Molecular aspects of the polysaccharides production in Azospirillum brasilense and its role in the establishment of the Azospirillum-plant association

JOFRÉ, E.; PRINCIPE, A..; CASTRO, M.; FISCHER, S.; LAGARES, A; MORI, G

Azospirillum sp: cell physiology, plant response, agronomic and environmental research in Argentina

Asociación Argentina de Microbiología

Lugar: Buenos Aires; Año: 2008; p. 111 - 124

Bacteria of the genus Azospirillum are free-living nitrogen-fixing rhizobacteria that are found in close association with plant roots, where they exert beneficial effects on plant growth and yield of many crops of agronomic importance. Azospirilla are Gram-negative, plump, slightly curved, and straight rods of about 1.0 m in diameter and 2.0 to 3.0 m in length, which under some conditions can form spirals. Upon inoculation, bacteria attach to plant roots, proliferate, and subsequently invade and colonize the internal tissues of the roots (Okon, Vanderleyden 1997). As result of this association, several studies have shown a positive effect of the bacteria on the plant growth. These effects are mainly derived from morphological and physiological changes of the inoculated plant roots, which lead to an enhancement of water and mineral uptake. Secretion of plant growth promoting substances such as auxins, gibberellins, and cytokinins by the bacteria seems to be at least partially responsible for these effects although others mechanisms such as the microbial biological nitrogen fixation, and nitrate reduction have been reported (Fallik et al. 1994; Kapulnik et al. 1985; Boddey, Döbereiner 1994). However, this last contribution was shown to be of little significance in these systems. Field trials, carried out at different locations, have demonstrated that under certain environmental and soil conditions, inoculation with Azospirillum has beneficial effects on plant yields (Döbereiner, Pedrosa 1987; Okon, Labandera-Gonzalez 1994). Even though much evidence has been accumulated during many years indicating the involvement of extracellular polysaccharides and proteins in both cell aggregation and root attachment processes, the precise mechanisms of these phenomena remain unexplained. The difficulty in elucidating these processes derives from their complexity, because it seems that they are mediated by various cell-surface components.