Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Soil beneficial bacteria and their role in plant growth promotion: a review

This review presents a critical and comprehensive documentation and analysis of the developments in agricultural, environmental, molecular, and physiological studies related to Azospirillum cells, and to Azospirillum interactions with plants, based solely on information published between 1997 and 2003. It was designed as an update of previous reviews (Bashan and Levanony 1990; Bashan and Holguin 1997a), with a similar scope of interest. Apart from an update and critical analysis of the current knowledge, this review focuses on the central issues of Azospirillum research today, such as, (i) physiological and molecular studies as a general model for rhizosphere bacteria; (ii) co-inoculation with other microorganisms; (iii) hormonal studies and re-consideration of the nitrogen contribution by the bacteria under specific environmental conditions; (iv) proposed Azospirillum as a non-specific plant-growth-promoting bacterium; (v) re-introduction of the "Additive Hypothesis," which suggests involvement of multip...

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Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003)

Introduction to Soil Microbiology

Biological N2 fixation (BNF) by associative diazotrophic bacteria is a spontaneous process where soil N is limited and adequate C sources are available. Yet the ability of these bacteria to contribute to yields in crops is only partly a result of BNF. A range of diazotrophic plant growth-promoting rhizobacteria participate in interactions with C3 and C4 crop plants (e.g. rice, wheat, maize, sugarcane and cotton), significantly increasing their vegetative growth and grain yield. We review the potential of these bacteria to contribute to yield increases in a range of field crops and outline possible strategies to obtain such yield increases more reliably. The mechanisms involved have a significant plant growth-promoting potential, retaining more soil organic-N and other nutrients in the plant– soil system, thus reducing the need for fertiliser N and P. Economic and environmental benefits can include increased income from high yields, reduced fertiliser costs and reduced emission of the greenhouse gas, N2O (with more than 300 times the global warming effect of CO2), as well as reduced leaching of NO3 –N to ground water. Obtaining maximum benefits on farms from diazotrophic, plant growth promoting biofertilisers will require a systematic strategy designed to fully utilise all these beneficial factors, allowing crop yields to be maintained or even increased while fertiliser applications are reduced. q 2004 Elsevier Ltd. All rights reserved.

Non-symbiotic bacterial diazotrophs in crop-farming systems : can their potential for plant growth promotion be better exploited?

The genus Azospirillum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Azospirillum inoculation on plants . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . Inoculation effects on root development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Root colonization by Azospirillum . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proposed mode of action of Azospirillum on plant growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrogen fixation by Azospirillum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hormonal effects of Azospirillum on plants . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Improvement of root development, mineral uptake, and plant-water relationships by Azospirillum.. . . . . . . . . . . . Azospirillum nitrate reductase in plants . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specificity and variability in Azospirillum. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . Interaction of Azospirillum with other soil-rhizosphere microflora . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . Azospirillum as a competitor in the rhizosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interaction of Azospirillum with soil particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Genetics and immunology of Azospirillum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agrotechnical aspects: inoculants and interaction with pesticides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Current status of Azospirillum inoculation technology: Azospirillum as a challenge for agriculture

Culturomics of the plant prokaryotic microbiome and the dawn of plant-based culture media – A review

The influence of agro-industrial effluents on River Nile pollution

Dinitrogen fixation associated with non-legumes has been well documented (Dobereiner, 1983; Hegazi, 1983; Fayez, 1990) and is now generally considered to make a significant contribution to the nitrogen economy of several ecosystems. This intensifies the use of field inoculation with various diazotrophs. Therefore, the need arises to monitor the populations and individuals of several associative diazotrophs inhabiting the plant-soil system. The ability of several diazotrophs to multiply and reduce acetylene on numerous carbon sources encourages the search for a single medium suitable for collective growth and biomass production of a number of diazotrophs. The present study evaluates a new medium modification based on basal salts and a mixture of five carbon sources (glucose, malic acid, mannitol, sodium lactate and sucrose) to fulfil the requirements for a single medium on which the majority of associative diazotrophs would exhibit good growth and biomass production.

Modified combined carbon N-deficient medium for isolation, enumeration and biomass production of diazotrophs

Diversity of bacteria nesting the plant cover of north Sinai deserts, Egypt

A pot experiment was designed to investigate the effects of inoculation with Azospirillum and (or) straw amendment on growth of plants grown in Giza soils. Inoculation caused increases in plant dry weight (200%) and total N content (157%) of plants. These characters were correlated with increases in ATP production in rhizosphere (492%), nitrogenase activity (438%), and densities of Azospirillum sp. (116-fold). Addition of straw only (5%, w/w) to the soil stimulated rhizosphere microorganisms (ATP, 410%), N2 fixation (nitrogenase activity, 392%), and also plant growth (plant dry weight, 176%; total N content, 149%). Simultaneous Azospirillum inoculation and straw amendment exerted the most favourable conditions for N2 fixation on roots (nitrogenase activity, 554% increase over control) leading to the greatest biological (numbers of azospirilla, 156-fold; ATP, 543%; nitrogenase activity 554%), as well as agronomic (total dry weight, 343%; total N content, 196%; leaf surface, 478%) effects. Under farming con...

Nabil A. Hegazi

Papers

Culturomics of the plant prokaryotic microbiome and the dawn of plant-based culture media – A review

The influence of agro-industrial effluents on River Nile pollution

Modified combined carbon N-deficient medium for isolation, enumeration and biomass production of diazotrophs

Diversity of bacteria nesting the plant cover of north Sinai deserts, Egypt

Response of maize plants to inoculation with azospirilla and (or) straw amendment in Egypt