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Book ChapterDOI

Development and function of Azospirillum -inoculated roots

01 Feb 1986-Plant and Soil (Springer Netherlands)-Vol. 21, Iss: 1, pp 3-16
TL;DR: Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains of Azospirillum caused morphological changes in root starting immediately after germination, which lead in many cases to higher crop yield.
Abstract: The surface distribution of Azospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs. Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains of Azospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO 3 - , K+ and H2PO 4 - uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher in Azospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.
Citations
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Journal ArticleDOI
TL;DR: It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms, and further elucidation of the different mechanisms involved will help to make associative Diazotrophic bacteria a valuable partner in future agriculture.
Abstract: Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of anti...

1,051 citations


Cites background or result from "Development and function of Azospir..."

  • ...Similarly to what was observed in several grasses and cereals (Okon and Kapulnik, 1986), inoculation with A....

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  • ...Similarly to what was observed in several grasses and cereals (Okon and Kapulnik, 1986), inoculation with A. brasilense was found to promote root hair formation of bean or alfalfa (Itzigsohn et al., 1993; Burdman et al., 1996)....

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  • ...Moreover, in several other studies the increased plant growth observed after inoculation with Azospirillum was proposed to be due to bacterial phytohormone production (Okon and Kapulnik, 1986; Harari et al., 1988)....

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Journal ArticleDOI
TL;DR: Four aspects of the Azospirillum-plant root interaction are highlighted: natural habitat, plant root interaction, nitrogen fixation and biosynthesis of plant growth hormones, and the relevance of each of these four aspects for plant growth promotion by AzospIRillum is discussed.
Abstract: Azospirillum represents the best characterized genus of plant growth-promoting rhizobacteria. Other free-living diazotrophs repeatedly detected in association with plant roots, include Acetobacter diazotrophicus, Herbaspirillum seropedicae, Azoarcus spp. and Azotobacter. Four aspects of the Azospirillum–plant root interaction are highlighted: natural habitat, plant root interaction, nitrogen fixation and biosynthesis of plant growth hormones. Each of these aspects is dealt with in a comparative way. Azospirilla are predominantly surface-colonizing bacteria, whereas A. diazotrophicus, H. seropedicae and Azoarcus sp. are endophytic diazotrophs. The attachment of Azospirillum cells to plant roots occurs in two steps. The polar flagellum, of which the flagellin was shown to be a glycoprotein, mediates the adsorption step. An as yet unidentified surface polysaccharide is believed to be essential in the subsequent anchoring phase. In Azoarcus sp. the attachment process is mediated by type IV pili. Nitrogen fixation structural genes (nif) are highly conserved among all nitrogen-fixing bacteria, and in all diazotrophic species of the class of proteobacteria examined, the transcriptional activator NifA is required for expression of other nif genes in response to two major environmental signals (oxygen and fixed N). However, the mechanisms involved in this control can vary in different organisms. In Azospirillum brasilense and H. seropedicae (α- and β-subgroup, respectively), NifA is inactive in conditions of excess nitrogen. Activation of NifA upon removal of fixed N seems to involve, either directly or indirectly, the signal transduction protein PII. The presence of four conserved cysteine residues in the NifA protein might be an indication that NifA is directly sensitive to oxygen. In Azotobacter vinelandii (γ-subgroup) nifA is cotranscribed with a second gene nifL. The nifL gene product inactivates NifA in response to high oxygen tension and cellular nitrogen-status. NifL was found to be a redox-sensitive flavoprotein. The relief of NifL inhibition on NifA activity, in response to N-limitation, is suggested to involve a PII-like protein. Moreover, nitrogenase activity is regulated according to the intracellular nitrogen and O2 level. In A. brasilense and Azospirillum lipoferum posttranslational control of nitrogenase, in response to ammonium and anaerobiosis, involves ADP-ribosylation of the nitrogenase iron protein, mediated by the enzymes DraT and DraG. At least three pathways for indole-3-acetic acid (IAA) biosynthesis in A. brasilense exist: two Trp-dependent (the indole-3-pyruvic acid and presumably the indole-3-acetamide pathway) and one Trp-independent pathway. The occurrence of an IAA biosynthetic pathway not using Trp (tryptophan) as precursor is highly unusual in bacteria. Nevertheless, the indole-3-pyruvate decarboxylase encoding ipdC gene is crucial in the overall IAA biosynthesis in Azospirillum. A number of genes essential for Trp production have been isolated in A. brasilense, including trpE(G) which codes for anthranilate synthase, the key enzyme in Trp biosynthesis. The relevance of each of these four aspects for plant growth promotion by Azospirillum is discussed.

901 citations


Cites background from "Development and function of Azospir..."

  • ...Bacterial phytohormone production is assumed to cause the detected changes in root morphology after Azospirillum inoculation, which in turn may be related to enhanced mineral uptake [35,36,187]....

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  • ...Since pectin is a major constituent of the primary cell wall and middle lamellae, and low levels of pectinolytic and cellulolytic activities have been detected in Azospirillum cultures, the bacteria may eventually enter the root cortex intercellular spaces via enzymatic degradation of host cell wall middle lamellae [32,36,47]....

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01 Jan 2011
TL;DR: Plant growth-promoting rhizobacteria (PGPRs) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion.
Abstract: 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.

846 citations

Journal ArticleDOI
TL;DR: The authors wish to acknowledge the European Commission for their support to the ENDEGRADE project (EU PROJECT: QLK3- CT2000-00164).
Abstract: 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.

671 citations


Cites background from "Development and function of Azospir..."

  • ...Azospirillum, for instance, is generally regarded as being a rhizosphere bacterium that colonizes mainly the elongation and root hair zones of roots (Okon and Kapulnik, 1986; Döbereinner et al., 1995b; Vande Broek et al., 1998; Bashan and Holguin, 1995)....

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Journal ArticleDOI
TL;DR: It can be concluded that these bacteria are capable of promoting the yield of agriculturally-important crops in different soils and climatic regions and the implementation by regulatory authorities of quality control on commercial Azospirillum inoculants is very strongly suggested.
Abstract: By evaluating worldwide data accumulated over the past 20 years on field inoculation experiments with Azospirillum, it can be concluded that these bacteria are capable of promoting the yield of agriculturally-important crops in different soils and climatic regions. Various strains of A. brasilense and A. lipoferum have been used to inoculate cultivars of different species of plants. It is however difficult to accurately estimate the percentage of success due to Azospirillum inoculation. The data indicates 60–70% occurrence of success with statistically significant increases in yield of the order of 5–30%. Successful inoculation experiments appear to be those in which the researchers have paid special attention to the optimal number of cells of Azospirillum in the inoculant, using inoculation methods where the optimal number of cells remained viable and available to colonize the roots. Furthermore, experiments taking into consideration the potentialities and limitations of this technology have been better able to explain successes and failures. The different formulations (analogous to those of rhizobia) of the genus Azospirillum, irrespective of their form of application and their mode of action on the plant, are indeed inoculants. The term biofertilizer is not appropriate as it does not replace fertilizer but improves their utilization. We very strongly suggest the implementation by regulatory authorities of quality control on commercial Azospirillum inoculants.

670 citations

References
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Journal ArticleDOI
TL;DR: Experiments with pure plant hormones showed that gibberellin causes increased production of lateral roots, andIndole acetic acid and indole lactic acid were produced by A. brasilense from tryptophan, and combinations of these substances produced changes in root morphology of pearl millet similar to those produced by inoculated plants.
Abstract: Azospirillum brasilense, a nitrogen-fixing bacterium found in the rhizosphere of various grass species, was investigated to establish the effect on plant growth of growth substances produced by the bacteria. Thin-layer chromatography, high-pressure liquid chromatography, and bioassay were used to separate and identify plant growth substances produced by the bacteria in liquid culture. Indole acetic acid and indole lactic acid were produced by A. brasilense from tryptophan. Indole acetic acid production increased with increasing tryptophan concentration from 1 to 100 μg/ml. Indole acetic acid concentration also increased with the age of the culture until bacteria reached the stationary phase. Shaking favored the production of indole acetic acid, especially in a medium containing nitrogen. A small but biologically significant amount of gibberellin was detected in the culture medium. Also at least three cytokinin-like substances, equivalent to about 0.001 μg of kinetin per ml, were present. The morphology of pearl millet roots changed when plants in solution culture were inoculated. The number of lateral roots was increased, and all lateral roots were densely covered with root hairs. Experiments with pure plant hormones showed that gibberellin causes increased production of lateral roots. Cytokinin stimulated root hair formation, but reduced lateral root production and elongation of the main root. Combinations of indole acetic acid, gibberellin, and kinetin produced changes in root morphology of pearl millet similar to those produced by inoculation with A. brasilense. Images

971 citations

Journal ArticleDOI

482 citations

Journal ArticleDOI
TL;DR: Methods are described for growing Spirillum lipoferum in quantities sufficient to serve as inoculant in field trials of its associative N(2)-fixing ability with higher plants and as a source of cells for the preparation of nitrogenase, cytochromes, respiratory enzymes, etc.
Abstract: Methods are described for growing Spirillum lipoferum in quantities sufficient to serve as inoculant in field trials of its associative N(2)-fixing ability with higher plants and as a source of cells for the preparation of nitrogenase, cytochromes, respiratory enzymes, etc A heavy inoculum of S lipoferum grown on NH(4) was transferred to a medium of minimal nitrogen content, and initial rapid growth at the expense of residual combined nitrogen was replaced later by slower growth on N(2) Conversion to N(2) fixation was prompt upon exhaustion of fixed nitrogen; growth on N(2) was most rapid at a pO(2) of 0005 to 0007 atm Numbers of S lipoferum can be estimated by diluting soil, crushed roots, or other material, and inoculating diluted samples into a stagnant semisolid medium Development of a characteristic subsurface layer of organisms and demonstration the these organisms can reduce C(2)H(2) are presumptive evidence that they are S lipoferum With most-probable-number tables the observations can be converted to numbers of S lipoferum in the samples The most-probable-number method indicated that numbers of S lipoferum may increase 100-fold or more in roots of maize removed from the plant and incubated for 24 h at 30 degrees C at a pO(2) initially adjusted to 001 atm

391 citations

Journal ArticleDOI
TL;DR: Corn and sorghum plants grown to maturity on limiting nutrients in the greenhouse showed improved growth from inoculation approaching that of plants grown on normal nutrient concentrations.
Abstract: Inoculation of corn (Zea mays) seeds with Azospirillum brasilense strain Cd or Sp 7 significantly enhanced (30 to 50% over controls) the uptake of NO3−, K+, and H2PO4− into 3- to 4-day- and 2-week-old root segments. No gross changes in root morphology were observed; altered cell arrangement in the outer four or five layers of the cortex was seen in photomicrographs of cross sections of inoculated corn roots. The surface activity involved in ion uptake probably increased, as shown by the darker staining by methylene blue of the affected area. Shoot dry weight increased 20 to 30% in inoculated plants after 3 weeks, presumably by enhancement of mineral uptake. Corn and sorghum plants grown to maturity on limiting nutrients in the greenhouse showed improved growth from inoculation approaching that of plants grown on normal nutrient concentrations. Enhanced ion uptake may be a significant factor in the crop yield enhancement reported for Azospirillum inoculation.

278 citations

Journal ArticleDOI
TL;DR: 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.
Abstract: 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.

262 citations