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

Nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Rhizobacteria-mediated induced systemic resistance (ISR) has been demonstrated against fungi, bacteria, and viruses in Arabidopsis, bean, carnation, cucumber, radish, tobacco, and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated. Bacterial strains differ in their ability to induce resistance in different plant species, and plants show variation in the expression of ISR upon induction by specific bacterial strains. Bacterial determinants of ISR include lipopolysaccharides, siderophores, and salicylic acid (SA). Whereas some of the rhizobacteria induce resistance through the SA-dependent SAR pathway, others do not and require jasmonic acid and ethylene perception by the plant for ISR to develop. No consistent host plant alterations are associated with the induced state, but upon challenge inoculation, resistance responses are accelerated and enhanced. ISR is effective under field conditions and offers a natural mechanism for biological control of plant disease.

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Systemic resistance induced by rhizosphere bacteria

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

Biological control of soilborne pathogens by introduced microorganisms has been studied for over 65 years (9, 49), but during most of that time it has not been considered commercially feasible. Since about 1 965, however, interest and research in this area have increased steadily (9), as reflected by the number of books (10, 47,49, 152) and reviews about it (11,26,30, 106, 143, 153, 173, 174, 183) that have appeared . Concurrently, there has been a shift to the opinion that biological control can have an important role in agriculture in the future, and it is encouraging that several companies now have programs to develop biocontrol agents as commercial products. This renewed interest in biocontrol is in part a response to public concern about hazards associated with chemical pesticides. Microorganisms that can grow in the rhizosphere are ideal for use as biocontrol agents, since the rhizosphere provides the front-line defense for roots against attack by pathogens. Pathogens encounter antagonism from rhizosphere microorganisms before and during primary infection and also during secondary spread on the root. In some soils described as microbiologi­ cally suppressive to pathogens (172), microbial antagonism of the pathogen is especially great, leading to substantial disease control. Although pathogen­ suppressive soils are rare, those identified are excellent examples of the full potential of biological control of soilborne pathogens.

Biological control of soilborne plant pathogens in the rhizosphere with bacteria

▪ Abstract Agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed. One of the best-described examples occurs in take-all decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetylphloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant patholog...

Microbial populations responsible for specific soil suppressiveness to plant pathogens.

Specific strains of the Pseudomonas fluorescens-putida group have recently been used as seed inoculants on crop plants to promote growth and increase yields. These pseudomonads, termed plant growth-promoting rhizobacteria (PGPR), rapidly colonize plant roots of potato, sugar beet and radish, and cause statistically significant yield increases up to 144% in field tests1–5. These results prompted us to investigate the mechanism by which plant growth was enhanced. A previous study indicated that PGPR increase plant growth by antagonism to potentially deleterious rhizoplane fungi and bacteria, but the nature of this antagonism was not determined6. We now present evidence that PGPR exert their plant growth-promoting activity by depriving native microflora of iron. PGPR produce extracellular siderophores (microbial iron transport agents)7 which efficiently complex environmental iron, making it less available to certain native microflora.

Enhanced plant growth by siderophores produced by plant growth-promoting rhizobacteria

The addition of either fluorescentPseudomonas strain B10, isolated from a take-all suppressive soil, or its siderophore, pseudobactin, to bothFusarium-wilt and take-all conducive soils inoculated withFusarium oxysporum f. sp.lini orGaeumannomyces graminis var.tritici, respectively, rendered them disease suppressive. Our findings suggest that disease suppressiveness is caused in part by microbial siderophores which efficiently complex iron(III) in soils, making it unavailable to pathogens, thus inhibiting their growth. Amendment of exogenous iron(III) to disease-suppressive soils converted them to conductive soils presumably by repressing siderophore production.

Pseudomonas siderophores: A mechanism explaining disease-suppressive soils

The structure of nonfluorescent pseudobactin A, one of two extracellular siderophores (microbial iron transport agents) produced by the plant growth promoting bacterium Pseudomonas B10, was determined by comparison of its 1H and 13C NMR spectra with those of yellow-green, fluorescent pseudobactin, the other siderophore. The molecular and crystal structure of ferric pseudobactin is reported in the preceding paper in this issue [Teintze, M., Hossain, M. B., Barnes, C. L., Leong J., & van der Helm, D. (1981) Biochemistry (preceding paper in this issue)]. The only structural difference between pseudobactin and pseudobactin A was that the latter was saturated at carbons 3 and 4 of the quinoline derivative, whereas pseudobactin is unsaturated at these positions. A mechanism is proposed for the observed conversion of pseudobactin A into pseudobactin in aqueous solution.

Structure of pseudobactin A, a second siderophore from plant growth promoting Pseudomonas B10.

When grown in iron-limiting culture medium, sugar beet deleterious Pseudomonas 7SR1 produced extra-cellularly the yellow-green, fluorescent siderophore pseudobactin 7SR1. Pseudobactin 7SR1 had a molecular formula of C46H63N13O23 and a molecular mass of 1166 g/mol. Pseudobactin 7SR1 contained a cyclic octapeptide with the amino acid sequence L-Ala-Gly-Ser-Ser-threo-beta-OH-Asp-Thr-Ser-N delta-OH-Orn. Since pseudobactin 7SR1 was not affected by nonspecific enzymes, it might contain D-amino acids. A yellow-green, fluorescent quinoline derivative is postulated to be attached via an ester bond to the serine residue following the glycine. A malamide group was attached to carbon 3 of the quinoline derivative. The three bidentate iron(III)-chelating groups consisted of an alpha-hydroxy acid group derived from beta-hydroxyaspartic acid, an omicron-dihydroxy aromatic group derived from the yellow-green, fluorescent chromophore, and a hydroxamate group derived from N delta-acetyl-N delta-hydroxyornithine. The chemical structure of pseudobactin 7SR1 is remarkably similar to that of pseudobactin, the siderophore of plant growth promoting Pseudomonas B10.

Structure of pseudobactin 7SR1, a siderophore from a plant-deleterious Pseudomonas

Bean-deleterious Pseudomonas A214 produced the extracellular yellow-green, fluorescent siderophore [microbial iron(III) transport agent] pseudobactin A214 under iron-limiting conditions. Pseudobactin A214 has a molecular formula of C46H64N13O22 and a molecular mass of 1151 g/mol. Pseudobactin A214 contained an N-blocked linear octapeptide with the amino acid sequence Ser-Ala-Gly-Ser-Ala-threo-beta-OH-Asp-L-allo-Thr-N delta-OH-Orn with a yellow-green, fluorescent quinoline derivative attached via an amide bond to the amino terminus. A succinamide group was linked to carbon 3 of the quinoline derivative. Sequencing was accomplished by two-dimensional NMR spectroscopy and by Edman degradation of smaller peptides obtained from partial acid hydrolysis. Since pseudobactin A214 was not affected by nonspecific proteolytic enzymes, it might contain D-amino acids. The three bidentate iron-(III)-chelating groups consisted of a 1,2-dihydroxy aromatic group in the quinoline chromophore, an alpha-hydroxy acid group present as beta-hydroxyaspartic acid, and a hydroxamate group derived from N delta-acetyl-N delta-hydroxyornithine. The chemical structure of pseudobactin A214 is remarkably similar to those of pseudobactin and pseudobactin 7SR1, the siderophores of plant growth promoting and plant-deleterious Pseudomonas B10 and Pseudomonas 7SR1, respectively.

John Leong

Papers

Enhanced plant growth by siderophores produced by plant growth-promoting rhizobacteria

Pseudomonas siderophores: A mechanism explaining disease-suppressive soils

Structure of pseudobactin A, a second siderophore from plant growth promoting Pseudomonas B10.

Structure of pseudobactin 7SR1, a siderophore from a plant-deleterious Pseudomonas

Structure of pseudobactin A214, a siderophore from a bean-deleterious Pseudomonas