Identification of Up-regulated Genes of <Emphasis Type="Italic">Bacillus amyloliquefaciens</Emphasis> B55 During the Early Stage of Direct Surface Contact with Rice R109 Root

The early stage of plant–rhizobacteria interaction, affected by plant root exudates and plant–rhizobacteria surface contact, is considered to be critical for plant growth-promoting rhizobacteria colonizing plant roots and initiating the beneficial effects on plant growth. However, little is known about the mechanisms of plant–rhizobacteria surface contact involved in early stage of plant–rhizobacteria interaction. In order to reveal the molecular mechanisms of the surface contact, a rhizobacterium Bacillus amyloliquefaciens B55 was interacted with plant roots of rice R109 and used to perform a cDNA-based suppression-subtractive hybridization. Seven differentially expressed DNA fragments were identified. Except for the two fragments showing no matches to any known sequences in the Genbank, the other five fragments were found to have high homologies with the genes encoding 2-oxoglutarate dehydrogenase E1 component OdhA, aspartate ammonia-lyase AnsB, and hypothetical protein proposed to be involved in surface adhesion, acetolactate decarboxylase AlsD, and DNA mismatch repair protein MutL, respectively. The induced RNA expression levels of two putative genes ansB and odhA and an unmatched DNA fragment BD33 were verified by RT-PCR analysis.     

Development of a Root Colonization Bioassay for Rapid Screening of Rhizobacteria for Potential Biocontrol Agents

The ability to colonize roots is a sine qua non condition for a rhizobacteria to be considered a true plant growth‐promoting rhizobacteria (PGPR). A simple screening method to detect such a potential ability of PGPR is described. Tomato seeds were surface sterilized for 30 s in 50% ethanol and this was followed by 3 min dipping in 2% NaClO. They were then washed three times in sterile water, left immersed in a propagule suspension of the rhizobacteria for 24 h, and transferred onto sterile 0.6% water‐agar in tubes. The young, developing root system shows a tendency to grow downwards in the agar‐gel column. When the rhizobacterium has a potential ability to colonize roots it is possible to visualize, by transparency, bacterial growth (turbid, milky and narrow zone) along and around roots. Testing 500 rhizobacteria isolated from tomato rhizosphere for their ability to induce systemic resistance against Pseudomonas syringae pv. tomato, 28 of them did reduce infection to less than 40% and all 28 colonized roots according to the described bioassay. Therefore the bioassay may turn into an important auxiliary tool for helping in selecting rhizobacteria with PGPR potentiality.     

A Novel Interaction between Plant-Beneficial Rhizobacteria and Roots: Colonization Induces Corn Resistance against the Root Herbivore Diabrotica speciosa

A number of soil-borne microorganisms, such as mycorrhizal fungi and rhizobacteria, establish mutualistic interactions with plants, which can indirectly affect other organisms. Knowledge of the plant-mediated effects of mutualistic microorganisms is limited to aboveground insects, whereas there is little understanding of what role beneficial soil bacteria may play in plant defense against root herbivory. Here, we establish that colonization by the beneficial rhizobacterium Azospirillum brasilense affects the host selection and performance of the insect Diabrotica speciosa. Root larvae preferentially orient toward the roots of non-inoculated plants versus inoculated roots and gain less weight when feeding on inoculated plants. As inoculation by A. brasilense induces higher emissions of (E)-β-caryophyllene compared with non-inoculated plants, it is plausible that the non-preference of D. speciosa for inoculated plants is related to this sesquiterpene, which is well known to mediate belowground insect-plant interactions. To the best of our knowledge, this is the first study showing that a beneficial rhizobacterium inoculant indirectly alters belowground plant-insect interactions. The role of A. brasilense as part of an integrative pest management (IPM) program for the protection of corn against the South American corn rootworm, D. speciosa, is considered.     

Colonization behavior of bacterium Burkholderia cepacia inside the Oryza sativa roots visualized using green fluorescent protein reporter

Colonization behavior of endophytic bacteria Burkholderia cepacia strains RRE-3 and RRE-5 was studied in the seedlings of rice variety NDR97 using confocal laser scanning microscopy under controlled laboratory and greenhouse conditions. For studying colonization pattern, bacterial strains were tagged with pHRGFPGUS plasmid. The role of bacterial strains (both gfp/gus-tagged and untagged) in growth promotion was also studied. After coming into contact with the host root system the bacteria showed an irregular spreading. Dense colonization was observed on the primary and secondary roots and also on the junction of emergence of the lateral roots. Results showed that the colonization pattern of Burkholderia cepacia strains was similar to that of other endophytic bacteria isolated from non-legumes. Burkholderia cepacia got entry inside the root at the sites of emergence of lateral roots, without formation of infection threads as in the case of symbiotic rhizobacteria. Observations suggested that the endophytic bacterial strains RRE-3 and RRE-5 entered inside the rice roots in a progressive manner. Bacteria were found to line up along the intercellular spaces of adjoining epidermal cells adjacent to the lateral root junction, indicating endophytic colonization pattern of Burkholderia cepacia strains. Experiments with the rice seedlings inoculated with RRE-3 and RRE-5 strains revealed that both strains enhanced plant growth considerably when observed under laboratory and greenhouse conditions and produced significantly higher plant biomass. No considerable difference was observed between the gfp/gus-tagged and non-gfp/gus-tagged strains in the plant growth experiments both in the laboratory and greenhouse conditions.     

Pre-inoculation of Ri T-DNA-transformed pea roots with Pseudomonas fluorescens inhibits colonization by Pythium ultimum Trow: an ultrastructural and cytochemical study

The influence exerted by Pseudomonas fluorescens, strain 63-28R, in stimulating plant defense reactions was investigated using an in-vitro system in which Ri T-DNA-transformed pea (Pisum sativum L.) roots were subsequently infected with Pythium ultimum. Cytological investigations of samples from P. fluorescens-inoculated roots revealed that the bacteria multiplied abundantly at the root surface and colonized a small number of epidermal and cortical cells. Penetration of the epidermis occurred through the openings made by the disruption of the fibrillar network at the junction of adjacent epidermal cell walls. Direct cell wall penetration was never observed and bacterial ingress into the root tissues proceeded via an intercellular route. Striking differences in the extent of fungal colonization were observed between bacterized and non-bacterized pea roots following inoculation with P. ultimum. In non-bacterized roots, the pathogen multiplied abundantly through most of the tissues while in bacterized roots, pathogen growth was restricted to the epidermis and the outer cortex. At the root surface, the bacteria interacted with the pathogen, in a way similar to that observed in dual culture tests. Most Pythium cells were severely damaged but fungal penetration by the bacteria was never observed. Droplets of the amorphous material formed upon interaction between the bacteria and the host root were frequently found at the fungal cell surface. Incubation of sections with a -1,4-exoglucanase-gold complex revealed that the cell wall of markedly altered Pythium hyphae was structurally preserved. Successful penetration of the root epidermis was achieved by the few hyphae of P. ultimum that could escape the first defensive line in the rhizosphere. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. The unusual occurrence of polymorphic wall appositions along the host epidermal cells was an indication that the host plant was signalled to defend itself through the elaboration of physical barriers.Abbreviations AGL Aplysia gonad lectin - PGPR plant growth-promoting rhizobacteria The authors wish to thank Sylvain Noël for excellent technical assistance. This study was supported by grants from the Fonds Québécois pour la formation de chercheurs et l'Aide à la Recherche (FCAR), the Natural Sciences and Engineering Council of Canada (NSERC) and the Ministère de l'Industrie, du Commerce, de la Science et de la Technologie (SYNERGIE).     

Wheat lectin as a factor in plant-microbial communication and a stress response protein

Wheat lectin (wheat germ agglutinin, WGA), a representative of a broad group of cereal lectins, is excreted by plant roots into the surrounding medium and interacts with both pathogenic microflora and growth-stimulating rhizobacteria. WGA was found to serve as a molecular signal for the rhizobacterium Azospirillum brasilense, which forms endophytic and associative symbioses with wheat plants. The bacterial response to the lectin was pleiotropic: WGA at concentrations from 10^?10 to 10^?6 M exerted a dose-dependent effect on a range of processes in the bacterium that are important for the establishment and functioning of symbiosis. Plants with different WGA content differed in their responses to severe nitrogen starvation and to seed treatment with Azospirillum.     

Rhizobacteria improve sugarcane growth and photosynthesis under well‐watered conditions

Morpho‐physiological changes caused by particular plant growth‐promoting rhizobacteria were evaluated in sugarcane plants under varying water availability. Under well‐watered conditions, we have found one rhizobacteria isolate (IAC‐RBcr5) able to enhance root dry matter and photosynthesis of sugarcane plants. The IAC‐RBcr5 genome was sequenced and high similarity was found with Pseudomonas putida GB‐1. Based on increased root system size of inoculated plants, we hypothesised that sugarcane plants inoculated with IAC‐RBcr5 would have improved performance under water deficit. Although IAC‐RBcr5 had improved plant leaf CO₂ assimilation under water shortage, inoculation caused reduction of biomass accumulation in sugarcane. The negative influence of water deficit on shoot growth rate and root traits such as volume, area, diameter, length and specific root area was higher in plants treated with IAC‐RBcr5 as compared to non‐inoculated ones. However, rhizobacteria‐induced improvements in leaf and root proline contents would represent a strategy for storing carbon and nitrogen during low water availability and helping both organisms to resume their metabolism after rehydration. In conclusion, we found and identified a rhizobacterium able to improve growth and photosynthesis of sugarcane plants. Such benefit for plant growth was lost under low water availability as a likely consequence of increased carbon‐energy demand by rhizobacteria and their sensitivity to drought.     

Ultrastructural localization and identification ofAzospirillum brasilense Cd on and within wheat root by immuno-gold labeling

Azospirillum brasilense Cd localization in wheat roots was studied by light microscopy, by scanning, and by transmission electron microscopy.A. brasilense Cd cells were specifically identified immunocytochemically around and within root tissues.A. brasilense Cd cells found both outside and inside inoculated roots were intensively labeled with colloidal gold. In non-axenic cultures other bacterial strains or plant tissue were not labeled, thereby providing a non-interfering background. The roots of axenic grown wheat plants were colonized both externally and internally byA. brasilense Cd after inoculation, whereas non-axenic cultures were colonized by other bacterial strains as well.A. brasilense Cd cells were located on the root surface along the following zones: the root tip, the elongation, and the root-hair zone. However, bacteria were located within the cortex only in the latter two zones. In a number of observations, an electron dense material mediated the binding of bacterial cells to outer surfaces of epidermal cells, or between adjacent bacterial cells.A. brasilense Cd were found in root cortical intercellular spaces, but were not detected in either the endodermal layer or in the vascular system. This study proposes that in addition to root surface colonization,A. brasilense Cd forms intercellular associations within wheat roots.     

Differential effectiveness of Serratia plymuthica IC1270-induced systemic resistance against hemibiotrophic and necrotrophic leaf pathogens in rice

Background  

Induced resistance is a state of enhanced defensive capacity developed by a plant reacting to specific biotic or chemical stimuli. Over the years, several forms of induced resistance have been characterized, including systemic acquired resistance, which is induced upon localized infection by an avirulent necrotizing pathogen, and induced systemic resistance (ISR), which is elicited by selected strains of nonpathogenic rhizobacteria. However, contrary to the relative wealth of information on inducible defense responses in dicotyledoneous plants, our understanding of the molecular mechanisms underlying induced resistance phenomena in cereal crops is still in its infancy. Using a combined cytomolecular and pharmacological approach, we analyzed the host defense mechanisms associated with the establishment of ISR in rice by the rhizobacterium Serratia plymuthica IC1270.     

Spatial and temporal distribution of a bioluminescent‐marked Pseudomonas putida on soybean root

The ability of rhizobacteria to compete with other microorganisms for root colonization may be critical for its establishment on a root. Over a 6 day period, visualization of the spatial and temporal rhizosphere distribution of a bioluminescent‐marked rhizobacterium, Pseudomonas putida, strain GR7.4lux, was examined on soybean grown in non‐sterile soil conditions. Luminometry technologies showed a rapid root distribution of rhizobacteria where bioluminescence was particularly intense on the seed and upper root parts. The results provide new information on rhizobial root distribution, where, using enrichment broth, 50% of the root tips were still colonized by rhizobacteria up to 6 days after sowing. This suggests that rhizobial enrichment is required to detect low populations at the root tip. Bioluminescent technology represents a promising alternative to previous methods for studying rhizobial growth and distribution on roots. Copyright © 2003 John Wiley & Sons, Ltd.     

The auxin-signaling pathway is required for the lateral root response of Arabidopsis to the rhizobacterium Phyllobacterium brassicacearum

Plant root development is highly responsive both to changes in nitrate availability and beneficial microorganisms in the rhizosphere. We previously showed that Phyllobacterium brassicacearum STM196, a plant growth-promoting rhizobacteria strain isolated from rapeseed roots, alleviates the inhibition exerted by high nitrate supply on lateral root growth. Since soil-borne bacteria can produce IAA and since this plant hormone may be implicated in the high nitrate-dependent control of lateral root development, we investigated its role in the root development response of Arabidopsis thaliana to STM196. Inoculation with STM196 resulted in a 50% increase of lateral root growth in Arabidopsis wild-type seedlings. This effect was completely abolished in aux1 and axr1 mutants, altered in IAA transport and signaling, respectively, indicating that these pathways are required. The STM196 strain, however, appeared to be a very low IAA producer when compared with the high-IAA-producing Azospirillum brasilense sp245 strain and its low-IAA-producing ipdc mutant. Consistent with the hypothesis that STM196 does not release significant amounts of IAA to the host roots, inoculation with this strain failed to increase root IAA content. Inoculation with STM196 led to increased expression levels of several IAA biosynthesis genes in shoots, increased Trp concentration in shoots, and increased auxin-dependent GUS staining in the root apices of DR5::GUS transgenic plants. All together, our results suggest that STM196 inoculation triggers changes in IAA distribution and homeostasis independently from IAA release by the bacteria.     

Pseudomonas fluorescens mediated suppression of Meloidogyne incognita infection of cowpea and tomato

Plant growth-promoting rhizobacterium, Pseudomonas fluorescens strain BICC602 suppresses root-knot nematode (Meloidogyne incognita) by enhancing defence mechanism leading to induced systemic resistance in cowpea (Vigna unguiculata) cv. L.Walp. and tomato (Solanum lycopersicum) cv. Pusa Ruby. In cowpea, the soil treatment proved more effective than foliar spray on root galling and eggs in roots. However, which factors are necessary in the induction of resistance response in plants against nematodes by BICC602 is not yet known. Salicylic acid (SA) production by some bacteria acts as endogenous signal for the activation of certain plant defence responses. In a split-root trial with tomato as a host plant and M. incognita as challenging parasite, BICC602 induces systemic resistance in tomato plants. Based on the results, it is assumed that P. fluorescens-induced resistance against M. incognita in cowpea and tomato is made either through SA-dependent or SA-independent transduction pathway.     

Gibberellins-producing rhizobacteria increase endogenous gibberellins content and promote growth of red peppers

The growth of red pepper plants was enhanced by treatment with the rhizobacterium, Bacillus cereus MJ-1. Red pepper shoots showed a 1.38-fold increase in fresh weight (fw) and roots showed a 1.28-fold fw gain. This plant growth-promoting rhizobacterium (PGPR) has been reported to produce gibberellins (GAs). Other GAs-producing rhizobacteria, Bacillus macroides CJ-29 and Bacillus pumilus CJ-69, also enhanced the fw of the plants. They were less effective than B. cereus MJ-1, though. The endogenous GAs content of pepper shoots inoculated with MJ-1 was also higher than in shoots inoculated with CJ-29 or CJ-69. When inoculated with MJ-1, bacterial colonization rate of the roots was higher than that of roots inoculated with CJ-29 or CJ-69. These results support the idea that the plant growth-promoting effect of the bacteria also positively related with the efficiency of root colonization by the bacteria. In addition, we identified the major endogenous GAs of the red pepper as originating from both the early C-13 hydroxylation and the early non C-13 hydroxylation pathways, with the latter being the predominant pathway of GA biosynthesis in red pepper shoots.     

Root-secreted malic acid recruits beneficial soil bacteria

Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated. Here, we provide biochemical evidence that the tricarboxylic acid cycle intermediate L-malic acid (MA) secreted from roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Root secretions of L-MA are induced by the foliar pathogen Pseudomonas syringae pv tomato (Pst DC3000) and elevated levels of L-MA promote binding and biofilm formation of FB17 on Arabidopsis roots. The demonstration that roots selectively secrete L-MA and effectively signal beneficial rhizobacteria establishes a regulatory role of root metabolites in recruitment of beneficial microbes, as well as underscores the breadth and sophistication of plant-microbial interactions.     

Non‐pathogenic rhizobacteria interfere with the attraction of parasitoids to aphid‐induced plant volatiles via jasmonic acid signalling

Beneficial soil‐borne microbes, such as mycorrhizal fungi or rhizobacteria, can affect the interactions of plants with aboveground insects at several trophic levels. While the mechanisms of interactions with herbivorous insects, that is, the second trophic level, are starting to be understood, it remains unknown how plants mediate the interactions between soil microbes and carnivorous insects, that is, the third trophic level. Using Arabidopsis thaliana Col‐0 and the aphid Myzus persicae, we evaluate here the underlying mechanisms involved in the plant‐mediated interaction between the non‐pathogenic rhizobacterium Pseudomonas fluorescens and the parasitoid Diaeretiella rapae, by combining ecological, chemical and molecular approaches. Rhizobacterial colonization modifies the composition of the blend of herbivore‐induced plant volatiles. The volatile blend from rhizobacteria‐treated aphid‐infested plants is less attractive to an aphid parasitoid, in terms of both olfactory preference behaviour and oviposition, than the volatile blend from aphid‐infested plants without rhizobacteria. Importantly, the effect of rhizobacteria on both the emission of herbivore‐induced volatiles and parasitoid response to aphid‐infested plants is lost in an Arabidopsis mutant (aos/dde2‐2) that is impaired in jasmonic acid production. By modifying the blend of herbivore‐induced plant volatiles that depend on the jasmonic acid‐signalling pathway, root‐colonizing microbes interfere with the attraction of parasitoids of leaf herbivores.     

Gram-positive rhizobacterium <Emphasis Type="Italic">Bacillus amyloliquefaciens</Emphasis> FZB42 colonizes three types of plants in different patterns

The colonization of three types of different plants, Zea mays, Arabidopsis thaliana, and Lemna minor, by GFP-labeled Gram-positive rhizobacterium Bacillus amyloliquefaciens FZB42 was studied in gnotobiotic systems using confocal laser scanning microscopy and electron microscopy. It was demonstrated that FZB42 was able to colonize all the plants. On one hand, similar to some Gram-negative rhizobacteria like Pseudomonas, FZB42 favored the areas such as the concavities in root surfaces and the junctions where lateral roots occurred from the primary roots; on the other hand, we clearly demonstrated that root hairs were a popular habitat to the Gram-positive rhizobacterium. FZB42 exhibited a specific colonization pattern on each of the three types of plants. On Arabidopsis, tips of primary roots were favored by FZB42 but not so on maize. On Lemna, FZB42 accumulated preferably along the grooves between epidermal cells of roots and in the concave spaces on ventral sides of fronds. The results suggested L. minor to be a promising tool for investigations on plant-microbial interaction due to a series of advantages it has. Colonization of maize and Arabidopsis roots by FZB42 was also studied in the soil system. Comparatively, higher amount of FZB42 inoculum (∼10⁸ CFU/ml) was required for detectable root colonization in the soil system, where the preference of FZB42 cells to root hairs were also observed.     

Paenibacillus polymyxa Invades Plant Roots and Forms Biofilms

Paenibacillus polymyxa is a plant growth-promoting rhizobacterium with a broad host range, but so far the use of this organism as a biocontrol agent has not been very efficient. In previous work we showed that this bacterium protects Arabidopsis thaliana against pathogens and abiotic stress (S. Timmusk and E. G. H. Wagner, Mol. Plant-Microbe Interact. 12:951-959, 1999; S. Timmusk, P. van West, N. A. R. Gow, and E. G. H. Wagner, p. 1-28, in Mechanism of action of the plant growth promoting bacterium Paenibacillus polymyxa, 2003). Here, we studied colonization of plant roots by a natural isolate of P. polymyxa which had been tagged with a plasmid-borne gfp gene. Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms. Accumulation of bacteria was observed in the intercellular spaces outside the vascular cylinder. Systemic spreading did not occur, as indicated by the absence of bacteria in aerial tissues. Studies were performed in both a gnotobiotic system and a soil system. The fact that similar observations were made in both systems suggests that colonization by this bacterium can be studied in a more defined system. Problems associated with green fluorescent protein tagging of natural isolates and deleterious effects of the plant growth-promoting bacteria are discussed.     

Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

Cellular interactions between arbuscular mycorrhizal fungi and rhizosphere bacteria

Summary We have investigated whether direct physical interactions occur between arbuscular mycorrhizal (AM) fungi and plant growth promoting rhizobacteria (PGPRs), some of which are used as biocontrol agents. Attachment of rhizobia and pseudomonads to the spores and fungal mycelium ofGigaspora margarita has been assessed in vitro and visualized by a combination of electron and confocal microscopy. The results showed that both rhizobia and pseudomonads adhere to spores and hyphae of AM fungi germinated under sterile conditions, although the degree of attachment depended upon the strain.Pseudomonas fluorescens strain WCS 365 andRhizobium leguminosarum strains B556 and 3841 were the most effective colonizers. Extracellular material of bacterial origin containing cellulose produced around the attached bacteria may mediate fungal/bacterial interactions. These results suggest that antagonistic and synergistic interactions between AM fungi and rhizosphere bacteria may be mediated by soluble factors or physical contact. They also support the view that AM fungi are a vehicle for the colonization of plant roots by soil rhizobacteria.Abbreviations AM arbuscular mycorrhiza - PGPR plant growth promoting rhizobacteria - CBH cellobiohydrolase - DAPG 2,4-(diacetyl-phloroglucinol - TY triptone-yeast - LB Lauria-Bertani Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

Multiple strategies of plant colonization by beneficial endophytic Enterobacter sp. SA187

Although many endophytic plant growth-promoting rhizobacteria have been identified, relatively little is still known about the mechanisms by which they enter plants and promote plant growth. The beneficial endophyte Enterobacter sp. SA187 was shown to maintain the productivity of crops in extreme agricultural conditions. Here we present that roots of its natural host (Indigofera argentea), alfalfa, tomato, wheat, barley and Arabidopsis are all efficiently colonized by SA187. Detailed analysis of the colonization process in Arabidopsis showed that colonization already starts during seed germination, where seed-coat mucilage supports SA187 proliferation. The meristematic zone of growing roots attracts SA187, allowing epiphytic colonization in the elongation zone. Unlike primary roots, lateral roots are significantly less epiphytically colonized by SA187. Root endophytic colonization was found to occur by passive entry of SA187 at lateral-root bases. However, SA187 also actively penetrates the root epidermis by enzymatic disruption of plant cell wall material. In contrast to roots, endophytic colonization of shoots occurs via stomata, whereby SA187 can actively re-open stomata similarly to pathogenic bacteria. In summary, several entry strategies were identified that allow SA187 to establish itself as a beneficial endophyte in several plant species, supporting its use as a plant growth-promoting bacterium in agriculture systems.