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

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

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

Evaluation of the use of prairie grasses for stimulating polycyclic aromatic hydrocarbon treatment in soil

Publisher Summary This chapter presents a detailed review of biotic interactions, principally microbe–plant, microbe–microbe, and microbe–fauna. The ecological concepts of stability and resilience in relation to food-web complexity and nutrient cycling are also discussed. The nutritional characteristics of wild plants grown under low-fertility conditions (by agronomic standards) are quite different from crop plants in fertile habitats. In most cases, plants have not adapted to nutrient stress by evolving an enhanced capacity to extract nutrients—absorption per unit weight of root—from the soil. The energy cost for producing absorbing surfaces by ectomycorrhizae (ECM) fungi, as well as vesicular-arbuscular mycorrhizal (VAM) fungi, is much less than that required for a comparable amount of absorbing surface as root tissue. Nutrient turnover is a valuable diagnostic feature of soil systems. The bulk of nutrients are present in large inactive pools versus small pools of biologically active components. Small pools, of rapidly cycling components, may account for a major proportion of the total available N and P taken up by plants in terrestrial ecosystems.

Biological Strategies of Nutrient Cycling in Soil Systems

Research in microbial growth and interactions on roots is like ly to increase in the future as the need for integration of plant pathology and soil microbiology is recognized. On the one hand, some diseases are suppressed by soil microorgan­ isms (4) and on the other hand, the soil microflora can also.suppress beneficial root symbionts; for example, the establishment of a selected mycorrhizal fungus is easier in sterile than in n onsterile soil (21, 143) . Also, poor responses to Rhizobium inoculation may result from competition from less effective indige­ nous strains and antagonists in soi l . Interest in controlling the rhizosphere comes also from the effects of certain rhizosphere microorgan isms themselves on plant growth; many organisms in the general soil microflora may be subclinical pathogens reducing root and root hair growth or acting directly on the plant (8 , 19). Inoculation of many plant species with certain soil bacte ria sometimes in­ creases yield, advances flowering, or increases internode extension (27, 93, 117). After a long period devoted largely to isolation of organisms from roots, there is new impetus into finding mechanisms of colonization of the root; also, ex­ perimental approaches to the population dynamics of microorganisms on roots are emerging. In this review we summarize the present information and suggest possible future research . Good data are sparse and conclusions have necessarily often been made on the basis of very few experiments.

and G D Bowen

Papers

Microbial Colonization of Plant Roots