Motility and attachment to the plant root
Research carried out by Stijn Spaepen coordinated by Jos Vanderleyden
In order to establish a successful association with plants, Azospirillum bacteria must first reach the roots and than attach to the roots. Attachment of A. brasilense to roots is a two-step process comprising a fast (within 2 h of incubation), weak and reversible adsorption phase, followed by a stronger and irreversible anchoring phase. We identified different genetic loci involved in motility and adsorption to wheat roots on the Azospirillum brasilense Sp7 90-MDa plasmid (Croes et al., 1991).
Two types of flagella are involved in the motility of A. brasilense : one polar flagellum is present under all growth conditions, while synthesis of lateral flagella is induced when cells are grown on solid media. The glycosylated polar flagellum is used for locomotion in liquid media (swimming) and is also involved in the adsorption to roots. The lateral flagella are involved in swarming, which can be described as active surface spreading. We isolated the gene encoding the lateral flagellin, laf1, and construct a knocked-out mutant (Moens et al., 1995) . We found that expression of laf1 is induced on solid medium or when rotation of the polar flagellum is hindered (Moens et al., 1996).
To visualize colonization of roots by Azospirillum, we developed cells expressing the beta-glucuronidase enzyme and fluorescently labeled Azospirillum cells (Vande Broek et al., 1993; Zhu et al., 2002).
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References:
Croes C, Van Bastelaere E, DeClercq E, Eyers M, Vanderleyden J, Michiels K (1991) Identification and mapping of loci involved in motility, adsorption to wheat roots, colony morphology, and growth in minimal medium on the Azospirillum brasilense Sp7 90-MDa plasmid. Plasmid 26: 83-93. PubMed
Moens S, Michiels K, Keijers V, Van Leuven F, Vanderleyden J (1995) Cloning, sequencing, and phenotypic analysis of laf1, encoding the flagellin of the lateral flagella of Azospirillum brasilense Sp7. J Bacteriol 177: 5419-5426. PubMed
Moens S, Schloter M, Vanderleyden J (1996) Expression of the structural gene, laf1, encoding the flagellin of the lateral flagella in Azospirillum brasilense Sp7. J Bacteriol 178: 5017-5019. PubMed
Vande Broek A, Michiels J, Van Gool A, Vanderleyden J (1993) Spatial-temporal colonization patterns of Azospirillum brasilense on the wheat root surface and expression of the bacterial nifH gene during the association. MPMI6: 592-600. APSnet
Zhu G, Dobbelaere S, Vanderleyden J (2002) Use of green fluorescent protein to visualize rice root colonization by Azospirillum irakense and Azospirillum brasilense. Funct Plant Biol 29: 1279-1285. CSIRIO Publishing
