Association of Azospirillum with Grass Roots

Abstract

The association between grass roots and Azospirillum brasilense Sp 7 was investigated by the Fahraeus slide technique, using nitrogen-free medium. Young inoculated roots of pearl millet and guinea grass produced more mucilaginous sheath (mucigel), root hairs, and lateral roots than did uninoculated sterile controls. The bacteria were found within the mucigel that accumulated on the root cap and along the root axes. Adherent bacteria were associated with granular material on root hairs and fibrillar material on undifferentiated epidermal cells. Significantly fewer numbers of azospirilla attached to millet root hairs when the roots were grown in culture medium supplemented with 5 mM potassium nitrate. Under these growth conditions, bacterial attachment to undifferentiated epidermal cells was unaffected. Aseptically collected root exudate from pearl millet contained substances which bound to azospirilla and promoted their adsorption to the root hairs. This activity was associated with nondialyzable and proteasesensitive substances in root exudate. Millet root hairs adsorbed azospirilla in significantly higher numbers than cells of Rhizobium, Pseudomonas, Azotobacter, Klebsiella, or Escherichia. Pectolytic activities, including pectin transeliminase and endopolygalacturonase, were detected in pure cultures of A. brasilense when this species was grown in a medium containing pectin. These studies describe colonization of grass root surfaces by A. brasilense and provide a possible explanation for the limited colonization of intercellular spaces of the outer root cortex.

Endophytic colonization ofTypha australisby a plant growth-promoting bacteriumKlebsiella oxytocastrain GR-3

“…Many endophytic diazotrophic bacteria including, Gluconacetobacter, Alcaligenes, Azoarcus, Azospirillum, Enterobacter, Pseudomonas, Xanthomonas, Bradyrhizobium and the like are known to produce IAA, which enhances root development and improves uptake of minerals and water (Umali-Garacia et al 1980). Additionally, these bacteria are known to influence root morphology with resultant increases in biomass thereby enabling the plant to exploit greater soil volume and resultant nutrients.…”

Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects

“…Recently, genes encoding a pectate lyase (pelA) and two aryl L-glucosidases (salA and salB) have been isolated in A. irakense by heterologous expression in E. coli [48,49]. Alternatively, Azospirillum species may enter the root through lysed root hairs and cracks, disrupted cortical tissues at lateral root junctions [32]. In conclusion, when analyzed for particular properties, such as mode of plant root colonization, strain-spe-ci¢c di¡erences for A. brasilense can be observed.…”

Endophytic Herbaspirillum seropedicae expresses nif genes in gramineous plants

“…Entry of Azospirillum spp. and P. solanacearum near lateral roots has also been observed [38,44], suggesting that taxonomically unrelated bacteria might share similar mechanisms of entry into both monocotyledonous and dicotyledonous plants [22].…”