Pseudomonas spp. are ubiquitous bacteria in agricultural soils and have many traits that make them well suited as biocontrol agents of soilborne pathogens. Tremendous progress has been made in characterizing the process of root colonization by pseudomonads, the biotic and abiotic factors affecting colonization, bacterial traits and genes contributing to rhizosphere competence, and the mechanisms of pathogen suppression. This review looks back over the last 30 years of Pseudomonas biocontrol research and highlights key studies, strains, and findings that have had significant impact on shaping our current understanding of biological control by bacteria and the direction of future research.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-97-2-0250

Pseudomonas biocontrol agents of soilborne pathogens: looking back over 30 years.

Biocontrol involves harnessing disease-suppressive microor? ganisms to improve plant health. Disease suppression by biocontrol agents is the sustained manifestation of interactions among the plant, the pathogen, the biocontrol agent, the microbial community on and around the plant, and the physi? cal environment. Even in model laboratory systems, the study of biocontrol involves interactions among a minimum of three organisms. Therefore, despite its potential in agricultural ap? plications, biocontrol is one of the most poorly understood areas of plant-microbe interactions. The complexity of these systems has influenced the accep? tance of biocontrol as a means of controlling plant diseases in two ways. First, practical results with biocontrol have been variable. Thus, despite some stunning successes with biocon? trol agents in agriculture, there remains a general skepticism born of past failures (Cook and Baker, 1983; Weiler, 1988). Second, progress in understanding an entire system has been slow. Recently, however, substantial progress has been made in a number of biocontrol systems through the application of genetic and mathematical approaches that accommodate the complexity. Biocontrol of soilborne diseases is particularly complex be? cause these diseases occur in the dynamic environment at the interface of root and soil known as the rhizosphere, which is defined as the region surrounding a root that is affected by it. The rhizosphere is typified by rapid change, intense microbial activity, and high populations of bacteria compared with nonrhizosphere soil. Plants release metabolically active cells from their roots and deposit as much as 20% of the carbon allocated to roots in the rhizosphere, suggesting a highly evolved relationship between the plant and rhizosphere microorgan? isms. The rhizosphere is subject to dramatic changes on a short temporal scale?rain events and daytime drought can result in fluctuations in salt concentration, pH, osmotic poten? tial, water potential, and soil particle structure. Over longer temporal scales, the rhizosphere can change due to root growth, interactions with other soil biota, and weathering processes. It is the dynamic nature of the rhizosphere that makes it an interesting setting for the interactions that lead to disease and biocontrol of disease (Rovira, 1965,1969,1991; Hawes, 1991; Waisel et al., 1991).

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Biocontrol of Soilborne Plant Pathogens.

Publisher Summary The chapter presents a discussion on both rhizosphere and rhizoplane. Rhizosphere is defined as the soil adjacent to roots with a different physical, chemical, and biological environment from the bulk soil. The rhizoplane is defined as the actual root surface or soil interface. The chapter describes plant responses, the nature and dynamics of rhizosphere populations and their determinants, the management of the plant, and management of microorganisms. The chapter also explores the impact of soil conditions and agronomic practices on the rhizoshere microflora and suggests areas that require more study in order to achieve effective management. The chapter also illustrates different methods that are used by agronomists, plant pathologists, and microbiologists in order to modify the rhizosphere. In most cases, when microbiologists and plant pathologists needs to change the rhizosphere in order to improve plant growth they often think in terms of introducing microorganisms that possess the desired characteristics, be they antibiotic or siderophore production, or the ability to stimulate plant growth.

The rhizosphere and its management to improve plant growth

Soilborne root diseases caused by plant pathogenic Pythium species cause serious losses in a number of agricultural production systems, which has led to a considerable effort devoted to the development of biological agents for disease control. In this article we review information on the ecology and biological control of these pathogens with the premise that a clear understanding of the ecology of the pathogen will assist in the development of efficacious biocontrol agents. The lifecycles of the pathogens and etiology of host infection also are reviewed, as are epidemiological concepts of inoculum-disease relationships and the influence of environmental factors on pathogen aggressiveness and host susceptibility. A number of fungal and bacterial biocontrol agents are discussed and parallels between their ecology and that of the target pathogens highlighted. The mechanisms by which these microbial agents suppress diseases caused by Pythium spp., such as interference with pathogen survival, disruption of the...

Soilborne plant diseases caused by pythium spp: ecology, epidemiology, and prospects for biological control

Certain rhizobacteria referred to as 'plant growth-promoting rhizobacteria' (PGPR) can contribute to the biological control of plant pathogens and improve plant growth. They enhance root development either directly by producing phytohormones, or indirectly by inhibiting pathogens through the synthesis of different compounds. PGPR are likely to be of great interest in sustainable crop protection and have drawn much attention in recent years. However, the use of these bacteria to protect crops sometimes fails because rhizobacteria are unable to recolonize the rhizosphere of inoculated plants. The colonization of roots by inoculated bacteria is an important step in the interaction between beneficial bacteria and the host plant. However, it is a complex phenomenon influenced by many biotic and abiotic parameters, some of which are now apparent. This paper summarises knowledge on rhizosphere colonization by PGPR.

Root Colonization by Inoculated Plant Growth-Promoting Rhizobacteria

Root colonization by introduced strains of Pseudomonas fluorescens was examined to determine if temperature and bacterial motility affect the distribution of bacteria along the taproot of peas. Seeds coated with strain PRA25rif were sown in soil held at a constant soil-water matric potential of −6 kPa and temperatures of 16, 20, 24 and 28°C. After 7 days, 1 cm segments of the epicotyl (0–1 cm above the seed) and taproot (0–5 cm below the seed) and the seed were assayed for bacterial populations by dilution plating. A significant ( P P = 0.68). Of the temperatures tested, the temperature optimal for root colonization was 16°C, whereas root growth was most rapid at 24°C. The bacterium was never recovered from taproot segments > 3 cm below the seed, indicating that transport of this strain with the growing root, even when roots grew slowly, was not a major means of dispersal in the pea rhizosphere. The effect of bacterial motility on root colonization was assessed with P. fluorescens strains WCS374(Tn5) (motile) and LWM74-30 (non-flagellated, non-motile mutant) at 24°C with both a constant soil-water matric potential of −6kPa and with 27.2 mm of water added 4 days after planting. Seven days after planting the epicotyl, seed and 1 cm taproot segments 0–18 cm below the seed were assayed as above for bacterial populations. Neither the motile nor the non-motile strain was recovered > 3 cm below the seed under tatic conditions. However, both strains were recovered 14–16 cm below the seed 3 days after water was added, indicating that passive movement with water flow, and not motility, played a large role in bacterial dispersal in this system.

J.H. Bowers

Papers

Colonization of pea (Pisum sativum L.) taproots by Pseudomonas fluorescens: effect of soil temperature and bacterial motility.