The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

/pdf/microbial-interactions-and-biocontrol-in-the-rhizosphere-1erj9kqzek.pdf

Microbial interactions and biocontrol in the rhizosphere

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

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phytostimulation. Reviewing the role of bacterial IAA in different microorganism–plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phytostimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.

Indole-3-acetic acid in microbial and microorganism-plant signaling.

Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Inoculation of crop plants with certain strains of PGPR at an early stage of development improves biomass production through direct effects on root and shoots growth. Inoculation of ornamentals, forest trees, vegetables, and agricultural crops with PGPR may result in multiple effects on early-season plant growth, as seen in the enhancement of seedling germination, stand health, plant vigor, plant height, shoot weight, nutrient content of shoot tissues, early bloom, chlorophyll content, and increased nodulation in legumes. PGPR are reported to influence the growth, yield, and nutrient uptake by an array of mechanisms. They help in increasing nitrogen fixation in legumes, help in promoting free-living nitrogen-fixing bacteria, increase supply of other nutrients, such as phosphorus, sulphur, iron and copper, produce plant hormones, enhance other beneficial bacteria or fungi, control fungal and bacterial diseases and help in controlling insect pests. There has been much research interest in PGPR and there is now an increasing number of PGPR being commercialized for various crops. Several reviews have discussed specific aspects of growth promotion by PGPR. In this review, we have discussed various bacteria which act as PGPR, mechanisms and the desirable properties exhibited by them.

Plant Growth Promoting Rhizobacteria: A Critical Review

The authors wish to acknowledge the European Commission for their support to the ENDEGRADE project (EU PROJECT: QLK3- CT2000-00164). DvdL and ST are presently being
supported by Laboratory Directed Research and Development funds at the Brookhaven National Laboratory under contract with the U.S. Department of Energy.

Endophytic Bacteria and Their Potential Applications

Our laboratory has focused on microbial production of plant hormones and precursor-inoculum interactions. Indole-3-acetic acid was detected in soils incubated with L-tryptophan (L-TRP). Inoculation with the ectomycorrhizae, Pisolithus tinctorius significantly stimulated the growth of Douglas fir when supplied with low concentrations of L-TRP to soil. Among three Azotobacter spp. and two Pseudomonas spp., the most prolific producer of cytokinins was A. chroococcum and among the precursors tested, adenine (ADE) and isopentyl alcohol (IA) were the most effective. Corn rhizosphere was found to be quite rich with microflora capable of producing ethylene from L-methionine (L-MET). Amino acids, carbohydrates and organic acids typically found in root exudates, were stimulatory to ethylene biosynthesis in soil. Etiolated pea seedlings exhibited the classical ‘triple’ response when L-MET and Acremonium falciforme were applied in combination to sterile soil or when L-MET was added to nonsterile soil.

Microbial production of plant hormones

Ethylene (C2H4) is a plant hormone that is involved in the regulation of many physiological processes. It is the simplest unsaturated carbon compound in nature which is a gas under physiological conditions of temperature and pressure, and exerts a major influence on many aspects of plant growth and development. Ethylene is produced by all higher plants and many soil microorganisms. In trace amounts, it interacts with the other plant hormones, particularly auxins.

W. T. Frankenberger

Papers

Microbial production of plant hormones

Microbial Biosynthesis of Ethylene and Its Influence on Plant Growth