Systematyka, genetyka i biologia bakterii z rodzaju azospirillum

Abstract

Bacteria of the genus Azospirillum (α-subclass of Proteobacteria) have been known for many years as plant growth promoting rhizobacteria (PGPR). Azospirillum species are plant-associated diazotrophs, Gram-negative, nitrogen fixing rhizosphere bacteria. Currently, 19 species of Azospirillum are known. They were isolated from the rhizosphere of many grasses and cereals all over the world, in tropical as well as in temperate climates. They occur in association with roots, stems and leaves of a large variety of gramineous and non-gramineous plants, e.g. numerous wild and cultivated grasses and cereals, legumes, vegetables and fruit trees. Bacteria of the genus Azospirillum were described in the mid 1970s as heterotrophic N2-fixing bacteria. They display a versatile C- and N metabolism, which makes them well adapted to establish in the competitive environment of the rhizosphere. Ammonium, nitrate, nitrite, amino acids and molecular nitrogen can serve as N-sources. In unfavorable conditions, such as desiccation and nutrient limitation, azospirilla can convert into enlarged cyst-like forms. This morphological change is accompanied by the development of an outer coat of polysaccharides and by the accumulation of abundant poly-L-hydroxybutyrate granules, which can serve as C- and energy sources under conditions of stress and starvation. Bacteria belonging to the genus Azospirillum are highly motile. A. brasilense, A. lipoferum and A. irakense display a mixed pattern of flagellation. The core genome of Azospirillum was established and consists of 2,328 protein-coding genes, representing between 30% to 38% of the total encoded proteins within a genome. Azospirillum strains possessed several megareplicons, some probably linear, and 16S ribosomal DNA hybridization indicated multiple chromosomes in genomes ranging in size from 4.8 to 9.7 Mbp.