Search

Bacteria of the genus Azospirillum are diazotrophs that colonize the roots of plants. Colonization patterns can be visualized by using strains equipped with a reporter gene (Vande Broek et al., 1993; Arsene et al., 1994). The initial steps of bacterial colonization are chemotaxis and adhesion to the root surface. In order to characterize the bacterial genes and signals that determine these processes genetic and biochemical approaches were used.

The surface of bacteria consists of polysaccharides, like the exopolysaccharides and lipopolysaccharides and, as it is the case for Azospirillum brasilense, glycoproteins like glycosylated flagella. The precursors for the synthesis of polysaccharides are sugarnucleotides. Several genes, coding for enzymes involved in the synthesis and in the modification of sugarnucleotides, have been characterised is Azospirillum brasilense Sp7.

Azospirillum is probably the best studied example of beneficial plant rhizosphere bacteria. Studies in our laboratory focus on the identification of bacterial genes and gene products that are of importance in the physical and metabolic interaction of Azospirillum brasilense with plant roots. Here we report for Azospirillum brasilense, flagellation, motility, the physical interaction with plant roots, the synthesis of indole-3-acetic acid, the expression of nif genes in plant-root associated bacteria, and the induction of gene expression with plant root exudates.

Breakdown of plant polysaccharides by root colonising bacteria can provide them with new carbon sources. Within the genus Azospirillum, breakdown of pectin (and polygalacturonic acid) is best documented for A. irakense. From a gene library of A. irakense KBC1 we have isolated several cosmid clones that express polygalacturonase and pectate lyase activity in E. coli and A. brasilense. Genes encoding these enzymes appear to be closely linked in the A. irakense genome.

Bacteria of the genus Azospirillum are diazotrophs associated with the root zone of grasses (for review see Dobereiner and Pedrosa 1987). The molecular genetic analysis of Azospirillum plant interaction is hampered by the lack of plant root differentiation at the site of interaction. The identification of bacterial genes possibly involved in Azospirillum rhizocoenoses has therefore been approached either by molecular hybridization with cloned genes from other rhizosphere bacteria (Fogher et al 1985; Waelkens et al 1987) or by genetic complementation of characterized mutants of Rhizobium meliloti with gene libraries of A. brasilense (Michiels et al 1988). By genetic complementation we initially isolated two A. brasilense Sp7 DNA fragments that correct R. meliloti exoB and exoC mutants respectively. These fragments are located on the p90 plasmid DNA of A. brasilense Sp7 (Michiels et al 1989b). In this paper we report a more extensive inventory of A. brasilense loci that are involved in exopolysaccharide synthesis. Furthermore we describe some experiments to study the nature and regulation of these genes, and finally we focus on the genetic map of the p90 plasmid of A. brasilense strain Sp7.

Bacteria of the genus Azospirillum (Tarrand et al 1978) are. free-living diazotrophs that can readily be isolated from the rhizosphere and roots of grasses. Little is known about the genetics of these bacteria or the molecular biology of their association with plants (for reviews, see Elmerich, 1984; Okon 1985).