The expression of flcA, a gene controlling 'Azospirillum'-plant interactions

Abstract

'Azospirillum brasilense' strains are known to be highly pleomorphic and to change their metabolic activities swiftly in response to changes in environmental conditions. Under low oxygen tension bacteria of the genus 'Azospirillum' are highly motile, half-curved or vibrioid, Gram-negative rods with a long polar flagellum in liquid medium and additional peritrichous flagella on solid medium. Under aerobic conditions, particularly in aged cultures, vegetative cells undergo a transition to round, non-motile, encapsulated forms that are considered to be cysts. Heavy capsulation gives the cells a particular adhesive nature, so that they aggregate in a matrix of polysaccharide material, forming large macroscopic clumps that flocculate in liquid cultures. Fructose induces flocculation in 'Azospirillum brasilense' to a greater extent than other carbon sources. ... Mutants of 'Azospirillum brasilense' unable to undergo transition from vegetative vibrioids into encapsulated forms are unable to colonize the surface of wheat roots, whereas the wild-type strain of 'A. brasilense' Sp7 forms a sheath of bacteria on the root surface. The attachment of 'Azospirillum' to wheat roots is mainly dependent on two factors: the existence of a polar flagellum, allowing the bacteria to adsorb to the roots, and the production of exopolysaccharides (BPS), allowing the bacteria to firmly attach to the root surface. The regulatory gene flcA was reported earlier by the author to have strong effects on the colonisation efficiency of wheat roots by 'Azospirillum', and to control the differentiation progress of these bacteria from vegetative state to cyst-like forms, both in cultures and on plants. Studying the regulation of expression of this gene will provide essential information on the conditions triggering these phenotypes in 'Azospirillum' (1, 2 for earlier work by the author).