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Transverse section of an olive twig 12 d.p.i., showing: cambium(c); differentiated new cambia (nc), in cortical parenchyma and phloem parenchyma, secondary xylem (sx); and centrifugally oriented xylem elements (x) (insert). Bar = 10µm.
Source publication
SUMMARY The development of the olive knot disease was stud- ied taking into consideration the anatomical changes in the different parts of inoculated twig tissues, the genesis of knots and the defense reactions of the plant. At the inoculation site, bacterial inoculum invaded the cortical parenchyma, moving into the intercellular spaces and degradi...
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Context 1
... cells in contact with bacterial cavities were also degraded. Oc- tonuclear coenocytic divisions of pith cells (Fig. 3B) and differentiation of parenchyma cells into xylem elements were observed, adjacent to bacterial cavities, Cell walls of parenchyma cells around bacterial cavities became ligni- fied. New cambia, independent of the cambium (Fig. 5) were produced by periclinal division of dedifferentiated Journal of Plant Pathology (2008) parenchyma cells of the cortex, phloem, xylem, and parenchyma ray cells. New cambia were also produced by dedifferentiated cells of new tissue masses. The new cambia produced xylem elements, some of which were not centripetally oriented, in ...
Context 2
... division of dedifferentiated Journal of Plant Pathology (2008) parenchyma cells of the cortex, phloem, xylem, and parenchyma ray cells. New cambia were also produced by dedifferentiated cells of new tissue masses. The new cambia produced xylem elements, some of which were not centripetally oriented, in contrast to those of the xylem of the twig (Fig. 5, insert). Fifteen d.p.i., the knot continued to develop because of the hyperplastic activity of parenchyma cells of differ- ent tissues, cortex and phloem in particular. New cam- bia became more abundant. Thirty d.p.i., the cambium extended towards the ex- ternal side of the knot, producing secondary xylem and phloem tissues close to the knot ...
Citations
... The pathogen colonizes the apoplast and produces phytohormones playing an active role in disease development (Surico et al., 1985). The virulence of the bacterium is closely related to the bacterial secretion of indole-3-acetic acid (IAA) and cytokinins at the site of infection stimulating olive cells to divide and develop new tissue and overgrowths (Surico et al., 1985;Powell & Morris, 1986;Glass & Kosuge, 1988;Temsah et al., 2008;Rodríguez-Moreno et al., 2009;Quesada et al., 2012;Ramos et al., 2012). Presence of the hrp/hrc gene cluster in the P. savastanoi pv. ...
... Besides type III and IV secretion systems, quorum sensing (QS) regulation, and some other metabolic adaptation mechanisms, phytohormones are among the outstanding virulence determinants of P. savastanoi pv. savastanoi (Surico et al., 1985;Powell & Morris, 1986;Glass & Kosuge, 1988;Sisto et al., 2004;Temsah et al., 2008;Rodríguez-Moreno et al., 2009;Perez-Martinez et al., 2010;Matas et al., 2012;Quesada et al., 2012;. In this study, the isolates produced significantly different amounts of IAA (Fig. 3). ...
Aim of study: To evaluate the virulence and indole-3-acetic acid (IAA) biosynthesis ability of several Turkish P. savastanoi pv. savastanoi isolates and the susceptibility of some native genotypes to olive knot.
Area of study: The Aegean, Marmara, and Mediterranean Regions of Turkey.
Material and methods: 101 isolated bacteria were identified on the basis of biochemical, PCR for amplification of the bacterial iaaL gene, and pathogenicity tests. The virulence of the isolates was determined in a randomized experimental trial carried out by stem inoculation of pot-grown seedlings of olive (cv. ‘Manzanilla’) in the growing chamber. The amounts of IAA produced by the isolates were determined colorimetrically. The susceptibility of native olive genotypes was evaluated on 2-yr old plants inoculated with two distinct strains.
Main results: Tested P. savastanoi pv. savastanoi isolates showed significant differences in virulence found to be associated with their geographical origin. The isolates produced IAA amounts varied from 148.67 to 0.3 μg mL-1. The geographical variation in IAA biosynthesis ability of the isolates was observed. No correlation (R=0.0225) was determined between virulence and IAA amounts of the isolates. Native olive genotypes indicated different susceptibility levels to the olive knot pathogen. No genotype tested had complete resistance. However, low susceptible genotypes (‘Memecik’, ‘Ayvalık’ and ‘Uslu’) were identified. Some genotypes had variable reactions depending on the isolate used.
Research highlights: The results undergird the differences in the virulence and IAA production of the isolates within the area and also between geographical locations. Genotypes with low susceptibility can be used as genitors in further breeding studies.
... This effect has been reported to be a consequence of the cooperation and competition of Pseudomonas species with other microorganisms 31,34 . Endophytically, Pss cells are organized in clusters, forming also biofilm layers 35 . The formation of this biofilm provides several advantages to certain bacteria, such as social cooperation, resource capture and protection from antimicrobials 36 , which may explain the increase of bacterial abundance in OK symptomatic twigs. ...
Plant-inhabiting microorganisms interact directly with each other affecting disease progression. However, the role of host plant and plant habitat in shaping pathobiome composition and their implication for host susceptibility/resistance to a particular disease are currently unknown. For the elucidation of these questions, both epiphytic and endophytic bacterial communities, present in asymptomatic and symptomatic twigs from olive cultivars displaying different susceptibilities to olive knot (OK) disease, were investigated using culturing methods. OK disease was the main driver of the bacterial community, causing changes on their diversity, abundance and composition. OK disease effect was most notorious on OK-susceptible cultivar and when considering the endophytic communities. Plant habitat (epiphytes vs. endophytes) also contributed to the bacterial community assembling, in particular on symptomatic twigs (knots) of OK-susceptible cultivar. In contrast, host cultivar had little effect on the bacterial community composition, but OK-symptomatic twigs (knots) revealed to be more affected by this driver. Overall, the pathobiome seems to result from an intricate interaction between the pathogen, the resident bacteria, and the plant host. Specific bacterial genera were associated to the presence or absence of OK disease in each cultivar. Their ability to trigger and/or suppress disease should be studied in the future.
... It grows, in vitro, at 1 to 32°C, with an optimum of 26°C. In vivo, it proliferates very well at room temperature with 70 to 100% of relative humidity ) Temsah et al., 2008). The minimal temperatures of 18-22°C are essential for the onset of the disease (Iannotta et al., 2007). ...
... Psav survives as an epiphyte in the phyllosphere penetrating its host through wounds (Lavermicocca and Surico, 1987). Once inside host plants, the bacterium colonizes the apoplast and due to its ability to secrete the plant hormones indole-3-acetic acid (IAA) and cytokinins, it stimulates olive cells to produce new tissue giving rise to knot development and tissue overgrowth (Glass and Kosuge, 1988;Powell and Morris, 1986;Quesada et al., 2012;Ramos et al., 2012;Rodríguez-Moreno et al., 2008;Surico et al., 1985;Temsah et al., 2008). The switch from an epiphytic to endophytic (apoplastic) life style is an abrupt transition for the bacterium that requires: (i) a remarkable adaptation to an environment that is extremely different in pH, osmotic pressure, carbon sources and oxygen availability, (ii) the ability to suppress basal and induced plant defences (Rico et al., 2009). ...
... In fact, the presence of glucose, fructose, sucrose or ATP inhibited calcium entry entirely. Thus far, Psav virulence was largely linked to the bacterial secretion of the phytohormones IAA and cytokinins at the site of infection, which stimulates olive cell activity to produce new tissue and gives rise to knot development (Glass and Kosuge, 1988;Powell and Morris, 1986;Rodríguez-Moreno et al., 2008;Quesada et al., 2012;Surico et al., 1985;Temsah et al., 2008). Our results demonstrate that the presence of L-tryptophan or IAA does not alter Ca 2+ entry into the Psav cells, suggesting that there is no feedback regulation by the auxin pathway during infection. ...
In a number of compatible plant-bacterium interactions, a rise in apoplastic Ca2+ levels is observed, suggesting that Ca2+ represents an important environmental clue, as reported for bacteria infecting mammalians. We demonstrate that Ca2+ entry in Pseudomonas savastanoi pv. savastanoi (Psav) strain DAPP-PG 722 is mediated by a Na+/Ca2+ exchanger critical for virulence. Using the fluorescent Ca2+ probe Fura 2-AM, we demonstrate that Ca2+ enters Psav cells foremost when they experience low levels of energy, a situation mimicking the apoplastic fluid. In fact, Ca2+ entry was suppressed in the presence of high concentrations of glucose, fructose, sucrose or ATP. Since Ca2+ entry was inhibited by nifedipine and LiCl, we conclude that the channel for Ca2+ entry is a Na+/Ca2+ exchanger. In silico analysis of the Psav DAPP-PG 722 genome revealed the presence of a single gene coding for a Na+/Ca2+ exchanger (cneA), which is a widely conserved and ancestral gene within the P. syringae complex based on gene phylogeny. Mutation of cneA compromised not only Ca2+ entry, but also compromised the Hypersensitive response (HR) in tobacco leaves and blocked the ability to induce knots in olive stems. The expression of both pathogenicity (hrpL, hrpA and iaaM) and virulence (ptz) genes was reduced in this Psav-cneA mutant. Complementation of the Psav-cneA mutation restored both Ca2+ entry and pathogenicity in olive plants, but failed to restore the HR in tobacco leaves. In conclusion, Ca2+ entry acts as a ‘host signal’ that allows and promotes Psav pathogenicity on olive plants.
... Psav survives as an epiphyte in the phyllosphere penetrating its host through wounds (Lavermicocca and Surico, 1987). Once inside host plants, the bacterium colonizes the apoplast and due to its ability to secrete the plant hormones indole-3-acetic acid (IAA) and cytokinins, it stimulates olive cells to produce new tissue giving rise to knot development and tissue overgrowth (Glass and Kosuge, 1988;Powell and Morris, 1986;Quesada et al., 2012;Ramos et al., 2012;Rodríguez-Moreno et al., 2008;Surico et al., 1985;Temsah et al., 2008). The switch from an epiphytic to endophytic (apoplastic) life style is an abrupt transition for the bacterium that requires: (i) a remarkable adaptation to an environment that is extremely different in pH, osmotic pressure, carbon sources and oxygen availability, (ii) the ability to suppress basal and induced plant defences (Rico et al., 2009). ...
... In fact, the presence of glucose, fructose, sucrose or ATP inhibited calcium entry entirely. Thus far, Psav virulence was largely linked to the bacterial secretion of the phytohormones IAA and cytokinins at the site of infection, which stimulates olive cell activity to produce new tissue and gives rise to knot development (Glass and Kosuge, 1988;Powell and Morris, 1986;Rodríguez-Moreno et al., 2008;Quesada et al., 2012;Surico et al., 1985;Temsah et al., 2008). Our results demonstrate that the presence of L-tryptophan or IAA does not alter Ca 2+ entry into the Psav cells, suggesting that there is no feedback regulation by the auxin pathway during infection. ...
In a number of compatible plant-bacterium interactions, a rise in apoplastic Ca2+ levels is observed, suggesting that Ca2+ represents an important environmental clue, as reported for bacteria infecting mammalians. We demonstrate that Ca2+ entry in Pseudomonas savastanoi pv. savastanoi (Psav) strain DAPP-PG 722 is mediated by a Na+/Ca2+ exchanger critical for virulence. Using the fluorescent Ca2+ probe Fura 2-AM, we demonstrate that Ca2+ enters Psav cells foremost when they experience low levels of energy, a situation mimicking the apoplastic fluid. In fact, Ca2+ entry was suppressed in the presence of high concentrations of glucose, fructose, sucrose or ATP. Since Ca2+ entry was inhibited by nifedipine and LiCl, we conclude that the channel for Ca2+ entry is a Na+/Ca2+ exchanger. In silico analysis of the Psav DAPP-PG 722 genome revealed the presence of a single gene coding for a Na+/Ca2+ exchanger (cneA), which is a widely conserved and ancestral gene within the P. syringae complex based on gene phylogeny. Mutation of cneA compromised not only Ca2+ entry, but also compromised the Hypersensitive response (HR) in tobacco leaves and blocked the ability to induce knots in olive stems. The expression of both pathogenicity (hrpL, hrpA and iaaM) and virulence (ptz) genes was reduced in this Psav-cneA mutant. Complementation of the Psav-cneA mutation restored both Ca2+ entry and pathogenicity in olive plants, but failed to restore the HR in tobacco leaves. In conclusion, Ca2+ entry acts as a ‘host signal’ that allows and promotes Psav pathogenicity on olive plants.
... In conclusion, we showed that de novo vascularization during leafy gall development includes activation of the vascular meristem and parenchyma cell transdifferentiation, which are the same developmental mechanisms used during the vascularization of callus-like galls induced by A. tumefaciens (Aloni et al. 1995), P. savastanoi (Temsah et al. 2008), P. agglomerans pv. gypsophilae (Chalupowicz et al. 2006), and an auxin-producing bacterial consortium (Best et al. 2004). ...
Main conclusion:
Extensive de novo vascularization of leafy galls emerging upon Rhodococcus fascians infection is achieved by fascicular/interfascicular cambium activity and transdifferentiation of parenchyma cells correlated with increased auxin signaling. A leafy gall consisting of fully developed yet growth-inhibited shoots, induced by the actinomycete Rhodococcus fascians, differs in structure compared to the callus-like galls induced by other bacteria. To get insight into the vascular development accompanying the emergence of the leafy gall, the anatomy of infected axillary regions of the inflorescence stem of wild-type Arabidopsis thaliana accession Col-0 plants and the auxin response in pDR5:GUS-tagged plants were followed in time. Based on our observations, three phases can be discerned during vascularization of the symptomatic tissue. First, existing fascicular cambium becomes activated and interfascicular cambium is formed giving rise to secondary vascular elements in a basipetal direction below the infection site in the main stem and in an acropetal direction in the entire side branch. Then, parenchyma cells in the region between both stems transdifferentiate acropetally towards the surface of the developing symptomatic tissue leading to the formation of xylem and vascularize the hyperplasia as they expand. Finally, parenchyma cells in the developing gall also transdifferentiate to vascular elements without any specific direction resulting in excessive vasculature disorderly distributed in the leafy gall. Prior to any apparent anatomical changes, a strong auxin response is mounted, implying that auxin is the signal that controls the vascular differentiation induced by the infection. To conclude, we propose the "sidetracking gall hypothesis" as we discuss the mechanisms driving the formation of superfluous vasculature of the emerging leafy gall.
... In conclusion, we showed that de novo vascularization during leafy gall development includes activation of the vascular meristem and parenchyma cell transdifferentiation, which are the same developmental mechanisms used during the vascularization of callus-like galls induced by A. tumefaciens (Aloni et al. 1995), P. savastanoi (Temsah et al. 2008), P. agglomerans pv. gypsophilae (Chalupowicz et al. 2006), and an auxin-producing bacterial consortium (Best et al. 2004). ...
Main conclusion:
Extensive de novo vascularization of leafy galls emerging upon Rhodococcus fascians infection is achieved by fascicular/interfascicular cambium activity and transdifferentiation of parenchyma cells correlated with increased auxin signaling. A leafy gall consisting of fully developed yet growth-inhibited shoots, induced by the actinomycete Rhodococcus fascians, differs in structure compared to the callus-like galls induced by other bacteria. To get insight into the vascular development accompanying the emergence of the leafy gall, the anatomy of infected axillary regions of the inflorescence stem of wild-type Arabidopsis thaliana accession Col-0 plants and the auxin response in pDR5:GUS-tagged plants were followed in time. Based on our observations, three phases can be discerned during vascularization of the symptomatic tissue. First, existing fascicular cambium becomes activated and interfascicular cambium is formed giving rise to secondary vascular elements in a basipetal direction below the infection site in the main stem and in an acropetal direction in the entire side branch. Then, parenchyma cells in the region between both stems transdifferentiate acropetally towards the surface of the developing symptomatic tissue leading to the formation of xylem and vascularize the hyperplasia as they expand. Finally, parenchyma cells in the developing gall also transdifferentiate to vascular elements without any specific direction resulting in excessive vasculature disorderly distributed in the leafy gall. Prior to any apparent anatomical changes, a strong auxin response is mounted, implying that auxin is the signal that controls the vascular differentiation induced by the infection. To conclude, we propose the "sidetracking gall hypothesis" as we discuss the mechanisms driving the formation of superfluous vasculature of the emerging leafy gall.
... A histological examination of the knots induced by P. savastanoi infections in different hosts reveals strong similarities among olive (Surico, 1977;Temsah et al., 2008;Rodríguez-Moreno et al., 2009), oleander (Wilson, 1965;Temsah et al., 2010), buckthorn (Temsah et al., 2007a), and myrtle (Temsah et al., 2007b). During the early stages of an infection, bacteria are localized in the intercellular spaces of the cortical parenchyma and in the vascular tissues damaged by the wound, where bacterial pectolytic and hemicellulolytic enzymatic activities cause cell wall degradation of adjacent plant cells, resulting in the plasmolysis of host cells and the generation of internal cavities. ...
The study of the molecular basis of tree diseases is lately receiving a renewed attention, especially with the emerging perception that pathogens require specific pathogenicity and virulence factors to successfully colonize woody hosts. Pathosystems involving woody plants are notoriously difficult to study, although the use of model bacterial strains together with genetically homogeneous micropropagated plant material is providing a significant impetus to our understanding of the molecular determinants leading to disease. The gammaproteobacterium Pseudomonas savastanoi belongs to the intensively studied Pseudomonas syringae complex, and includes three pathogenic lineages causing tumorous overgrowths (knots) in diverse economically relevant trees and shrubs. As it occurs with many other bacteria, pathogenicity of P. savastanoi is dependent on a type III secretion system, which is accompanied by a core set of at least 20 effector genes shared among strains isolated from olive, oleander, and ash. The induction of knots of wild-type size requires that the pathogen maintains adequate levels of diverse metabolites, including the phytohormones indole-3-acetic acid and cytokinins, as well as cyclic-di-GMP, some of which can also regulate the expression of other pathogenicity and virulence genes and participate in bacterial competitiveness. In a remarkable example of social networking, quorum sensing molecules allow for the communication among P. savastanoi and other members of the knot microbiome, while at the same time are essential for tumor formation. Additionally, a distinguishing feature of bacteria from the P. syringae complex isolated from woody organs is the possession of a 15 kb genomic island (WHOP) carrying four operons and three other genes involved in degradation of phenolic compounds. Two of these operons mediate the catabolism of anthranilate and catechol and, together with another operon, are required for the induction of full-size tumors in woody hosts, but not in non-woody micropropagated plants. The use of transposon mutagenesis also uncovered a treasure trove of additional P. savastanoi genes affecting virulence and participating in diverse bacterial processes. Although there is still much to be learned on what makes a bacterium a successful pathogen of trees, we are already untying the knots.
... eriobotrya, P. syringae pv. syringae, P. savastonoi and B. nigrifluens (M enard & Delort, 2004;Temsah et al., 2008). The related P. avellanae causes bacterial canker of European hazelnut (Scortichini, 2002), but was not included in the phylogenetic tree. ...
A survey of bleeding canker disease, caused by Pseudomonas syringae pv. aesculi, was undertaken across Ireland. Incidence has become severe and can be considered epidemic, as 61% of the 1,587 trees surveyed showed symptoms of the disease. Bacteria were isolated from a sample of trees and characterised using gyrB DNA sequencing. DNA was also extracted directly from wound tissue. The Irish P. syringae pv. aesculi genotype was identical to genotypes previously sequenced with gyrB from the UK and some other locations in Europe. Real-time PCR using existing primers and a more pathovar specific primer set, designed here, was assessed for disease screening. A total of 11 trees from a sample of 55 tested positive for P. syringae pv. aesculi in Ireland with molecular screening. It was more efficient to extract DNA directly from wound tissue, especially fresh bark, for disease detection than to undertake bacterial isolation with subsequent molecular analysis. A further set of sequencing primers was developed for the amplification of the gyrB gene from P. syringae pv. aesculi and their specificity shown using a diverse sample of bacterial isolate DNAs. The study also isolated and identified other bacterial species from diseased material. Some of these are known pathogens (Brenneria nigrifluens, P. marginalis and P. syringae) or have previously been identified as potentially beneficial endophytes of host trees (Erwinia billingiae, E. tolentana, P. fluorescens, P. putida and Raoultella). This article is protected by copyright. All rights reserved.
... Among the structural induced defenses, Temsah et al. (2008) observed lignin deposits on cell walls of parenchyma olive cells around cavities and injured tissues caused by Psv. They also reported that new periderm was produced at the surface of the olive knots, which constitutes a barrier impairing the entrance of other pathogens and saprophytes. ...
There is an increasing interest in studying interspecies bacterial interactions in diseases of animals and plants as it is believed that the great majority of bacteria found in nature live in complex communities. Plant pathologists have thus far mainly focused on studies involving single species or on their interactions with antagonistic competitors. A bacterial disease used as model to study multispecies interactions is the olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi (Psv). Knots caused by Psv in branches and other aerial parts of the olive trees are an ideal niche not only for the pathogen but also for many other plant-associated bacterial species, mainly belonging to the genera Pantoea, Pectobacterium, Erwinia, and Curtobacterium. The non-pathogenic bacterial species Erwinia toletana, Pantoea agglomerans, and Erwinia oleae, which are frequently isolated inside the olive knots, cooperate with Psv in modulating the disease severity. Co-inoculations of these species with Psv result in bigger knots and better bacterial colonization when compared to single inoculations. Moreover, harmless bacteria co-localize with the pathogen inside the knots, indicating the formation of stable bacterial consortia that may facilitate the exchange of quorum sensing signals and metabolites. Here we discuss the possible role of bacterial communities in the establishment and development of olive knot disease, which we believe could be taking place in many other bacterial plant diseases.
