroxylase (CH3H) from Cosmos sulphureus we show that F3'H and CH3H accept phloretin to some extent but higher conversion rates are obtained with CH3H. To test whether CH3H catalyzes the hydroxylation of dihydrochalcones in planta and if this could be of physiological relevance, we created transgenic apple trees harbouring CH3H from C. sulphureus. The three transgenic lines obtained showed lower polyphenol concentrations but no shift between the main polyphenol classes dihydrochalcones, flavonols, hydroxycinnamic acids and flavan 3-ols. Increase of 3-hydroxyphloridzin within the dihydrochalcones and of epicatechin/catechin within soluble flavan 3-ols were observed. Decreased activity of dihydroflavonol 4-reductase and chalcone synthase/chalcone isomerase could partially explain the lower polyphenol concentrations. In comparison to the parent line, the transgenic CH3H-lines showed a lower disease susceptibility to fire blight and apple scab that correlated with the increased 3-hydroxyphlorizin contents.

Development of the First Cisgenic Apple with Increased Resistance to Fire Blight

The generation and selection of novel fire blight resistant apple genotypes would greatly improve the management of this devastating disease, caused by Erwinia amylovora. Such resistant genotypes are currently developed by conventional breeding, but novel breeding technologies including cisgenesis could be an alternative approach. A cisgenic apple line C44.4.146 was regenerated using the cisgene FB_MR5 from wild apple Malus ×robusta 5 (Mr5), and the previously established method involving A. tumefaciens-mediated transformation of the fire blight susceptible cultivar 'Gala Galaxy' using the binary vector p9-Dao-FLPi. The line C44.4.146 was shown to carry only the cisgene FB_MR5, controlled by its native regulatory sequences and no transgenes were detected by PCR or Southern blot following heat induced recombinase-mediated elimination of the selectable markers. Although this line contains up to 452 bp of vector sequences, it still matches the original definition of cisgenesis. A single insertion of T-DNA into the genome of 'Gala Galaxy' in chromosome 16 was identified. Transcription of FB_MR5 in line C44.4.146 was similar to the transcription in classically bred descendants of Mr5. Three independent shoot inoculation experiments with a Mr5 avirulent strain of Erwinia amylovora were performed using scissors or syringe. Significantly lower disease symptoms were detected on shoots of the cisgenic line compared to those of untransformed 'Gala Galaxy'. Despite the fact that the pathogen can overcome this resistance by a single nucleotide mutation, this is, to our knowledge, the first prototype of a cisgenic apple with increased resistance to fire blight.

Virulence Factors of Erwinia amylovora: A Review

Erwinia amylovora, a Gram negative bacteria of the Enterobacteriaceae family, is the causal agent of fire blight, a devastating plant disease affecting a wide range of host species within Rosaceae and a major global threat to commercial apple and pear production. Among the limited number of control options currently available, prophylactic application of antibiotics during the bloom period appears the most effective. Pathogen cells enter plants through the nectarthodes of flowers and other natural openings, such as wounds, and are capable of rapid movement within plants and the establishment of systemic infections. Many virulence determinants of E. amylovora have been characterized, including the Type III secretion system (T3SS), the exopolysaccharide (EPS) amylovoran, biofilm formation, and motility. To successfully establish an infection, E. amylovora uses a complex regulatory network to sense the relevant environmental signals and coordinate the expression of early and late stage virulence factors involving two component signal transduction systems, bis-(3'-5')-cyclic di-GMP (c-di-GMP) and quorum sensing. The LPS biosynthetic gene cluster is one of the relatively few genetic differences observed between Rubus- and Spiraeoideae-infecting genotypes of E. amylovora. Other differential factors, such as the presence and composition of an integrative conjugative element associated with the Hrp T3SS (hrp genes encoding the T3SS apparatus), have been recently described. In the present review, we present the recent findings on virulence factors research, focusing on their role in bacterial pathogenesis and indicating other virulence factors that deserve future research to characterize them.

Conserved aspartate and lysine residues of RcsB are required for amylovoran biosynthesis, virulence, and DNA binding in Erwinia amylovora

In Erwinia amylovora, the Rcs phosphorelay system is essential for amylovoran production and virulence. To further understand the role of conserved aspartate residue (D56) in the phosphor receiver (PR) domain and lysine (K180) residue in the function domain of RcsB, amino acid substitutions of RcsB mutant alleles were generated by site-directed mutagenesis and complementation of various rcs mutants were performed. A D56E substitution of RcsB, which mimics the phosphorylation state of RcsB, complemented the rcsB mutant, resulting in increased amylovoran production and gene expression, reduced swarming motility, and restored pathogenicity. In contrast, D56N and K180A or K180Q substitutions of RcsB did not complement the rcsB mutant. Electrophoresis mobility shift assays showed that D56E, but not D56N, K180Q and K180A substitutions of RcsB bound to promoters of amsG and flhD, indicating that both D56 and K180 are required for DNA binding. Interestingly, the RcsBD56E allele could also complement rcsAB, rcsBC and rcsABCD mutants with restored virulence and increased amylovoran production, indicating that RcsB phosphorylation is essential for virulence of E. amylovora. In addition, mutations of T904 and A905, but not phosphorylation mimic mutation of D876 in the PR domain of RcsC, constitutively activate the Rcs system, suggesting that phosphor transfer is required for activating the Rcs system and indicating both A905 and T904 are required for the phosphatase activity of RcsC. Our results demonstrated that RcsB phosphorylation and dephosphorylation, phosphor transfer from RcsC are essential for the function of the Rcs system, and also suggested that constitutive activation of the Rcs system could reduce the fitness of E. amylovora.

Using molecular tools to decipher the complex world of plant resistance inducers: an apple case study

Exogenous application of plant resistance inducers (PRIs) able to activate plant defenses is an interesting approach for new integrated pest management practices. The full integration of PRIs into agricultural practices requires methods for the fast and objective upstream screening of efficient PRIs and optimization of their application. To select active PRIs, we used a molecular tool as an alternative to methods involving plant protection assays. The expressions of 28 genes involved in complementary plant defense mechanisms were simultaneously determined by quantitative real-time PCR in PRI-treated tissues. Using a set of 10 commercial preparations and considering the pathosystem apple/Erwinia amylovora, this study shows a strong correlation between defense activation and protection efficiency in controlled conditions, thus enabling the easy identification of promising PRIs in fire blight protection. Hence this work clearly highlights the benefits of using a molecular tool to discriminate nonactive PRI preparations and provides useful molecular markers for the optimization of their use in orchard.

Engineering fire blight resistance into the apple cultivar 'Gala' using the FB_MR5 CC-NBS-LRR resistance gene of Malus × robusta 5

The fire blight susceptible apple cultivar Malus × domestica Borkh. cv. 'Gala' was transformed with the candidate fire blight resistance gene FB_MR5 originating from the crab apple accession Malus × robusta 5 (Mr5). A total of five different transgenic lines were obtained. All transgenic lines were shown to be stably transformed and originate from different transgenic events. The transgenic lines express the FB_MR5 either driven by the constitutive CaMV 35S promoter and the ocs terminator or by its native promoter and terminator sequences. Phenotyping experiments were performed with Mr5-virulent and Mr5-avirulent strains of Erwinia amylovora, the causal agent of fire blight. Significantly less disease symptoms were detected on transgenic lines after inoculation with two different Mr5-avirulent E. amylovora strains, while significantly more shoot necrosis was observed after inoculation with the Mr5-virulent mutant strain ZYRKD3_1. The results of these experiments demonstrated the ability of a single gene isolated from the native gene pool of apple to protect a susceptible cultivar from fire blight. Furthermore, this gene is confirmed to be the resistance determinant of Mr5 as the transformed lines undergo the same gene-for-gene interaction in the host-pathogen relationship Mr5-E. amylovora.

[Erwinia amylovora--the fire blight pathogen of trees in Ukraine]

Niduses of fire blight of fruit and ornamental trees have been found in the Kyiv and Vinnitsa regions of Ukraine. Pathogen Erwinia amylovora was isolated between April and October. The pathogen was often accompanied by bacteria Pseudomonas syringae pv. syringae. Artificial infection with a mixture of bacteria E. amylovora and P. syringae pv. syringae accelerates and enhances the disease process in the laboratory.

Membrane-targeted HrpNEa can modulate apple defense gene expression

Fire blight caused by Erwinia amylovora is the major bacterial disease of tribe Maleae, including apple. Among the proteins secreted by this bacterium, HrpNEa, also called harpin, is known to induce hypersensitive response in nonhost plants and to form amyloid oligomers leading to pore opening in the plasma membrane and alteration of membrane homeostasis. To better understand the physiological effects of HrpNEa in the host plant, we produced transgenic apple plants expressing HrpNEa with or without a secretion signal peptide (SP). HrpNEa expressed with a SP was found to be associated within the membrane fraction, in accordance with amyloidogenic properties and the presence of transmembrane domains revealed by in silico analysis. Expression analysis of 28 apple defense-related genes revealed gene modulations in the transgenic line expressing membrane-targeted HrpNEa. While apple transgenic trees displaying a high constitutive expression level of SP-HrpNEa showed a slight reduction of infection frequency after E. amylovora inoculation, there was no decrease in the disease severity. Thus HrpNEa seems to act as an elicitor of host defenses, when localized in the host membrane.

Small-molecule inhibitors suppress the expression of both type III secretion and amylovoran biosynthesis genes in Erwinia amylovora

The type III secretion system (T3SS) and exopolysaccharide (EPS) amylovoran are two essential pathogenicity factors in Erwinia amylovora, the causal agent of the serious bacterial disease fire blight. In this study, small molecules that inhibit T3SS gene expression in E. amylovora under hrp (hypersensitive response and pathogenicity)-inducing conditions were identified and characterized using green fluorescent protein (GFP) as a reporter. These compounds belong to salicylidene acylhydrazides and also inhibit amylovoran production. Microarray analysis of E. amylovora treated with compounds 3 and 9 identified a total of 588 significantly differentially expressed genes. Among them, 95 and 78 genes were activated and suppressed by both compounds, respectively, when compared with the dimethylsulphoxide (DMSO) control. The expression of the majority of T3SS genes in E. amylovora, including hrpL and the avrRpt2 effector gene, was suppressed by both compounds. Compound 3 also suppressed the expression of amylovoran precursor and biosynthesis genes. However, both compounds induced significantly the expression of glycogen biosynthesis genes and siderophore biosynthesis, regulatory and transport genes. Furthermore, many membrane, lipoprotein and exported protein-encoding genes were also activated by both compounds. Similar expression patterns were observed for compounds 1, 2 and 4. Using crab apple flower as a model, compound 3 was capable of reducing disease development in pistils. These results suggest a common inhibition mechanism shared by salicylidene acylhydrazides and indicate that small-molecule inhibitors that disable T3SS function could be explored to control fire blight disease.

Alternative sigma factor RpoN and its modulation protein YhbH are indispensable for Erwinia amylovora virulence

In Erwinia amylovora, ECF (extracytoplasmic functions) alternative sigma factor HrpL regulates the transcription of hrp (hypersensitive response and pathogenicity)-type III secretion system (T3SS) genes by binding to a consensus sequence known as the hrp box in hrp gene promoters. In turn, the expression of hrpL has been proposed to be positively controlled by alternative sigma factor 54 (σ(54)) (RpoN) and HrpS, a member of the σ(54) enhancer-binding proteins (EBPs). However, the function of RpoN has not been characterized genetically in E. amylovora. In this study, we investigated the role of RpoN, a nitrogen limitation sigma factor, and its modulation protein YhbH, a novel ribosome-associated protein, in E. amylovora virulence. Our results showed that mutations in hrpS, hrpL, rpoN and yhbH, but not yfiA and rmf3, resulted in a nonpathogenic phenotype on immature pear fruits and apple shoots. Consistently, the expression of T3SS genes, including hrpL, dspE, hrpN and hrpA, was barely detected in hrpS, hrpL, rpoN and yhbH mutants. These mutants were also not capable of eliciting a hypersensitive response (HR) on tobacco; however, the overexpression of hrpL using an inducible promoter rescued the HR-eliciting abilities of these mutants. These results suggest that a sigma factor cascade exists in the regulatory networks of E. amylovora and regulates important virulence factors. On the basis of this study and previously reported data, a model is proposed for the regulation of T3SS in E. amylovora.

The fire blight pathogen Erwinia amylovora requires the rpoN gene for pathogenicity in apple

RpoN is a σ(54) factor regulating essential virulence gene expression in several plant pathogenic bacteria, including Pseudomonas syringae and Pectobacterium carotovorum. In this study, we found that mutation of rpoN in the fire blight pathogen Erwinia amylovora caused a nonpathogenic phenotype. The E. amylovora rpoN Tn5 transposon mutant rpoN1250::Tn5 did not cause fire blight disease symptoms on shoots of mature apple trees. In detached immature apple fruits, the rpoN1250::Tn5 mutant failed to cause fire blight disease symptoms and grew to population levels 12 orders of magnitude lower than the wild-type. In addition, the rpoN1250::Tn5 mutant failed to elicit a hypersensitive response when infiltrated into nonhost tobacco plant leaves, and rpoN1250::Tn5 cells failed to express HrpN protein when grown in hrp (hypersensitive response and pathogenicity)-inducing liquid medium. A plasmid-borne copy of the wild-type rpoN gene complemented all the rpoN1250::Tn5 mutant phenotypes tested. The rpoN1250::Tn5 mutant was prototrophic on minimal solid and liquid media, indicating that the rpoN1250::Tn5 nonpathogenic phenotype was not caused by a defect in basic metabolism or growth. This study provides clear genetic evidence that rpoN is an essential virulence gene of E. amylovora, suggesting that rpoN has the same function in E. amylovora as in P. syringae and Pe. carotovorum.

Morphological and biochemical characterization of Erwinia amylovora-induced hypersensitive cell death in apple leaves

In attached apple leaves, spot-inoculated with Erwinia amylovora, the phenotypic appearance of the hypersensitive response (HR) and the participation of ethylene, reactive oxygen species (ROS) and of vacuolar processing enzyme (VPE) (a plant caspase-1-like protease) were analysed. The HR in both the resistant and susceptible genotypes expressed a similar pattern of distinguishable micro HR lesions that progressed into confined macro HR lesions. The HR symptoms in apple were compared to those in non-host tobacco. The morphology of dead cells (protoplast shrinkage and retraction from cell wall) in apple leaves resembled necrotic programmed cell death (PCD). Lesion formation in both cv. Free Redstar (resistant) and cv. Idared (highly susceptible) was preceded by ROS accumulation and elevation of ethylene levels. Treatment of infected leaves with an inhibitor of ethylene synthesis led to a decrease of ethylene emission and suppression of lesion development in both cultivars. In the resistant but not in the susceptible apple cultivar an early and late increase in VPE gene expression was detected. This suggests that VPE might be an underlying component of the response to E. amylovora in resistant apple cultivars. The findings show that in the studied pathosystem the cell death during the HR proceeds through a signal transduction cascade in which ROS, ethylene and VPE pathways play a role.

Potent and specific bactericidal effect of juglone (5-hydroxy-1,4-naphthoquinone) on the fire blight pathogen Erwinia amylovora

A screening of plant quinones for inhibiting effects on the bacterial fire blight pathogen Erwinia amylovora was performed. The most active compound, juglone from walnuts, has a potent and specific bactericidal effect on E. amylovora and minimal inhibitory concentrations of only 2.5-10 μM, with stronger effects at lower, but still physiological, pH values. In vitro tests with juglone and inoculated flowers of apple (Malus domestica) showed an efficacy of 67% in preventing infection. In two years of field tests juglone had variable degrees of efficacy ranging from 40 to 82%, seemingly due to environmental conditions. A phytotoxic reaction to juglone, which is known for its allelopathic effect on plants, was restricted to browning of petals; later fruit russeting was not observed. Juglone is a promising candidate for the development of a new environmentally friendly plant protectant to replace the antibiotic streptomycin currently used in fire blight control.

AmyR is a novel negative regulator of amylovoran production in Erwinia amylovora

In this study, we attempted to understand the role of an orphan gene amyR in Erwinia amylovora, a functionally conserved ortholog of ybjN in Escherichia coli, which has recently been characterized. Amylovoran, a high molecular weight acidic heteropolymer exopolysaccharide, is a virulent factor of E. amylovora. As reported earlier, amylovoran production in an amyR knockout mutant was about eight-fold higher than that in the wild type (WT) strain of E. amylovora. When a multicopy plasmid containing the amyR gene was introduced into the amyR mutant or WT strains, amylovoran production was strongly inhibited. Furthermore, amylovoran production was also suppressed in various amylovoran-over-producing mutants, such as grrSA containing multicopies of the amyR gene. Consistent with amylovoran production, an inverse correlation was observed between in vitro expression of amyR and that of amylovoran biosynthetic genes. However, both the amyR knockout mutant and over-expression strains showed reduced levan production, another exopolysaccharide produced by E. amylovora. Virulence assays demonstrated that while the amyR mutant was capable of inducing slightly greater disease severity than that of the WT strain, strains over-expressing the amyR gene did not incite disease on apple shoots or leaves, and only caused reduced disease on immature pear fruits. Microarray studies revealed that amylovoran biosynthesis and related membrane protein-encoding genes were highly expressed in the amyR mutant, but down-regulated in the amyR over-expression strains in vitro. Down-regulation of amylovoran biosynthesis genes in the amyR over-expression strain partially explained why over-expression of amyR led to non-pathogenic or reduced virulence in vivo. These results suggest that AmyR plays an important role in regulating exopolysaccharide production, and thus virulence in E. amylovora.

T3SS-dependent differential modulations of the jasmonic acid pathway in susceptible and resistant genotypes of Malus spp. challenged with Erwinia amylovora

Fire blight is a bacterial disease of Maloideae caused by Erwinia amylovora (Ea). This necrogenic enterobacterium uses a type III secretion system (T3SS) to inject type III effectors into the plant cells to cause disease on its susceptible hosts, including economically important crops like apple and pear. The expressions of marker genes of the salicylic acid (SA) and jasmonic acid (JA) defense regulation pathways were monitored by RT-qPCR in leaves of two apple genotypes, one susceptible and one resistant, challenged with a wild type strain, a T3SS-deficient strain or water. The transcriptional data taken together with hormone level measurements indicated that the SA pathway was similarly induced in both apple genotypes during infection by Ea. On the contrary, the data clearly showed a strong T3SS-dependent down-regulation of the JA pathway in leaves of the susceptible genotype but not in those of the resistant one. Accordingly, methyl-jasmonate treated susceptible plants displayed an increased resistance to Ea. Bacterial mutant analysis indicated that JA manipulation by Ea mainly relies on the type III effector DspA/E. Taken together, our data suggest that the T3SS-dependent down-regulation of the JA pathway is a critical step in the infection process of Malus spp. by Ea.

Erwinia amylovora expresses fast and simultaneously hrp/dsp virulence genes during flower infection on apple trees

BACKGROUND

Pathogen entry through host blossoms is the predominant infection pathway of the gram-negative bacterium Erwinia amylovora leading to manifestation of the disease fire blight. Like in other economically important plant pathogens, E. amylovora pathogenicity depends on a type III secretion system encoded by hrp genes. However, timing and transcriptional order of hrp gene expression during flower infections are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Using quantitative real-time PCR analyses, we addressed the questions of how fast, strong and uniform key hrp virulence genes and the effector dspA/E are expressed when bacteria enter flowers provided with the full defense mechanism of the apple plant. In non-invasive bacterial inoculations of apple flowers still attached to the tree, E. amylovora activated expression of key type III secretion genes in a narrow time window, mounting in a single expression peak of all investigated hrp/dspA/E genes around 24-48 h post inoculation (hpi). This single expression peak coincided with a single depression in the plant PR-1 expression at 24 hpi indicating transient manipulation of the salicylic acid pathway as one target of E. amylovora type III effectors. Expression of hrp/dspA/E genes was highly correlated to expression of the regulator hrpL and relative transcript abundances followed the ratio: hrpA>hrpN>hrpL>dspA/E. Acidic conditions (pH 4) in flower infections led to reduced virulence/effector gene expression without the typical expression peak observed under natural conditions (pH 7).

CONCLUSION/SIGNIFICANCE

The simultaneous expression of hrpL, hrpA, hrpN, and the effector dspA/E during early floral infection indicates that speed and immediate effector transmission is important for successful plant invasion. When this delicate balance is disturbed, e.g., by acidic pH during infection, virulence gene expression is reduced, thus partly explaining the efficacy of acidification in fire blight control on a molecular level.

EDS1 contributes to nonhost resistance of Arabidopsis thaliana against Erwinia amylovora

Erwinia amylovora causes fire blight in rosaceous plants. In nonhost Arabidopsis thaliana, E. amylovora triggers necrotic symptoms associated with transient bacterial multiplication, suggesting either that A. thaliana lacks a susceptibility factor or that it actively restricts E. amylovora growth. Inhibiting plant protein synthesis at the time of infection led to an increase in necrosis and bacterial multiplication and reduced callose deposition, indicating that A. thaliana requires active protein synthesis to restrict E. amylovora growth. Analysis of the callose synthase-deficient pmr4-1 mutant indicated that lack of callose deposition alone did not lead to increased sensitivity to E. amylovora. Transcriptome analysis revealed that approximately 20% of the genes induced following E. amylovora infection are related to defense and signaling. Analysis of mutants affected in NDR1 and EDS1, two main components of the defense-gene activation observed, revealed that E. amylovora multiplied ten times more in the eds1-2 mutant than in the wild type but not in the ndr1-1 mutant. Analysis of mutants affected in three WRKY transcription factors showing EDS1-dependent activation identified WRKY46 and WRKY54 as positive regulators and WRKY70 as a negative regulator of defense against E. amylovora. Altogether, we show that EDS1 is a positive regulator of nonhost resistance against E. amylovora in A. thaliana and hypothesize that it controls the production of several effective defenses against E. amylovora through the action of WRKY46 and WRKY54, while WRKY70 acts as a negative regulator.

HopX1 in Erwinia amylovora functions as an avirulence protein in apple and is regulated by HrpL

Fire blight is a devastating disease of rosaceous plants caused by the Gram-negative bacterium Erwinia amylovora. This pathogen delivers virulence proteins into host cells utilizing the type III secretion system (T3SS). Expression of the T3SS and of translocated and secreted substrates is activated by the alternative sigma factor HrpL, which recognizes hrp box promoters upstream of regulated genes. A collection of hidden Markov model (HMM) profiles was used to identify putative hrp boxes in the genome sequence of Ea273, a highly virulent strain of E. amylovora. Among potential virulence factors preceded by putative hrp boxes, two genes previously known as Eop3 and Eop2 were characterized. The presence of functionally active hrp boxes upstream of these two genes was confirmed by β-glucuronidase (GUS) assays. Deletion mutants of the latter candidate genes, renamed hopX1(Ea) and hopAK1(Ea), respectively, did not differ in virulence from the wild-type strain when assayed in pear fruit and apple shoots. The hopX1(Ea) deletion mutant of Ea273, complemented with a plasmid overexpressing hopX1(E)(a), suppressed the development of the hypersensitivity response (HR) when inoculated into Nicotiana benthamiana; however, it contributed to HR in Nicotiana tabacum and significantly reduced the progress of disease in apple shoots, suggesting that HopX1(Ea) may act as an avirulence protein in apple shoots.

Silencing of flavanone-3-hydroxylase in apple (Malus × domestica Borkh.) leads to accumulation of flavanones, but not to reduced fire blight susceptibility

Transgenic antisense flavanone-3-hydroxylase apple plants were produced to mimic the effect of the agrochemical prohexadione-Ca on apple leaves. This enzyme inhibitor for 2-oxoglutarate dependent dioxygenases is used as a growth retardant and for control of secondary fire blight of leaves. Like using the agent, silencing of flavanone-3-hydroxylase leads to an accumulation of flavanones in leaves, but in contrast not to the formation of 3-deoxyflavonoids. In prohexadione-Ca treated leaves the 3-deoxyflavonoid luteoforol is formed from accumulating flavanones, acting as an antimicrobial compound against the fire blight pathogen Erwinia amylovora. Seemingly, the silencing of just one of the 2-oxoglutarate dependent dioxygenases (in apple also flavonol synthase and anthocyanidin synthase take part downstream in the pathway) does not provide a sufficiently high ratio of flavanones to dihydroflavonols. This seems to be needed to let the dihydroflavonol-4-reductase/flavanone-4-reductase enzyme reduce flavanones to luteoforol, and to let this be reduced by the leucoanthocyanidin-4-reductase/3-deoxyleucoanthocyanidin-4-reductase, each acting with their respective weak secondary activities. Accordingly, also the intended inducible resistance to fire blight by prohexadione-Ca is not observed with the antisense flavanone-3-hydroxylase apple plants. On the other hand, for most transgenic lines with strong flavanone-4-reductase down-regulation, up-regulation of gene expression for the other flavonoid genes was found. This provides further evidence for the feedback regulation of flavonoid gene expression having been previously reported for the prohexadione-Ca inhibited apple plants.

Genetic characterization of the HrpL regulon of the fire blight pathogen Erwinia amylovora reveals novel virulence factors

The bacterial pathogen Erwinia amylovora is the causal agent of fire blight, an economically significant disease of apple and pear. Disease initiation by E. amylovora requires the translocation of effector proteins into host cells via the hypersensitive response and pathogenicity (hrp) type III secretion system (T3SS). The alternative sigma factor HrpL positively regulates the transcription of structural and translocated components of the T3SS via hrp promoter elements. To characterize genome-wide HrpL-dependent gene expression in E. amylovora Ea1189, wild-type and Ea1189ΔhrpL strains were cultured in hrp-inducing minimal medium, and total RNA was compared using a custom microarray designed to represent the annotated genes of E. amylovora ATCC 49946. The results revealed 24 genes differentially regulated in Ea1189ΔhrpL relative to Ea1189 with fold-change expression ratios greater than 1.5; of these, 19 genes exhibited decreased transcript abundance and five genes showed increased transcript abundance relative to Ea1189. To expand our understanding of the HrpL regulon and to elucidate direct versus indirect HrpL-mediated effects on gene expression, the genome of E. amylovora ATCC 49946 was examined in silico using a hidden Markov model assembled from known Erwinia spp. hrp promoters. This technique identified 15 putative type III novel hrp promoters, seven of which were validated with quantitative polymerase chain reaction based on expression analyses. It was found that HrpL-regulated genes encode all known components of the hrp T3SS, as well as five putative type III effectors. Eight genes displayed apparent indirect HrpL regulation, suggesting that the HrpL regulon is connected to downstream signalling networks. The construction of deletion mutants of three novel HrpL-regulated genes resulted in the identification of additional virulence factors as well as mutants displaying abnormal motility and biofilm phenotypes.

Fire blight: applied genomic insights of the pathogen and host

The enterobacterial phytopathogen Erwinia amylovora causes fire blight, an invasive disease that threatens a wide range of commercial and ornamental Rosaceae host plants. The response elicited by E. amylovora in its host during disease development is similar to the hypersensitive reaction that typically leads to resistance in an incompatible host-pathogen interaction, yet no gene-for-gene resistance has been described for this host-pathogen system. Comparative genomic analysis has found an unprecedented degree of genetic uniformity among strains of E. amylovora, suggesting that the pathogen has undergone a recent genetic bottleneck. The genome of apple, an important host of E. amylovora, has been sequenced, creating new opportunities for the study of interactions between host and pathogen during fire blight development and for the identification of resistance genes. This review includes recent advances in the genomics of both host and pathogen.

Cell surface attachment structures contribute to biofilm formation and xylem colonization by Erwinia amylovora

Biofilm formation plays a critical role in the pathogenesis of Erwinia amylovora and the systemic invasion of plant hosts. The functional role of the exopolysaccharides amylovoran and levan in pathogenesis and biofilm formation has been evaluated. However, the role of biofilm formation, independent of exopolysaccharide production, in pathogenesis and movement within plants has not been studied previously. Evaluation of the role of attachment in E. amylovora biofilm formation and virulence was examined through the analysis of deletion mutants lacking genes encoding structures postulated to function in attachment to surfaces or in cellular aggregation. The genes and gene clusters studied were selected based on in silico analyses. Microscopic analyses and quantitative assays demonstrated that attachment structures such as fimbriae and pili are involved in the attachment of E. amylovora to surfaces and are necessary for the production of mature biofilms. A time course assay indicated that type I fimbriae function earlier in attachment, while type IV pilus structures appear to function later in attachment. Our results indicate that multiple attachment structures are needed for mature biofilm formation and full virulence and that biofilm formation facilitates entry and is necessary for the buildup of large populations of E. amylovora cells in xylem tissue.

Eop1 from a Rubus strain of Erwinia amylovora functions as a host-range limiting factor

Strains of Erwinia amylovora, the bacterium causing the disease fire blight of rosaceous plants, are separated into two groups based on host range: Spiraeoideae and Rubus strains. Spiraeoideae strains have wide host ranges, infecting plants in many rosaceous genera, including apple and pear. In the field, Rubus strains infect the genus Rubus exclusively, which includes raspberry and blackberry. Based on comparisons of limited sequence data from a Rubus and a Spiraeoideae strain, the gene eop1 was identified as unusually divergent, and it was selected as a possible host specificity factor. To test this, eop1 genes from a Rubus strain and a Spiraeoideae strain were cloned and mutated. Expression of the Rubus-strain eop1 reduced the virulence of E. amylovora in immature pear fruit and in apple shoots. Sequencing the orfA-eop1 regions of several strains of E. amylovora confirmed that forms of eop1 are conserved among strains with similar host ranges. This work provides evidence that eop1 from a Rubus-specific strain can function as a determinant of host specificity in E. amylovora.

Characterization of the RcsC sensor kinase from Erwinia amylovora and other Enterobacteria

RcsC is a hybrid sensor kinase which contains a sensor domain, a histidine kinase domain, and a receiver domain. We have previously demonstrated that, although the Erwinia amylovora rcsC mutant produces more amylovoran than the wild-type (WT) strain in vitro, the mutant remains nonpathogenic on both immature pear fruit and apple plants. In this study, we have comparatively characterized the Erwinia RcsC and its homologs from various enterobacteria. Results demonstrate that expression of the Erwinia rcsC gene suppresses amylovoran production in various amylovoran overproducing WT and mutant strains, thus suggesting the presence of a net phosphatase activity of Erwinia RcsC. Findings have also demonstrated that rcsC homologs from other enterobacteria could not rescue amylovoran production of the Erwinia rcsC mutant in vitro. However, virulence of the Erwinia rcsC mutant is partially restored by rcsC homologs from Pantoea stewartii, Yersinia pestis, and Salmonella enterica but not from Escherichia coli on apple shoots. Domain-swapping experiments have indicated that replacement of the E. coli RcsC sensor domain by those of Erwinia and Yersinia spp. partially restores virulence of the Erwinia rcsC mutant, whereas chimeric constructs containing the sensor domain of E. coli RcsC could not rescue virulence of the Erwinia rcsC mutant on apple. Interestingly, only chimeric constructs containing the histidine kinase and receiver domains of Erwinia RcsC are fully capable of rescuing amylovoran production. These results suggest that the sensor domain of RcsC may be important in regulating bacterial virulence, whereas the activity of the histidine kinase and receiver domains of Erwinia RcsC may be essential for amylovoran production in vitro.

Improvement of fitness and efficacy of a fire blight biocontrol agent via nutritional enhancement combined with osmoadaptation

The efficacy of Pseudomonas fluorescens EPS62e in the biocontrol of Erwinia amylovora was improved by a procedure of physiological adaptation to increase colonization and survival in the phytosphere of rosaceous plants. The procedure consisted of osmoadaptation (OA) and nutritional enhancement (NE). OA was based on saline stress and osmolyte amendment of the growth medium during inoculum preparation. NE consisted of addition of glycine and Tween 80 to the formulation. NE and OA increased the growth rate and carrying capacity of EPS62e under high-relative-humidity (RH) conditions and improved survival at low RH on flowers under controlled environmental conditions. NE did not promote growth or affect infection capacity of E. amylovora. The effect of both methods was tested in the field by following the population of EPS62e using quantitative PCR (Q-PCR) (total population) and CFU counting (culturable population) methods. Following field application, EPS62e colonized blossoms, but it was stressed, as indicated by a sharp decrease in culturable compared to total population levels. However, once established in flowers and at the end of bloom, almost all the total population was culturable. The physiological adaptation treatments increased population levels of EPS62e over those of nonadapted cells during the late stage of the flowering period. Control of fire blight infections in flowers and immature fruits was tested by field application of EPS62e and subsequent inoculation with E. amylovora under controlled-environment conditions. The efficacy of fire blight control increased significantly with the combination of nutritional enhancement and osmoadaptation, in comparison with the absence of physiological adaptation.

Multiple treatment meta-analysis of products evaluated for control of fire blight in the eastern United States

The aim of this analysis was to estimate the effect sizes and consistency of products evaluated for fire blight control in the eastern United States over the last decade. Because only 3% of the 69 studies published from 2000 to 2008 explicitly presented a measure of within-study variability, a method for estimating the least significant difference (LSD) and, hence the sampling variance, for studies with at least two significant mean separations in the presented mean multiple comparisons was developed. Lin's concordance analysis indicated that the estimated LSD was an accurate predictor of the actual LSD based on 35 studies in a calibration evaluation (ρ(c) = 0.997). Separate multi-treatment random-effects meta-analyses were performed for three control categories: antibiotics, biological control, and plant defense-activating products and mean log response ratios relative to the nontreated controls ([Formula: see text]) were computed for each treatment and then back-transformed to obtain the mean percent disease control. None of the products evaluated performed as well as streptomycin, the standard product for fire blight control, for which the mean disease control was 68.6%. As a group, experimental antibiotics provided the best fire blight control with mean effect sizes ranging from 59.7 to 61.7%. Among the biological controls, the best control was noted for treatments combining the antibiotic streptomycin with a product based on Pantoea agglomerans (55.0% mean disease reduction) or Bacillus subtilis (53.9%). Mean disease control was 31.9, 25.7, and 22.6%, respectively, for products based on B. subtilis, Pantoea agglomerans, and Pseudomonas fluorescens without an antibiotic, suggesting that the higher efficacy of the combination treatments was due to the antibiotic. Among the plant defense-activating products, prohexadione calcium had the highest and most consistent effect size (50.7% control), while other products provided modest mean disease control of between 6.1 and 25.8%. Percent control values were significantly moderated by study location and cultivar used in the study, and were smaller, but more variable, when products were tested under high disease intensity compared with low disease intensity. Results indicate that wide-scale use of biological control and plant defense-activating products in the eastern United States is likely to remain low.

Iron homeostasis and fire blight susceptibility in transgenic pear plants overexpressing a pea ferritin gene

The bacterial pathogen Erwinia amylovora causes the devastating disease known as fire blight in some rosaceous plants including apple and pear. One of the pathogenicity factors affecting fire blight development is the production of a siderophore, desferrioxamine, which overcomes the limiting conditions in plant tissues and also protects bacteria against active oxygen species. In this paper we examine the effect of an iron chelator protein encoded by the pea ferritin gene on the fire blight susceptibility of pear (Pyrus communis). Transgenic pear clones expressing this gene controlled either by the constitutive promoter CaMV 35S or by the inducible promoter sgd24 promoter were produced. The transgenic clones produced were analysed by Q-RT-PCR to determine the level of expression of the pea transgene. A pathogen-inducible pattern of expression of the pea transgene was observed in sgd24-promoter transformants. Adaptation to iron deficiency in vitro was tested in some transgenic clones and different iron metabolism parameters were measured. No strong effect on iron and chlorophyll content, root reductase activity and fire blight susceptibility was detected in the transgenic lines tested. No transformants showed a significant reduction in susceptibility to fire blight in greenhouse conditions when inoculated with E. amylovora.

The HrpN effector of Erwinia amylovora, which is involved in type III translocation, contributes directly or indirectly to callose elicitation on apple leaves

Erwinia amylovora is responsible for fire blight of apple and pear trees. Its pathogenicity depends on a type III secretion system (T3SS) mediating the translocation of effectors into the plant cell. The DspA/E effector suppresses callose deposition on apple leaves. We found that E. amylovora and Pseudomonas syringae DC3000 tts mutants or peptide flg22 do not trigger callose deposition as strongly as the dspA/E mutant on apple leaves. This suggests that, on apple leaves, callose deposition is poorly elicited by pathogen-associated molecular patterns (PAMPs) such as flg22 or other PAMPs harbored by tts mutants and is mainly elicited by injected effectors or by the T3SS itself. Callose elicitation partly depends on HrpW because an hrpW-dspA/E mutant elicits lower callose deposition than a dspA/E mutant. Furthermore, an hrpN-dspA/E mutant does not trigger callose deposition, indicating that HrpN is required to trigger this plant defense reaction. We showed that HrpN plays a general role in the translocation process. Thus, the HrpN requirement for callose deposition may be explained by its role in translocation: HrpN could be involved in the translocation of other effectors inducing callose deposition. Furthermore, HrpN may also directly contribute to the elicitation process because we showed that purified HrpN induces callose deposition.