ABSTRACT

Bacteria of the genus Azospirillum (alpha-proteobacteria) have been known for many years as plant growth-promoting rhizobacteria (PGPR) (Döbereiner and Day 1976). These free-living nitrogen-fi xing highly motile bacteria can be isolated from the rhizosphere of many grasses and cereals under tropical and temperate climates (Döbereiner et al. 1976, Patriquin et al. 1983). Azospirilla are predominantly surface-colonizing bacteria, whereas other diazotrophs such as Azotobacter diazotrophicus, Herbaspirillum seropedicae and Azoarcus sp. are endophytic; only a few Azospirillum strains are able to penetrate roots (Baldani et al. 1986, Assmus et al. 1995), probably via localized enzymatic degradation of plant cell wall or via root cracks.