The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

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Microbial interactions and biocontrol in the rhizosphere

Certain strains of fluorescent pseudomonads are important biological components of agricultural soils that are suppressive to diseases caused by pathogenic fungi on crop plants. The biocontrol abilities of such strains depend essentially on aggressive root colonization, induction of systemic resistance in the plant, and the production of diffusible or volatile antifungal antibiotics. Evidence that these compounds are produced in situ is based on their chemical extraction from the rhizosphere and on the expression of antibiotic biosynthetic genes in the producer strains colonizing plant roots. Well-characterized antibiotics with biocontrol properties include phenazines, 2,4-diacetylphloroglucinol, pyoluteorin, pyrrolnitrin, lipopeptides, and hydrogen cyanide. In vitro, optimal production of these compounds occurs at high cell densities and during conditions of restricted growth, involving (i) a number of transcriptional regulators, which are mostly pathway-specific, and (ii) the GacS/GacA two-component system, which globally exerts a positive effect on the production of extracellular metabolites at a posttranscriptional level. Small untranslated RNAs have important roles in the GacS/GacA signal transduction pathway. One challenge in future biocontrol research involves development of new strategies to overcome the broad toxicity and lack of antifungal specificity displayed by most biocontrol antibiotics studied so far.

Regulation of antibiotic production in root-colonizing pseudomonas spp. and relevance for biological control of plant disease

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The Vegetation of Wisconsin: An Ordination of Plant Communities.

Environmental drivers in mangrove establishment and early development: A review

Understanding the environmental factors that regulate the biosynthesis of antimicrobial compounds by disease-suppressive strains of Pseudomonas fluorescens is an essential step toward improving the level and reliability of their biocontrol activity. We used liquid culture assays to identify several minerals and carbon sources which had a differential influence on the production of the antibiotics 2,4-diacetylphloroglucinol (PHL), pyoluteorin (PLT), and pyrrolnitrin and the siderophores salicylic acid and pyochelin by the model strain CHA0, which was isolated from a natural disease-suppressive soil in Switzerland. Production of PHL was stimulated by Zn2+, NH4Mo2+, and glucose; the precursor compound mono-acetylphloroglucinol was stimulated by the same factors as PHL. Production of PLT was stimulated by Zn2+, Co2+, and glycerol but was repressed by glucose. Pyrrolnitrin production was increased by fructose, mannitol, and a mixture of Zn2+ and NH4Mo2+. Pyochelin production was increased by Co2+, fructose, mannitol, and glucose. Interestingly, production of its precursor salicylic acid was increased by different factors, i.e., NH4Mo2+, glycerol, and glucose. The mixture of Zn2+ and NH4Mo2+ with fructose, mannitol, or glycerol further enhanced the production of PHL and PLT compared with either the minerals or the carbon sources used alone, but it did not improve siderophore production. Extending fermentation time from 2 to 5 days increased the accumulation of PLT, pyrrolnitrin, and pyochelin but not of PHL. When findings with CHA0 were extended to an ecologically and genetically diverse collection of 41 P. fluorescens biocontrol strains, the effect of certain factors was strain dependent, while others had a general effect. Stimulation of PHL by Zn2+ and glucose was strain dependent, whereas PLT production by all strains that can produce this compound was stimulated by Zn2+ and transiently repressed by glucose. Inorganic phosphate reduced PHL production by CHA0 and seven other strains tested but to various degrees. Production of PLT but not pyrrolnitrin by CHA0 was also reduced by 100 mM phosphate. The use of 1/10-strength nutrient broth-yeast extract, compared with standard nutrient broth-yeast extract, amended with glucose and/or glycerol resulted in dramatically increased accumulations of PHL (but not PLT), pyochelin, and salicylic acid, indicating that the ratio of carbon source to nutrient concentration played a key role in the metabolic flow. The results of this study (i) provide insight into the biosynthetic regulation of antimicrobial compounds, (ii) limit the number of factors for intensive study in situ, and (iii) indicate factors that can be manipulated to improve bacterial inoculants.

Environmental Factors Modulating Antibiotic and Siderophore Biosynthesis by Pseudomonas fluorescens Biocontrol Strains

The antibacterial activity of the leaves and bark of mangrove plants, Avicennia marina, A. officinalis, Bruguiera sexangula, Exoecaria agallocha, Lumnitzera racemosa, and Rhizophora apiculata was evaluated against antibiotic resistant pathogenic bacteria, Staphylococcus aureus and Proteus sp. Soxhlet extracts of petroleum ether, ethyl acetate, ethanol and water were prepared and evaluated the antibacterial activity using agar diffusion method. Most of the plant extracts showed promising antibacterial activity against both bacterial species. However, higher antibacterial activity was observed for Staphylococcus aureus than Proteus sp. The highest antibacterial activity was shown by ethyl acetate of mature leaf extracts of E. agallocha for Staphylococcus aureus. All ethyl acetate extracts showed higher inhibition against S. aureus while some extracts of chloroform, ethyl acetate and ethanol gave inhibition against Proteus sp. None of the petroleum ether and aqueous extracts showed inhibition against Proteus sp. All fresh plant materials did also show more antibacterial activity against both bacterial strains than did dried plant extracts. Antibacterial activity of fresh and dried plant materials reduced for both bacterial strains with time after extraction. Since L. racemosa and A. marina gave the best inhibition for bacterial species, they were used for further investigations. Charcoal treated plant extracts of L. racemosa and A. marina were able to inhibit both bacterial strains more than those of untreated plant extracts. Phytochemical screening of mature leaf, bark of L. racemosa and leaf extracts of A. marina has been carried out and revealed that leaf and bark contained alkaloids, steroids, triterpenoids and flavonoids. None of the above extracts indicate the presence of saponins and cardiac glycosides. Separated bands of extracts by TLC analysis showed antibacterial activity against S. aureus.

https://www.ijpsonline.com/articles/antibacterial-activity-of-some-medicinal-mangroves-against-antibiotic-resistant-pathogenic-bacteria.pdf

Antibacterial Activity of some Medicinal Mangroves against Antibiotic Resistant Pathogenic Bacteria.

The disjunct zonation pattern of Avicennia marina (Forsk.) Vierh., frequently observed along the Kenyan coast as a landward and a seaward zone, has been studied on a morphological level and complemented with preliminary genetic data. The objective was to describe the two zones in-depth in order to provide a background ecological setting that may be used in explanations on the emergence of such a bimodal zonation. The vegetation structure in the two zones is described by means of the Braun-Blanquet releve and PCQ-methods, root- and leaf-morphological characteristics, and environmental variables to provide a background ecological setting. The two zones show considerable differences in physiognomy of A. marina trees, and indicate that the differences may constitute an environmental response. Results show that in the landward A. marina zone, tree heights, leaf sizes, density, and total length of pneumatophores tend to be smaller as compared to the same attributes of the trees in the seaward zone. Differences in leaf stomatal density and in above- and below-ground pneumatophore length, the latter closely related to the effect of inundation, were observed. This suggests a considerable adaptability and plasticity with respect to environmental conditions. The hypothesis for the genetic analysis was that mangrove fringes as close as 105 m from one another constitute a same genetic pool. Preliminary genetic analysis using 48 amplified DNA fragments (RAPD - Randomly Amplified Polymorphic DNA) from 37 landward and seaward trees revealed significantly different frequencies for four DNA fragments, indicating that there might be less contact and genetic interchange between the two zones than within each zone. Whether this phenomenon, coupled to the different environmental conditions, can lead to further divergence cannot be answered yet. Of all mangrove zonation patterns described worldwide by various authors, that of Avicennia marina (Forsk.) Vierh. provides an incentive for particular analysis because of its disjunct nature (Watson, 1928; Macnae, 1968, 1969; Johnstone, 1983; Smith, 1987a, 1992). In Kenya this species displays a disjunct zonation pattern across the intertidal zone (Dahdouh-Guebas et al., 2002). Avicennia marina is present in the most landward as well as seaward fringe of the mangrove forest at certain locations along the Kenyan coast. The same pattern has been reported for other specific regions of the world; however, it has been described visually or superficially without intensive sampling or in-depth comparisons. Walter and Steiner (1936) described the higher tidal A. marina trees as “exceptions” of seaward specimens. Other authors (e.g., Macnae, 1968; Johnstone, 1983; Smith, 1992) reported a “double zonation” of A. marina, but did not go beyond stating this observation or mentioning as personal observations that tree height or leaf size varies between the upper and lower intertidal zones. This paper provides a more rigorous comparison of the landward and seaward A. marina fringe on a morphological level (such as tree height, leaf size, root density, etc.), as an ecological setting framing the emergence of the two disjunct zones. We also provide preliminary findings on genetic differentiation between the observed zones. Rather than aiming at a generalization from

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Comparative study of the disjunct zonation pattern of the grey mangrove Avicennia marina (Forsk.) Vierh. in Gazi Bay (Kenya)

Antibacterial activity of mature leaves, tender leaves and bark extracts of Avicennia marina, Avicennia officinalis and Bruguiera sexangula were evaluated using Soxhelt extraction method. Petroleum ether, chloroform, ethyl acetate and ethanol were used as solvents in order to get the plant extracts. The antibacterial activity was screened by using agar diffusion technique against pathogenic bacteria species of Staphylococcus sp. (from urine), Proteus sp. (from a wound), Escherichia coli (from infected blood), Shigella sp. (from a wound) and Pseudomonas sp. (from a wound). The length of inhibition zone was measured in millimeters from the edge of the well to the edge of the inhibition zone. Twelve different plant extracts in A. marina, A. officinalis and B. sexangula exhibited different degree of growth inhibition against tested bacterial strains. Mature leaf extracts of A. marina and tender leaf extracts of A. officinalis in ethyl acetate exhibited promising antibacterial activity than other plant extracts. All plant extracts in ethyl acetate showed strong inhibition compared to other extracts on all tested bacterial strains. Among all bacterial strains, Pseudomonas sp. and E. coli showed considerable growth inhibition against almost all plant extracts. Mature leaf extract of only A. marina was used for further investigations since a large amount of tender leaves of A. officinalis was not readily available. Charcoal treated mature leaf extracts of A. marina showed more inhibition for all tested bacterial strains than untreated plant extracts. Combinations of mature leaf extracts of A. marina in different solvents failed to exhibit synergistic activity against tested bacterial strains. Most combinations showed antagonistic effect against Proteus sp., E. coli and Shigella sp. Antibacterial activity of plant extracts of A. marina gradually declined during 4 months after extraction, for all tested bacterial strains. Components of mature leaf extracts of A. marina in chloroform, ethyl acetate and ethanol were separated by Analytical Thin Layer Chromatography (ATLC) when the eluents were hexane and diethyl ether. Isolated components in Preparative Thin Layer Chromatography (PTLC) exhibited moderate antibacterial activity against Pseudomonas sp., Shigella sp. and E. coli. Separated components did not show any antibacterial activity against Proteus sp. and Staphylococcus sp. Phytochemical screening revealed that mature leaf of A. marina contained alkaloids, steroids, triterpenoids and flavonoids.

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Evaluation of antibacterial activity of different mangrove plant extracts

Genetic and geographic variation of the mangrove tree Bruguiera in Sri Lanka

Pseudomonas fluorescens ATCC 17400 shows in vitro activity against Pythium debaryanum under conditions of iron limitation. A lacZ reporter gene introduced by transposon mutagenesis into the P. fluorescens ATCC 17400 trehalase gene (treA) was induced by a factor released by the phytopathogen Pythium debaryanum. The induction of the lacZ gene was lost upon treatment of the Pythium supernatant with commercial trehalase. A trehalose concentration as low as 1 microM could induce the expression of treA. The mutation did not affect the wild-type potential for fungus antagonism but drastically decreased the osmotolerance of the mutant in liquid culture and suppressed the ability of P. fluorescens ATCC 17400 to utilize trehalose as a carbon source. A subsequent transposon insertion in treP, one of the trehalose phosphotransferase genes upstream of treA, silenced the lacZ gene. This double mutant restricted fungal growth only under conditions of high osmolarity, which probably results in internal trehalose accumulation. These data confirm the role of the disaccharide trehalose in osmotolerance, and they indicate its additional role as an initiator of or a signal for fungal antagonism.

P. D. Abeysinghe

Papers

Antibacterial Activity of some Medicinal Mangroves against Antibiotic Resistant Pathogenic Bacteria.

Comparative study of the disjunct zonation pattern of the grey mangrove Avicennia marina (Forsk.) Vierh. in Gazi Bay (Kenya)

Evaluation of antibacterial activity of different mangrove plant extracts

Genetic and geographic variation of the mangrove tree Bruguiera in Sri Lanka

Trehalose induces antagonism towards Pythium debaryanum in Pseudomonas fluorescens ATCC 17400.