Several microbes promote plant growth, and many microbial products that stimulate plant growth have been marketed. In this review we restrict ourselves to bacteria that are derived from and exert this effect on the root. Such bacteria are generally designated as PGPR (plant-growth-promoting rhizobacteria). The beneficial effects of these rhizobacteria on plant growth can be direct or indirect. This review begins with describing the conditions under which bacteria live in the rhizosphere. To exert their beneficial effects, bacteria usually must colonize the root surface efficiently. Therefore, bacterial traits required for root colonization are subsequently described. Finally, several mechanisms by which microbes can act beneficially on plant growth are described. Examples of direct plant growth promotion that are discussed include (a) biofertilization, (b) stimulation of root growth, (c) rhizoremediation, and (d) plant stress control. Mechanisms of biological control by which rhizobacteria can promote plant growth indirectly, i.e., by reducing the level of disease, include antibiosis, induction of systemic resistance, and competition for nutrients and niches.

Plant-growth-promoting rhizobacteria.

▪ Abstract Biofilms can be defined as communities of microorganisms attached to a surface. It is clear that microorganisms undergo profound changes during their transition from planktonic (free-swimming) organisms to cells that are part of a complex, surface-attached community. These changes are reflected in the new phenotypic characteristics developed by biofilm bacteria and occur in response to a variety of environmental signals. Recent genetic and molecular approaches used to study bacterial and fungal biofilms have identified genes and regulatory circuits important for initial cell-surface interactions, biofilm maturation, and the return of biofilm microorganisms to a planktonic mode of growth. Studies to date suggest that the planktonic-biofilm transition is a complex and highly regulated process. The results reviewed in this article indicate that the formation of biofilms serves as a new model system for the study of microbial development.

Biofilm Formation as Microbial Development

Trichoderma spp. are free-living fungi that are common in soil and root ecosystems. Recent discoveries show that they are opportunistic, avirulent plant symbionts, as well as being parasites of other fungi. At least some strains establish robust and long-lasting colonizations of root surfaces and penetrate into the epidermis and a few cells below this level. They produce or release a variety of compounds that induce localized or systemic resistance responses, and this explains their lack of pathogenicity to plants. These root-microorganism associations cause substantial changes to the plant proteome and metabolism. Plants are protected from numerous classes of plant pathogen by responses that are similar to systemic acquired resistance and rhizobacteria-induced systemic resistance. Root colonization by Trichoderma spp. also frequently enhances root growth and development, crop productivity, resistance to abiotic stresses and the uptake and use of nutrients.

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Trichoderma species--opportunistic, avirulent plant symbionts.

The formation of complex bacterial communities known as biofilms begins with the interaction of planktonic cells with a surface in response to appropriate environmental signals. We report the isolation and characterization of mutants of Pseudomonas aeruginosa PA14 defective in the initiation of biofilm formation on an abiotic surface, polyvinylchloride (PVC) plastic. These mutants are designated surface attachment defective (sad ). Two classes of sad mutants were analysed: (i) mutants defective in flagellar-mediated motility and (ii) mutants defective in biogenesis of the polar-localized type IV pili. We followed the development of the biofilm formed by the wild type over 8 h using phase-contrast microscopy. The wild-type strain first formed a monolayer of cells on the abiotic surface, followed by the appearance of microcolonies that were dispersed throughout the monolayer of cells. Using time-lapse microscopy, we present evidence that microcolonies form by aggregation of cells present in the monolayer. As observed with the wild type, strains with mutations in genes required for the synthesis of type IV pili formed a monolayer of cells on the PVC plastic. However, in contrast to the wild-type strain, the type IV pili mutants did not develop microcolonies over the course of the experiments, suggesting that these structures play an important role in microcolony formation. Very few cells of a non-motile strain (carrying a mutation in flgK) attached to PVC even after 8 h of incubation, suggesting a role for flagella and/or motility in the initial cell-to-surface interactions. The phenotype of these mutants thus allows us to initiate the dissection of the developmental pathway leading to biofilm formation.

Flagellar and twitching motility are necessary for Pseudomonas aeruginosa biofilm development

The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. Th...

The enhancement of plant growth by free-living bacteria

Inhibition of root cell energy metabolism is suggested to be responsible for potato yield reductions in short potato-rotation soils. Hydrogen cyanide is the microbial metabolile possibly involved in inhibition of energy metabolism. This is supported by the following observations: (1) approximately 50% of potato rhizosphere pseudomonads was shown to produce cyanide in vitro; (2) 5 μM HCN inhibited cytochrome oxidase respiration by at least 40% in intact potato roots in vitro; (3) cyanide production in vitro by Pseudomonas sp. isolate WCS361 depended on the Fe3+ concentration of the medium. Growth promoting fluorescent Pseudomonas spp isolates WCS374 and WCS358 did not produce cyanide in vitro. A hypothesis, that potato plant growth is depressed in short potato rotation soils by the microbial production of cyanide in the rhizosphere is discussed. In such soils, bacteria producing specific siderophores increase growth by competing with cyanide-producing organisms for Fe3+.

Microbial cyanide production in the rhizosphere in relation to potato yield reduction and Pseudomonas SPP-mediated plant growth-stimulation

The involvement of mechanisms other than competition for iron in biological control of Fusarium oxysporum f. sp. dianthi of carnation by Pseudomonas sp. strain WCS417r was investigated. Pathogen and antagonist were spatially separated by bacterizing the roots while inoculating the stem with the pathogen. When the roots were bacterized with strain WCS417r 1 wk prior to stem inoculation with F. o. dianthi, the number of diseased plants in all experiments with cultivar Pallas was reduced from about 50 to 20%, and in one experiment with cultivar Lena from 69 to 38% (...)

Induced resistance and phytoalexin accumulation in biological control of Fusarium wilt of carnation by Pseudomonas sp. strain WCS417r

Evidence is increasing that the saprophytic microflora of the rhizosphere includes both deleterious and beneficial elements that have the potential to influence plant growth and crop yields significantly (13, 16, 25, 27, 57, 72, 77, 78). The deleterious microorganisms affect plant growth negatively, but they do not necessarily parasitize the plant tissue. Their deleterious activities include alterations of the supply of water, ions, and plant growth substances by changing root functions and/or by limiting root growth (1,57, 67, 77). The mechanisms by which beneficial nonsymbiotic microorganisms are considered to affect plant growth positively include promotion of the availability and uptake of mineral nutrients (6, 7, 27), provision of plant-growth substances (27, 56, 57), and, most of all, suppression of deleterious rhizosphere microorganisms (13, 27, 28, 72, 77). Beneficial rhizobacteria that promote plant growth, supposedly by competing for iron with deleterious rhizosphere microorganisms, have recently become known as plant growth-promoting rhizobacteria (PGPR) (13, 41, 67, 77). This paper reviews recent knowledge on deleterious and beneficial rhizosphere bacteria, their interactions, and their effect on yield as related to cropping practices.

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Interactions of Deleterious and Beneficial Rhizosphere Microorganisms and the Effect of Cropping Practices

The role of motility in the colonization of potato roots by Pseudomonas bacteria was studied. Four Tn5-induced flagella-less mutants of the plant-growth-stimulating P. fluorescens WCS374 appeared to be impaired in their ability to colonize growing potato roots.

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Flagella of a plant-growth-stimulating Pseudomonas fluorescens strain are required for colonization of potato roots.

A special bioassay on rock wool was used to study the influence of iron availability on the induction of systemic resistance in radish (Raphanus sativus L.) against Fusarium wilt mediated by Pseudomonas fluorescens. In this bioassay, the pathogen (Fusarium oxysporum f. sp. raphani) and a strain of Pseudomonas, salicylic acid (SA), or a pseudobactin were applied at separate locations on the plant root. Strain WCS374 of P. fluorescens and its pseudobactin-minus Tn5 mutant gave greater disease control in the induced systemic resistance bioassay when iron availability in the radish nutrient solution was low than when it was high. Mutants of P. fluorescens strains WCS374 and WCS417 lacking the O-antigenic side chain of the lipopolysaccharide induced resistance at low but not at high iron availability. The purified pseudobactin of strain WCS374, but not the pseudobactins of strains WCS358 and WCS417, induced resistance. Gas chromatography and spectrophotometry were used to detect and measure production of SA by these strains. Strains WCS374 and WCS417 produced 47 and 8 μg of SA per milliliter, respectively, at low iron availability in vitro ; the production of SA decreased rapidly with increasing iron availability. P putida WCS358 did not induce resistance, either at low or at high iron availability, and did not produce SA in vitro. Commercial SA induced resistance at concentrations as low as 100 fg per root. High concentrations (> 1 mg/ml) of SA reduced growth of hyphae from germinated conidia of E oxysporum f. sp. raphani. We hypothesize that the Fe 3+ -chelating SA, produced by selected P. fluorescens strains at low iron availability, is involved in the induction of systemic resistance to Fusarium wilt of radish. The pseudobactin produced by WCS374 may be involved as well. Given these results, it seems appropriate to reevaluate the role of siderophore-mediated competition for iron in the suppression of disease by fluorescent Pseudomonas spp.

Bob Schippers

Papers

Microbial cyanide production in the rhizosphere in relation to potato yield reduction and Pseudomonas SPP-mediated plant growth-stimulation

Induced resistance and phytoalexin accumulation in biological control of Fusarium wilt of carnation by Pseudomonas sp. strain WCS417r

Interactions of Deleterious and Beneficial Rhizosphere Microorganisms and the Effect of Cropping Practices

Flagella of a plant-growth-stimulating Pseudomonas fluorescens strain are required for colonization of potato roots.

Iron availability affects induction of systemic resistance to Fusarium wilt of radish by Pseudomonas fluorescens.