Phenotypic variation in Azospirillum brasilense Sp7 does not influence plant growth promotion effects

• Published in: Soil biology & biochemistry 2013-12, Vol.67, p.255-262
• Main Authors: Volfson, Victoria, Fibach-Paldi, Sharon, Paulucci, Natalia Soledad, Dardanelli, Marta Susana, Matan, Ofra, Burdman, Saul, Okon, Yaacov
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Arachis hypogaea; Azospirillum; Azospirillum brasilense; Biochemistry and biology; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; Exopolysaccharides; Fundamental and applied biological sciences. Psychology; Genomic changes; Phase variation; Phenotypic variation; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Plant growth promotion; Soil science; Triticum aestivum; Zea mays
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2013.09.008

The Azospirillum genus comprises free-living, plant growth-promoting, nitrogen-fixing bacteria found in the rhizosphere of plant roots. Azospirilla are able to promote plant growth mainly through improvement of root development. Bacterial surface components, such as extracellular polysaccharides and proteins, are involved in root colonization. Phase variation – or phenotypic variation – is one of the mechanisms by which microorganisms adapt to environmental changes. This phenomenon is characterized by the presence of a sub-population of the bacteria presenting a different phenotype relative to the major population. In this study we characterized phenotypic variation of Azospirillum brasilense Sp7. When plated on solid media, some A. brasilense colonies were shown to possess a much more mucoid morphology, producing 7.5–8 times more exopolysaccharide with different monosaccharide composition than the parental strain Sp7. The rate of appearance of this kind of variant colonies was 1 in 5000, in agreement with the accepted rate for the phase/phenotypic variation phenomenon. The variants were significantly more resistant to heat and UV-exposure than the parental strain and displayed genomic changes as seen by the different band patterns following ERIC-PCR, BOX-PCR and RAPD analyses. In plant inoculation experiments under greenhouse conditions, with maize, wheat, soybean and peanuts, the EPS overproducing variants performed as similar as the parental strain. Therefore, EPS overproduction did not confer an apparent advantage to A. brasilense in terms of induction of plant growth promotion. •We characterized phenotypic variation of the bacterium Azospirillum brasilense Sp7.•Some variant colonies produced 7.5–8 times more EPS than the parental strain Sp7.•The variants were more resistant to heat and UV-exposure than the parental strain.•The variants had genomic changes as assessed by repetitive-PCR and RAPD analyses.•The variants had similar plant growth promotion effects than the parental strain.