Soil Chemical Analysis

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

All plants are inhabited internally by diverse microbial communities comprising bacterial, archaeal, fungal, and protistic taxa. These microorganisms showing endophytic lifestyles play crucial roles in plant development, growth, fitness, and diversification. The increasing awareness of and information on endophytes provide insight into the complexity of the plant microbiome. The nature of plant-endophyte interactions ranges from mutualism to pathogenicity. This depends on a set of abiotic and biotic factors, including the genotypes of plants and microbes, environmental conditions, and the dynamic network of interactions within the plant biome. In this review, we address the concept of endophytism, considering the latest insights into evolution, plant ecosystem functioning, and multipartite interactions.

The Hidden World within Plants: Ecological and Evolutionary Considerations for Defining Functioning of Microbial Endophytes

Soil salinity: A serious environmental issue and plant growth promoting bacteria as one of the tools for its alleviation.

A . K H A L I D , M . A R S H A D A N D Z . A . Z A H I R . 2004. Aims: Plant growth promoting rhizobacteria (PGPR) are commonly used as inoculants for improving the growth and yield of agricultural crops, however screening for the selection of effective PGPR strains is very critical. This study focuses on the screening of effective PGPR strains on the basis of their potential for in vitro auxin production and plant growth promoting activity under gnotobiotic conditions. Methods and Results: A large number of bacteria were isolated from the rhizosphere soil of wheat plants grown at different sites. Thirty isolates showing prolific growth on agar medium were selected and evaluated for their potential to produce auxins in vitro. Colorimetric analysis showed variable amount of auxins (ranging from 1AE 1t o 12AE 1m g l )1 ) produced by the rhizobacteria in vitro and amendment of the culture media with L-tryptophan (L-TRP), further stimulated auxin biosynthesis (ranging from 1AE 8t o 24AE 8m g l )1 ). HPLC analysis confirmed the presence of indole acetic acid (IAA) and indole acetamide (IAM) as the major auxins in the culture filtrates of these rhizobacteria. A series of laboratory experiments conducted on two cv. of wheat under gnotobiotic (axenic) conditions demonstrated increases in root elongation (up to 17AE3%), root dry weight (up to 13AE5%), shoot elongation (up to 37AE7%) and shoot dry weight (up to 36AE3%) of inoculated wheat seedlings. Linear positive correlation (r ¼ 0AE99) between in vitro auxin production and increase in growth parameters of inoculated seeds was found. Based upon auxin biosynthesis and growth-promoting activity, four isolates were selected and designated as plant growth-promoting rhizobacteria (PGPR). Auxin biosynthesis in sterilized vs nonsterilized soil inoculated with selected PGPR was also monitored that revealed superiority of the selected PGPR over indigenous microflora. Peatbased seed inoculation with selected PGPR isolates exhibited stimulatory effects on grain yields of tested wheat cv. in pot (up to 14AE7% increase over control) and field experiments (up to 27AE5% increase over control); however, the response varied with cv. and PGPR strains. Conclusions: It was concluded that the strain, which produced the highest amount of auxins in nonsterilized soil, also caused maximum increase in growth and yield of both the wheat cv. Significance and Impact of Study: This study suggested that potential for auxin biosynthesis by rhizobacteria could be used as a tool for the screening of effective PGPR strains.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2003.02161.x

Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat.

The role of mycorrhizae and plant growth promoting rhizobacteria (PGPR) in improving crop productivity under stressful environments.

Rhizobacteria that exert beneficial effects on plant growth and development are referred to as plant growth promoting rhizobacteria (PGPR) In recent years, the use of PGPR to promote plant growth has increased in various parts of the world PGPR can affect plant growth by production and release of secondary metabolites (plant growth regulators/phytohormones/biologically active substances), lessening or preventing deleterious effects of phytopathogenic organisms in the rhizosphere and/or facilitating the availability and uptake of certain nutrients from the root environment Selection of effective PGPR is the most critical aspect to have maximum benefits from this technology Researchers are using different approaches for screening rhizobacteria to select effective PGPR including promotion of root/shoot growth under gnotobiotic conditions, in vitro production of plant growth regulators/biologically active substances and assessing of ACC-deaminase activity of the rhizobacteria However, the combined use of two or more approaches for screening of rhizobacteria could be more useful to select effective PGPR These screening approaches and practical applications of PGPR in agriculture are the major focus of this review

Plant Growth Promoting Rhizobacteria: Applications and Perspectives In Agriculture

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils.

Aims: This study was conducted to test the hypothesis that the bacterial strains possessing 1-aminocyclopropane-1-carboxylic acid (ACC)-deaminase activity may also promote growth of inoculated plants and could increase nodulation in legumes upon co-inoculation with rhizobia.



Methods and Results:  Several rhizobacteria were isolated from maize rhizosphere through enrichment on ACC as a sole N source. Purified isolates were screened for growth promotion in maize under axenic conditions and for in vitro ACC-deaminase activity. A significant positive correlation was observed between in vitro ACC-deaminase activity of bacterial cells and root elongation. None of the isolates produced auxins. Bradyrhizobiumjaponicum produced less amount of auxins but did not carry ACC-deaminase activity. Results of pot experiment revealed that co-inoculation with Bradyrhizobium and plant growth promoting rhizobacteria (PGPR) isolates enhanced the nodulation in mung bean compared with inoculation with Bradyrhizobium alone.



Conclusions:  It is highly expected that inoculation with rhizobacteria containing ACC-deaminase hydrolysed endogenous ACC into ammonia and α-ketobutyrate instead of ethylene. Consequently, root and shoot growth as well as nodulation were promoted.



Significance and Impact of the Study:  The ACC-deaminase trait could be employed as an efficient tool to screen effective PGPR, which could be successfully used as biofertilizers to increase the growth of inoculated plants as well as nodulation in legumes.

Effect of plant growth promoting rhizobacteria containing ACC-deaminase on maize (Zea mays L.) growth under axenic conditions and on nodulation in mung bean (Vigna radiata L.).

Zahir Ahmad Zahir

Papers

Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat.

The role of mycorrhizae and plant growth promoting rhizobacteria (PGPR) in improving crop productivity under stressful environments.

Plant Growth Promoting Rhizobacteria: Applications and Perspectives In Agriculture

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils.

Effect of plant growth promoting rhizobacteria containing ACC-deaminase on maize (Zea mays L.) growth under axenic conditions and on nodulation in mung bean (Vigna radiata L.).