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

The plant-growth-promoting rhizobacteria (PGPR), Bacillus pumilus and Bacillus licheniformis, isolated from the rhizosphere of alder (Alnus glutinosa [L.] Gaertn.) have a strong growth-promoting activity. Bioassay data showed that the dwarf phenotype induced in alder seedlings by paclobutrazol (an inhibitor of gibberellin [GA] biosynthesis) was effectively reversed by applications of extracts from media incubated with both bacteria and also by exogenous GA 3 , Full-scan gas chromatography-mass spectrometry analyses on extracts of these media showed the presence of GA 1 , GA 3 , GA 4 and GA 20 , in addition to the isomers 3-epi-GA 1 and iso-GA 3 . Isotope dilution analysis indicated that epi-GA 1 was an artefact. Likewise, iso-GA 3 is also probably an artifact spontaneously formed during extraction and/or analysis. In both culture media, GA 1 was present in higher concentrations (130-150 ng ml -1 ) than GA 3 (50-60 ng ml -1 ), GA 4 (8-12 ng ml -1 ) and GAS (2-3 ng ml -1 ). The data indicated that culture of both bacteria accumulate bioactive C 19 -gibberellins in relative high amounts and that these GAs appear to be physiologically active in the host plant. The evidence suggests that the promotion of stem elongation induced by the PGPR could be mediated by bacterial GAs.

The plant‐growth‐promoting rhizobacteria Bacillus pumilus and Bacillus licheniformis produce high amounts of physiologically active gibberellins

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

This review focuses on studies with bacteria for which biosynthesis/production of the plant hormones gibberellins have been demonstrated. Actual data on gibberellin metabolism by bacteria are analyzed in comparison with the biosynthetic pathways known for vascular plants and fungi. The potential involvement of gibberellins produced by symbiotic and soil-endophytic microorganisms in plant growth promotion and yield increase is also discussed.

Gibberellin production by bacteria and its involvement in plant growth promotion and yield increase.

Population growth and increase in food requirement is the global problem. It is inevitable to introduce new practices that help to increase agricultural productivity. Use of plant growth promoting rhizobacteria (PGPR) has shown potentials to be a promising technique in the practice of sustainable agriculture. A group of natural soil microbial flora acquire dwelling in the rhizosphere and on the surface of the plant roots which impose beneficial effect on the overall well-being of the plant are categorized as PGPR. Researchers are actively involved in understanding plant growth promoting mechanics employed by PGPR. Broadly, these are divided into direct and indirect mechanics. Any mechanism that directly enhances plant growth either by providing nutrients or by producing growth regulators are portrayed as direct mechanics. Whereas, any mechanisms that protects plant from acquiring infections (biotic stress) or helps plant to grow healthily under environmental stresses (abiotic stress) are considere...

Portraying mechanics of plant growth promoting rhizobacteria (PGPR): A review

Similar ranges of gibberellins (GAs) were detected by high-performance liquid chromatography (HPLC)-immunoassay procedures in ten cultures of wild-type and mutant strains of Rhizobium phaseoli. The major GAs excreted into the culture medium were GA1 and GA4. These identifications were confirmed by combined gas chromatographymass spectrometry. The HPLC-immunoassays also detected smaller amounts of GA9- as well as GA20-like compounds, the latter being present in some but not all cultures. In addition to GAs, all strains excreted indole-3-acetic acid (IAA) but there was no obvious relationship between the amounts of GA and IAA that accumulated. The Rhizobium strains studied included nod
− and fix
− mutants, making it unlikely that the IAA- and GA-biosynthesis genes are closely linked to the genes for nodulation and nitrogen fixation. The HPLC-immunoassay analyses showed also that nodules and non-nodulated roots of Phaseolus vulgaris L. contained similar spectra of GAs to R. phaseoli culture media. The GA pools in roots and nodules were of similar size, indicating that Rhizobium does not make a major contribution to the GA content of the infected tissue.

R. Atzorn

Papers

Production of gibberellins and indole-3-acetic acid by Rhizobium phaseoli in relation to nodulation of Phaseolus vulgaris roots.