However, mechanisms underlying this protection are still poorly understood. Under standard conditions, our results indicated that Bp PsJN inoculation led to growth promotion of Arabidopsis plants without significant modification on photosynthesis parameters and chloroplast organization. However, bacterial colonization induced a cell wall strengthening in the mesophyll. Following low temperatures stress, a decrease of photosynthesis parameters was observed.

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Essaid A. Artois, Univ. The genus Burkholderia includes at least 90 species including pathogenic strains, plant pathogens, as well as plant beneficial species as those related to Paraburkholderia, which has been reported to be associated with plants and exerts a positive effect on plant growth and fitness.

Paraburkholderia phytofirmans PsJN, a beneficial endophyte able to colonize a wide range of plants, is an established model for plant-associated endophytic bacteria. Indeed, in addition to its plant growth promoting ability, it can also induce plant resistance against biotic as well as abiotic stresses. Here, we summarized an inventory of knowledge on PsJN-plant interaction, from the perception to the resistance mechanisms induced in the plant by a way of the atypical colonization mode of this endophyte.

We also have carried out an extensive genome analysis to identify all gene clusters which contribute to the adaptive mechanisms under different environments and partly explaining the high ecological competence of P. Introduction Historically, plant diseases have been controlled by the application of chemical pesticides, commonly leading to residual contamination and pathogen resistance Couderchet, Therefore, the use of plant-associated bacteria has shown a great promise for controlling diseases and thereby reducing the use of agrochemicals in the agriculture Pieterse et al.

Plant growth-promoting bacteria PGPB are naturally associated with plant roots and confer positive effects to host plants Shameer and Prasad, Among PGPB, endophytes are defined as those bacteria able to colonize the internal tissue of the plant without causing negative effects or external symptoms of infection on their host Compant et al. They can enter the plant via different sites including tissue wounds Agarwal, , stomata Roos and Hattingh, , penetration of root hairs Huang, and secretion of cell wall degradative enzymes Huang, ; Reinhold-Hurek et al.

Endophytes, some of which are belonged to the common soil bacteria, such as Bacillus, Pseudomonas, and Burkholderia, are applied to different plant species to promote their growth and control their diseases Hardoim et al.

Some members of Burkholderia have attracted a great deal of interest in biotechnology such as plant growth promotion, bioremediation, and biocontrol of plant diseases Depoorter et al. Among Burkholderia strains, Paraburkholderia phytofirmans strain PsJN is a Gram-negative rod-shaped, non-sporulating and motile bacterium.

This bacterium was first isolated from surface-sterilized onion roots infected with the mycorrhizal fungus Glomus vesiculiferum Frommel et al.

It has been recently classified as a member of Paraburkholderia, a group of formerly named Burkholderia species, mostly reported to be associated with plants and have biocontrol and bioremediation properties Sawana et al. Furthermore, P. These mechanisms act either directly, by providing adequate plant nutrition, and producing plant hormones, or indirectly, by reducing susceptibility to diseases Yang et al.

The bacterium is also able to decrease the ethylene level in host plants through production of the 1- aminocyclopropanecarboxylate ACC deaminase enzyme Glick et al. Moreover, it confers plants resistance against a broad spectrum of phytopathogens by the induction of plant-mediated resistance response in above ground parts of plants Miotto-Vilanova et al.

The strain also has been shown to induce tolerance toward different abiotic stresses including high temperature, cold, drought, and salinity Bensalim et al. The complete genome sequences 8. Previous comparative analysis of this strain revealed numerous biosynthetic gene clusters, secretion system, and metabolic potentials involved in endophytic behavior with diverse beneficial effects Mitter et al. Compared to other PGPB, less information is available about the mechanisms attributing to biocontrol effect and endophyte lifestyle of P.

Here, together with previously published data, we summarize an inventory of knowledge on P. Gene clusters contribution to the adaptive mechanisms and beneficial effects on plant growth and biocontrol under different environments are also highlighted. Perception of P. A successful colonization of plant tissue by beneficial bacteria is influenced by the excretion of organic acids by their host plant Kost et al. It has been reported that plant roots secrete a significant mixture of organic compounds known as exudates, which attract complex microbial populations present in the rhizosphere and initiate the first communication between host plants and endophytes Kawasaki et al.

The contribution of carbon sources in the recruitment of endophytic strain PsJN by host plants has been reported Kost et al. Microbial quorum sensing molecules presented by acylated homoserine lactones AHLs have also been shown to act as targets for host recognition and are likely implicated in communication with host plants, and subsequently colonization process Mathesius et al.

Moreover, endophytic bacteria move toward the plant roots, using chemotactic affinity for root exudates, and subsequently followed by adhesion to the plant root surfaces. The attachment of endophytes to the root surfaces can be enhanced through the production of cellulose or exopolysaccahrides EPS by bacteria Kandel et al. Furthermore, genome mining of endophytic strain PsJN revealed that many gene clusters involved in motility, biofilm production, adhesion and genes encoding for chemotactic activity and siderophore synthesis may reflect the efficient plant colonization and endophytic lifestyle of this bacterium Mitter et al.

In response to microbial perception, plants evolve different strategies to recognize and respond to microbial signal exposure Pieterse et al. Therefore, endophytes have developed strategies to minimize the plant immunity by secretion of proteins called effectors that cross the first layer of defense and suppress the PTI signaling or avoid the recognition by the host.

In addition, plants have a second layer of perception in which resistance receptors mediate the recognition of effectors compounds leading to effectors-triggered immunity ETI which plays an essential role to control the pathogen progress Pieterse et al. Plant perception of endophytic strain PsJN begins after an exposure to bacterial signals such as flagellin leading to the plant response characterized by early and long-term responses in plant immunity and plant growth regulation and morphogenesis Bordiec et al.

It has been shown that P. However, no significant accumulation of H2O2 neither cell death was observed in grapevine after P. Plant phytohormones play a crucial role as key regulators in plant defense-signaling pathways Pieterse et al. The expression levels of related defense genes after endophyte perception is a key point as endophytes are firstly known as potential invaders. Therefore, active interference with the plant immune responses is essential for the initiation of a compatible relationship with the host plant.

In case of strain PsJN, the low level of induced defense genes in the grapevine may give insight into the ability of this strain to colonize the rhizoplane and to transfer into the entire tissue leading to the endophytic behavior of this bacterium Bordiec et al.

Moreover, the distribution of different secretion system gene clusters Figure 1 in the PsJN genome might play a crucial role in the plant-P.

Distribution of secretion systems in the genome of Paraburkholderia phytofirmans PsJN. T3SS and T4SS clusters are located in chromosome 2 position — and —, respectively. T6SS is represented by two clusters located on chromosomes 1 and 2 position — and —, respectively.

Secretion systems in PsJN might inject effector proteins which cross the cell wall and might act on host plant to modulate plant signal transduction and elicit host defense responses. Plant colonization by endophytes can occur either in intracellular or intercellular spaces in plant tissues.

The colonization depends on the strain and different colonization routes and specific interactions have been described James et al. Some endophytic bacteria are able to penetrate the endodermis, which represents an obstacle for colonization, and inhabits the internal plant compartment Compant et al. One of the most frequently raised questions related to endophytic bacteria is how do they enter plant tissue?

In the early stages of colonization, P. The secretion of root exudates by the plant initiates the first communication between endophytes and host plants Kandel et al. The attachment of endophytes to the root surfaces is considered the first colonization step which is essential in getting access to the main entry points. The production of EPS by endophytes as well as the presence of bacterial flagella and cell surface polysaccharides may help the adhesion of endophytes onto the host roots and may facilitate the colonization process Kandel et al.

Depending on the strain, endophytes can get into plant tissues through wounds, stomata, and hydathodes, which are considered as the main entry sites Hardoim et al.

The emergence zone of secondary roots and injuries constitute also a natural opening allowing the entry of the endophytes inside the plant Kandel et al. Moreover, some bacterial endophytes deploy a wide range of catabolic activities that allow them to break down different selections of organic compounds and modify the plant cell wall compositions Preston, ; Taghavi et al.

The PsJN genome harbors a total of 41 putative plant polymer genes, encoding for putative hydrolytic enzymes Mitter et al. Among them, 14 genes represent glycoside hydrolyses GH that are involved in cell wall and sugar metabolism. In silico analysis of the PsJN genome also revealed the presence of genes involved in malonate metabolizing such as malonate decarboxylase which is important for symbiosis between endophytes and the plant Kim, Genes related to cupin superfamily involved in the modification of plant cell wall carbohydrates Dunwell et al.

Furthermore, it has been reported that bacteria, mostly related to plant pathogens, produce different extracellular enzymes or cell wall degrading factors such as cellulases, hemicellulases, and endoglucanases which enable endophytes to penetrate plant cell wall and colonize the interspatial region between plant cells Taghavi et al. In the course of a search of the PsJN genome, one gene cluster Figure 2A responsible for bacterial cellulose biosynthesis has been found in the chromosome 2 and gene encoding for endo 1, 4 glucanase was found as a part of the cluster.

Beside the cluster for cellulose synthesis, another one involved in pectin degradation was also found Figure 2B. The latter includes genes coding for polygalcturonate and galactarate dehydratase, which are involved in the degradation of pectin and could play a crucial role in colonizing the interspatial part between plant cells Taghavi et al. Biosynthetic genes clusters located in chromosome 2 of P. Phytofirmans PsJN genome.

Paraburkholderia phytofirmans PsJN establishes rhizosphere and endophytic colonization in different plants such as potato Bensalim et al. The bacterium colonizes the grapevine rhizoplane immediately after the inoculation and transmits to the root interior 3 h after inoculation and then systemically migrates from the rhizoplane to aerial tissues Compant et al.

It has been shown that the highest level of bacterial populations on grapevine rhizoplane, and aerial parts achieved at 24 and 84 h after inoculation, respectively, and the colonization level in leaves was significantly greater than stem Compant et al. Compant et al. Furthermore, Mitter et al. In potato plantlets, strain PsJN was observed in the first epidermal layers of roots and in the xylem tissue of stem Frommel et al.

In maize, under normal conditions, strain PsJN was observed in root and shoot interior and the maximum population density CFU g-1 dry weight reached 5. The ability of PsJN to produce EPS and form a biofilm as well as the presence of genes implicated in the biosynthesis of flagella, cellulose and genes encoding for chemotactic activity in the genome of PsJN may reflect the efficient endophytic colonization of the plant by this bacterium Mitter et al.

Moreover, the capacity of strain PsJN to colonize different plant hosts is probably due to large genome size 8. The endophytic P. The different beneficial effects of strain PsJN on different host plants are reported in Table 1. The mechanisms behind the observed positive effect are linked to the production of plant phytohormones, ACC deaminase, siderophores and other secondary metabolites which contribute as signaling molecules for better bacteria-plant communication leading to an efficient colonization of plant roots Sun et al.

Beneficial effects provided by the endophytic strain Paraburkholderia phytofirmans PsJN on different plants. Indeed, ACC prevents ethylene signaling by cleaving the ethylene precursor ACC deaminase to ammonia and 2-oxobutanoate. Moreover, the ACC deaminase-expressing bacteria were reported to enhance the plant growth under different biotic and abiotic stresses, including pathogen attack, drought, salinity, organic and inorganic contaminants Glick, The plant growth promoting effect in tomato associated with the strain PsJN was suggested to be linked to the expression of ACC deaminase, which plays a crucial role to enhance the growth performance of tomato plants Onofre-Lemus et al.

Sun et al. Complete genome analysis of strain PsJN revealed the presence of relevant genes involved in the indoleacetamide and the tryptophan side chain oxidase pathways Weilharter et al. Furthermore, this study demonstrated that the different plant growth parameters plant height and biomass, photosynthesis, and chlorophyll content of maize were significantly improved when applying PsJN inoculum supplemented with L-TRP.

The ability of strain PsJN to produce and degrade IAA as a sole carbon source gave an insight into the ability of this bacterium to resist under different stresses and lead to understand its friendly interaction with host plant Donoso et al.

The availability of iron in the environment to living microbes is very low due to its poor solubility at neutral pH 7 Andrews et al. Therefore, bacteria including endophytes have evolved several pathways including siderophore with high affinity to scavenge and transport iron from the environments Loaces et al. Beside their ability to produce siderophores, some endophytes have membrane receptor proteins for the uptake of siderophores produced by other endophytes Cornelis and Bodilis, The ability of endophytes to produce or capture siderophores, under iron stress condition, is one the most traits which provide iron to host plants Johnson et al.

It also contributes to protect host plant against phytopathogenic infection Miethke and Marahiel, , to activate the ISR Van Loon et al. The main mechanism of siderophore synthesis is achieved through modular megaenzymes called non-ribosomal peptides synthetases NRPSs , and others are assembled by various enzymes known as NRPS independent siderophore NIS Challis, NRPSs organized in modules, arranged in sets of primary domains including adenylation A , thiolation T , condensation C , and thioesterase TE domains, which synthesize basic peptides.

Moreover, secondary domains such as epimerization E are also contributing in the modifying peptides into more structurally complex peptides Esmaeel et al.



Search Menu Abstract Plant growth-promoting rhizobacteria PGPR are beneficial microorganisms that colonize the rhizosphere of many plant species and confer beneficial effects, such as an increase in plant growth. PGPR are also well known as inducers of systemic resistance to pathogens in plants. However, the molecular mechanisms involved locally after direct perception of these bacteria by plant cells still remain largely unknown. Burkholderia phytofirmans strain PsJN is an endophytic PGPR that colonizes grapevine and protects the plant against the grey mould disease caused by Botrytis cinerea.


Expression of flowering key regulator genes after inoculation of Arabidopsis with Burkholderia phytofirmans PsJN. A single event of inoculation during the germination phase had effects on growth parameters during the early and late stages of plant development. The first observable effects of bacteria in plants were changes in root length and a higher number and length of root hairs. Also, higher plant weight and chlorophyll content was found in these inoculated plants. Different initial bacterial concentrations were tested, inducing different levels of plant responses.


Strain PsJN was chronologically detected on the root surfaces, in the endorhiza, inside grape inflorescence stalks, not inside preflower buds and flowers but rather as an endophyte inside young berries. Data demonstrated low endophytic populations of strain PsJN in inflorescence organs, i. Nevertheless, endophytic colonization of inflorescences by strain PsJN was substantial for some plants. Microscopic analysis revealed PsJN as a thriving endophyte in inflorescence organs after the colonization process. Strain PsJN was visualized colonizing the root surface, entering the endorhiza and spreading to grape inflorescence stalks, pedicels and then to immature berries through xylem vessels. In parallel to these observations, a natural microbial communities was also detected on and inside plants, demonstrating the colonization of grapevine by strain PsJN in the presence of other microorganisms.

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