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

A TIR-NBS-LRR Gene MdTNL1 Regulates Resistance to Glomerella Leaf Spot in Apple

Glomerella leaf spot (GLS), caused by the fungus Colletotrichum fructicola, is one of the most devastating apple diseases. Our previous study reported that the GLS resistance locus was defined on the chromosome 15 region. Here, we further found a single-nucleotide polymorphism (SNP) site (SNP_7309212) in the GLS resistance that was able to distinguish resistant cultivars (lines) from susceptible ones. On the basis of the SNP site, we cloned a TNL gene from the GLS resistant locus and named it MdTNL1 (NCBI Accession Number: ON402514). This gene contains a toll/interleukin-1 receptor transmembrane domain (TIR), nucleotide-binding sites (NBS), and leucine-rich repeat (LRR) domain. Subcellular location indicated that MdTNL1 was expressed in the nucleus and cell membrane. Ectopic overexpression of MdTNL1 in Nicotiana benthamiana caused cell death. We further demonstrated allelic polymorphisms in MdTNL1. It is noteworthy that NBS and LRR domains of the MdTNL1 protein serve as the repository for generating allelic diversity. Quantitative real-time PCR (qRT-PCR) assay revealed that MdTNL1 was highly expressed in resistant apple cultivar ‘Fuji’ after inoculation with C. fructicola, whereas susceptible cultivar ‘Golden Delicious’ exhibited low expression after inoculation. Over-expression of MdTNL1-1 in susceptible apple fruits and leaves improved disease resistance, while in ‘Orin’ calli, silencing the MdTNL1-1 gene conversely decreased GLS resistance. In conclusion, we identified a GLS associated with SNP_7309212 and demonstrated that a TIR-NBS-LRR gene MdTNL1-1 positively regulates GLS resistance in apple.

Pathogens pulling the strings: Effectors manipulating salicylic acid and phenylpropanoid biosynthesis in plants

During evolution, plants have developed sophisticated ways to cope with different biotic and abiotic stresses. Phytohormones and secondary metabolites are known to play pivotal roles in defence responses against invading pathogens. One of the key hormones involved in plant immunity is salicylic acid (SA), of which the role in plant defence is well established and documented. Plants produce an array of secondary metabolites categorized in different classes, with the phenylpropanoids as major players in plant immunity. Both SA and phenylpropanoids are needed for an effective immune response by the plant. To successfully infect the host, pathogens secrete proteins, called effectors, into the plant tissue to lower defence. Secreted effectors can interfere with several metabolic or signalling pathways in the host to facilitate infection. In this review, we will focus on the different strategies pathogens have developed to affect the levels of SA and phenylpropanoids to increase plant susceptibility.

Genetic Dissection of the Erwinia amylovora Disease Cycle

Fire blight, caused by the bacterial phytopathogen Erwinia amylovora, is an economically important and mechanistically complex disease that affects apple and pear production in most geographic production hubs worldwide. We compile, assess, and present a genetic outlook on the progression of an E. amylovora infection in the host. We discuss the key aspects of type III secretion-mediated infection and systemic movement, biofilm formation in xylem, and pathogen dispersal via ooze droplets, a concentrated suspension of bacteria and exopolysaccharide components. We present an overall outlook on the genetic elements contributing to E. amylovora pathogenesis, including an exploration of the impact of floral microbiomes on E. amylovora colonization, and summarize the current knowledge of host responses to an incursion and how this response stimulates further infection and systemic spread. We hope to facilitate the identification of new, unexplored areas of research in this pathosystem that can help identify evolutionarily susceptible genetic targets to ultimately aid in the design of sustainable strategies for fire blight disease mitigation.

Transcriptional profile of AvrRpt2EA-mediated resistance and susceptibility response to Erwinia amylovora in apple

Most of the commercial apple cultivars are highly susceptible to fire blight, which is the most devastating bacterial disease affecting pome fruits. Resistance to fire blight is described especially in wild Malus accessions such as M. × robusta 5 (Mr5), but the molecular basis of host resistance response to the pathogen Erwinia amylovora is still largely unknown. The bacterial effector protein AvrRpt2_EA was found to be the key determinant of resistance response in Mr5. A wild type E. amylovora strain and the corresponding avrRpt2_EA deletion mutant were used for inoculation of Mr5 to induce resistance or susceptible response, respectively. By comparison of the transcriptome of both responses, 211 differentially expressed genes (DEGs) were identified. We found that heat-shock response including heat-shock proteins (HSPs) and heat-shock transcription factors (HSFs) are activated in apple specifically in the susceptible response, independent of AvrRpt2_EA. Further analysis on the expression progress of 81 DEGs by high-throughput real-time qPCR resulted in the identification of genes that were activated after inoculation with E. amylovora. Hence, a potential role of these genes in the resistance to the pathogen is postulated, including genes coding for enzymes involved in formation of flavonoids and terpenoids, ribosome-inactivating enzymes (RIPs) and a squamosa promoter binding-like (SPL) transcription factor.

The guanine-hypoxanthine permease GhxP of Erwinia amylovora facilitates the influx of the toxic guanine derivative 6-thioguanine

AIM

Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. This study determines whether the E. amylovora guanine-hypoxanthine transporter (EaGhxP) is required for virulence and if it can import the E. amylovora produced toxic analogue 6-thioguanine (6TG) into cells.

METHODS AND RESULTS

Characterization of EaGhxP in guanine transport deficient Escherichia coli reveals that it can transport guanine, hypoxanthine and the toxic analogues 8-azaguanine (8AG) and 6TG. Similarly, EaGhxP transports 8AG and 6TG into E. amylovora cells. EaGhxP has a high affinity for 6TG with a K_i of 3·7 µmol l^-1 . An E. amylovora ⊿ghxP::Cam^r strain shows resistance to growth on 8AG and 6TG. Although EaGhxP is expressed during active disease propagation, it is not necessary for virulence as determined on immature apple and pear assays.

CONCLUSIONS

EaGhxP is not required for virulence, but it does import 6TG into E. amylovora cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

As part of the disease establishment process, E. amylovora synthesizes and exports a toxic guanine derivative 6TG. Our results are counter intuitive and show that EaGhxP, an influx transporter, can move 6TG into cells raising questions regarding the role of 6TG in disease establishment.

Genome-wide identification of the Sec-dependent secretory protease genes in Erwinia amylovora and analysis of their expression during infection of immature pear fruit

The general secretory (Sec) pathway represents a common mechanism by which bacteria secrete proteins, including virulence factors, into the extracytoplasmic milieu. However, there is little information about this system, as well as its associated secretory proteins, in relation to the fire blight pathogen Erwinia amylovora. In this study, data mining revealed that E. amylovora harbors all of the essential components of the Sec system. Based on this information, we identified putative Sec-dependent secretory proteases in E. amylovora on a genome-wide scale. Using the programs SignalP, LipoP, and Phobius, a total of 15 putative proteases were predicted to contain the N-terminal signal peptides (SPs) that might link them to the Sec-dependent pathway. The activities of the predicted SPs were further validated using an Escherichia coli-based alkaline phosphatase (PhoA) gene fusion system that confirmed their extracytoplasmic property. Transcriptional analyses showed that the expression of 11 of the 15 extracytoplasmic protease genes increased significantly when E. amylovora was used to inoculate immature pears, suggesting their potential roles in plant infection. The results of this study support the suggestion that E. amylovora might employ the Sec system to secrete a suite of proteases to enable successful infection of plants, and shed new light on the interaction of E. amylovora with host plants.

An Erwinia amylovora uracil transporter mutant retains virulence on immature apple and pear fruit

Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. A previous study revealed that an E. amylovora uracil auxotroph was still virulent and can cause disease, suggesting that uracil can be obtained from the host environment. The E. amylovora genome contains a locus encoding for a uracil transporter belonging to the nucleobase cation symporter 2 family, displaying a high level of amino acid sequence similarity to the Escherichia coli UraA. Expression of E. amylovora UraA in nucleobase transporter-deficient E. coli strains, coupled with radiolabeled uptake studies reveal that E. amylovora UraA is a high affinity uracil transporter with a K_m of 0.57 μM. Both E. coli and E. amylovora carrying extra copies of E. amylovora UraA are sensitive to growth on the toxic analog 5-fluorouracil. An E. amylovora ΔuraA::Cam^r mutant is still able to grow and cause disease symptoms on immature pears and apples.

A host target of a bacterial cysteine protease virulence effector plays a key role in convergent evolution of plant innate immune system receptors

Some virulence effectors secreted from pathogens target host proteins and induce biochemical modifications that are monitored by nucleotide-binding and leucine-rich repeat (NLR) immune receptors. Arabidopsis RIN4 protein (AtRIN4: RPM1-interacting protein 4) homologs are present in diverse plant species and targeted by several bacterial type III effector proteins including the cysteine protease AvrRpt2. RIN4 is 'guarded' by several independently evolved NLRs from various plant species, including Arabidopsis RPS2. Recently, it was shown that the MR5 NLR from a wild apple relative can recognize the AvrRpt2 effector from Erwinia amylovora, but the details of this recognition remained unclear. The present contribution reports the mechanism of AvrRpt2 recognition by independently evolved NLRs, MR5 from apple and RPS2, both of which require proteolytically processed RIN4 for activation. It shows that the C-terminal cleaved product of apple RIN4 (MdRIN4) but not AtRIN4 is necessary and sufficient for MR5 activation. Additionally, two polymorphic residues in AtRIN4 and MdRIN4 are identified that are crucial in the regulation of and physical association with NLRs. It is proposed that polymorphisms in RIN4 from distantly related plant species allow it to remain an effector target while maintaining compatibility with multiple NLRs.

The structure of Erwinia amylovora AvrRpt2 provides insight into protein maturation and induced resistance to fire blight by Malus × robusta 5

The AvrRpt2 protein of the phytopathogenic bacterium Erwinia amylovora (AvrRpt2_EA) is a secreted type III effector protein, which is recognised by the FB_MR5 resistance protein of Malus × robusta 5, the only identified resistance protein from a Malus species preventing E. amylovora infection. The crystal structure of the immature catalytic domain of AvrRpt2_EA, a C70 family cysteine protease and type III effector, was determined to a resolution of 1.85 Å. The structure provides insights into the cyclophilin-dependent activation of AvrRpt2, and identifies a cryptic leucine of a non-canonical cyclophilin binding motif. The structure also suggests that residue Cys156, responsible for the gene induced resistance, is not involved in substrate determination, and hints that recognition by FB_MR5 is due to direct interaction.

Malus Hosts-Erwinia amylovora Interactions: Strain Pathogenicity and Resistance Mechanisms

The bacterium, Erwinia amylovora, deposits effector proteins such as AvrRpt2_EA into hosts through the type III secretion pathogenicity island to cause fire blight in susceptible Malus genotypes. A single nucleotide polymorphism in the AvrRpt2_EA effector plays a key role in pathogen virulence on Malus hosts by exchanging one cysteine to serine in the effector protein sequence. Fire blight resistance quantitative trait loci (QTLs) were detected in a few apple cultivars and wild Malus genotypes with the resistance of wild apples generally found to be stronger than their domestic relatives. The only candidate and functionally analyzed fire blight resistance genes proposed are from wild apple genotypes. Nevertheless, the aforementioned AvrRpt2_EA SNP and a couple of effector mutants of E. amylovora are responsible for the breakdown of resistance from a few Malus donors including detected QTLs and underlying R-genes. This review summarizes a key finding related to the molecular basis underpinning an aspect of virulence of E. amylovora on Malus genotypes, as well as mechanisms of host recognition and specificity, and their implications on the results of genetic mapping and phenotypic studies within the last 5-6 years. Although the knowledge gained has improved our understanding of the Malus-E. amylovora system, more research is required to fully grasp the resistance mechanisms in this genus especially as they pertain to direct interactions with pathogen effectors.