Molecular Studies on the Role of a Root Surface Agglutinin in Adherence and Colonization by Pseudomonas putida

Isolation of soybean-specific plant growth-promoting rhizobacteria using soybean agglutin and evaluation of their effects to improve soybean growth, yield, and soil nutritional status

The basic requirements of plant growth-promoting rhizobacteria (PGPR) for field applications are that they have an affinity for the host plant and that they can colonize the rhizosphere. Here, a new technique was established using soybean agglutin (SBA) as a tool to isolate soybean-specific PGPR. Thirty-three PGPR strains with an affinity for soybean were obtained via the screening method with SBA. All 33 isolates were able to produce indole acetic acid and solubilize inorganic phosphate and potassium. Most isolates (93.94%) were able to solubilize organic phosphate and almost half (45.45%) were able to produce siderophores. More than 40% of the isolates exhibited all five plant growth-promoting traits. The isolate Enterobacter sp. strain DN9 was selected for further analyses of its rhizosphere colonization and soybean growth-promoting effects because of its excellent activity in phosphate and potassium solubilization. The luciferase luxAB gene was electrotransformed into DN9, and the labelled DN9 (DN9-L) was able to survive in the soybean rhizosphere and colonize new spaces as the soybean roots elongated. This strain positively affected root system development and soybean seedling growth. In pot and field experiments, the isolates DN9, DW1, and DW13 significantly increased the nutrient contents in rhizosphere soil and soybean leaves. On average, the seed number per plant and the seed weight per plant were increased by 20% and 24% respectively, in plants inoculated with these PGPR strains in the pot experiment. In a field experiment, compared with uninoculated plants, those inoculated with DW1 showed 46.78% higher pod number per plant and 5.23% higher seed oil content; those inoculated with DW13 showed 79.82% higher seed number per plant and 65.10% higher seed weight per plant; and those inoculated with DN9 showed 9.13% higher 100-seed weight. These results show that SBA can be used as a tool to isolate efficient PGPR to enhance soybean production.

Catalase and superoxide dismutase of root-colonizing saprophytic fluorescent pseudomonads

Root-colonizing, saprophytic fluorescent pseudomonads of the Pseudomonas putida-P. fluorescens group express similar levels of catalase and superoxide dismutase activities during growth on a sucrose- and amino acid-rich medium. Increased specific activities of catalase but not superoxide dismutase were observed during growth of these bacteria on components washed from root surfaces. The specific activities of both enzymes were also regulated during contact of these bacteria with intact bean roots. Increased superoxide dismutase and decreased catalase activities were observed rapidly, by 10 min upon inoculation of cells onto intact bean roots. Catalase specific activity increased with time to peak at 12 h before declining. By 48 h, the cells displayed this low catalase but maintained high superoxide dismutase specific activities. Catalase with a low specific activity and a high superoxide dismutase activity also were present in extracts of cells obtained from 7-day-old roots colonized from inoculum applied to seed. This specific activity of superoxide dismutase of root-contacted cells was about fourfold-higher in comparison to cells grown on rich medium, whereas the specific activity for catalase was reduced about fivefold. A single catalase isozyme, isozyme A, and one isozyme of superoxide dismutase, isozyme 1, were detected during growth of the bacteria on root surface components and during exposure of cells to intact bean roots for 1 h. An additional catalase, isozyme B, was detected from bacteria after exposure to the intact bean roots for 12 h. Catalase isozyme A and superoxide dismutase isozyme 1 were located in the cytoplasm and catalase band B was located in the membrane of P. putida.

Effects of fungal root pathogens on the population dynamics of biocontrol strains of fluorescent pseudomonads in the wheat rhizosphere

The influences of Gaeumannomyces graminis var. tritici (which causes take-all of wheat), Rhizoctonia solani AG-8 (which causes rhizoctonia root rot of wheat), Pythium irregulare, P. aristosporum, and P. ultimum var. sporangiiferum (which cause pythium root rot of wheat) on the population dynamics of Pseudomonas fluorescens 2-79 and Q72a-80 (bicontrol strains active against take-all and pythium root rot of wheat, respectively) in the wheat rhizosphere were examined. Root infection by either G. graminis var. tritici or R. solani resulted in populations of both bacterial strains that were equal to or significantly larger than their respective populations maintained on roots in the absence of these pathogens. In contrast, the population of strain 2-79 was significantly smaller on roots in the presence of any of the three Pythium species than on noninfected roots and was often below the limits of detection (50 CFU/cm of root) on Pythium-infected roots after 40 days of plant growth. In the presence of either P. aristosporum or P. ultimum var. sporangiiferum, the decline in the population of Q72a-80 was similar to that observed on noninfected roots; however, the population of this strain declined more rapidly on roots infected by P. irregulare than on noninfected roots. Application of metalaxyl (which is selectively inhibitory to Pythium spp.) to soil naturally infestated with Pythium spp. resulted in significantly larger rhizosphere populations of the introduced bacteria over time than on plants grown in the same soil without metalaxyl. It is apparent that root infections by fungal pathogens may either enhance or depress the population of fluorescent pseudomonads introduced for their control, with different strains of pseudomonads reacting differentially to different genera and species of the root pathogens.

Mutations Affecting Hyphal Colonization and Pyoverdine Production in Pseudomonads Antagonistic toward Phytophthora parasitica

In previous studies, Pseudomonas putida 06909 and Pseudomonas fluorescens 09906 suppressed populations of Phytophthora parasitica in the citrus rhizosphere, suggesting that these bacteria may be useful in biological control of citrus root rot. In this study we investigated the mechanisms of antagonism between the bacteria and the fungus. Both bacteria colonized Phytophthora hyphae and inhibited the fungus on agar media. A hyphal column assay was developed to measure the colonization of bacteria on fungal hyphae and to enrich for colonization-deficient mutants. In this way we identified Tn5 mutants of each pseudomonad that were not able to colonize the hyphae and inhibit fungal growth in vitro. Colonization-deficient mutants were nonmotile and lacked flagella. Survival of nonmotile mutants in a citrus soil was similar to survival of a random Tn5 mutant over a 52-day period. Additional screening of random Tn5 mutants of both pseudomonads for loss of fungal inhibition in vitro yielded two distinct types of mutants. Mutants of the first type were deficient in production of pyoverdines and in inhibition of the fungus in vitro, although they still colonized fungal hyphae. Mutants of the second type lacked flagella and were not able to colonize the hyphae or inhibit fungal growth. No role was found for antibiotic production by the two bacteria in the inhibition of the fungus. Our results suggest that both hyphal colonization and pyoverdine production are important in the inhibition of Phytophthora parasitica by P. fluorescens and P. putida in vitro.

Agglutination, adherence, and root colonization by fluorescent pseudomonads

Two fractions of agglutination activity towards fluorescent pseudomonads were detected in root washes of potato, tomato, wheat, and bean. High-molecular-mass (>10 Da) components in crude root washes agglutinated only particular saprophytic, fluorescent Pseudomonas isolates. Ion-exchange treatment of the crude root washes resulted in preparations of lower-molecular-mass (10 to 10 Da) fractions which agglutinated almost all Pseudomonas isolates examined. Also, components able to suppress agglutination reactions of pseudomonads with the lower-molecular-mass root components were detected in crude root washes of all crops studied. Pseudomonas isolates were differentially agglutinated by both types of root components. The involvement of these two types of root components in short-term adherence and in colonization was studied in potato, tomato, and grass, using Pseudomonas isolates from these crops. Short-term adherence of isolates to roots was independent of their agglutination with either type of root components. With agglutination-negative mutants, the high-molecular-mass components seemed to be involved in adherence of Pseudomonas putida Corvallis to roots of all crops studied. Short-term adherence to roots of four Pseudomonas isolates could be influenced by addition of both crude and ion-exchange-treated root washes, depending on their agglutination phenotype with these root wash preparations. Potato root colonization by 10 different isolates from this crop, over a period of 7 days, was not correlated with their agglutination phenotype. Agg mutants of P. putida Corvallis were not impaired in root colonization. It is concluded that the root agglutinins studied can be involved in short-term adherence of pseudomonads to roots but do not play a decisive role in their root colonization.

Attachment of bacteria to the roots of higher plants

Attachment of soil bacteria to plant cells is supposedly the very early step required in plant-microbe interactions. Attachment also is an initial step for the formation of microbial biofilms on plant roots. For the rhizobia-legume symbiosis, various mechanisms and diverse surface molecules of both partners have been proposed to mediate in this process. The first phase of attachment is a weak, reversible, and unspecific binding in which plant lectins, a Ca(+2)-binding bacterial protein (rhicadhesin), and bacterial surface polysaccharide appear to be involved. The second attachment step requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. Cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment of Agrobacterium to plant cells. Attachment of Azospirillum brasilense to cereals roots also can be divided in two different steps. Bacterial surface proteins, capsular polysaccharide and flagella appear to govern the first binding step while extracellular polysaccharide is involved in the second step. Outer cell surface proteins and pili are implicated in the adherence of Pseudomonas species to plant roots.

Plant growth-promoting Pseudomonas sp. Strains reduce natural occurrence of anthracnose in soybean (Glycine max L.) in central Himalayan region

Biological control is an accepted important component of current plant disease management strategies. Introduction of bacterized seeds carrying bacterial isolates with proven growth-promotion capabilities and antagonistic characteristics offer a valid alternative to chemical protectants. Root colonization of disease-susceptible (PS 1024) and moderately resistant (PS1042) varieties of soyabean (Glycine Max L) by fluorescent pseudomonad (FLPs) strains GRP3, PEn-4, PRS1, and WRS-24 was studied in relation to natural occurrence of anthracnose caused by Colletotrichum dematium (Pers Ex Fr.) Grove. Rhizoplane population of FLPs was maintained at a critical level (5.3 cfu) up to 30 days of plant growth, followed by a steep decline. Indigenous FLPs population, however, remained nearly unchanged (3.0 to 2.4 log g(-1) root) between 30 days and 75 days of plant growth. The relative FLPs population in rhizosphere was lower than that in rhizoplane. Although intervarietal difference was observed, the root/shoot length remained unaffected. Compared to nonbacterized control, dry root weight was improved by FLPs treatment. Severity of foliar anthracnose was reduced significantly after FLPs treatment in the variety PS 1042. Because the point of FLPs treatment (seed bacterization) was away from the site of disease appearance (leaf), operation of induced systemic resistance in strains PEn-4 and GRP3 appears imminent.

United States Department of Agriculture-Agricultural Research Service research programs in biological control of plant diseases

A number of USDA-ARS programs directed at overcoming impediments to the use of biocontrol agents on a commercial scale are described. These include improvements in screening techniques, taxonomic studies to identify beneficial strains more precisely, and studies on various aspects of the large-scale production of biocontrol agents. Another broad area of studies covers the ecological aspects of biocontrol agents-their interaction with the pathogen, with the plant and with other aspects of the environmental complex. Examples of these studies are given and their relevance to the further development and expansion of biocontrol agents is discussed.

Mutation of rpiA in Enterobacter cloacae decreases seed and root colonization and biocontrol of damping-off caused by Pythium ultimum on cucumber

Strains of Enterobacter cloacae show promise as biocontrol agents for Pythium ultimum-induced damping-off on cucumber and other crops. E. cloacae A145 is a mini-Tn5 Km transposon mutant of strain 501R3 that was significantly reduced in suppression of damping-off on cucumber caused by P. ultimum. Strain A145 was deficient in colonization of cucumber, sunflower, and wheat seeds and significantly reduced in colonization of corn and cowpea seeds relative to strain 501R3. Populations of strain A145 were also significantly lower than those of strain 501R3 at all sampling times in cucumber, wheat, and sunflower rhizosphere. Populations of strain A145 were not detectable in any rhizosphere after 42 days, while populations of strain 501R3 remained at substantial levels throughout all experiments. Molecular characterization of strain A145 indicated mini-Tn5 Km was inserted in a region of the E. cloacae genome with a high degree of DNA and amino acid sequence similarity to rpiA, which encodes ribose-5-phosphate isomerase. In Escherichia coli, RpiA catalyzes the interconversion of ribose-5-phosphate and ribulose-5-phosphate and is a key enzyme in the pentose phosphate pathway. Ribose-5-phosphate isomerase activity in cell lysates from strain A145 was approximately 3.5% of that from strain 501R3. In addition, strain A145 was a ribose auxotroph, as expected for an rpiA mutant. Introduction of a 1.0-kb DNA fragment containing only the rpiA homologue into strain A145 restored ribose phosphate isomerase activity, prototrophy, seedling colonization, and disease suppression to levels similar to those associated with strain 501R3. Experiments reported here indicate a key role for rpiA and possibly the pentose phosphate pathway in suppression of damping-off and colonization of subterranean portions of plants by E. cloacae.

Root mucilage from pea and its utilization by rhizosphere bacteria as a sole carbon source

Plant roots secrete a complex polysaccharide mucilage that may provide a significant source of carbon for microbes that colonize the rhizosphere. High molecular weight mucilage was separated by high-pressure liquid chromatography gel filtration from low molecular weight components of pea root exudate. Purified pea root mucilage generally was similar in sugar and glycosidic linkage composition to mucilage from cowpea, wheat, rice, and maize, but appeared to contain an unusually high amount of material that was similar to arabinogalactan protein. Purified pea mucilage was used as the sole carbon source for growth of several pea rhizosphere bacteria, including Rhizobium leguminosarum 8401 and 4292, Burkholderia cepacia AMMD, and Pseudomonas fluorescens PRA25. These species grew on mucilage to cell densities of three- to 25-fold higher than controls with no added carbon source, with cell densities of 1 to 15% of those obtained on an equal weight of glucose. Micromolar concentrations of nod gene-inducing flavonoids specifically stimulated mucilage-dependent growth of R. leguminosarum 8401 to levels almost equaling the glucose controls. R. leguminosarum 8401 was able to hydrolyze p-nitrophenyl glycosides of various sugars and partially utilize a number of purified plant polysaccharides as sole carbon sources, indicating that R. leguminosarum 8401 can make an unexpected variety of carbohydrases, in accordance with its ability to extensively utilize pea root mucilage.

The role of motility in the in vitro attachment of Pseudomonas putida PaW8 to wheat roots

The attachment of motile and non-motile strains of Pseudomonas putida PaW8 to sterile wheat roots was assessed in both non-competitive and intra-specific competitive assays. The motile strain showed significantly greater attachment to wheat roots than non-motile strains in phosphate buffer. Overall, the motile strain attached better than the non-motile strain at 10(6), 10(7) and 10(8) cfu ml(-1) in competitive assays and at 10(6) and 10(7) cfu ml(-1) in non-competitive assays. When attachment was studied in Luria broth no significant difference between motile and non-motile strains was detected. P. putida PaW8 cells marked with the luxAB genes were used to compare direct detection of attached cells by luminometry with indirect detection by dilution plate counts following extraction from root material. Although direct detection permitted a rapid assessment (60 s) of attachment to surfaces, dilution plate counts provided a more sensitive method for quantification of bacteria. The detection limits were approximately 10 cfu root(-1) using dilution plate counts compared with 1000 cfu root(-1) using luminometry. All results highlighted the importance of motility for the attachment of P. putida to plant roots in simple model systems. To take this work further, studies to assess the role of motility using complex non-sterile systems are needed.

Importance of pfkA for rapid growth of Enterobacter cloacae during colonization of crop seeds

Enterobacter cloacae A-11 is a prototrophic, glycolytic mutant of strain 501R3 with a single transposon insertion in pfkA. The populations of strain A-11 on cucumber and radish seeds were smaller than the populations of strain 501R3 in natural soil, but the populations of these two strains on pea, soybean, sunflower, and sweet corn seeds were similar (D. P. Roberts, P. D. Dery, I. Yucel, J. Buyer, M. A. Holtman, and D. Y. Kobayashi, Appl. Environ. Microbiol. 65:2513-2519, 1999). The net effect of the mutation in pfkA in vitro was a shift from rapid growth on certain carbohydrates detected in seed exudates to much slower growth on other carbohydrates, amino acids, and organic acids. The impact of the mutation in pfkA was greatest on the growth rate of E. cloacae on the seeds that released the smallest quantities of fructose, other carbohydrates, and amino acids. Corn, pea, soybean, and sunflower seeds released total amounts of carbohydrates and amino acids at rates that were approximately 10- to 100-fold greater than the rates observed with cucumber and radish seeds for the first 24 h after inhibition began. The growth rate of strain A-11 was significantly less (50% less) than the growth rate of strain 501R3 on radish seeds, and the growth rate of strain A-11 was too low to estimate on cucumber seeds in sterile sand for the first 24 h after inhibition began. The growth rate of strain A-11 was also significantly lower on soybean seeds, but it was only 17% lower than the growth rate of strain 501R3. The growth rates of strains 501R3 and A-11 were similar on pea, sunflower, and corn seeds in sterile sand for the first 30 h after imbibition began. Large reductions in the growth rates of strain A-11 on seeds were correlated with subsequent decreased levels of colonization of seeds compared to the levels of colonization of strain 501R3. The strain A-11 populations were significantly smaller than the strain 501R3 populations only on radish and cucumber seeds. The mutation in pfkA appears to decrease the level of colonization by E. cloacae for seeds that release small quantities of reduced carbon compounds by decreasing the size of the pool of compounds that support rapid growth by this bacterium.

Cell envelope mutants of Pseudomonas putida: physiological characterization and analysis of their ability to survive in soil

To generate mutants with altered lipopolysaccharides (LPS) of the wild-type Pseudomonas putida KT2442, we used the mini-Tn5luxAB-Km transposon. A mutant was found among luminescent colonies and selected as a negative clone in enzyme-linked immunosorbent assay (ELISA) with monoclonal antibody (mAb) 7.3B, which recognizes the O-antigen of P. putida LPS. The DNA region of the LPS mutant interrupted by the minitransposon insertion was cloned and sequenced. Comparison of the deduced amino acid sequence with protein sequence databases showed similarity to the O-antigen polymerase (Wzy) of Salmonella enterica (muenchen). The wild-type gene was rescued by polymerase chain reaction (PCR), cloned into a broad-host-range plasmid and used to carry out complementation assays. The cloned gene was able to restore the wild-type phenotype of the P. putida wzy mutant. We constructed an isogenic mutant of the luminescent wzy mutant to which an oprL mutation was transferred by homologous recombination with an oprL::xylE cassette. The wzy mutants of P. putida were more sensitive to SDS, deoxycholate and EDTA than the corresponding parental strains. We analysed the ability of wzy, oprL and wzy oprL mutants of P. putida to colonize soil. In comparison with the wild-type strain, the ability of single mutants to colonize soil decreased; this characteristic was more evident for the double mutant, especially at high temperatures.

Role of pfkA and general carbohydrate catabolism in seed colonization by Enterobacter cloacae

Enterobacter cloacae A-11 is a transposon mutant of strain 501R3 that was deficient in cucumber spermosphere colonization and in the utilization of certain carbohydrates (D. P. Roberts, C. J. Sheets, and J. S. Hartung, Can. J. Microbiol. 38:1128-1134, 1992). In vitro growth of strain A-11 was reduced or deficient on most carbohydrates that supported growth of strain 501R3 but was unaffected on fructose, glycerol, and all amino acids and organic acids tested. Colonization by strain A-11 was significantly reduced (P Collapse

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MESH Headings

• Carbohydrate Metabolism
• DNA, Bacterial/genetics
• Enterobacter cloacae/genetics
• Enterobacter cloacae/physiology
• Genes, Bacterial
• Molecular Sequence Data
• Phosphofructokinase-1/genetics
• Phosphofructokinase-1/metabolism
• Plasmids/genetics
• Seeds/chemistry
• Seeds/metabolism
• Seeds/microbiology
• Vegetables/growth & development
• Vegetables/microbiology

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Affiliation(s)

D P Roberts Biocontrol of Plant Diseases Laboratory, USDA Agricultural Research Service, Beltsville, Maryland 20705, USA.
P D Dery
I Yucel
J Buyer
M A Holtman
D Y Kobayashi

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Role of the O-antigen of lipopolysaccharide, and possible roles of growth rate and of NADH:ubiquinone oxidoreductase (nuo) in competitive tomato root-tip colonization by Pseudomonas fluorescens WCS365

Colonization-defective, transposon-induced mutants of the efficient root colonizer Pseudomonas fluorescens WCS365 were identified with a gnotobiotic system. Most mutants were impaired in known colonization traits, i.e., prototrophy for amino acids, motility, and synthesis of the O-antigen of LPS (lipopolysaccharide). Mutants lacking the O-antigen of LPS were impaired in both colonization and competitive growth whereas one mutant (PCL1205) with a shorter O-antigen chain was defective only in colonization ability, suggesting a role for the intact O-antigen of LPS in colonization. Eight competitive colonization mutants that were not defective in the above-mentioned traits colonized the tomato root tip well when inoculated alone, but were defective in competitive root colonization of tomato, radish, and wheat, indicating they contained mutations affecting host range. One of these eight mutants (PCL1201) was further characterized and contains a mutation in a gene that shows homology to the Escherichia coli nuo4 gene, which encodes a subunit of one of two known NADH:ubiquinone oxidoreductases. Competition experiments in an oxygen-poor medium between mutant PCL1201 and its parental strain showed a decreased growth rate of mutant PCL1201. The requirement of the nuo4 gene homolog for optimal growth under conditions of oxygen limitation suggests that the root-tip environment is micro-aerobic. A mutant characterized by a slow growth rate (PCL1216) was analyzed further and contained a mutation in a gene with similarity to the E. coli HtrB protein, a lauroyl transferase that functions in lipid A biosynthesis.

A novel cell surface polysaccharide in Pseudomonas putida WCS358, which shares characteristics with Escherichia coli K antigens, is not involved in root colonization

Previously we have shown that flagella and the O-specific polysaccharide of lipopolysaccharide play a role in colonization of the potato root by plant growth-promoting Pseudomonas strains WCS374 and WCS358. In this paper, we describe a novel cell surface-exposed structure in Pseudomonas putida WCS358 examined with a specific monoclonal antibody. This cell surface structure appeared to be a polysaccharide, which was accessible to the monoclonal antibody at the outer cell surface. Further study revealed that it does not contain 2-keto-3-deoxyoctonate, heptose, or lipid A, indicating that it is not a second type of lipopolysaccharide. Instead, the polysaccharide shared some characteristics with K antigen described for Escherichia coli. From a series of 49 different soil bacteria tested, only one other potato plant growth-promoting Pseudomonas strain reacted positively with the monoclonal antibody. Mutant cells lacking the novel antigen were efficiently isolated by an enrichment method involving magnetic antibodies. Mutant strains defective in the novel antigen contained normal lipopolysaccharide. One of these mutants was affected in neither its ability to adhere to sterile potato root pieces nor its ability to colonize potato roots. We conclude that the bacterial cell surface of P. putida WCS358 contains at least two different polysaccharide structures. These are the O-specific polysaccharide of lipopolysaccharide, which is relevant for potato root colonization, and the novel polysaccharide, which is not involved in adhesion to or colonization of the potato root.

Utilization of heterologous siderophores and rhizosphere competence of fluorescent Pseudomonas spp

In this study, the potential of different Pseudomonas strains to utilize heterologous siderophores was compared with their competitiveness in the rhizosphere of radish. This issue was investigated in interactions between Pseudomonas putida WCS358 and Pseudomonas fluoresceins WCS374 and in interactions between strain WCS358 and eight indigenous Pseudomonas strains capable of utilizing pseudobactin 358. During four successive plant growth cycles of radish, strain WCS358 significantly reduced rhizosphere population densities of the wild-type strain WCS374 by up to 30 times, whereas derivative strain WCS374(pMR), harboring the siderophore receptor PupA for ferric pseudobactin 358, maintained its population density. Studies involving interactions between strain WCS358 and eight different indigenous Pseudomonas strains demonstrated that despite the ability of these indigenous isolates to utilize pseudobactin 358, their rhizosphere population densities were significantly reduced by strain WCS358 by up to 20 times. Moreover, rhizosphere colonization by WCS358 was not affected by any of these indigenous strains, even though siderophore-mediated growth inhibition of WCS358 by a majority of these strains was demonstrated in a plate bioassay. In conclusion, it can be stated that siderophore-mediated competition for iron is a major determinant in interactions between WCS358 and WCS374 in the rhizosphere. Moreover, our findings support the common assumption that cloning of siderophore receptor genes from one Pseudomonas strain into another can confer a competitive advantage in interactions in the rhizosphere. Interactions between WCS358 and the selected indigenous rhizosphere isolates, however, indicate that other traits also contribute to the rhizosphere competence of fluorescent Pseudomonas spp.Key words: siderophore, siderophore receptors, root colonization, fluorescent Pseudomonas.

Traits of fluorescent Pseudomonas spp. involved in suppression of plant root pathogens

Certain members of the fluorescent pseudomonad group have been shown to be potential agents for the biocontrol of plant root diseases. The major problems with the commercialization of these beneficial strains are that few wild-type strains contain all the desired characteristics for this process and the performance of strains in different soil and climatic conditions is not reproducible. Consequently, prior to selection and/or improvement of suitable strains for biocontrol purposes, it is necessary to understand the important traits required for this purpose. The production of fluorescent siderophores (iron-binding compounds) and antibiotic compounds has been recognized as important for the inhibition of plant root pathogens. Efficient root colonization is also a prerequisite for successful biocontrol strains. This review discusses some of the characteristics of fluorescent pseudomonads that have been suggested to be important for biocontrol. The genetic organization and regulation of these processes is also examined. This information is necessary for attempts aimed at the improvement of strains based on deregulating pathways or introducing traits from one strain to another. The release of genetically engineered organisms into the environment is governed by regulations, and this aspect is summarized. The commercialization of fluorescent pseudomonads for the biological control of plant root diseases remains an exciting possibility. The understanding of the relevant characteristics will facilitate this process by enabling the direct selection and/or construction of strains which will perform under a variety of environmental conditions.

Molecular mechanisms of attachment of Rhizobium bacteria to plant roots

Attachment of bacteria to plant cells is one of the earliest steps in many plant-bacterium interactions. This review covers the current knowledge on one of the best-studied examples of bacterium-plant attachment, namely the molecular mechanism by which Rhizobium bacteria adhere to plant roots. Despite differences in several studies with regard to growth conditions of bacteria and plants and to methods used for measuring attachment, an overall consensus can be drawn from the available data. Rhizobial attachment to plant root hairs appears to be a two-step process. A bacterial Ca(2+)-binding protein, designated as rhicadhesin, is involved in direct attachment of bacteria to the surface of the root hair cell. Besides this step, there is another step which results mainly in accumulation and anchoring of the bacteria to the surface of the root hair. This leads to so-called firm attachment. Depending on the growth conditions of the bacteria, the latter step is mediated by plant lectins and/or by bacterial appendages such as cellulose fibrils and fimbriae. The possible role of these adhesions in root nodule formation is discussed.

Contribution of phenazine antibiotic biosynthesis to the ecological competence of fluorescent pseudomonads in soil habitats

Phenazine antibiotics produced by Pseudomonas fluorescens 2-79 and Pseudomonas aureofaciens 30-84, previously shown to be the principal factors enabling these bacteria to suppress take-all of wheat caused by Gaeumannomyces graminis var. tritici, also contribute to the ecological competence of these strains in soil and in the rhizosphere of wheat. Strains 2-79 and 30-84, their Tn5 mutants defective in phenazine production (Phz-), or the mutant strains genetically restored for phenazine production (Phz+) were introduced into Thatuna silt loam (TSL) or TSL amended with G. graminis var. tritici. Soils were planted with three or five successive 20-day plant-harvest cycles of wheat. Population sizes of Phz- derivatives declined more rapidly than did population sizes of the corresponding parental or restored Phz+ strains. Antibiotic biosynthesis was particularly critical to survival of these strains during the fourth and fifth cycles of wheat in the presence of G. graminis var. tritici and during all five cycles of wheat in the absence of take-all. In pasteurized TSL, a Phz- derivative of strain 30-84 colonized the rhizosphere of wheat to the same extent that the parental strain did. The results indicate that production of phenazine antibiotics by strains 2-79 and 30-84 can contribute to the ecological competence of these strains and that the reduced survival of the Phz- strains is due to a diminished ability to compete with the resident microflora.

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.

Mutational changes in physiochemical cell surface properties of plant-growth-stimulating Pseudomonas spp. do not influence the attachment properties of the cells

Bacteriophage-resistant mutant strains of the root-colonizing Pseudomonas strains WCS358 and WCS374 lack the O-antigenic side chain of the lipopolysaccharide, as was shown by the loss of the typical lipopolysaccharide ladder pattern after analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These strains differed from their parent strains in cell surface hydrophobicity and in cell surface charge. The observed variation in these physicochemical characteristics could be explained by the differences in sugar composition. The mutant strains had no altered properties of adherence to sterile potato roots compared with their parental strains, nor were differences observed in the firm adhesion to hydrophilic, lipophilic, negatively charged, or positively charged artificial surfaces. These results show that neither physicochemical cell surface properties nor the presence of the O-antigenic side chain plays a major role in the firm adhesion of these bacterial cells to solid surfaces, including potato roots.

Fusarium Wilt Suppression and Agglutinability of Pseudomonas putida

Mutants of Pseudomonas putida (Agg − ) that lack the ability to agglutinate with components present in washes of bean and cucumber roots showed limited potential to protect cucumber plants against Fusarium oxysporum f. sp. cucumerinum . However, a higher level of protection was observed against Fusarium wilt in cucumber plants coinoculated with the parental bacterium (Agg + ), which was agglutinable. The Agg − mutants did not colonize the roots of cucumber plants as extensively as the Agg + parental isolate did. In competition experiments involving bean roots inoculated with a mixture of Agg + and Agg − bacteria, the Agg + strains colonized roots to a greater extent than the Agg − cells did. These data suggest that the Agg + phenotype provides additional interactions that aid in the beneficial character of P. putida.