The VirPphA/AvrPtoB family of type III effectors in Pseudomonas syringae

Integrated transmission electron microscopy and proteomic analyses reveal the cytoarchitectural response to cucumber mosaic virus infection in tobacco

Cucumber mosaic virus (CMV) causes huge economic losses to agriculture every year; thus, understanding the mechanism of plant resistance to CMV is imperative. In this study, an integrated analysis of transmission electron microscopy (TEM) observations and proteomic results was used to identify cytoarchitectural differences in Nicotiana tabacum cv. NC82 (susceptible) and cv. Taiyan 8 (T.T.8; resistant) following infection with CMV. The TEM observations showed that the structure of the chloroplasts and mitochondria was severely damaged at the late stage of infection in NC82. Moreover, the chloroplast stroma and mitochondrial cristae were reduced and disaggregated. However, in T.T.8, organelle structure remained largely intact Selective autophagy predominated in T.T.8, whereas non-selective autophagy dominated in NC82, resembling cellular disorder. Proteomic analysis of T.T.8 revealed differentially expressed proteins (DEPs) mostly associated with photosynthesis, respiration, reactive oxygen species (ROS) scavenging, and cellular autophagy. Biochemical analyses revealed that ROS-related catalase, autophagy-related disulfide isomerase, and jasmonic acid and antioxidant secondary metabolite synthesis-related 4-coumarate:CoA ligase (Nt4CL) exhibited different trends and significant differences in expression in the two cultivars after CMV inoculation. Furthermore, mutant phenotyping verified that reduced Nt4CL expression impaired resistance in T.T.8. The identified DEPs are crucial for maintaining intracellular homeostatic balance and likely contribute to the mechanism of CMV resistance in tobacco. These findings increase our understanding of plant cytological mechanisms conferring resistance to CMV infection.

Molecular and Genomic Characterization of the Pseudomonas syringae Phylogroup 4: An Emerging Pathogen of Arabidopsis thaliana and Nicotiana benthamiana

Environmental fluctuations such as increased temperature, water availability, and air CO2 concentration triggered by climate change influence plant disease dynamics by affecting hosts, pathogens, and their interactions. Here, we describe a newly discovered Pseudomonas syringae strain found in a natural population of Arabidopsis thaliana collected from the southwest of France. This strain, called Psy RAYR-BL, is highly virulent on natural Arabidopsis accessions, Arabidopsis model accession Columbia 0, and tobacco plants. Despite the severe disease phenotype caused by the Psy RAYR-BL strain, we identified a reduced repertoire of putative Type III virulence effectors by genomic sequencing compared to P. syringae pv tomato (Pst) DC3000. Furthermore, hopBJ1Psy is found exclusively on the Psy RAYR-BL genome but not in the Pst DC3000 genome. The plant expression of HopBJ1Psy induces ROS accumulation and cell death. In addition, HopBJ1Psy participates as a virulence factor in this plant-pathogen interaction, likely explaining the severity of the disease symptoms. This research describes the characterization of a newly discovered plant pathogen strain and possible virulence mechanisms underlying the infection process shaped by natural and changing environmental conditions.

Integrated regulatory network in Pseudomonas syringae reveals dynamics of virulence

Pseudomonas syringae, a Gram-negative plant pathogen, expresses multitudinous transcriptional regulators to control the type III secretion system (T3SS) and response to diverse environmental challenges. Although the mechanisms of virulence-associated regulators of P. syringae have been studied for decades, the overall crosstalk underlying these regulators is still elusive. Here, we identify five T3SS regulators (EnvZ-OmpR, CbrAB2, PhoPQ, PilRS, and MgrA), and find that the two-component systems EnvZ-OmpR and CbrAB2 negatively regulate the T3SS. To elucidate crosstalk between 16 virulence-associated regulators in P. syringae, we map an online intricate network called "PSRnet" (Pseudomonas syringae regulatory network) by combining the differentially expressed genes (DEGs) of these 16 regulators by RNA sequencing (RNA-seq) and their binding loci by chromatin immunoprecipitation sequencing (ChIP-seq). Consequently, we identify 238 and 153 functional genes involved in the T3SS and other virulence-related pathways in KB and MM media, respectively. Our results provide insights into the mechanism of plant infections caused by P. syringae.

Transcriptional studies of the hrpM/opgH gene in Pseudomonas syringae during biofilm formation and in response to different environmental challenges

Pseudomonas syringae pv. syringae strain FF5 is a phytopathogen that causes a rapid dieback on ornamental pear trees. In the present study, the transcriptional expression of hrpM/opgH, algD, hrpR and rpoD was evaluated in P. syringae FF5 and FF5.M2 (hrpM/opgH mutant). The temporal expression of these genes was evaluated during biofilm formation, the hypersensitive reaction (HR) on tobacco plants, and when the bacteria were subjected to different environmental stresses. The results indicate that mutations in hrpM negatively impair several traits including biofilm formation, the ability to cause disease in host plants and the HR in non-host plants, and the expression of hrpR, a regulatory gene modulating the latter two traits. Furthermore, FF5.M2 was decreased in swarming motility and unable to respond to different environmental challenges. Interestingly, FF5.M2 showed an exponential increase in the expression of algD, which is the first gene to be transcribed during the biosynthesis of the alginate, a virulence factor in P. syringae. The expression of both hrpM and algD were required for biofilm formation, and hrpM was expressed earlier than algD during biofilm development. These findings indicate that hrpM expression is required for several traits in P. syringae and plays an important role in how this bacterium responds to environmental challenges.

Visualization of novel virulence activities of the Xanthomonas type III effectors AvrBs1, AvrBs3 and AvrBs4

Xanthomonas campestris pv. vesicatoria secretes at least 20 effector proteins through the type III secretion system directly into plant cells. In this study, we uncovered virulence activities of the effector proteins AvrBs1, AvrBs3 and AvrBs4 using Agrobacterium-mediated transient expression of the corresponding genes in Nicotiana benthamiana, followed by microscopic analyses. We showed that, in addition to the nuclear-localized AvrBs3, the effector AvrBs1, which localizes to the plant cell cytoplasm, also induces a morphological change in mesophyll cells. Comparative analyses revealed that avrBs3-expressing plant cells contain highly active nuclei. Furthermore, plant cells expressing avrBs3 or avrBs1 show a decrease in the starch content in chloroplasts and an increased number of vesicles, indicating an enlargement of the central vacuole and the cell wall. Both AvrBs1 and AvrBs3 cause an increased ion efflux when expressed in N. benthamiana. By contrast, expression of the avrBs3 homologue avrBs4 leads to large catalase crystals in peroxisomes, suggesting a possible virulence function of AvrBs4 in the suppression of the plant defence responses. Taken together, our data show that microscopic inspection can uncover subtle and novel virulence activities of type III effector proteins.

Plant pattern-recognition receptor FLS2 is directed for degradation by the bacterial ubiquitin ligase AvrPtoB

BACKGROUND

An important layer of active defense in plant immunity is the detection of pathogen-associated molecular patterns (PAMPs) mediated by cell-surface receptors. For the establishment of disease, pathogens depend on the ability to overcome PAMP perception and disable plant signaling pathways activated in response to PAMPs. Pattern recognition receptors (PRRs) are therefore prime targets for pathogen effectors. FLS2, its coreceptor BAK1, and EFR encode receptor-like kinases that play a role in immunity against bacterial pathogens.

RESULTS

Here, we report that virulence of Pseudomonas syringae pv tomato DC3000 (PtoDC3000) in Arabidopsis is enhanced through the action of its effector AvrPtoB, which promotes degradation of FLS2. We show that AvrPtoB, through its N terminus, associates with FLS2 and BAK1, of which interaction with FLS2 is enhanced by flg22 activation. In vitro, AvrPtoB is active as an E3 ligase to catalyze polyubiquitination of the kinase domain of FLS2, a process confirmed in planta. Full enhancement of PtoDC3000 virulence appears to require the E3 ligase activity of AvrPtoB.

CONCLUSIONS

AvrPtoB, initially identified through its activation of hypersensitive resistance in tomato cultivars expressing the Pto kinase, is composed of at least two functional domains: the N terminus is responsible for interaction with Pto, and the C terminus carries an E3 ligase activity. Based on our findings, we propose that both domains of AvrPtoB act together to support the virulence of PtoDC3000 in Arabidopsis through their ability to eliminate FLS2 from the cell periphery, and probably also other PAMP sensors that are constitutively expressed or induced after pathogen challenge.

CorR regulates multiple components of virulence in Pseudomonas syringae pv. tomato DC3000

The phytotoxin coronatine (COR) is produced by various pathovars of Pseudomonas syringae, including P. syringae pv. tomato DC3000, which is pathogenic on crucifers and tomato, and P. syringae pv. glycinea PG4180, a soybean pathogen. The COR molecule contains two distinct components, coronafacic acid (CFA) and coronamic acid (CMA), which are intermediates in the COR biosynthetic pathway. In P. syringae pv. tomato DC3000, it is not clear whether corR, which encodes a response regulator, positively regulates CFA and CMA synthesis as it does in P. syringae pv. glycinea PG4180. In this study, a corR mutant of P. syringae pv. tomato DC3000 was constructed and was shown to be defective in the production of COR, CFA, and CMA. Furthermore, disease severity was greatly reduced in tomato plants inoculated with the corR mutant compared with wild-type P. syringae pv. tomato DC3000. We also showed that a mutation in hrpL, which encodes an alternate RNA polymerase sigma factor (sigmaL) required for the expression of genes encoding components of the type III secretion system, abrogated production of COR in P. syringae pv. tomato DC3000. The presence of a potential hrp box, the recognition site for sigmaL, upstream of corR suggested that corR might be regulated by hrpL. This was confirmed in reverse-transcription polymerase chain reaction experiments showing that the upstream effector gene holPtoAA, which was associated with the hrp box, was cotranscribed with corR. Furthermore, studies also were conducted to investigate whether mutations in corR had effects on the expression of hrpL. The corR mutant of P. syringae pv. tomato DC3000 showed both a reduction and delay in the expression of hrpL and was impaired in its ability to elicit a hypersensitive response on Nicotiana benthamiana. A putative CorR-binding site was identified upstream of hrpL, and gel shift studies confirmed the binding of CorR to this region. These results indicate that corR directly impacts the expression of the hrp regulon in P. syringae.