Chemotaxis of Rhizobium spp. to Plant Root Exudates

Rhizobium spp. show chemotaxis to plant root exudates. Both legumes and non-legume root exudates attract the different rhizobia studied. However, the bacteria show a differential response in that they are attracted to the root exudates of some plants and show no attraction toward others. An example of negative chemotaxis was also observed. The trefoil strain of Rhizobium shows chemotaxis which is qualitatively different from that observed in other bacteria in that simple sugars, di-and trisaccharides, dextrans, and amino acids do not attract this bacterium.     

Tomato root exudates and their effect on the growth and antifungal activity of Pseudomonas strains

The study of the effect of the root exometabolites of tomato plants on the growth and antifungal activity of the plant growth-promoting Pseudomonas strains showed that the antifungal activity of plant growth-promoting rhizobacteria in the plant rhizosphere may depend on the sugar and organic acid composition of root exudates.     

Diversity and chemotaxis of soil bacteria with antifungal activity against Fusarium wilt of banana

The chemotactic response of bacteria to root exudates plays an important role in the colonization of bacteria in the rhizosphere. In this study, 420 strains of antifungal bacteria against Fusarium oxysporum f. sp. cubense (Foc) were screened for chemotaxis based on a cheA molecular diagnostic method. A total of 124 strains with antifungal efficiencies of 27.26-67.14?% generated a characteristic band of cheA. The chemotaxis of 97 bacterial strains producing a cheA band was confirmed using the drop assay and swarm plate assay using catechol, p-hydroxybenzoic acid, salicylic acid, and asparagine as the attractants. A phylogenetic analysis based on restriction fragment length polymorphisms (RFLPs) and 16S rDNA sequences indicated that the 124 chemotactic antagonists of Foc were affiliated with 18 species of Paenibacillaceae, Bacillaceae, Streptomycineae, Enterobacteriaceae, and Pseudomonadaceae. The chemical composition of banana root exudates were analyzed by GC-MS, and 62 compounds, including alkanes, alkenes, naphthalenes, benzenes, and alcohols, were evaluated. Five representative antagonists of Foc showed 1.76- to 7.75-fold higher chemotactic responses than the control to seven compounds in banana root exudates, as determination by capillary assays.     

Cultivar-specific impairment of strawberry growth,photosynthesis, carbohydrate and nitrogen accumulation by ozone

In a 2-year study, fruiting plants of strawberry (Fragaria × ananassa Duch.) cv. ‘Korona’ and ‘Elsanta’ were exposed for 2 months to 78 ppb ozone on average or filtered air without ozone in controlled environment chambers. Plant growth, photosynthesis, carbohydrate accumulation, and macronutrient concentrations were investigated in order to demonstrate cultivar-specific differences in the ozone sensitivity of ‘Korona’ and ‘Elsanta’ on the whole plant level. Moreover, the hypothesis was tested whether properties of the root system in strawberry were involved in ozone tolerance, for example, the roots’ ability to store or make available carbohydrates and their capacity to secure plants’ supply with nitrogen during a stress situation. In strawberry, ozone reduced leaf area by reducing leaf number. Moreover, specific leaf area (SLA) and relative leaf water content were reduced. Net photosynthesis was only slightly impaired, but activity of Rubisco and chlorophyll content in older leaves of cv. ‘Elsanta’ were significantly reduced. The most important, indirect impairment of photosynthesis was the reduction of plants’ total leaf area, which resulted in a decrease in plant biomass. The reduction of root biomass, the root/shoot ratio, and also the distribution of carbohydrates indicated a partitioning priority of the shoot at expense of the root system. Cultivar ‘Elsanta’ was characterized by significantly lower carbohydrate levels in ozone-exposed leaves, whereas levels remained fairly stable in ‘Korona’ leaves. In addition, nitrogen concentrations in leaves and roots decreased significantly in ‘Elsanta’, not in ‘Korona’. The reduced nitrogen concentration in leaves may be related with the more distinct reduction in Rubisco activity and chlorophyll content in older leaves of ‘Elsanta’.     

Promotion of Sorghum Callus Growth by the s-Triazine Herbicides

Growth-promoting action of simazine and other s-triazine herbicides was detected by the use of sorghum (Sorghum bicolor [L]. Moench) callus tissue and the chlorophyll retention test. Soil application of simazine [2-chloro-4, 6-bis(ethylamino)-s-triazine] at sublethal levels nearly doubled the growth-promoting action of sorghum root exudates. Treated plants yielded up to 26% more total protein than untreated plants. This indicated that the level of callus growth-promoting action in the root exudate of the plant has a positive effect on its final total protein yield and confirms a positive effect of simazine on total protein content in certain instances. The results may provide a new understanding of the mode of action of s-triazines applied at sublethal levels in increasing protein content and certain enzymic activities of treated plants. It is speculated that the growth-promoting action of these herbicides is hormonal in nature and most likely kinetin-like.     

Recognition of dominant attractants by key chemoreceptors mediates recruitment of plant growth-promoting rhizobacteria

Chemotaxis to plant root exudates is supposed to be a prerequisite for efficient root colonization by rhizobacteria. This is a highly multifactorial process since root exudates are complex compound mixtures of which components are recognized by different chemoreceptors. Little information is available as to the key components in root exudates and their receptors that drive colonization related chemotaxis. We present here the first global assessment of this issue using the plant growth-promoting rhizobacterium (PGPR) Bacillus velezensis SQR9 (formerly B. amyloliquefaciens). This strain efficiently colonizes cucumber roots, and here, we show that chemotaxis to cucumber root exudates was essential in this process. We conducted chemotaxis assays using cucumber root exudates at different concentrations, individual exudate components as well as recomposed exudates, taking into account their concentrations detected in root exudates. Results indicated that two key chemoreceptors, McpA and McpC, were essential for root exudate chemotaxis and root colonization. Both receptors possess a broad ligand range and recognize most of the exudate key components identified (malic, fumaric, gluconic and glyceric acids, Lys, Ser, Ala and mannose). The remaining six chemoreceptors did not contribute to exudate chemotaxis. This study provides novel insight into the evolution of the chemotaxis system in rhizobacteria.     

Effects of different plant root exudates and their organic acid components on chemotaxis, biofilm formation and colonization by beneficial rhizosphere-associated bacterial strains

Aim

It is necessary to understand the roles of root exudates involved in plant-microbe interactions to inform practical application of beneficial rhizosphere microbial strains.

Methods

Colonization of Bacillus amyloliquefaciens SQR9 (isolated from cucumber rhizosphere) and Bacillus subtilis N11 (isolated from banana rhizosphere) of their original host was found to be more effective as compared to the colonization of the non-host plant. Organic acids in the root exudates of the two plants were identified by High performance liquid chromatography (HPLC). The chemotactic response and effects on biofilm formation were assessed for SQR9 and N11 in response to cucumber and banana root exudates, as well as their organic acids components.

Results

Citric acid detected exclusively in cucumber exudates could both attract SQR9 and induce its biofilm formation, whereas only chemotactic response but not biofilm formation was induced in N11. Fumaric acid that was only detected in banana root exudates revealed both significant roles on chemotaxis and biofilm formation of N11, while showing only effects on biofilm formation but not chemotaxis of SQR9.

Conclusion

The relationship between PGPR strain and root exudates components of its original host might contribute to preferential colonization. This study advances a clearer understanding of the mechanisms relevant to application of PGPR strains in agricultural production.     

广西葛根内生细菌的分离鉴定及其促生特性

【背景】植物内生菌长期与宿主共生,对宿主生长发育产生影响。葛根作为重要的药食两用作物,葛根内生菌的研究具有重要实践意义。【目的】对广西葛根根部内生细菌进行分离、鉴定及促植物生长特性分析,旨在了解该药食同源植物内生细菌种群结构及其促生特性,为分析内生菌群体在药食同源植物产量和品质形成的作用及其内生细菌资源的开发利用提供参考。【方法】采用6种不同的培养基从广西葛根的根瘤、根系和根愈伤组织分离内生细菌,16S rRNA基因测序和系统发育分析内生细菌的分布特征和遗传多样性,采用生理生化方法测定分离菌株的固氮活性、溶磷特性、产生嗜铁素、分泌吲哚乙酸(indole-3-aceticacid,IAA)等促生特性。【结果】从葛根根瘤、根系和根部愈伤组织中共分离得到223个菌株,16S rRNA基因测序鉴定这些菌株隶属于2门4纲10科19属,其中芽孢杆菌属、假单胞菌属、土壤杆菌属、肠杆菌属为葛根优势菌群;内生细菌数量和群落组成存在明显的组织特异性,其数量表现为根瘤>根系>根愈伤组织,但其种群多样性表现为根愈伤组织>根系>根瘤。不同培养基分离出的细菌种群丰富度有差异。从供试菌株中筛...     

Evaluation of potential biocontrol rhizobacteria from different host plants of Verticillium dahliae Kleb

AIMS: A screening approach was developed to assess the potential of rhizobacterial strains to control Verticillium wilt caused by Verticillium dahliae Kleb. METHODS AND RESULTS: Sixty randomly chosen antagonistic bacterial strains originally isolated from rhizosphere of three different host plants of V. dahliae--strawberry, potato and oilseed rape--were evaluated for biocontrol and plant growth promotion by analysing in vitro antagonism towards V. dahliae and other plant pathogenic fungi, production of fungal cell wall-degrading enzymes and plant growth-promoting effects on strawberry seedlings. To test the plant growth-promoting effect, a microplate assay with strawberry seedlings was developed. Although the rhizobacterial strains were isolated from different plants they showed effects on the growth of strawberry seedlings. According to the in vitro biocontrol and plant growth-promoting activity, the three best candidates Pseudomonas putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Serratia plymuthica R12 (oilseed rape rhizosphere) were selected for greenhouse experiments to verify the in vitro screening results. Under greenhouse conditions the isolates selected according to this strategy were as effective, or more effective than commercial biocontrol agents and may therefore possibly be valuable as antagonists of V. dahliae. CONCLUSIONS: In this study, the screening strategy resulted in a selection of three interesting biocontrol candidates against Verticillium: Ps. putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Ser. plymuthica R12 (oilseed rape rhizosphere). SIGNIFICANCE AND IMPACT OF THE STUDY: A new combination of in vitro screening methods including a microplate assay with strawberry seedlings to test the plant growth promoting effect which allow to more efficiently select potential biological control agents was developed successfully.     

Strain-specific chemotaxis of Azospirillum spp.

Chemotactic responses of three Azospirillum strains originating from different host plants were compared to examine the possible role of chemotaxis in the adaptation of these bacteria to their respective hosts. The chemotaxis to several sugars, amino acids, and organic acids was determined qualitatively by an agar plate assay and quantitatively by a channeled-chamber technique. High chemotactic ratios, up to 40, were obtained with the latter technique. The chemotactic response did not rely upon the ability of the bacteria to metabolize the attractant. Rather, it depended on the attractant concentration and stereoconfiguration. Chemotaxis was found to be strain specific. Differences were particularly observed between a wheat isolate and strains originating from the C4-pathway plants maize and Leptochloa fusca. In contrast to the other two strains, the wheat isolate was strongly attracted to D-fructose, L-aspartate, citrate, and oxalate. The other strains showed maximal attraction to L-malate. The chemotactic responses to organic acids partially correlate with the exudation of these acids by the respective host plants. Additionally, a heat-labile, high-molecular-weight attractant was found in the root exudates of L. fusca, which specifically attracted the homologous Azospirillum strain. It is proposed that strain-specific chemotaxis probably reflects an adaptation of Azospirillum spp. to the conditions provided by the host plant and contributes to the initiation of the association process.     

Characterization of Root Surface and Endorhizosphere Pseudomonads in Relation to Their Colonization of Roots

An extensive colonization of the endorhizosphere by fluorescent pseudomonads was observed in tomato plants grown on artificial substrates. These studies reveal that a significantly higher percentage of pseudomonads obtained from the endorhizosphere (30%) reduced plant growth than those obtained from the root surface (4%). Lipopolysaccharide patterns, cell envelope protein patterns, and other biochemical characteristics indicated that Pseudomonas isolates obtained from the endorhizosphere are distinct from Pseudomonas isolates obtained from the root surface. Isolates from the endorhizosphere especially were able to recolonize the endorhizosphere of both sterile and nonsterile tomato roots. The ability of the endorhizosphere isolates to colonize the endorhizosphere significantly correlated with their agglutination by tomato root agglutinin but did not correlate with chemotaxis to seed exudates of tomato. No correlation between colonization of the endorhizosphere and agglutination by root agglutinin could be demonstrated for the root surface isolates. We propose that agglutination of specific Pseudomonas strains by root agglutinin is of importance in the initial phase of adherence of bacteria to the root surface.     

Root Exudates Regulate Soil Fungal Community Composition and Diversity

Plants are in constant contact with a community of soil biota that contains fungi ranging from pathogenic to symbiotic. A few studies have demonstrated a critical role of chemical communication in establishing highly specialized relationships, but the general role for root exudates in structuring the soil fungal community is poorly described. This study demonstrates that two model plant species (Arabidopsis thaliana and Medicago truncatula) are able to maintain resident soil fungal populations but unable to maintain nonresident soil fungal populations. This is mediated largely through root exudates: the effects of adding in vitro-generated root exudates to the soil fungal community were qualitatively and quantitatively similar to the results observed for plants grown in those same soils. This effect is observed for total fungal biomass, phylotype diversity, and overall community similarity to the starting community. Nonresident plants and root exudates influenced the fungal community by both positively and negatively impacting the relative abundance of individual phylotypes. A net increase in fungal biomass was observed when nonresident root exudates were added to resident plant treatments, suggesting that increases in specific carbon substrates and/or signaling compounds support an increased soil fungal population load. This study establishes root exudates as a mechanism through which a plant is able to regulate soil fungal community composition.     

Taxonomic and functional diversity of pseudomonads isolated from the roots of field-grown canola

Among the most important rhizosphere bacteria are the pseudomonads, which are aggressive colonizers and utilize a wide range of substrates as carbon sources. The objective of this study was to determine if the taxonomic or metabolic diversity of pseudomonads differed among field-grown canola cultivars. Bacteria (n=2257) were isolated from the rhizosphere and root interior of six cultivars of field-grown canola, including three transgenic varieties. The bacteria were identified by fatty acid methyl ester (FAME) analysis, and about 35% were identified as Pseudomonas species. The most abundant species were Pseudomonas putida and Pseudomonas chlororaphis. Dendrograms based on FAME analysis revealed that many pseudomonad strains were found in all of the canola cultivars. Pseudomonads of the same strain were found in both the rhizosphere and the root interior of canola plants, suggesting that endophytic bacteria were a subset of the rhizosphere community. Because metabolic profiling provides more useful information than taxonomy, P. putida and P. chlororaphis isolates were characterized for their ability to utilize carbon substrates and produce several enzymes. Bacteria isolated from different plant cultivars had different carbon utilization profiles, but when only carbon substrates found in root exudates were analyzed, the cultivar effect was less pronounced. These characterizations also demonstrated that bacteria that were determined by FAME to be the same strain were metabolically different, suggesting functional redundancy among Pseudomonas isolates. The results of this study suggest that pseudomonads were functionally diverse. They differed in their metabolic potential among the canola cultivars from which they were isolated. Because bacteria capable of using many substrates can effectively adapt to new environments, these results have implications for the use of pseudomonads as biofertilizers, biological control agents and plant growth-promoting bacteria in canola.     

Effect of <Emphasis Type="Italic">Pseudomonas</Emphasis> Bacteria on Peroxidase Activity in Wheat Plants when Infected with <Emphasis Type="Italic">Bipolaris sorokiniana</Emphasis>

We investigated the effect of treating soft wheat seeds (Triticum aestivum L.) with two Pseudomonas bacteria strains, isolated from earthworm coprolites, showing a significant antifungal and growth-promoting action in preliminary screening on the activity of guaiacol-dependant peroxidase under phytopathogenic load in the presence of Bipolaris sorokiniana (Sacc.) Shoemaker as a mechanism for inducing plant resistance to the pathogen. We established a statistically significant decrease (P < 0.05) in root rot disease incidence and severity during bacterization, which is indicative both of antifungal activity of the used bacterial isolates and of their successful colonizing the rhizosphere of wheat plants. We noted a response of free and weakly bound peroxidase of wheat plants to infection with B. sorokiniana: the enzyme activity increased during pathogenesis. Bacterization also increased peroxidase activity in plant leaves and roots, the greatest differences from non-bacterized plants being observed in wheat roots in the presence of the pathogen. We detected a direct link between peroxidase activity in wheat roots and leaf tissues in the absence of the pathogen and the feedback between peroxidase activity and plant infestation by the root rot pathogen. In the presence of the phytopathogen, there is a lack of correlation between peroxidase activity in wheat roots and leaves, and there is a shift of activity towards its increase in roots, which plays an important role in the development of systemic resistance against the root rot pathogen that penetrates into plants through the roots and root collar.     

Achieving similar root microbiota composition in neighbouring plants through airborne signalling

The ability to recognize and respond to environmental signals is essential for plants. In response to environmental changes, the status of a plant is transmitted to other plants in the form of signals such as volatiles. Root-associated bacteria trigger the release of plant volatile organic compounds (VOCs). However, the impact of VOCs on the rhizosphere microbial community of neighbouring plants is not well understood. Here, we investigated the effect of VOCs on the rhizosphere microbial community of tomato plants inoculated with a plant growth-promoting rhizobacterium Bacillus amyloliquefaciens strain GB03 and that of their neighbouring plants. Interestingly, high similarity (up to 69%) was detected in the rhizosphere microbial communities of the inoculated and neighbouring plants. Leaves of the tomato plant treated with strain GB03-released β-caryophyllene as a signature VOC, which elicited the release of a large amount of salicylic acid (SA) in the root exudates of a neighbouring tomato seedling. The exposure of tomato leaves to β-caryophyllene resulted in the secretion of SA from the root. Our results demonstrate for the first time that the composition of the rhizosphere microbiota in surrounding plants is synchronized through aerial signals from plants.Subject terms: Microbial ecology, Soil microbiology     

Relationships between cell membrane stability, exudate content and infectivity of Bradyrhizobium japonicum strain 639 to boron starved soybean plants

The influence of boron starvation on the root exudates content in soybean seedlings (Glycine max. L. Merr.) and the effect of exudates pretreatment on the pre-infection processes in symbiotic system Br. japonicum strain 636 and soybean were investigated. Root cell membrane stability of boron starved soybean plants (-B) decreased compared to the control. The concentrations of all analyzed metabolites (reducing sugars, free amino acids, organic acids, soluble phenols and total flavonoids) from root exudates of -B plants were lower than the control concentrations. Analysis of polyphenols after HPLC chromatography of root exudates showed significant difference of peak numbers between chromatograms of exudates obtained from boron starved and from control plants. Bacterial culture treatment with root exudates from -B plants showed decreased growth, chemotaxis and attachment ability toward the host root compared to the control exudate treatments. These changes were accompanied by decreased nodulation and acetylene reduction activity of boron starved soybean plants.     

Natural occurrence of Azospirillum brasilense in strawberry plants

Azospirillum species are free-living nitrogen-fixing bacteria commonly found in soil and in association with roots of different plant species. For their capacity to stimulate growth they are known as plant growth-promoting bacteria (PGPB). In this work, we demonstrate the natural occurrence and colonization of different parts of strawberry plants by Azospirillum brasilense in the cropping area of Tucumán, Argentina. Although bacteria isolations were carried out from two strawberry cultivars, e.g., Camarosa and Pájaro, attempts were successful only with the cultivar Camarosa. Whereas different strains of Azospirillum were isolated from the root surface and inner tissues of roots and stolons of the cultivar Camarosa, we have not obtained Azospirillum isolates from the cultivar Pájaro. After microbiological and molecular characterization (ARDRA) we determined that the isolates belonged to the species A. brasilense. All isolates showed to have the capacity to fix nitrogen, to produce siderophores and indoles. Local isolates exhibited different yields of indoles production when growing in N-free NFb semisolid media supplemented or not with tryptophan (0.1 mg ml⁻¹). This is the first report on the natural occurrence of A. brasilense in strawberry plants, especially colonizing inner tissues of stolons, as well as roots. The local isolates showed three important characteristics within the PGPB group: N₂-fixation, siderophores, and indoles production.     

Identification of interior salt-tolerant bacteria from ice plant <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> and evaluation of their promoting effects

Endophytic bacteria can stimulate host plant development. Insufficient information is available about NaCl-tolerant bacteria that colonize ice plants (Mesembryanthemum crystallinum) in their habitats. In this study, a culture-dependent method was used to isolate endophytic nitrogen-fixing bacteria from ice plants, and the resulting cultures were screened for salt-stress tolerance in vitro. A total of 17 salt-tolerant bacteria were obtained. The majority of the isolates grew well in 2.05 M NaCl with a maximum tolerance at 3.59 M. Most of the strains were Gram-positive bacteria with various plant growth-promoting traits. The 16S rRNA gene sequences revealed that the 17 isolates were distributed within three genera and corresponded to the bacterial species Halomonas sp., Bacillus sp., and Planococcus sp. Inoculation of cabbage (Brassica olereacea) seeds with selected strains showed that the strain MC1 promoted seed germination, and the same strain significantly increased root dry weight under saline stress by 24.5%. Our study suggests that ice plants naturally accommodate a variety of salt-tolerant endophytic bacteria and that these bacteria are able to relieve abiotic stress during plant growth.