POPULATION-DYNAMICS OF A MOTILE AND A NONMOTILE AZOSPIRILLUM-LIPOFERUM STRAIN DURING RICE ROOT COLONIZATION AND MOTILITY VARIATION IN THE RHIZOSPHERE

Azospirillum lipoferum 4B and non-motile A. lipoferum 4T have been sim ultaneously isolated from rice rhizosphere at the same frequency. A. l ipoferum 4T showed stable morphological and metabolic traits which are atypical for A. lipoferum species such as lack of motility, carbohydr ate metabolism and laccase activity. Inoculation experiments showed th at A. lipoferum 4T, but not A. lipoferum 4B, needed rice roots to stab ilize in sterile soil. Both strains were able to colonize efficiently rice roots (10(8) cfu g(-1) flesh roots) but motile form 4B remained d ominant. In spite of their phenotypical differences, A. lipoferum 4B a nd 4T co-existed without exclusion in sterile soil (planted or not) an d rice rhizosphere. Inoculation of rice roots with A. lipoferum 4B sho wed that rice rhizosphere enhanced the frequency of appearance of stab le non-motile forms (40%). This percentage was weaker in plantlet grow th medium (4%). However, these non-motile bacteria kept the same bioch emical traits than the motile parental strain 4B (carbohydrates metabo lism, laccase activity).