This is more than most orchardist want to know https://academic.oup.com/femsle/article/253/2/185/505649
" JOURNAL ARTICLE
Molecular genetics of Erwinia amylovora involved in the development of fire blight
[Chang-Sik Oh](javascript:;), [Steven V. Beer]
FEMS Microbiology Letters, Volume 253, Issue 2, December 2005, Pages 185–192, https://doi.org/10.1016/j.femsle.2005.09.051
Published:
01 December 2005
(https://academic.oup.com/femsle/article/253/2/185/505649#)
Abstract
The bacterial plant pathogen, Erwinia amylovora, causes the devastating disease known as fire blight in some Rosaceous plants like apple, pear, quince, raspberry and several ornamentals. Knowledge of the factors affecting the development of fire blight has mushroomed in the last quarter century. On the molecular level, genes encoding a Hrp type III secretion system, genes encoding enzymes involved in synthesis of extracellular polysaccharides and genes facilitating the growth of E. amylovora in its host plants have been characterized. The Hrp pathogenicity island, delimited by genes suggesting horizontal gene transfer, is composed of four distinct regions, the hrp/hrc region, the HEE (Hrp effectors and elicitors) region, the HAE (Hrp-associated enzymes) region, and the IT (Island transfer) region. The Hrp pathogenicity island encodes a Hrp type III secretion system (TTSS), which delivers several proteins from bacteria to plant apoplasts or cytoplasm. E. amylovora produces two exopolysaccharides, amylovoran and levan, which cause the characteristic fire blight wilting symptom in host plants. In addition, other genes, and their encoded proteins, have been characterized as virulence factors of E. amylovora that encode enzymes facilitating sorbitol metabolism, proteolytic activity and iron harvesting. This review summarizes our understanding of the genes and gene products of E. amylovora that are involved in the development of the fire blight disease.
[Pathogenicity island](javascript:;), [hrp](javascript:;), [Bacterial pathogenesis](javascript:;), [Exopolysaccharide](javascript:;), [Virulence factor](javascript:;), [TTSS](javascript:
Issue Section:
Minireview
1 Introduction
Erwinia amylovora causes the devastating disease called fire blight in some Rosaceous plants like apple, pear, raspberry, cotoneaster and pyracantha [[1](javascript:;)]. The bacterial etiology of the disease was recognized in 1880. Koch’s postulates were completed for the causal bacterium, the first bacterial plant pathogen, a few years later. Thus, E. amylovora is considered the first proven bacterial plant pathogen [[2](javascript:;)]. E. amylovora is Gram-negative and belongs to the family Enterobacteriaceae, which includes Escherichia coli, Yersinia spp., Shigella spp. and Salmonella spp., which are human and animal pathogens.
E. amylovora infects host plants primarily through nectarthodes in flowers or wounds in succulent tissues. Bacteria are transferred from overwintering cankers by crawling insects and splashing rain, and from flower to flower by pollinating insects like bees [[1](javascript:;)]. Once bacteria become established in the plant, they can move within the vascular system. As bacteria accumulate in the xylem, distal plant parts blight and are killed as a result of blocked water flow ([Fig. 1](javascript:;)). In addition, bacterial ooze, a characteristic sign of fire blight, composed of bacteria, polysaccharides, and plant sap, is produced in the infection sites ([Fig. 1](javascript:;)). E. amylovora can infect leaves, shoots, rootstocks and fruits as well. From its origin in North America, fire blight has spread to New Zealand, Europe, the Middle East, and Japan [[3](javascript:;)].
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Disease symptoms of fire blight in apple (A), and characterization of mutants of E. amylovora in terms of induction of the hypersensitive response (B) and disease development (C). (A) The picture at the top shows an apple orchard severely infected by E. amylovora. The smaller pictures at the bottom show symptoms of fire blight affecting different plant parts; all of these were present in this young orchard near Geneva, New York, USA (photo and that of rootstock infection courtesy of H.S. Aldwinckle). Left to right: Rootstock infection; note that the lesion extends around the periphery of the rootstock girdling the tree, a fatal development. Vegetative shoot infection; note the characteristic wilting and “shepherd’s crook” of the affected succulent shoot and the several drops of ooze on the stem. Blossom cluster infection; note the necrotic flower parts, including the fruiting cups, sepals and peduncles and the drops of ooze on the affected parts. Fruit infection, which is characterized by copious ooze and necrosis. (B) The hypersensitive response (HR) induced by the wild-type (WT) and an hrpL mutant strain of E. amylovora in leaves of two tobacco species, Nicotiana tabacum cv. Xanthi and N. benthamiana. Each panel of the leaves of tobacco was infiltrated, by needleless syringe, with ca. 50 μl of suspension of strains of E. amylovora (2 × 108 colony forming U/ml). The leaves were photographed 24 h after infiltration. (C) Immature pear fruit pathogenicity assay. The pictures at the top show disease progress following inoculation with E. amylovora (photos courtesy of S.C.D. Carpenter). With increasing incubation time, bacterial ooze and necrosis appears in and surrounding the sites of inoculation. The pictures at the bottom, which were taken 3 days after inoculation, show no symptoms induced by either the hrpL or dspE mutant, compared to the WT.
During the past quarter century, many genes and gene products have been identified and characterized as being involved in the ability of E. amylovora to cause fire blight in host plants. This review summarizes our current understanding of these factors and the genetics and mechanisms of pathogenesis of E. amylovora.
2 The Hrp pathogenicity island (PAI) of E. amylovora
In the mid 1980s, mutants of E. amylovora, which had lost both pathogenicity in apple and ability to elicit the hypersensitive response (HR) in tobacco were generated by transposon mutagenesis ([Fig. 1](javascript:;)) [[4](javascript:;)]. The HR refers to a rapid and localized cell death of leaf tissue previously infiltrated with a suspension of bacteria above a threshold level of concentration [[5](javascript:;)]. Those mutated genes were designated “hrp” for hypersensitive response and pathogenicity, based on similar phenotypes induced in plants by P. syringae [[6](javascript:;)]. Mutation of some genes abolished only pathogenicity in host plants, but not HR-elicitation in tobacco, these were designated “dsp” (disease-specific) genes [[7](javascript:;)]. The hrp and dsp genes exist in genomic DNA as the hrp/dsp gene cluster.
Since Hacker and his colleagues introduced the concept of a pathogenicity island (PAI) in pathogenic Escherichia coli in the late 1980s, PAIs have been identified in many bacterial pathogens [[8](javascript:;)]. The entire Hrp PAI of E. amylovora strain Ea321, including the hrp/dsp gene cluster, has been sequenced and characterized [[9](javascript:;)]. The PAI of E. amylovora contains ca. 60 genes in ca. 62-kb of genomic DNA, which can be divided into four distinct DNA regions as shown in [Fig. 2](javascript:;); the hrp/hrc region, the Hrp effectors and elicitors (HEE) region, the Hrp-associated enzymes (HAE) region, and the island transfer (IT) region. The hrp/dsp gene cluster, previously described, includes the hrp/hrc region and the HEE region. The IT region has a tRNAPhe, a non-functional integrase gene homolog and three homologs of phage genes. The right border of the PAI of E. amylovora is not clear, but based on homology to “housekeeping genes” of E. coli, the PAI ends at or near orf19, which has a promoter controlled by HrpL, an alternative sigma factor [[9](javascript:;)].
2
![The Hrp pathogenicity island of E. amylovora strain Ea321. It consists of four DNA regions: the hrp/hrc region, the HEE region, the HAE region, and the IT region. The hrp/dsp gene cluster includes the hrp/hrc region and the HEE region. The genes having significant functions or homology with other significant genes are color-coded as indicated. The % G + C graph is the result of a sliding window of 500 nucleotides. This figure was adapted from Fig. 1 of Oh et al. [9].](https://d55v7rs15ikf5.cloudfront.net/original/3X/6/8/684f4c04c0b9b4343bc06f570491ac20bfa67e63.gif)
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The Hrp pathogenicity island of E. amylovora strain Ea321. It consists of four DNA regions: the hrp/hrc region, the HEE region, the HAE region, and the IT region. The hrp/dsp gene cluster includes the hrp/hrc region and the HEE region. The genes having significant functions or homology with other significant genes are color-coded as indicated. The % G + C graph is the result of a sliding window of 500 nucleotides. This figure was adapted from [Fig. 1](javascript: of Oh et al. [[9](javascript:;)].
2.1 hrp/dsp gene cluster
The hrp/dsp gene cluster of E. amylovora consists of the hrp/hrc region and the HEE region ([Fig. 2](javascript:;)). The hrp/hrc region contains 25 genes, including four regulatory genes, hrpL, hrpS, and hrpXY, which control the expression of other hrp genes, and nine “hrc” (for HR and conserved) genes. The latter were renamed because they are highly conserved among pathogenic bacteria [[10](javascript:;)]. The HEE region contains seven genes; two (hrpN and hrpW) encode harpins, two are dsp genes (dspA/E and dspB/F), one is a yopJ homolog (eopB), and two encode putative chaperones (orfA and orfC) [[9](javascript:;)]. Proteins encoded by genes in the HEE region are discussed in detail later.
The most important role of hrp genes is to form a protein secretion/translocation pathway, called the “Hrp TTSS”, to secrete and deliver proteins from bacteria to plant apoplasts or cytoplasm. Nine hrc genes are believed to constitute the core structural components of the Hrp TTSS. In E. amylovora, the structure of the Hrp TTSS and morphology of a Hrp pilus have not been characterized, but based on homology of Hrc proteins to those of P. syringae, hrcC encodes an outer membrane protein [[11](javascript:;)], while other genes encode inner membrane proteins, which form the basal structure of the Hrp TTSS. The Hrp TTSS extends outside the bacterial cell as a pilus, which may reach the host cell membrane. In E. amylovora, hrpA encodes a pilin protein as does hrpA of P. syringae [[11](javascript:;)]. He and his colleagues reported that a pilus was densely labeled by HrpA antibody and this pilus did not appear in an hrpA mutant [[12](javascript:;)]. They also showed that this Hrp pilus is required for secretion of HrpN and DspA/E.
In the TTSS of animal bacterial pathogens, translocator proteins, which are required for translocation of effector proteins from bacterial cytoplasm into host cells, are thought to function in pore formation in the host plasma membrane, for example, YopB and YopD of Yersinia spp. [[13](javascript:;)]. Translocators have not been clearly identified in plant-pathogenic bacteria including E. amylovora, although two candidates, HrpF of X. campestris pv. vesicatoria [[14](javascript:;)] and HrpK of P. syringae pv. tomato [[15](javascript:;)] have been suggested. Recently, harpins have been considered as “helper proteins” [[16](javascript:;)] that may facilitate the translocation of effector proteins into plant cells. However, evidence to support a role for harpins in the translocation process is needed.
In E. amylovora, HrpL controls expression of all known hrp and hrc genes, as it does in P. syringae [[17](javascript:;)]. HrpL recognizes promoters with a hrp box, which consists of specific DNA sequences. Expression of the hrpL gene is controlled by HrpS, a member of the NtrC family of sigma 54 enhancer-binding proteins and sigma 54 [[18](javascript:;)]. In addition, HrpX, a putative sensor protein and HrpY, a potential response regulator, which together constitute a two-component regulatory system, are involved in expression of the hrpL gene [[18](javascript:;)]. The regulatory genes of E. amylovora are expressed in planta only under conditions of low nutrients and low pH; they are repressed in rich media [[19](javascript:;)]. However, the identity of the molecules that actually are responsible for stimulating expression of hrp and hrc genes in planta (if any) remains to be determined.
2.2 Hrp-associated enzyme (HAE) region
The HAE region includes five genes situated next to the hrpJ operon in the hrp/dsp gene cluster ([Fig. 2](javascript:;)) [[9](javascript:;)]. These genes are homologous to genes encoding enzymes and may be involved in peptide synthesis. For example, the hsv (hrp-associated systemic virulence) genes, which were designated recently because they were involved in systemic infection of E. amylovora in apple [[9](javascript:;)], are homologous to genes involved in the biosynthesis of phaseolotoxin in P. syringae pv. phaseolicola [[20](javascript:;)]. Although no phaseolotoxin activity was detected in E. amylovora [[9](javascript:;)], whether this bacterium produces a phaseolotoxin-like compound is still unclear. A small peptide that differs structurally and biologically from phaseolotoxin may be produced by E. amylovora.
2.3 Island transfer (IT) region
The IT region includes 17 genes; three are homologous to phage genes [[9](javascript:;)]. The G + C content over the IT region is highly variable as shown in [Fig. 2](javascript:;). Because a mutant in which the whole IT region was deleted still caused disease symptoms in immature pear fruit slices and induced an HR in tobacco leaves, it is unlikely that the IT region contributes to pathogenicity in immature pear and elicitation of the HR in tobacco [[21](javascript:;)]. However, the virulence of this mutant in apple shoots was not determined. Moreover, within this region, there are two putative virulence genes, orfL and orfO, which are 83% identical to each other [[9](javascript:;)]. They both have a potential bipartite nuclear localization signal (NLS), which is a characteristic of nuclear proteins in eukaryotic cells. In addition, ORFL and ORFO have potential Ca+±binding EF-hand motifs, which suggests that these proteins require Ca++ for activation. These characteristics indicate that ORFL and ORFO are likely targeted to plant cells.
The left border of the PAI of E. amylovora, located in the IT region, is highly conserved with a PAI of Y. pseudotuberculosis [[9](javascript:;)]. The five genes next to a tRNAPhe gene are nearly identical to genes in a PAI of Y. pseudotuberculosis. Moreover, the G + C content of the five genes is significantly higher than that of other genes in the IT region and the average, 53.2%, of the whole E. amylovora genome. These features suggest that this DNA region, which is present in both animal and plant pathogens, may have been mobile through horizontal gene transfer.
3 Proteins secreted through the Hrp TTSS
3.1 Harpins; HrpN and HrpW
Harpins are proteins that they are glycine-rich, heat-stable, lack cysteine and have few aromatic amino acids. To date, two harpins, HrpN and HrpW, have been found in E. amylovora. HrpN was considered involved in disease development based on mutational analysis, and it was described as the first cell-free elicitor of a HR in the early 1990s [[22](javascript:;)]. Two research groups showed separately that hrpN mutants of E. amylovora dramatically lost virulence in host plants [[22](javascript:;),[23](javascript:;)]. HrpW has a putative pectate lyase domain in its C-terminus; no virulence function for HrpW has been detected [[24](javascript:;)]. At least four harpin-like proteins, including HrpZ and HrpW, were found in P. syringae pv. tomato DC3000, based on analysis of the whole genome sequence [[25](javascript:;)]. These findings indicate that plant-pathogenic bacteria produce multiple harpins; however, their functions in disease development remain to be determined. In addition, unlike effector proteins, which are translocated to the plant cytoplasm, harpins are secreted and targeted to the intercellular spaces of plant tissues through the Hrp TTSS [[26](javascript:;)]. Barny and her colleagues [[27](javascript:;)] found through electron microscopic studies that HrpN is secreted and localized only in apoplasts in planta during the infection process.
Both HrpN and HrpW of E. amylovora induce the HR following infiltration of partially purified proteins into the leaves of tobacco [[22](javascript:;),[24](javascript:;)]. However, how harpins induce the HR and the nature of their interactors are unknown. Interestingly, HrpN stimulates the K+/H+ exchange reaction in tobacco cell suspension [[22](javascript:;)] and K+ ion efflux in Arabidopsis suspension cells [[28](javascript:;)], indicating that it affects ion channels in plasma membranes. HrpN also may indirectly disrupt mitochondrial function because the HrpN protein inhibits ATP synthesis in tobacco cell cultures [[29](javascript:;)]. However, whether these activities are related to HR elicitation in plants is not clear. In addition, only extracellularly targeted HrpN induces an HR in tobacco, and HrpN-induced HR was suppressed by AvrPtoB, a known HR suppressor [[30](javascript:;)]. This means that HrpN may activate signal transduction pathways involved in HR induction outside the plant cells.
Under disease conditions in pear, a host plant, Brisset and her colleagues [[31](javascript:;)] found that E. amylovora induces oxidative stress responses, including accumulation of superoxide, lipid peroxidation, and electrolyte leakage; these responses are typical characteristics of incompatible interactions. In addition, they found that the Hrp TTSS is required for these responses because a hrp secretion mutant failed to induce them. They also found that elicitation of oxidative stress responses resulted from the combined action of two proteins, HrpN and DspA/E, but not from HrpW [[32](javascript:;)].
HrpN not only induces an HR, but it has other pleiotropic effects in plants. First, it induces the salicylic acid (SA)-dependent and the jasmonate (JA)-dependent pathways in Arabidopsis. After spraying plants with less than 15 μg/ml of HrpN protein, expression of the PR-1 and PDF1.2 genes, typical marker genes for SA-, and JA-dependent pathways in plants, respectively, is enhanced [[33](javascript:;)]. Moreover, HrpN-treated plants have increased resistance against pathogens [[34](javascript:;),[35](javascript:;)], and aphids [[33](javascript:;)]. Furthermore, treating plants with HrpN results in enhanced growth and increased productivity in Arabidopsis, tomato and cotton [[33](javascript:;)]. How HrpN induces these pleiotropic effects in planta remains to be determined.
The importance of HrpN to disease development and its targeting to apoplasts of plant tissues suggest that HrpN is functionally active outside plant cells and that HrpN-interacting proteins may be located in the plasma membrane or outside plant cells. Researchers in the Beer lab at Cornell University and at EDEN Bioscience Corporation separately identified plant proteins that interact with HrpN. A HrpN-interacting protein from Malus (HIPM) was found in apple [[30](javascript:;)], and a HrpN-binding protein (HrBP1) was found in Arabidopsis (United States Patent Application 20040034554). HIPM has a functional signal peptide and is associated with plasma membranes, but HrBP1 fractionated with cell walls (Zhongmin Wei, personal communication). The relationship between the plant proteins and HrpN to disease development is not clear. Nevertheless, AtHIPM, an Arabidopsis HIPM ortholog, is involved at least in the enhanced growth of Arabidopsis responding to HrpN [[30](javascript:;)], suggesting that HrpN–HIPM interaction is important in the interaction of HrpN with plants.
3.2 DspA/E
DspA/E, a homolog of AvrE of P. syringae, is known as a pathogenicity factor in E. amylovora because dspA/E mutants are not pathogenic to apple shoots, immature pear fruit slices ([Fig. 1](javascript:;)) [[36](javascript:;)] or pear seedlings [[37](javascript:;)]. Moreover, DspA/E functions as an avirulence factor when it is expressed in P. syringae pv. glycinea race 4; it makes this otherwise pathogenic bacterium avirulent in soybean. Initially, DspA/E was not considered as an HR elicitor, but recently, DspA/E was shown to elicit the HR following its transient expression in Nicotiana benthamiana [[30](javascript:;)].
He and his colleagues [[38](javascript:;)] found that a dspA/E mutant of E. amylovora activated SA-dependent callose deposition in wild-type Arabidopsis and apple as does a CEL deletion mutant of P. syringae pv. tomato; the wild-type strains suppressed callose deposition. This indicates that DspA/E contributes to disease development by inhibiting SA-dependent innate immunity.
DspA/E is secreted through the Hrp TTSS [[39](javascript:;)], and its secretion is dependent on DspB/F, which seems to be a DspA/E-specific chaperone [[40](javascript:;)]. A dspB/F mutant was greatly reduced in virulence in apple shoots [[40](javascript:;)]. This indicates that a small amount of DspA/E may be secreted without DspB/F, or another chaperone may be involved in this process. Recently, direct evidence for delivery of DspA/E into plant cells has been presented using a DspA/E-CyaA fusion protein [[41](javascript:;)].
Recently, two groups of DspA/E-interacting proteins from apple, a host plant, were found in the Beer lab at Cornell University using yeast two-hybrid assays. One group is comprised of four serine/threonine protein receptor kinases, which were detected by using the N-terminal half of DspA/E as bait [[42](javascript:;)]. These kinases were designated “DspE-interacting proteins from Malus (DIPM)”. Each contains a leucine-rich repeat (LRR), a transmembrane ™, and a kinase domain. The second protein that interacts with DspA/E is a preferredoxin, the cytoplasmic precursor of ferredoxin; it was detected using the C-terminal half of DspA/E as bait [[43](javascript:;)]. Chloroplastic ferredoxin is involved in electron transfer in Photosystem I; it does not interact with DspA/E in yeast two-hybrid assays. However, it is still unclear how these interacting proteins from Malus sp. are involved in functions of DspA/E in planta.
3.3 EopB
EopB (Erwinia outer protein B) belongs to the YopJ (Yersinia outer protein J from Y. pseudotuberculosis) family of proteins, based on overall homology and the conservation of three amino acids, which in YopJ constitute a catalytic triad critical for cysteine protease activity [[44](javascript:;)]. EopB was first characterized as one of several secreted proteins of E. amylovora by comparing the protein profiles of a wild-type strain and a TTSS-deficient mutant strain grown in hrp-inducing medium (Riitta Nissinen and Steven V. Beer, unpublished). Recently, using the AvrRpt2 reporter system, translocation of EopB into plant cells was demonstrated [[30](javascript:;)].
Whether EopB is involved in virulence remains to be determined in apple. However, EopB probably does not play a role in virulence or pathogenicity of E. amylovora in immature pear fruit and in HR elicitation because its mutant’s responses did not differ from that of the wild-type strain in pear fruit and N. tabacum cv. Xanthi, respectively [[45](javascript:;)].
3.4 Other proteins
HrpJ and HrpK also were characterized as secreted proteins by the same methods as used for EopB. HrpJ was considered a homolog of YopN [[46](javascript:;)]; YopN may function in gating the TTSS for secretion of Yop proteins and prevent their secretion in the presence of calcium. If HrpJ functions like YopN, hrpJ mutants should induce HR in tobacco. However, hrpJ mutants do not [[47](javascript:;)], suggesting that HrpJ differs in function from YopN.
HrpK of P. syringae pv. tomato DC3000 was characterized as a putative translocator for delivery of Hop (Hrp outer protein) proteins, which may function to create channels in the plasma membranes of plant cells [[15](javascript:;)]. The virulence of hrpK mutants of P. syringae pv. tomato DC3000 in tomato was reduced significantly relative to the wild type, and HrpK was translocated into plant cells based on assays using the CyaA reporter fusion system [[15](javascript:;)]. However, a hrpK mutant of E. amylovora did not differ from the wild-type parent in terms of disease development in apple shoots and immature pear fruit slices [[9](javascript:;)]. Therefore, the function of HrpK of E. amylovora seems quite different from HrpK in P. syringae, and its true function(s) in the fire blight pathogen remains to be determined.
Genome sequencing of E. amylovora was launched in late 2004. Once the genome sequence is available, other Eop proteins might be discovered using several screening methods and bioinformatic tools.
4 Other factors involved in pathogenicity or virulence
4.1 EPS; amylovoran and levan
Studies on bacterial ooze, which contains bacteria, polysaccharides and plant sap were begun in the 1930s. Intensive work in the 1970s revealed that bacteria-free preparations from ooze or tissues infected by E. amylovora induce wilting of detached shoots of host plants. Goodman and his colleagues [[48](javascript:;)] isolated a high-molecular-weight polysaccharide from bacterial ooze, designated amylovoran, which they considered a toxin. Subsequent work indicated that amylovoran affects plants primarily by plugging the vascular tissues, thus inducing wilt of shoots [[49](javascript:;)]. Amylovoran is now known as a pathogenicity factor because amylovoran-deficient mutants of E. amylovora lack pathogenic ability [[50](javascript:;)]. Later, levan, a second EPS was shown to be produced by E. amylovora; levan is involved in virulence [[51](javascript:;)].
The biosynthesis of amylovoran in E. amylovora requires a cluster of 12 ams genes [[52](javascript:;)]. Expression of the ams genes is controlled by RcsA and RcsB, which are conserved regulatory proteins, first identified in E. coli as regulators of capsule synthesis (cps). In E. amylovora, RcsA and RcsB bind to the promoter region of the ams operon and control expression of ams genes [[53](javascript:;)]. Moreover, mutation in rcsB affects both amylovoran biosynthesis and virulence [[54](javascript:;)]. The lsc gene, which encodes levansucrase, controls the biosynthesis of levan [[55](javascript:;)]. Levan synthesis is positively regulated by RlsA [[56](javascript:;)]. The rlsA gene is located next to dspB/F, which is in the HEE region of the PAI.
4.2 Sorbitol metabolism
Sorbitol is the dominant sugar alcohol in rosaceous plants and is used for carbohydrate transport rather than sucrose, which is used in many other plants. The srl operon, which is necessary for sorbitol metabolism, was identified in E. amylovora by functional complementation using E. coli strains mutated in the gut genes [[57](javascript:;)]. The srl operon consists of six genes; three are needed for sorbitol uptake (srlA, srlB, and srlE), one encodes a protein that converts sorbitol to fructose (srlD), and two other genes are regulatory (srlM and srlR). The components and gene order of the srl operon in E. amylovora are quite similar to those of the gut operon in E. coli. srl mutants are virulent in immature pear fruit slices, but are not virulent in apple shoots [[57](javascript:;)]. This indicates that the capability of E. amylovora to use sorbitol may be an important factor affecting disease-causing activity in apple shoots. The srl mutant phenotype is similar to that of hsv gene mutants.
4.3 Protease
In contrast to many species of Erwinia, E. amylovora lacks the ability to degrade cell wall components by the action of carbohydrate-degrading enzymes [[58](javascript:;)]. However, E. amylovora produces and secretes PrtA, a metalloprotease, in minimal medium [[59](javascript:;)]. PrtA is secreted by the type I secretion system, which is comprised of three structural proteins, PrtD, PrtE, and PrtF. The type I secretion system is similar to the system used for the secretion of lipase in Serratia marcescens. A strain of E. amylovora mutated in the prtD gene was unable to secrete PrtA, which resulted in reduced colonization of apple leaves by E. amylovora [[59](javascript:;)].
4.4 Desferrioxamine (siderophore)
As iron often limits survival, bacteria produce siderophores, compounds that have high affinity for iron, to aid in iron acquisition [[60](javascript:;)]. Under iron-limiting conditions, siderophores bind to Fe+++. Iron-bound siderophores are taken-up through receptors in the outer membrane, and iron is delivered to the cells. Under iron-poor conditions, E. amylovora produces and secretes cyclic desferroxamines (DFOs), hydroxamate-type siderophores [[61](javascript:;)]. For uptake of these siderophores, E. amylovora produces siderophore receptors, for example, FoxR for DFOE [[62](javascript:;)]. Two types of mutants have been characterized; dfoA mutants and foxR mutants are disrupted in DFO synthesis and DFO uptake, respectively. DfoA is 58% identical to AlcA of Bordetella bronchiseptica, which is necessary for alcaligin production; FoxR is 65% identical to FoxA, the ferrioxamine receptor of Y. enterocolitica [[63](javascript:;)]. On apple seedlings, only a foxR mutant of E. amylovora resulted in less necrosis, but on flowers, both the dfoA and foxR mutants elicited fewer necrotic symptoms and showed less bacterial growth than the wild-type strain [[63](javascript:;)]. These results indicate that iron acquisition systems are important for virulence in E. amylovora.
Interestingly, a second role of desferrioxamine was determined as a major factor for protection of E. amylovora against oxidative conditions (See Section 3.1 in this text). Brisset and her colleagues [[32](javascript:;)] found that a DFOE-deficient mutant is more sensitive to H2O2 in vitro.
5 Conclusion
Over the past 20 years, many virulence factors of E. amylovora and the genes encoding them have been revealed and characterized. These include the Hrp PAI, several proteins delivered from bacteria to plant apoplasts or cytoplasm, and EPS. When the whole genome sequence of E. amylovora becomes available, more comprehensive and extensive experiments will be possible that will likely expand our understanding of the virulence factors of E. amylovora. Recently, experiments to identify host (apple) factors necessary for E. amylovora to cause disease were initiated. Perhaps, by blocking their normal functions, or by enhancing the activity of host proteins, disease development might be thwarted or reduced. These findings may well facilitate the development of improved methods for control of fire blight."
