Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

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Biological control of soil-borne pathogens by fluorescent pseudomonads

Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop

Use of Plant Growth-Promoting Bacteria for Biocontrol of Plant Diseases: Principles, Mechanisms of Action, and Future Prospects

Endophytic bacteria are ubiquitous in most plant species, residing latently or actively colonizing plant tissues locally as well as systemically Several definitions have been proposed for endophyt

Bacterial endophytes in agricultural crops

Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Soil beneficial bacteria and their role in plant growth promotion: a review

Plant-associated microorganisms fulfill important functions for plant growth and health. Direct plant growth promotion by microbes is based on improved nutrient acquisition and hormonal stimulation. Diverse mechanisms are involved in the suppression of plant pathogens, which is often indirectly connected with plant growth. Whereas members of the bacterial genera Azospirillum and Rhizobium are well-studied examples for plant growth promotion, Bacillus, Pseudomonas, Serratia, Stenotrophomonas, and Streptomyces and the fungal genera Ampelomyces, Coniothyrium, and Trichoderma are model organisms to demonstrate influence on plant health. Based on these beneficial plant–microbe interactions, it is possible to develop microbial inoculants for use in agricultural biotechnology. Dependent on their mode of action and effects, these products can be used as biofertilizers, plant strengtheners, phytostimulators, and biopesticides. There is a strong growing market for microbial inoculants worldwide with an annual growth rate of approximately 10%. The use of genomic technologies leads to products with more predictable and consistent effects. The future success of the biological control industry will benefit from interdisciplinary research, e.g., on mass production, formulation, interactions, and signaling with the environment, as well as on innovative business management, product marketing, and education. Altogether, the use of microorganisms and the exploitation of beneficial plant–microbe interactions offer promising and environmentally friendly strategies for conventional and organic agriculture worldwide.

Plant–microbe interactions promoting plant growth and health: perspectives for controlled use of microorganisms in agriculture

Biological Control of Fusarium Wilt on Cotton by Use of Endophytic Bacteria

Inorganic fertilizers containing ammoniacal nitrogen or formulations releasing this form of N in the soil are most effective for suppressing nematode populations. Anhydrous ammonia has been shown to reduce soil populations of Tylenchorhynchus claytoni, Helicotylenchus dihystera, and Heterodera glycines. The rates required to obtain significant suppression of nematode populations are generally in excess of 150 kg N/ha. Urea also suppresses several nematode species, including Meloidogyne spp., when applied at rates above 300 kg N/ha. Additional available carbon must be provided with urea to permit soil microorganisms to metabolize excess N and avoid phytotoxic effects. There is a direct relation between the amount of "protein" N in organic amendments and their effectiveness as nematode population suppressants. Most nematicidal amendments are oil cakes, or animal excrements containing 2-7% (w:w) N; these materials are effective at rates of 4-10 t/ha. Organic soil amendments containing mucopolysaccharides (e.g., mycelial wastes, chitinous matter) are also effective nematode suppressants. Key words: amendments, biological control, fertilizers, microbial ecology, nonchemical control, pest management, waste management.

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Organic and inorganic nitrogen amendments to soil as nematode suppressants.

Plant-associated bacteria reside in the rhizosphere, phyllosphere, and inside tissues of healthy plants. This chapter discusses concepts and examples of how naturally occurring and introduced bacteria may contribute to management of soilborne and foliar diseases. Introduced bacteria which have demonstrated biological control activity against soil-borne pathogenic fungi and nematodes include rhizobacteria (root-colonising bacteria) and endophytic bacteria (bacteria isolated from within healthy plant tissues). Recently, some introduced rhizobacteria have been found to enhance plant defences, leading to systemic protection against foliar pathogens upon seed or root-treatments with the rhizobacteria. In these cases, introduction of the rhizobacteria results in reduced damage to multiple pathogens, including viruses, fungi and bacteria. An alternative strategy to the introduction of specific antagonists is the augmentation of existing antagonists in the root environment. This augmentation may result from the use of specific organic amendments, such as chitin, which stimulate populations of antagonists, thereby inducing suppressiveness. Inter-cropping or crop rotation with some tropical legumes, including velvetbean (Mucuna deeringiana), lead to management of phytoparasitic nematodes, partly through stimulation of antagonistic microorganisms. Some biorational nematicides, such as specific botanical aromatic compounds, also appear to induce suppressiveness through alterations in the soil microbial community.

Plant root-bacterial interactions in biological control of soilborne diseases and potential extension to systemic and foliar diseases

Changes in microbial communities associated with nematode control were studied by comparing population numbers of fungi and bacteria in the soil and in internal root tissues (endorhiza) in non-amended and chitin-amended soils Addition of chitin to soil at 1% (w/w) eliminated plant-parasitic nematodes in a first planting of cotton cv `Rowden' and significantly reduced Meloidogyne incognita infestation in a second planting, confirming long-term nematode suppressiveness induced by this organic amendment The chitin amendment was associated with an increase in fungal and bacterial populations, especially those with chitinolytic activity The bacterial communities of soil, rhizosphere and endorhiza were assessed by examining the taxonomic diversity of recoverable bacteria based on identification with fatty acid analysis of sample sizes of 35 soil and rhizosphere bacteria and 25 endophytic bacteria All major bacterial species which formed at least 2% of the total population in non-amended soils and rhizospheres also occurred with chitin amendment In contrast, chitin-amended soils and rhizospheres yielded several species which were not found without chitin amendment, including Aureobacterium testaceum, Corynebacterium aquaticum and Rathayibacter tritici Burkholderia cepacia was recovered from both amended and non-amended soils and rhizospheres, but it was most abundant with chitin amendment at the end of the first cotton planting Soil was probably the major source for bacterial endophytes of cotton roots, since nearly all endophytic bacteria were also found in the soil or rhizosphere However, two dominant genera in the soil and rhizosphere, Bacillus and Arthrobacter, were not detected as endophytes Chitin amendment exhibited a further specific influence on the endophytic bacterial community; Phyllobacterium rubiacearum was not a common endophyte following chitin amendment, even though chitin amendment stimulated its populations in non-planted soil Burkholderia cepacia, found in similar numbers in the soil of both treatments, was the dominant endophyte in plants grown in chitin-amended soil but rarely colonized cotton roots grown in non-amended soil These results indicate that application of an organic amendment can lead to modifications of the bacterial communities of the soil, rhizosphere and endorhiza

Chitin-mediated changes in bacterial communities of the soil, rhizosphere and within roots of cotton in relation to nematode control

Organic matter amendments to soil can be used to manage phytoparasitic nematodes. The most effective amendments are those with narrow C:N ratios and high protein or amine-type N content. For soil with 1.0% (w/w) organic matter amendment there is a direct relation between extent of nematode control and the N content of amendments. A special group of amendments are those containing chitinous materials. Chitin addition to soil results in stimulation of a select microflora capable of degrading the polymer. Several microbial species are known to destroy the eggs of phytonematodes (Meloidogyne spp.). Organic matter can be modified by addition of specific compounds or by inoculation with particular microbial species to produce an amendment that will induce suppressiveness.

Rodrigo Rodriguez-Kabana

Papers

Biological Control of Fusarium Wilt on Cotton by Use of Endophytic Bacteria

Organic and inorganic nitrogen amendments to soil as nematode suppressants.

Plant root-bacterial interactions in biological control of soilborne diseases and potential extension to systemic and foliar diseases

Chitin-mediated changes in bacterial communities of the soil, rhizosphere and within roots of cotton in relation to nematode control

Biological control of nematodes: Soil amendments and microbial antagonists