Root Colonization Ability Research Articles

The ColRS Two-Component System Regulates Membrane Functions and Protects Pseudomonas putida against Phenol

As reported, the two-component system ColRS is involved in two completely different processes. It facilitates the root colonization ability of Pseudomonas fluorescens and is necessary for the Tn4652 transposition-dependent accumulation of phenol-utilizing mutants in Pseudomonas putida. To determine the role of the ColRS system in P. putida, we searched for target genes of response regulator ColR by use of a promoter library. Promoter screening was performed on phenol plates to mimic the conditions under which the effect of ColR on transposition was detected. The library screen revealed the porin-encoding gene oprQ and the alginate biosynthesis gene algD occurring under negative control of ColR. Binding of ColR to the promoter regions of oprQ and algD in vitro confirmed its direct involvement in regulation of these genes. Additionally, the porin-encoding gene ompA(PP0773) and the type I pilus gene csuB were also identified in the promoter screen. However, it turned out that ompA(PP0773) and csuB were actually affected by phenol and that the influence of ColR on these promoters was indirect. Namely, our results show that ColR is involved in phenol tolerance of P. putida. Phenol MIC measurement demonstrated that a colR mutant strain did not tolerate elevated phenol concentrations. Our data suggest that increased phenol susceptibility is also the reason for inhibition of transposition of Tn4652 in phenol-starving colR mutant bacteria. Thus, the current study revealed the role of the ColRS two-component system in regulation of membrane functionality, particularly in phenol tolerance of P. putida.

Screening rhizobacteria for biological control of Fusarium root and crown rot of sorghum in Ethiopia

Fusarium oxysporum Schlectend causes root and crown rot in several crops including sorghum that results in low grain yield in Ethiopia and other East African countries. Seventy-eight bacterial isolates were obtained and subsequently tested both in vitro and in the greenhouse. Of the 78 isolates tested, 23 displayed between 30 and 66.3% inhibition of in vitro mycelial growth of F. oxysporum and also showed significant root colonization ability on sorghum seedlings. These isolates were further tested for their biocontrol ability against F. oxysporum in the greenhouse. Four isolates viz. KBE5-7, KBE5-1, KBE2-5 and NAE5-5 resulted in 100% disease suppression and no symptoms of root and crown rot were observed compared to the control. The complete suppression of F. oxysporum by these isolates was also confirmed by root plating on Fusarium-selective medium. The most effective isolates were identified by means of the API system as members of the Genus Bacillus including B. cereus , B. subtilis , B. circulans , B. licheniformis and B. stearothermophilus . Two other isolates, which colonized the sorghum rhizosphere and resulted in more than 70% disease suppression, have been identified as Chromobacterium violaceum . The study demonstrated effective biological control by the rhizobacterial isolates tested, thereby indicating the possibility of application of rhizobacteria for control of soilborne diseases of sorghum in Ethiopia and other countries.

Heterozygosis drives maize hybrids to select elite 2,4-diacethylphloroglucinol-producing Pseudomonas strains among resident soil populations

By comparing the distribution of two genomic markers among Pseudomonas strains recovered from the rhizosphere of two maize hybrids with those of strains recovered from the rhizosphere of their four respective parental lines, we showed that both hybrids supported more elite probiotic strains than the parents. Elite Pseudomonas strains showed genomic potential for both an appropriate in vitro 2,4-diacetylphloroglucinol (DAPG) productivity, and a superior root-colonization ability. The actual biocontrol and root-colonization abilities of these strains were confirmed by bioassays on five fungal strains and on axenic maize plants. Furthermore, results on the abundance and genetic diversity of resident DAPG+ Pseudomonas strains indicated that each hybrid was able to select its own specific DAPG+ population, whereas the four parental lines were not. The evidence that heterozygosis can drive maize plants to select elite probiotic rhizospheric DAPG+ Pseudomonas strains opens the way to a new strategy in the set up of plant breeding for low-input and organic agriculture.

Quorum-sensing system influences root colonization and biological control ability in Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a biocontrol agent isolated from a wheat take-all decline soil in China. This strain produces several antifungal compounds, such as 2,4-diacetylphloroglucinol (2,4-DAPG), hydrogen cyanide and siderophore(s). Our recent work revealed that strain 2P24 employs a quorum-sensing system to regulate its biocontrol activity. In this study, we identified a quorum-sensing system consisting of PcoR and PcoI of the LuxR-LuxI family from strain 2P24. Deletion of pcoI from 2P24 abolishes the production of the quorum-sensing signals, but does not detectably affect the production of antifungal metabolites. However, the mutant is significantly defective in biofilm formation, colonization on wheat rhizosphere and biocontrol ability against wheat take-all, whilst complementation of pcoI restores the biocontrol activity to the wild-type level. Our data indicate that quorum sensing is involved in regulation of biocontrol activity in P. fluorescens 2P24.

Exopolysaccharide Structure Is Not a Determinant of Host-Plant Specificity in Nodulation of Vicia sativa Roots

Exopolysaccharide (EPS)-deficient strains of the root nodule symbiote Rhizobium leguminosarum induce formation of abortive infection threads in Vicia sativa subsp. nigra roots. As a result, the nodule tissue remains uninfected. Formation of an infection thread can be restored by coinoculation of the EPS-deficient mutant with a Nod factor-deficient strain, which produces a similar EPS structure. This suggests that EPS contributes to host-plant specificity of nodulation. Here, a comparison was made of i) coinoculation with heterologous strains with different EPS structures, and ii) introduction of the pRL1JI Sym plasmid or a nod gene-encoding fragment in the same heterologous strains. Most strains not complementing in coinoculation experiments were able to nodulate V. sativa roots as transconjugants. Apparently, coinoculation is a delicate approach in which differences in root colonization ability or bacterial growth rate easily affect successful infection-thread formation. Obviously, lack of infection-thread formation in coinoculation studies is not solely determined by EPS structure. Transconjugation data show that different EPS structures can allow infection-thread formation and subsequent nodulation of V. sativa roots.

The Type III Secretion System of Biocontrol Pseudomonas fluorescens KD Targets the Phytopathogenic Chromista Pythium ultimum and Promotes Cucumber Protection

The type III secretion system (TTSS) is used by Proteobacteria for pathogenic or symbiotic interaction with plant and animal hosts. Recently, TTSS genes thought to originate from the phytopathogen Pseudomonas syringae were evidenced in Pseudomonas fluorescens KD, which protects cucumber from the oomycete Pythium ultimum (kingdom Chromista/Stramenopila). However, it is not known whether the TTSS contributes to plant protection by the bacterium and, if so, whether it targets the plant or the phytopathogen. Inactivation of TTSS gene hrcV following the insertion of an omega cassette strongly reduced the biocontrol activity of the pseudomonad against P. ultimum on cucumber when compared with the wild type, but had no effect on its root-colonization ability. Analysis of a plasmid-based transcriptional hrpJ'-inaZ reporter fusion revealed that expression in strain KD of the operon containing hrcV was strongly stimulated in vitro and in situ by the oomycete and not by the plant. In vitro, both strain KD and its hrcV mutant reduced the activity level of the pectinase polygalacturonase (a key pathogenicity factor) from P. ultimum, but the reduction was much stronger with the wild type. Together, these results show that the target range of bacterial TTSS is not restricted to plants and animals but also can include members of Chromista/Stramenopila, and suggest that virulence genes acquired horizontally from phytopathogenic bacteria were functionally recycled in biocontrol saprophytic Pseudomonas spp., resulting in enhanced plant protection by the latter.

Colonization and Population Changes of a Biocontrol Agent, Paenibacillus polymyxa E681, in Seeds and Roots

Paenibacillus polymyxa E681, with its plant growth promotion and root colonization ability, has been proven to be a promising biocontrol agent of cucumber and barley. This study investigated the attributes related to the movement of bacteria from the seed to the radicle and to the whole root system. It also illustrated the existing form and population changes of the bacteria on seed and root using the scanning electron microscope and confocal laser scanning microscopy. The bacteria invaded and colonized the inside of the seed coat while the seeds were soaked in bacterial suspension. Almost the same number of bacteria on seed surface invaded the inside of the seed coat right after seed soaking. The population densities of E681 increased greatly inside as well as on the surface of the seed before the radicle emerged. The bacteria attached on the emerging radicle directly affected the initial population of newly emerg-ing root. The colonized cells on the root were arranged linearly toward the elongation of the root axis. In addition to colonizing the root surface, strain E681 was found inside the roots, where cells colonized the inter-cellular space between certain epidermal and cortical cells. When the cucumber seeds were soaked in bacterial suspension and sown in pot, the bacterial populations attached on both the surface and inside of the root were sustained up to harvesting time. This means that E681 successfully colonized the root of cucumber and sustained its population density up to harvesting time through seed treatment.

Suppression of Rhizoctonia solani diseases of sugar beet by antagonistic and plant growth-promoting yeasts.

Isolates of Candida valida, Rhodotorula glutinis and Trichosporon asahii from the rhizosphere of sugar beet in Egypt were examined for their ability to colonize roots, to promote plant growth and to protect sugar beet from Rhizoctonia solani AG-2-2 diseases, under glasshouse conditions. Root colonization abilities of the three yeast species were tested using the root colonization plate assay and the sand-tube method. In the root colonization plate assay, C. valida and T. asahii colonized 95% of roots after 6 days, whilst Rhod. glutinis colonized 90% of roots after 8 days. Root-colonization abilities of the three yeast species tested by the sand-tube method showed that roots and soils attached to roots of sugar beet seedlings were colonized to different degrees. Population densities showed that the three yeast species were found at all depths of the rhizosphere soil adhering to taproots up to 10 cm, but population densities were significantly (P < 0.05) greater in the first 4 cm of the root system compared with other root depths. The three yeast species, applied individually or in combination, significantly (P < 0.05) promoted plant growth and reduced damping off, crown and root rots of sugar beet in glasshouse trials. The combination of the three yeasts (which were not inhibitory to each other) resulted in significantly (P < 0.05) better biocontrol of diseases and plant growth promotion than plants exposed to individual species. Isolates of C. valida, Rhod. glutinis and T. asahii were capable of colonizing sugar beet roots, promoting growth of sugar beet and protecting the seedlings and mature plants from R. solani diseases. This is the first successful attempt to use yeasts as biocontrol agents against R. solani which causes root diseases. Yeasts were shown to provide significant protection to sugar beet roots against R. solani, a serious soil-borne root pathogen. Yeasts also have the potential to be used as biological fertilizers.

Identification and characterization of rhizobacteria isolated by enrichment culture on spinach roots

Rhizobacteria can be used for biological control and environmental restoration. In this study, we performed enrichment culture of rhizobacteria, identified isolates, and investigated the physiological properties of the bacterial isolates. Five bacteria differing in their colony morphology were isolated from spinach roots as enriched rhizobacteria. Four isolates were identified by sequencing of 16S rDNA as β and γ-Proteobacteria; 16S rDNA sequencing was not completed on one isolate. Based on microscopic observation, we determined that at least two types of bacteria differing in their morphology co-existed in this isolate, and that it may not be possible to culture the two types separately. Based on tests of substrate utilization, we could not find the characteristics that were common to the isolates. One of the five isolates was inoculated into non-sterile soil, and we examined its root-colonizing ability. The test strain which was not detected in the non-rhizosphere soil, accounted for about 20% of the total bacteria on the roots. These results suggested that enrichment culture might be useful for isolating bacteria with a high root-colonizing ability

Characterization of NADH dehydrogenases of Pseudomonas fluorescens WCS365 and their role in competitive root colonization.

The excellent-root-colonizing Pseudomonas fluorescens WCS365 was selected previously as the parental strain for the isolation of mutants impaired in root colonization. Transposon mutagenesis of WCS365 and testing for root colonization resulted in the isolation of mutant strain PCL1201, which is approximately 100-fold impaired in competitive tomato root colonization. In this manuscript, we provide evidence that shows that the lack of NADH dehydrogenase I, an enzyme of the aerobic respiratory chain encoded by the nuo operon, is responsible for the impaired root-colonization ability of PCL1201. The complete sequence of the nuo operon (ranging from nuoA to nuoN) of P. fluorescens WCS365 was identified, including the promoter region and a transcriptional terminator consensus sequence downstream of nuoN. It was shown biochemically that PCL1201 is lacking NADH dehydrogenase I activity. In addition, the presence and activity of a second NADH dehydrogenase, encoded by the ndh gene, was identified to our knowledge for the first time in the genus Pseudomonas. Since it was assumed that low-oxygen conditions were present in the rhizosphere, we analyzed the activity of the nuo and the ndh promoters at different oxygen tensions. The results showed that both promoters are up-regulated by low concentrations of oxygen and that their levels of expression vary during growth. By using lacZ as a marker, it was shown that both the nuo operon and the ndh gene are expressed in the tomato rhizosphere. In contrast to the nuo mutant PCL1201, an ndh mutant of WCS365 appeared not to be impaired in competitive root tip colonization.

Effect of lac ZY-marking of the 2,4-diacetyl-phloroglucinol producing Pseudomonas fluorescens-strain 5-2/4 on its physiological performance and root colonization ability

Transgenic Pseudomonas fluorescens 5-2/4 with reinforced 2,4-diacetyl phloroglucinol (phl) production had shown increased biocontrol ability towards Pythium ultimum (Pu), but inferior root colonization ability compared to its wild type 5.014. Therefore, enhanced root colonization ability of the transgenic strain by repeated inoculation and reisolation on tomato plants was suggested. As a preparation for repeated inoculation and reisolation cycles, the construction of a negative control of the transgenic strain 5-2/4 by marking with lacZY and screening for a mutant possessing qualities comparable to 5-2/4 was performed. Morphologically, colonies of all of the 11 selected mutants were similar on MLXgal medium. The root colonization ability of two of the lacZY-marked strains (mutants 1 and 10) was comparable to the parental strain. These were also able to compete with the resident microflora of tomato seedlings to the same extent as the parental strain. Five mutants were excluded due to lower growth rates on Yeast Malt, King's B Medium (KB) and 0.1 Tryptic Soy Agar (mutant 4, 5 and 8), excessive growth and higher siderophore production on KB (mutant 10) and increased protease production (mutant 2). With respect to in vitro-antagonism of Pu, no differences could be found between the target strain and mutants 1, 3, 6, 7 and 9. Examination of sole carbon source utilization of these five lacZY-marked strains revealed a significantly higher utilization of alpha-D-lactose and lactulose compared to 5-2/4. However, significant differences could be found for 51% of the utilized carbon sources. Cluster analysis showed a high degree of similarity between 5-2/4 and mutant 1 both when analyzed with and without alpha-D-lactose. As mutant 1 also represented the colonization pattern most similar to the parental strain 5-2/4, it presents a presumptive subject for a negative control in the following inoculation and reisolation studies on tomato.

Isolation and Phenotypic Characterization of Plant Growth–Promoting Rhizobacteria with High Antiphytopathogenic Activity and Root-Colonizing Ability

Some bacterial strains isolated from the plant rhizosphere showed high root-colonizing ability and antiphytopathogenic activity against 6 fungal species. The antifungal activity was species-specific, which could be accounted for by the fact that the isolates differed in the ability to produce lytic enzymes (chitinases, proteases, and lipases) and to secrete cyanide. The possibility of using there rhizobacteria to control phytopathogens is discussed.

Using GUS expression in a nonpathogenic Fusarium oxysporum strain to measure fungal biomass

Nonpathogenic Fusarium oxysporum strain SA70, a biological control agent against tomato fusarium wilt, was genetically transformed with both hygromycin B resistance (Hph) and p-glucuronidase (GusA) genes. Transformant 70T01 was selected for further testing based on its stable expression of (3-glucuronidase (GUS) activity, resistance to hygromycin B (HmB), and retention of wild-type characteristics, including growth rate, root colonization ability, and disease control efficacy. Single copies of each of the GusA and Hph genes were stably integrated into the 70T01 genomic DNA. Expression of GUS, as measured by fluorometry, was highly correlated with mycelial dry weight. Assayable GUS activity was highest in young mycelia and was detectable in as little as 1 ng of mycelia (dry weight). GUS activity was extracted from protoplasts generated from germinating spores and the activity in the extracts related to the number of viable protoplasts (colony-forming protoplasts, CFPs). Fungal biomass in 70T01 -infected tomato plant roots, as determined by the GUS-CFP method, was compared with that determined by the conventional method of plating macerated, infected plant material. The results demonstrated that while similar changes in the relative levels of fungal biomass could be detected with both methods, GUS-CFP biomass estimates were typically 6- to 50-fold higher than those determined by the plating method. The GUS-CFP assay provides a rapid and sensitive technique for determining fungal biomass in natural ecological settings.

Selection of Tn5::lacZ mutants isogenic to wild type Azospirillum brasilense strains capable of growing at sub-optimal temperature

Azospirillum brasilense strains, CDJA and A40, capable of growing at sub-optimal temperature were tagged with stable chromogenic marker Tn5-lacZ. Mutants were screened for plant growth promoting activities at 20, 22, 25, 30 and 37 °C. Mutants MC48 and MA3 were found to fix nitrogen upto 85% and produced indole acetic acid (IAA) and siderophore in isogenic manner to their respective wild type strains, CDJA and A40, at sub-optimal temperatures. Co-inoculation of mutants with their respective parent (1:1 ratio) to the wheat revealed that colonization potential of the mutants was affected greatly. Tn5-lacZ tagged mutants MC48 and MA3 were found isogenic to their respective wild type Azospirillum strain, with regards to plant growth promoting activities and root colonization ability.

Tomato seed and root exudate sugars: composition, utilization by Pseudomonas biocontrol strains and role in rhizosphere colonization.

The role of tomato seed and root exudate sugars as nutrients for Pseudomonas biocontrol bacteria was studied. To this end, the major exudate sugars of tomato seeds, seedlings and roots were identified and quantified using high-performance liquid chromatographic (HPLC) analysis. Glucose, fructose and maltose were present in all studied growth stages of the plant, but the ratios of these sugars were strongly dependent on the developmental stage. In order to study the putative role of exudate sugar utilization in rhizosphere colonization, two approaches were adopted. First, after co-inoculation on germinated tomato seeds, the root-colonizing ability of the efficient root-colonizing P. fluorescens strain WCS365 in a gnotobiotic quartz sand-plant nutrient solution system was compared with that of other Pseudomonas biocontrol strains. No correlation was observed between the colonizing ability of a strain and its ability to use the major exudate sugars as the only carbon and energy source. Secondly, a Tn5lacZ mutant of P. fluorescens strain WCS365, strain PCL1083, was isolated, which is impaired in its ability to grow on simple sugars, including those found in exudate. The mutation appeared to reside in zwf, which encodes glucose-6-phosphate dehydrogenase. The mutant grows as well as the parental strain on other media, including tomato root exudate. After inoculation of germinated sterile tomato seeds, the mutant cells reached the same population levels at the root tip as the wild-type strain, both alone and in competition, indicating that the ability to use exudate sugars does not play a major role in tomato root colonization, despite the fact that sugars have often been reported to represent the major exudate carbon source. This conclusion is supported by the observation that the growth of mutant PCL1083 in vitro is inhibited by glucose, a major exudate sugar, at a concentration of 0.001%, which indicates that the glucose concentration in the tomato rhizosphere is very low.

Effect of soil-spraying time on root-colonization ability of antagonistic Streptomyces griseoviridis

The root-colonization ability of Streptomyces griseoviridis Anderson et al. was tested on turnip rape (Brassica rapa subsp. oleifera DC.) and carrot (Daucus carota L.) by the sand-tube method. Nonsterile sand was sprayed with a microbial suspension immediately or 7 days after the seed had been sown. Results expressed as population frequencies and densities indicated that S. griseoviridis effectively colonizes the rhizosphere when the microbe is applied immediately after sowing but less effectively when it is applied 7 days later. Detection values of S. griseoviridis were higher for turnip rape than for carrot. In sterile sand, S. griseoviridis invaribly colonized the rhizosphere of turnip rape after each of the two applications. These findings indicate that S. griseoviridis can compete with indigenous soil microbes in the rhizosphere if it is sufficiently abundant in the soil before the seed emerges. If applied later, however, it competes rather poorly. In root-free nonsterile sand, S. griseoviridis dispersed and survived well.

Role of root colonization ability of plant growth promoting fungi in the suppression of take-all and common root rot of wheat

Plant growth promoting fungal (PGPF) isolates — GS6-1, GS7-4 ( Phoma sp.) and GU23-3 (non-sporulating fungus), from zoysiagrass rhizosphere, suppressed take-all and common root rot of spring wheat caused by Gaeumannomyces graminis var. tritici (Ggt) and Cochliobolus sativus (Cs), respectively, in the greenhouse. Barley kernels colonized with pathogens (0.1%, 0.5% or 1.0% w w ) and PGPF isolates (1.0%, 1.5% or 2.0% w w ) were added to soil individually and in different combinations of pathogen and PGPF. With an increase in the rate of PGPF amendment, all three PGPF isolates suppressed take-all and common root rot ( P = 0.05), irrespective of the pathogen inoculum level. Among the PGPF isolates, GS6-1 and GS7-4 were more effective than GU23-3. When added to soil individually, PGPF isolates were recovered from 75 to 95% of root segments, and Ggt and Cs from 88–95% of root segments. Both Ggt and Cs were isolated less frequently from roots when the PGPF concentration was high (2.0%); however, the PGPF isolates were isolated from roots in high frequencies even when the pathogen inoculum level was high. Isolate GS6-1 promoted plant growth even when the pathogen inoculum rate was high, as did GS7-4, but to a lesser amount. These fungal isolates suppressed take-all and common root rot, and two of them also increased plant growth. The suppression of take-all and common root rot appeared to be due to the competitive root colonization by PGPF isolates, thus blocking sites available for infection by pathogens.

Responses of cucumber cultivars to induction of systemic resistance against anthracnose by plant growth promoting fungi

Initial experiment on the reactions of five Japanese cultivars of cucumber toColletotrichum orbiculare infection in the greenhouse revealed that cv Suyo and Gibai were susceptible and moderately susceptible, respectively, while cv Shogoin fushinari and Sagami hanjiro were resistant to infection byC. orbiculare; cv Ochiai fushinari was moderately resistant. The ability of 16 plant growth promoting fungi (some isolates belonged to species ofPhoma and some non-sporulating isolates) isolated from zoysiagrass rhizospheres to induce systemic resistance in the above five cucumber cultivars was tested by growing plants in potting medium infested with barley grain inocula of PGPF in the greenhouse. The second true leaves of 21-day-old plants were challenge inoculated withC. orbiculare and disease assessed. Nine, out of 16 isolates, caused significant reduction of disease caused byC. orbiculare in at least two cultivars.Phoma isolates (GS8-1 and GS8-2) and non-sporulating isolates (GU21-2, GU23-3, and GU24-3) significantly reduced the disease in all the five cultivars. The disease suppression in cucumber was due to the induction of systemic resistance, since the inducer(s) and the pathogen were separated spatially and that the inducer did not colonize aerial portions. The resistance induced by certain isolates in a susceptible cultivar was less than that in a resistant cultivar. Disease suppression caused by isolate GU21-2 was similar to theC. orbiculare induced control in certain cultivars. The average rate of expansion of lesion diameter on leaves due toC. orbiculare was slower due to induction with the selected plant growth promoting fungi compared to the uninduced control plants. Roots of four cultivars were colonized by only three isolates, however, roots of one cultivar (Suyo) was colonized by five isolates suggesting the cultivar-specific root colonization ability.

Root Development in Sweet Corn Inoculated with the Biocontrol Fungus Trichoderma harzianum

Production of shrunken-2 sweet corn is often limited by poor establishment. Good root development is necessary for establishment, and it can be limited by stress or disease. Trichoderma harzianum strain 1295-22 was developed as a biocontrol fungus with particularly strong root-colonizing ability. In addition to acting as a biocontrol agent, it stimulates root growth. In greenhouse experiments using field soil, root dry weight 21 days after planting was 500 mg, greater compared with 320 mg in uninoculated controls, an average increase of &gt;50%. The thoroughness of soil exploration more than doubled, from 31% (control) to 70% (Trichoderma) of the soil being within 1 cm of a root. The difference in performance was not attributable to disease: no disease symptoms were evident, the occurrence of disease organisms was low, it was the same in both treatments, and it was not associated with smaller plants. Furthermore, the greatest differences were noted in steam-sterilized soil. Colonization of the roots by Trichoderma was related to the age of the root. The oldest part of the radicle had 106 cfu/g root DW. Branched seminal roots had 105.5 cfu/g. Even the rapidly growing tips of the first-whorl roots were well-colonized (104.7 cfu/g). The mechanism of increased root growth has not been identified, but colonized roots acidify about 0.1 pH units more than control roots, which could cause both faster acid-growth and increased ion uptake. Ion leakage into distilled water is about 25% lower in colonized roots.

Root-colonization ability of antagonistic Streptomyces griseoviridis

Root-colonization ability of Streptomyces griseoviridis was tested on turnip rape (Brassica rapa subsp. oleifera) and carrot (Daucus carota) by the plate test and the sand-tube method. In the plate test, colonized root length of total root length was highly significantly greater for turnip rape roots (72%) from those for carrot roots (1%). In the sand-tube method, root-colonization ability was examined in nonsterile soil, and no water was added after sowing. Seeds were treated with spores of S. griseoviridis or the biofungicide Mycostop. Roots were cut into 2-cm segments, and the root segments and the rhizosphere soil were studied separately. Root-colonization frequencies and population densities of the microbe in the rhizosphere soil indicated that S. griseoviridis successfully colonized turnip rape but weakly colonized carrot. Root-colonization of turnip rape is accounted for as proliferation of S. griseoviridis in the rhizosphere of turnip rape seedlings and is not due to the movement of microbe through the rhizosphere by water infiltration.