Pseudomonas syringae addresses distinct environmental challenges during plant infection through the coordinated deployment of polysaccharides

Prior to infection, phytopathogenic bacteria face a challenging environment on the plant surface, where they are exposed to nutrient starvation and abiotic stresses. Pathways enabling surface adhesion, stress tolerance and epiphytic survival are important for successful plant pathogenesis. Understanding the roles and regulation of these pathways is therefore crucial to fully understand bacterial plant infections. The phytopathogen Pseudomonas syringae pv. tomato (Pst) encodes multiple polysaccharides that are implicated in biofilm formation, stress survival and virulence in other microbes. To examine how these polysaccharides impact Pst epiphytic survival and pathogenesis, we analysed mutants in multiple polysaccharide loci to determine their intersecting contributions to epiphytic survival and infection. In parallel, we used qRT-PCR to analyse the regulation of each pathway. Pst polysaccharides are tightly coordinated by multiple environmental signals. Nutrient availability, temperature and surface association strongly affect the expression of different polysaccharides under the control of the signalling proteins ladS and cbrB and the second messenger cyclic-di-GMP. Furthermore, functionally redundant, combinatorial phenotypes were observed for several polysaccharides. Exopolysaccharides and WapQ-mediated lipopolysaccharide production are important for leaf adhesion, while α-glucan and alginate together confer desiccation tolerance. Our results suggest that polysaccharides play important roles in overcoming environmental challenges to Pst during plant infection.

Biological role of EPS from Pseudomonas syringae pv. syringae UMAF0158 extracellular matrix, focusing on a Psl-like polysaccharide

Pseudomonas syringae is a phytopathogenic model bacterium that is used worldwide to study plant–bacteria interactions and biofilm formation in association with a plant host. Within this species, the syringae pathovar is the most studied due to its wide host range, affecting both, woody and herbaceous plants. In particular, Pseudomonas syringae pv. syringae (Pss) has been previously described as the causal agent of bacterial apical necrosis on mango trees. Pss exhibits major epiphytic traits and virulence factors that improve its epiphytic survival and pathogenicity in mango trees. The cellulose exopolysaccharide has been described as a key component in the development of the biofilm lifestyle of the P. syringae pv. syringae UMAF0158 strain (PssUMAF0158). PssUMAF0158 contains two additional genomic regions that putatively encode for exopolysaccharides such as alginate and a Psl-like polysaccharide. To date, the Psl polysaccharide has only been studied in Pseudomonas aeruginosa, in which it plays an important role during biofilm development. However, its function in plant-associated bacteria is still unknown. To understand how these exopolysaccharides contribute to the biofilm matrix of PssUMAF0158, knockout mutants of genes encoding these putative exopolysaccharides were constructed. Flow-cell chamber experiments revealed that cellulose and the Psl-like polysaccharide constitute a basic scaffold for biofilm architecture in this bacterium. Curiously, the Psl-like polysaccharide of PssUMAF0158 plays a role in virulence similar to what has been described for cellulose. Finally, the impaired swarming motility of the Psl-like exopolysaccharide mutant suggests that this exopolysaccharide may play a role in the motility of PssUMAF0158 over the mango plant surface.

Chemical Structure, Biological Roles, Biosynthesis and Regulation of the Yellow Xanthomonadin Pigments in the Phytopathogenic Genus Xanthomonas

Xanthomonadins are membrane-bound yellow pigments that are typically produced by phytopathogenic bacterial Xanthomonas spp., Xylella fastidiosa, and Pseudoxanthomonas spp. They are also produced by a diversity of environmental bacterial species. Considerable research has revealed that they are a unique group of halogenated, aryl-polyene, water-insoluble pigments. Xanthomonadins have been shown to play important roles in epiphytic survival and host-pathogen interactions in the phytopathogen Xanthomonas campestris pv. campestris, which is the causal agent of black rot in crucifers. Here, we review recent advances in the understanding of xanthomonadin chemical structures, physiological roles, biosynthetic pathways, regulatory mechanisms, and crosstalk with other signaling pathways. The aim of the present review is to provide clues for further in-depth research on xanthomonadins from Xanthomonas and other related bacterial species.

Epiphytic Survival of Pantoea ananatis on Richardia scabra L. in Georgia

Pantoea ananatis, the causal organism of center rot of onion (Allium cepa L.), can survive on different weeds but, in a previous survey, it was most commonly found on Florida pusley (Richardia scabra L.). The epiphytic survival of P. ananatis on R. scabra under different temperature and moisture regimes was investigated. Weed seedlings were spray inoculated with rifampicin-resistant strain PNA 97-1rif at either 103 or 108 CFU/ml and incubated in a growth chamber at 15.5 or 21.1°C at 65% relative humidity for 96 h postinoculation (hpi), which represented the mean environmental conditions during mid-March to mid-May in Vidalia, GA when onion production and R. scabra presence overlap. For plants inoculated with P. ananatis at 103 CFU/ml, the bacterium survived for 96 hpi when incubated at 21.1°C, with mean populations of 1.7 × 102 CFU/g of leaf tissue. In contrast, no viable bacteria were detected after 72 hpi at 15.5°C. For plants inoculated with P. ananatis at 108 CFU/ml, the bacterium survived for 96 hpi at 21.1°C (3.8 × 105 CFU/g) whereas, during the sample time period, viable bacterial populations were not detected at 15.5°C. Survival of P. ananatis on R. scabra was also monitored during alternating 12 h wet and 12 h dry periods, or continuous wet or dry periods for 96 hpi at 15.5 or 21.1°C. Compared with initial or continuous dry periods, P. ananatis survived significantly better with a 12 h wet/12 h dry cycle or a continuous 96 hpi wet period at both 15.5 and 21.1°C. Unlike at 15.5°C, P. ananatis populations (7.4 × 102 CFU/g) survived for 96 hpi at 21.1°C under a cycle of 12 h dry and 12 h wet. These results demonstrate that P. ananatis can survive on R. scabra leaves under conditions of 21.1°C and prolonged leaf wetness and may potentially serve as a source of inoculum to onion.

Distribution of Agrobacterium vitis in Grapevines and Its Relevance to Pathogen Elimination

Agrobacterium vitis, the cause of crown gall disease on grapevine, survives internally in vines and can be spread in cuttings for propagation. The possibility of generating pathogen-free vines through tissue culture makes it essential to understand the distribution of the pathogen in grapevines. A highly sensitive magnetic capture hybridization procedure along with real-time polymerase chain reaction were used to measure the distribution of tumorigenic A. vitis in dormant canes and green shoots of grapevines. Tumorigenic A. vitis was distributed from the basal to apical nodal and internodal tissues of canes as well as in nonlignified green shoots. In experiments conducted in 2013, A. vitis was detected in up to 17% of shoot tips and 52% of meristems of greenhouse-grown plants initiated from known A. vitis-contaminated cuttings. A lower frequency of detection was observed from surface-disinfected shoot tips (7%) as compared with nondisinfected tips (37%), suggesting epiphytic survival on green tissues. In 2014, vines propagated from cuttings collected from crown gall-infected vines from a different vineyard yielded lower incidences of A. vitis from shoot tips, and the bacterium was not detected in meristems. Tumorigenic A. vitis was also detected in cuttings of wild grapevines (Vitis riparia) that were collected both adjacent to and far removed from commercial vineyards.

Identification and Characterization of Two Novel DSF-Controlled Virulence-Associated Genes Within the nodB-rhgB Locus of Xanthomonas oryzae pv. oryzicola Rs105

Xanthomonas oryzae pv. oryzicola and X. oryzae pv. oryzae are two pathovars of X. oryzae that cause leaf streak and blight in rice, respectively. These two bacterial pathogens cause different disease symptoms by utilizing different infection sites on rice. Compared with X. oryzae pv. oryzae, the molecular virulence mechanism of X. oryzae pv. oryzicola remains largely unknown. Previously, we identified a unique diffusible signal factor (DSF)-controlled virulence-related gene (hshB) in X. oryzae pv. oryzicola Rs105 located in the nodB-rghB locus, which is absent in X. oryzae pv. oryzae PXO99A. In the present study, we identified two additional genes within this locus (hshA and hshC) that were unique to X. oryzae pv. oryzicola Rs105 compared with X. oryzae pv. oryzae PXO99A, and we found that the transcription of these genes was regulated by DSF signaling in X. oryzae pv. oryzicola. The mutation of these genes impaired the virulence of the wild-type Rs105 when using a low inoculation density of X. oryzae pv. oryzicola. In contrast to hshB, the mutation of these genes did not have any visible effect on characterized virulence-related functions, including in vitro growth, extracellular polysaccharide production, extracellular protease activity, and antioxidative ability. However, we found that mutation of hshA or hshC significantly reduced the in planta growth ability and epiphytic survival level of X. oryzae pv. oryzicola cells, which was the probable mechanisms of involvement of these two genes in virulence. Collectively, our studies of X. oryzae pv. oryzicola have identified two novel DSF-controlled virulence-associated genes (hshA and hshC), which will add to our understanding of the regulatory mechanisms of conserved DSF virulence signaling in Xanthomonas species.

Xanthomonas albilineans OmpA1 Appears to be Functionally Modular and Both the OMC and C-like Domains Are Necessary for Leaf Scald Disease of Sugarcane

Several EZ-Tn5 insertions in gene locus XALc_0557 (OmpA1) of the sugarcane leaf scald pathogen Xanthomonas albilineans XaFL07-1 were previously found to strongly affect pathogenicity and endophytic stalk colonization. XALc_0557 has a predicted OmpA N-terminal outer membrane channel (OMC) domain and an OmpA C-like domain. Further analysis of mutant M468, with an EZ-Tn5 insertion in the upstream OMC domain coding region, revealed impaired epiphytic and endophytic leaf survival, impaired resistance to sodium dodecyl sulfate (SDS), structural defects in the outer membrane (OM), and hyperproduction of OM vesicles. Cloned full-length XALc_0557 complemented M468 for all phenotypes tested, including pathogenicity, resistance to SDS, and ability to survive both endophytically and epiphytically. Another construct, pCT47.3, which expressed only the C-like domain of XALc_0557, restored resistance to SDS in M468 but failed to complement any other mutant phenotype, indicating that the C-like domain functioned independently of the OMC domain to help maintain OM integrity. pCT47.3 also complemented pathogenicity, resistance to SDS, and stalk colonization in mutant M1152, which carries an EZ-Tn5 insert in the C-like coding region, indicating that both predicted domains are modular and necessary but neither is sufficient for X. albilineans pathogenicity, endophytic survival in, and epiphytic survival on sugarcane.

Recent progress on detecting understanding and controlling Pseudomonas syringae pv actinidiae a short review

In the last few years the causal agent of bacterial canker of kiwifruit Pseudomonas syringae pv actinidiae (Psa) has become a global pathogen of economic importance Since the beginning of this global outbreak many laboratories in the world have been working on Psa Today it is known that Psa is not a homogeneous pathovar and tools that allow the distinction between biovars (subpathovar classification) have been developed The whole genome sequence of several strains of Psa has now been published Some of the assumptions on the life cycle (ports of entry epiphytic survival etc) made in the early days of the outbreak have now been confirmed Although few new methods have been found to control Psa there is now a better understanding of how to reduce the incidence of this disease This paper reviews the progress made in understanding the pathogen the disease and how to control it

Epiphytic Survival of Erwinia tracheiphila on Muskmelon (Cucumis melo L.)

Erwinia tracheiphila, the causal agent of bacterial wilt of cucurbits, is transmitted by striped (Acalymma vittatum) and spotted (Diabrotica undecimpunctata howardi) cucumber beetles. Transmission occurs when infested frass with E. tracheiphila is deposited on plant surfaces with fresh feeding wounds. However, it is unclear whether the pathogen can survive as an epiphyte on leaves. Experiments were conducted in controlled environments to monitor E. tracheiphila survival on muskmelon (Cucumis melo) leaves under various temperature and moisture conditions. In the first experiment, muskmelon seedlings that had been spray inoculated with a rifampicin-resistant strain of E. tracheiphila were incubated at 10, 15, 20, 25, 30, or 35°C (±2°C) at ≥95% relative humidity, and E. tracheiphila populations were monitored for 72 h. In the second experiment, E. tracheiphila was monitored during alternating 12-h wet and dry periods, or continuous wet or dry conditions for 48 h at 20°C. Survival of E. tracheiphila on wet muskmelon leaves depended on temperature (P < 0.01), with the greatest survival at 10 and 15°C and least at 30 and 35°C. Leaf wetness also impacted survival; an initial 12-h dry period resulted in a 1,000- to 10,000-fold reduction in population size, followed by stabilization of the surviving population. These results demonstrate that E. tracheiphila can survive on muskmelon leaves under a wide range of environmental conditions, suggesting that epiphytic populations might serve as a reservoir of inoculum for infections.