The associative symbiosis between Plant Growth Promoting Rhizobacteria of the genus Azospirillum and cereals have mainly been studied from an agronomic and economic point of view, and several studies showed that plant morphological and metabolic changes depend on both bacterial and plant genotypes. However, if the specificity in the Rhizobium-legume symbiosis has been well characterized, the question of whether specificity occurs in the Azospirillum-plant associative symbiosis remains controversial. In this context, the overall gene expression of A. lipoferum 4B during its interaction with roots of two rice varieties (cv. Cigalon, cv. Nipponbare) was analyzed in order to characterize (i) genes differentially regulated in response to plant regardless of the variety and (ii) genes displaying a varietydependent regulation. Results of the transcriptomic analysis show that presence of the host plant triggers stress response systems, a large number of putative transcriptional regulators, signal transduction pathways, and many proteins of unknown function. This indicates a reprogramming of bacterial gene expression, due to adaptation to host plant. Genes specifically expressed during the interaction with one of the two varieties could be identified, suggesting the existence of specificity in the associative symbiosis between Azospirillum and cereals. (Texte intégral)